Merge branch 'ah/doc-wserrorhighlight' into maint
[gitweb.git] / refs / files-backend.c
index ea78ce9d90f0bb02fcd471c1c113e22053a3ba94..0404f2c2333c0fadeb57d225deedc6f0d4afadad 100644 (file)
 #include "../cache.h"
+#include "../config.h"
 #include "../refs.h"
 #include "refs-internal.h"
+#include "ref-cache.h"
+#include "../iterator.h"
+#include "../dir-iterator.h"
 #include "../lockfile.h"
 #include "../object.h"
 #include "../dir.h"
 
 struct ref_lock {
        char *ref_name;
-       char *orig_ref_name;
        struct lock_file *lk;
        struct object_id old_oid;
 };
 
-struct ref_entry;
-
-/*
- * Information used (along with the information in ref_entry) to
- * describe a single cached reference.  This data structure only
- * occurs embedded in a union in struct ref_entry, and only when
- * (ref_entry->flag & REF_DIR) is zero.
- */
-struct ref_value {
-       /*
-        * The name of the object to which this reference resolves
-        * (which may be a tag object).  If REF_ISBROKEN, this is
-        * null.  If REF_ISSYMREF, then this is the name of the object
-        * referred to by the last reference in the symlink chain.
-        */
-       struct object_id oid;
-
-       /*
-        * If REF_KNOWS_PEELED, then this field holds the peeled value
-        * of this reference, or null if the reference is known not to
-        * be peelable.  See the documentation for peel_ref() for an
-        * exact definition of "peelable".
-        */
-       struct object_id peeled;
-};
-
-struct ref_cache;
-
-/*
- * Information used (along with the information in ref_entry) to
- * describe a level in the hierarchy of references.  This data
- * structure only occurs embedded in a union in struct ref_entry, and
- * only when (ref_entry.flag & REF_DIR) is set.  In that case,
- * (ref_entry.flag & REF_INCOMPLETE) determines whether the references
- * in the directory have already been read:
- *
- *     (ref_entry.flag & REF_INCOMPLETE) unset -- a directory of loose
- *         or packed references, already read.
- *
- *     (ref_entry.flag & REF_INCOMPLETE) set -- a directory of loose
- *         references that hasn't been read yet (nor has any of its
- *         subdirectories).
- *
- * Entries within a directory are stored within a growable array of
- * pointers to ref_entries (entries, nr, alloc).  Entries 0 <= i <
- * sorted are sorted by their component name in strcmp() order and the
- * remaining entries are unsorted.
- *
- * Loose references are read lazily, one directory at a time.  When a
- * directory of loose references is read, then all of the references
- * in that directory are stored, and REF_INCOMPLETE stubs are created
- * for any subdirectories, but the subdirectories themselves are not
- * read.  The reading is triggered by get_ref_dir().
- */
-struct ref_dir {
-       int nr, alloc;
-
-       /*
-        * Entries with index 0 <= i < sorted are sorted by name.  New
-        * entries are appended to the list unsorted, and are sorted
-        * only when required; thus we avoid the need to sort the list
-        * after the addition of every reference.
-        */
-       int sorted;
-
-       /* A pointer to the ref_cache that contains this ref_dir. */
-       struct ref_cache *ref_cache;
-
-       struct ref_entry **entries;
-};
-
-/*
- * Bit values for ref_entry::flag.  REF_ISSYMREF=0x01,
- * REF_ISPACKED=0x02, REF_ISBROKEN=0x04 and REF_BAD_NAME=0x08 are
- * public values; see refs.h.
- */
-
-/*
- * The field ref_entry->u.value.peeled of this value entry contains
- * the correct peeled value for the reference, which might be
- * null_sha1 if the reference is not a tag or if it is broken.
- */
-#define REF_KNOWS_PEELED 0x10
-
-/* ref_entry represents a directory of references */
-#define REF_DIR 0x20
-
-/*
- * Entry has not yet been read from disk (used only for REF_DIR
- * entries representing loose references)
- */
-#define REF_INCOMPLETE 0x40
-
-/*
- * A ref_entry represents either a reference or a "subdirectory" of
- * references.
- *
- * Each directory in the reference namespace is represented by a
- * ref_entry with (flags & REF_DIR) set and containing a subdir member
- * that holds the entries in that directory that have been read so
- * far.  If (flags & REF_INCOMPLETE) is set, then the directory and
- * its subdirectories haven't been read yet.  REF_INCOMPLETE is only
- * used for loose reference directories.
- *
- * References are represented by a ref_entry with (flags & REF_DIR)
- * unset and a value member that describes the reference's value.  The
- * flag member is at the ref_entry level, but it is also needed to
- * interpret the contents of the value field (in other words, a
- * ref_value object is not very much use without the enclosing
- * ref_entry).
- *
- * Reference names cannot end with slash and directories' names are
- * always stored with a trailing slash (except for the top-level
- * directory, which is always denoted by "").  This has two nice
- * consequences: (1) when the entries in each subdir are sorted
- * lexicographically by name (as they usually are), the references in
- * a whole tree can be generated in lexicographic order by traversing
- * the tree in left-to-right, depth-first order; (2) the names of
- * references and subdirectories cannot conflict, and therefore the
- * presence of an empty subdirectory does not block the creation of a
- * similarly-named reference.  (The fact that reference names with the
- * same leading components can conflict *with each other* is a
- * separate issue that is regulated by verify_refname_available().)
- *
- * Please note that the name field contains the fully-qualified
- * reference (or subdirectory) name.  Space could be saved by only
- * storing the relative names.  But that would require the full names
- * to be generated on the fly when iterating in do_for_each_ref(), and
- * would break callback functions, who have always been able to assume
- * that the name strings that they are passed will not be freed during
- * the iteration.
- */
-struct ref_entry {
-       unsigned char flag; /* ISSYMREF? ISPACKED? */
-       union {
-               struct ref_value value; /* if not (flags&REF_DIR) */
-               struct ref_dir subdir; /* if (flags&REF_DIR) */
-       } u;
-       /*
-        * The full name of the reference (e.g., "refs/heads/master")
-        * or the full name of the directory with a trailing slash
-        * (e.g., "refs/heads/"):
-        */
-       char name[FLEX_ARRAY];
-};
-
-static void read_loose_refs(const char *dirname, struct ref_dir *dir);
-static int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len);
-static struct ref_entry *create_dir_entry(struct ref_cache *ref_cache,
-                                         const char *dirname, size_t len,
-                                         int incomplete);
-static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry);
-
-static struct ref_dir *get_ref_dir(struct ref_entry *entry)
-{
-       struct ref_dir *dir;
-       assert(entry->flag & REF_DIR);
-       dir = &entry->u.subdir;
-       if (entry->flag & REF_INCOMPLETE) {
-               read_loose_refs(entry->name, dir);
-
-               /*
-                * Manually add refs/bisect, which, being
-                * per-worktree, might not appear in the directory
-                * listing for refs/ in the main repo.
-                */
-               if (!strcmp(entry->name, "refs/")) {
-                       int pos = search_ref_dir(dir, "refs/bisect/", 12);
-                       if (pos < 0) {
-                               struct ref_entry *child_entry;
-                               child_entry = create_dir_entry(dir->ref_cache,
-                                                              "refs/bisect/",
-                                                              12, 1);
-                               add_entry_to_dir(dir, child_entry);
-                               read_loose_refs("refs/bisect",
-                                               &child_entry->u.subdir);
-                       }
-               }
-               entry->flag &= ~REF_INCOMPLETE;
-       }
-       return dir;
-}
-
-static struct ref_entry *create_ref_entry(const char *refname,
-                                         const unsigned char *sha1, int flag,
-                                         int check_name)
-{
-       struct ref_entry *ref;
-
-       if (check_name &&
-           check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
-               die("Reference has invalid format: '%s'", refname);
-       FLEX_ALLOC_STR(ref, name, refname);
-       hashcpy(ref->u.value.oid.hash, sha1);
-       oidclr(&ref->u.value.peeled);
-       ref->flag = flag;
-       return ref;
-}
-
-static void clear_ref_dir(struct ref_dir *dir);
-
-static void free_ref_entry(struct ref_entry *entry)
-{
-       if (entry->flag & REF_DIR) {
-               /*
-                * Do not use get_ref_dir() here, as that might
-                * trigger the reading of loose refs.
-                */
-               clear_ref_dir(&entry->u.subdir);
-       }
-       free(entry);
-}
-
-/*
- * Add a ref_entry to the end of dir (unsorted).  Entry is always
- * stored directly in dir; no recursion into subdirectories is
- * done.
- */
-static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry)
-{
-       ALLOC_GROW(dir->entries, dir->nr + 1, dir->alloc);
-       dir->entries[dir->nr++] = entry;
-       /* optimize for the case that entries are added in order */
-       if (dir->nr == 1 ||
-           (dir->nr == dir->sorted + 1 &&
-            strcmp(dir->entries[dir->nr - 2]->name,
-                   dir->entries[dir->nr - 1]->name) < 0))
-               dir->sorted = dir->nr;
-}
-
 /*
- * Clear and free all entries in dir, recursively.
+ * Return true if refname, which has the specified oid and flags, can
+ * be resolved to an object in the database. If the referred-to object
+ * does not exist, emit a warning and return false.
  */
-static void clear_ref_dir(struct ref_dir *dir)
+static int ref_resolves_to_object(const char *refname,
+                                 const struct object_id *oid,
+                                 unsigned int flags)
 {
-       int i;
-       for (i = 0; i < dir->nr; i++)
-               free_ref_entry(dir->entries[i]);
-       free(dir->entries);
-       dir->sorted = dir->nr = dir->alloc = 0;
-       dir->entries = NULL;
-}
-
-/*
- * Create a struct ref_entry object for the specified dirname.
- * dirname is the name of the directory with a trailing slash (e.g.,
- * "refs/heads/") or "" for the top-level directory.
- */
-static struct ref_entry *create_dir_entry(struct ref_cache *ref_cache,
-                                         const char *dirname, size_t len,
-                                         int incomplete)
-{
-       struct ref_entry *direntry;
-       FLEX_ALLOC_MEM(direntry, name, dirname, len);
-       direntry->u.subdir.ref_cache = ref_cache;
-       direntry->flag = REF_DIR | (incomplete ? REF_INCOMPLETE : 0);
-       return direntry;
-}
-
-static int ref_entry_cmp(const void *a, const void *b)
-{
-       struct ref_entry *one = *(struct ref_entry **)a;
-       struct ref_entry *two = *(struct ref_entry **)b;
-       return strcmp(one->name, two->name);
-}
-
-static void sort_ref_dir(struct ref_dir *dir);
-
-struct string_slice {
-       size_t len;
-       const char *str;
-};
-
-static int ref_entry_cmp_sslice(const void *key_, const void *ent_)
-{
-       const struct string_slice *key = key_;
-       const struct ref_entry *ent = *(const struct ref_entry * const *)ent_;
-       int cmp = strncmp(key->str, ent->name, key->len);
-       if (cmp)
-               return cmp;
-       return '\0' - (unsigned char)ent->name[key->len];
-}
-
-/*
- * Return the index of the entry with the given refname from the
- * ref_dir (non-recursively), sorting dir if necessary.  Return -1 if
- * no such entry is found.  dir must already be complete.
- */
-static int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len)
-{
-       struct ref_entry **r;
-       struct string_slice key;
-
-       if (refname == NULL || !dir->nr)
-               return -1;
-
-       sort_ref_dir(dir);
-       key.len = len;
-       key.str = refname;
-       r = bsearch(&key, dir->entries, dir->nr, sizeof(*dir->entries),
-                   ref_entry_cmp_sslice);
-
-       if (r == NULL)
-               return -1;
-
-       return r - dir->entries;
-}
-
-/*
- * Search for a directory entry directly within dir (without
- * recursing).  Sort dir if necessary.  subdirname must be a directory
- * name (i.e., end in '/').  If mkdir is set, then create the
- * directory if it is missing; otherwise, return NULL if the desired
- * directory cannot be found.  dir must already be complete.
- */
-static struct ref_dir *search_for_subdir(struct ref_dir *dir,
-                                        const char *subdirname, size_t len,
-                                        int mkdir)
-{
-       int entry_index = search_ref_dir(dir, subdirname, len);
-       struct ref_entry *entry;
-       if (entry_index == -1) {
-               if (!mkdir)
-                       return NULL;
-               /*
-                * Since dir is complete, the absence of a subdir
-                * means that the subdir really doesn't exist;
-                * therefore, create an empty record for it but mark
-                * the record complete.
-                */
-               entry = create_dir_entry(dir->ref_cache, subdirname, len, 0);
-               add_entry_to_dir(dir, entry);
-       } else {
-               entry = dir->entries[entry_index];
-       }
-       return get_ref_dir(entry);
-}
-
-/*
- * If refname is a reference name, find the ref_dir within the dir
- * tree that should hold refname.  If refname is a directory name
- * (i.e., ends in '/'), then return that ref_dir itself.  dir must
- * represent the top-level directory and must already be complete.
- * Sort ref_dirs and recurse into subdirectories as necessary.  If
- * mkdir is set, then create any missing directories; otherwise,
- * return NULL if the desired directory cannot be found.
- */
-static struct ref_dir *find_containing_dir(struct ref_dir *dir,
-                                          const char *refname, int mkdir)
-{
-       const char *slash;
-       for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
-               size_t dirnamelen = slash - refname + 1;
-               struct ref_dir *subdir;
-               subdir = search_for_subdir(dir, refname, dirnamelen, mkdir);
-               if (!subdir) {
-                       dir = NULL;
-                       break;
-               }
-               dir = subdir;
-       }
-
-       return dir;
-}
-
-/*
- * Find the value entry with the given name in dir, sorting ref_dirs
- * and recursing into subdirectories as necessary.  If the name is not
- * found or it corresponds to a directory entry, return NULL.
- */
-static struct ref_entry *find_ref(struct ref_dir *dir, const char *refname)
-{
-       int entry_index;
-       struct ref_entry *entry;
-       dir = find_containing_dir(dir, refname, 0);
-       if (!dir)
-               return NULL;
-       entry_index = search_ref_dir(dir, refname, strlen(refname));
-       if (entry_index == -1)
-               return NULL;
-       entry = dir->entries[entry_index];
-       return (entry->flag & REF_DIR) ? NULL : entry;
-}
-
-/*
- * Remove the entry with the given name from dir, recursing into
- * subdirectories as necessary.  If refname is the name of a directory
- * (i.e., ends with '/'), then remove the directory and its contents.
- * If the removal was successful, return the number of entries
- * remaining in the directory entry that contained the deleted entry.
- * If the name was not found, return -1.  Please note that this
- * function only deletes the entry from the cache; it does not delete
- * it from the filesystem or ensure that other cache entries (which
- * might be symbolic references to the removed entry) are updated.
- * Nor does it remove any containing dir entries that might be made
- * empty by the removal.  dir must represent the top-level directory
- * and must already be complete.
- */
-static int remove_entry(struct ref_dir *dir, const char *refname)
-{
-       int refname_len = strlen(refname);
-       int entry_index;
-       struct ref_entry *entry;
-       int is_dir = refname[refname_len - 1] == '/';
-       if (is_dir) {
-               /*
-                * refname represents a reference directory.  Remove
-                * the trailing slash; otherwise we will get the
-                * directory *representing* refname rather than the
-                * one *containing* it.
-                */
-               char *dirname = xmemdupz(refname, refname_len - 1);
-               dir = find_containing_dir(dir, dirname, 0);
-               free(dirname);
-       } else {
-               dir = find_containing_dir(dir, refname, 0);
-       }
-       if (!dir)
-               return -1;
-       entry_index = search_ref_dir(dir, refname, refname_len);
-       if (entry_index == -1)
-               return -1;
-       entry = dir->entries[entry_index];
-
-       memmove(&dir->entries[entry_index],
-               &dir->entries[entry_index + 1],
-               (dir->nr - entry_index - 1) * sizeof(*dir->entries)
-               );
-       dir->nr--;
-       if (dir->sorted > entry_index)
-               dir->sorted--;
-       free_ref_entry(entry);
-       return dir->nr;
-}
-
-/*
- * Add a ref_entry to the ref_dir (unsorted), recursing into
- * subdirectories as necessary.  dir must represent the top-level
- * directory.  Return 0 on success.
- */
-static int add_ref(struct ref_dir *dir, struct ref_entry *ref)
-{
-       dir = find_containing_dir(dir, ref->name, 1);
-       if (!dir)
-               return -1;
-       add_entry_to_dir(dir, ref);
-       return 0;
-}
-
-/*
- * Emit a warning and return true iff ref1 and ref2 have the same name
- * and the same sha1.  Die if they have the same name but different
- * sha1s.
- */
-static int is_dup_ref(const struct ref_entry *ref1, const struct ref_entry *ref2)
-{
-       if (strcmp(ref1->name, ref2->name))
+       if (flags & REF_ISBROKEN)
                return 0;
-
-       /* Duplicate name; make sure that they don't conflict: */
-
-       if ((ref1->flag & REF_DIR) || (ref2->flag & REF_DIR))
-               /* This is impossible by construction */
-               die("Reference directory conflict: %s", ref1->name);
-
-       if (oidcmp(&ref1->u.value.oid, &ref2->u.value.oid))
-               die("Duplicated ref, and SHA1s don't match: %s", ref1->name);
-
-       warning("Duplicated ref: %s", ref1->name);
-       return 1;
-}
-
-/*
- * Sort the entries in dir non-recursively (if they are not already
- * sorted) and remove any duplicate entries.
- */
-static void sort_ref_dir(struct ref_dir *dir)
-{
-       int i, j;
-       struct ref_entry *last = NULL;
-
-       /*
-        * This check also prevents passing a zero-length array to qsort(),
-        * which is a problem on some platforms.
-        */
-       if (dir->sorted == dir->nr)
-               return;
-
-       qsort(dir->entries, dir->nr, sizeof(*dir->entries), ref_entry_cmp);
-
-       /* Remove any duplicates: */
-       for (i = 0, j = 0; j < dir->nr; j++) {
-               struct ref_entry *entry = dir->entries[j];
-               if (last && is_dup_ref(last, entry))
-                       free_ref_entry(entry);
-               else
-                       last = dir->entries[i++] = entry;
-       }
-       dir->sorted = dir->nr = i;
-}
-
-/* Include broken references in a do_for_each_ref*() iteration: */
-#define DO_FOR_EACH_INCLUDE_BROKEN 0x01
-
-/*
- * Return true iff the reference described by entry can be resolved to
- * an object in the database.  Emit a warning if the referred-to
- * object does not exist.
- */
-static int ref_resolves_to_object(struct ref_entry *entry)
-{
-       if (entry->flag & REF_ISBROKEN)
-               return 0;
-       if (!has_sha1_file(entry->u.value.oid.hash)) {
-               error("%s does not point to a valid object!", entry->name);
-               return 0;
-       }
-       return 1;
-}
-
-/*
- * current_ref is a performance hack: when iterating over references
- * using the for_each_ref*() functions, current_ref is set to the
- * current reference's entry before calling the callback function.  If
- * the callback function calls peel_ref(), then peel_ref() first
- * checks whether the reference to be peeled is the current reference
- * (it usually is) and if so, returns that reference's peeled version
- * if it is available.  This avoids a refname lookup in a common case.
- */
-static struct ref_entry *current_ref;
-
-typedef int each_ref_entry_fn(struct ref_entry *entry, void *cb_data);
-
-struct ref_entry_cb {
-       const char *base;
-       int trim;
-       int flags;
-       each_ref_fn *fn;
-       void *cb_data;
-};
-
-/*
- * Handle one reference in a do_for_each_ref*()-style iteration,
- * calling an each_ref_fn for each entry.
- */
-static int do_one_ref(struct ref_entry *entry, void *cb_data)
-{
-       struct ref_entry_cb *data = cb_data;
-       struct ref_entry *old_current_ref;
-       int retval;
-
-       if (!starts_with(entry->name, data->base))
+       if (!has_sha1_file(oid->hash)) {
+               error("%s does not point to a valid object!", refname);
                return 0;
-
-       if (!(data->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
-             !ref_resolves_to_object(entry))
-               return 0;
-
-       /* Store the old value, in case this is a recursive call: */
-       old_current_ref = current_ref;
-       current_ref = entry;
-       retval = data->fn(entry->name + data->trim, &entry->u.value.oid,
-                         entry->flag, data->cb_data);
-       current_ref = old_current_ref;
-       return retval;
-}
-
-/*
- * Call fn for each reference in dir that has index in the range
- * offset <= index < dir->nr.  Recurse into subdirectories that are in
- * that index range, sorting them before iterating.  This function
- * does not sort dir itself; it should be sorted beforehand.  fn is
- * called for all references, including broken ones.
- */
-static int do_for_each_entry_in_dir(struct ref_dir *dir, int offset,
-                                   each_ref_entry_fn fn, void *cb_data)
-{
-       int i;
-       assert(dir->sorted == dir->nr);
-       for (i = offset; i < dir->nr; i++) {
-               struct ref_entry *entry = dir->entries[i];
-               int retval;
-               if (entry->flag & REF_DIR) {
-                       struct ref_dir *subdir = get_ref_dir(entry);
-                       sort_ref_dir(subdir);
-                       retval = do_for_each_entry_in_dir(subdir, 0, fn, cb_data);
-               } else {
-                       retval = fn(entry, cb_data);
-               }
-               if (retval)
-                       return retval;
-       }
-       return 0;
-}
-
-/*
- * Call fn for each reference in the union of dir1 and dir2, in order
- * by refname.  Recurse into subdirectories.  If a value entry appears
- * in both dir1 and dir2, then only process the version that is in
- * dir2.  The input dirs must already be sorted, but subdirs will be
- * sorted as needed.  fn is called for all references, including
- * broken ones.
- */
-static int do_for_each_entry_in_dirs(struct ref_dir *dir1,
-                                    struct ref_dir *dir2,
-                                    each_ref_entry_fn fn, void *cb_data)
-{
-       int retval;
-       int i1 = 0, i2 = 0;
-
-       assert(dir1->sorted == dir1->nr);
-       assert(dir2->sorted == dir2->nr);
-       while (1) {
-               struct ref_entry *e1, *e2;
-               int cmp;
-               if (i1 == dir1->nr) {
-                       return do_for_each_entry_in_dir(dir2, i2, fn, cb_data);
-               }
-               if (i2 == dir2->nr) {
-                       return do_for_each_entry_in_dir(dir1, i1, fn, cb_data);
-               }
-               e1 = dir1->entries[i1];
-               e2 = dir2->entries[i2];
-               cmp = strcmp(e1->name, e2->name);
-               if (cmp == 0) {
-                       if ((e1->flag & REF_DIR) && (e2->flag & REF_DIR)) {
-                               /* Both are directories; descend them in parallel. */
-                               struct ref_dir *subdir1 = get_ref_dir(e1);
-                               struct ref_dir *subdir2 = get_ref_dir(e2);
-                               sort_ref_dir(subdir1);
-                               sort_ref_dir(subdir2);
-                               retval = do_for_each_entry_in_dirs(
-                                               subdir1, subdir2, fn, cb_data);
-                               i1++;
-                               i2++;
-                       } else if (!(e1->flag & REF_DIR) && !(e2->flag & REF_DIR)) {
-                               /* Both are references; ignore the one from dir1. */
-                               retval = fn(e2, cb_data);
-                               i1++;
-                               i2++;
-                       } else {
-                               die("conflict between reference and directory: %s",
-                                   e1->name);
-                       }
-               } else {
-                       struct ref_entry *e;
-                       if (cmp < 0) {
-                               e = e1;
-                               i1++;
-                       } else {
-                               e = e2;
-                               i2++;
-                       }
-                       if (e->flag & REF_DIR) {
-                               struct ref_dir *subdir = get_ref_dir(e);
-                               sort_ref_dir(subdir);
-                               retval = do_for_each_entry_in_dir(
-                                               subdir, 0, fn, cb_data);
-                       } else {
-                               retval = fn(e, cb_data);
-                       }
-               }
-               if (retval)
-                       return retval;
-       }
-}
-
-/*
- * Load all of the refs from the dir into our in-memory cache. The hard work
- * of loading loose refs is done by get_ref_dir(), so we just need to recurse
- * through all of the sub-directories. We do not even need to care about
- * sorting, as traversal order does not matter to us.
- */
-static void prime_ref_dir(struct ref_dir *dir)
-{
-       int i;
-       for (i = 0; i < dir->nr; i++) {
-               struct ref_entry *entry = dir->entries[i];
-               if (entry->flag & REF_DIR)
-                       prime_ref_dir(get_ref_dir(entry));
        }
-}
-
-struct nonmatching_ref_data {
-       const struct string_list *skip;
-       const char *conflicting_refname;
-};
-
-static int nonmatching_ref_fn(struct ref_entry *entry, void *vdata)
-{
-       struct nonmatching_ref_data *data = vdata;
-
-       if (data->skip && string_list_has_string(data->skip, entry->name))
-               return 0;
-
-       data->conflicting_refname = entry->name;
        return 1;
 }
 
-/*
- * Return 0 if a reference named refname could be created without
- * conflicting with the name of an existing reference in dir.
- * See verify_refname_available for more information.
- */
-static int verify_refname_available_dir(const char *refname,
-                                       const struct string_list *extras,
-                                       const struct string_list *skip,
-                                       struct ref_dir *dir,
-                                       struct strbuf *err)
-{
-       const char *slash;
-       const char *extra_refname;
-       int pos;
-       struct strbuf dirname = STRBUF_INIT;
-       int ret = -1;
-
-       /*
-        * For the sake of comments in this function, suppose that
-        * refname is "refs/foo/bar".
-        */
-
-       assert(err);
-
-       strbuf_grow(&dirname, strlen(refname) + 1);
-       for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
-               /* Expand dirname to the new prefix, not including the trailing slash: */
-               strbuf_add(&dirname, refname + dirname.len, slash - refname - dirname.len);
-
-               /*
-                * We are still at a leading dir of the refname (e.g.,
-                * "refs/foo"; if there is a reference with that name,
-                * it is a conflict, *unless* it is in skip.
-                */
-               if (dir) {
-                       pos = search_ref_dir(dir, dirname.buf, dirname.len);
-                       if (pos >= 0 &&
-                           (!skip || !string_list_has_string(skip, dirname.buf))) {
-                               /*
-                                * We found a reference whose name is
-                                * a proper prefix of refname; e.g.,
-                                * "refs/foo", and is not in skip.
-                                */
-                               strbuf_addf(err, "'%s' exists; cannot create '%s'",
-                                           dirname.buf, refname);
-                               goto cleanup;
-                       }
-               }
-
-               if (extras && string_list_has_string(extras, dirname.buf) &&
-                   (!skip || !string_list_has_string(skip, dirname.buf))) {
-                       strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
-                                   refname, dirname.buf);
-                       goto cleanup;
-               }
-
-               /*
-                * Otherwise, we can try to continue our search with
-                * the next component. So try to look up the
-                * directory, e.g., "refs/foo/". If we come up empty,
-                * we know there is nothing under this whole prefix,
-                * but even in that case we still have to continue the
-                * search for conflicts with extras.
-                */
-               strbuf_addch(&dirname, '/');
-               if (dir) {
-                       pos = search_ref_dir(dir, dirname.buf, dirname.len);
-                       if (pos < 0) {
-                               /*
-                                * There was no directory "refs/foo/",
-                                * so there is nothing under this
-                                * whole prefix. So there is no need
-                                * to continue looking for conflicting
-                                * references. But we need to continue
-                                * looking for conflicting extras.
-                                */
-                               dir = NULL;
-                       } else {
-                               dir = get_ref_dir(dir->entries[pos]);
-                       }
-               }
-       }
-
-       /*
-        * We are at the leaf of our refname (e.g., "refs/foo/bar").
-        * There is no point in searching for a reference with that
-        * name, because a refname isn't considered to conflict with
-        * itself. But we still need to check for references whose
-        * names are in the "refs/foo/bar/" namespace, because they
-        * *do* conflict.
-        */
-       strbuf_addstr(&dirname, refname + dirname.len);
-       strbuf_addch(&dirname, '/');
-
-       if (dir) {
-               pos = search_ref_dir(dir, dirname.buf, dirname.len);
-
-               if (pos >= 0) {
-                       /*
-                        * We found a directory named "$refname/"
-                        * (e.g., "refs/foo/bar/"). It is a problem
-                        * iff it contains any ref that is not in
-                        * "skip".
-                        */
-                       struct nonmatching_ref_data data;
-
-                       data.skip = skip;
-                       data.conflicting_refname = NULL;
-                       dir = get_ref_dir(dir->entries[pos]);
-                       sort_ref_dir(dir);
-                       if (do_for_each_entry_in_dir(dir, 0, nonmatching_ref_fn, &data)) {
-                               strbuf_addf(err, "'%s' exists; cannot create '%s'",
-                                           data.conflicting_refname, refname);
-                               goto cleanup;
-                       }
-               }
-       }
-
-       extra_refname = find_descendant_ref(dirname.buf, extras, skip);
-       if (extra_refname)
-               strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
-                           refname, extra_refname);
-       else
-               ret = 0;
-
-cleanup:
-       strbuf_release(&dirname);
-       return ret;
-}
-
 struct packed_ref_cache {
-       struct ref_entry *root;
+       struct ref_cache *cache;
 
        /*
         * Count of references to the data structure in this instance,
-        * including the pointer from ref_cache::packed if any.  The
-        * data will not be freed as long as the reference count is
-        * nonzero.
+        * including the pointer from files_ref_store::packed if any.
+        * The data will not be freed as long as the reference count
+        * is nonzero.
         */
        unsigned int referrers;
 
-       /*
-        * Iff the packed-refs file associated with this instance is
-        * currently locked for writing, this points at the associated
-        * lock (which is owned by somebody else).  The referrer count
-        * is also incremented when the file is locked and decremented
-        * when it is unlocked.
-        */
-       struct lock_file *lock;
-
        /* The metadata from when this packed-refs cache was read */
        struct stat_validity validity;
 };
@@ -869,20 +52,23 @@ struct packed_ref_cache {
  * Future: need to be in "struct repository"
  * when doing a full libification.
  */
-static struct ref_cache {
-       struct ref_cache *next;
-       struct ref_entry *loose;
+struct files_ref_store {
+       struct ref_store base;
+       unsigned int store_flags;
+
+       char *gitdir;
+       char *gitcommondir;
+       char *packed_refs_path;
+
+       struct ref_cache *loose;
        struct packed_ref_cache *packed;
+
        /*
-        * The submodule name, or "" for the main repo.  We allocate
-        * length 1 rather than FLEX_ARRAY so that the main ref_cache
-        * is initialized correctly.
+        * Lock used for the "packed-refs" file. Note that this (and
+        * thus the enclosing `files_ref_store`) must not be freed.
         */
-       char name[1];
-} ref_cache, *submodule_ref_caches;
-
-/* Lock used for the main packed-refs file: */
-static struct lock_file packlock;
+       struct lock_file packed_refs_lock;
+};
 
 /*
  * Increment the reference count of *packed_refs.
@@ -899,7 +85,7 @@ static void acquire_packed_ref_cache(struct packed_ref_cache *packed_refs)
 static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
 {
        if (!--packed_refs->referrers) {
-               free_ref_entry(packed_refs->root);
+               free_ref_cache(packed_refs->cache);
                stat_validity_clear(&packed_refs->validity);
                free(packed_refs);
                return 1;
@@ -908,22 +94,22 @@ static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
        }
 }
 
-static void clear_packed_ref_cache(struct ref_cache *refs)
+static void clear_packed_ref_cache(struct files_ref_store *refs)
 {
        if (refs->packed) {
                struct packed_ref_cache *packed_refs = refs->packed;
 
-               if (packed_refs->lock)
-                       die("internal error: packed-ref cache cleared while locked");
+               if (is_lock_file_locked(&refs->packed_refs_lock))
+                       die("BUG: packed-ref cache cleared while locked");
                refs->packed = NULL;
                release_packed_ref_cache(packed_refs);
        }
 }
 
-static void clear_loose_ref_cache(struct ref_cache *refs)
+static void clear_loose_ref_cache(struct files_ref_store *refs)
 {
        if (refs->loose) {
-               free_ref_entry(refs->loose);
+               free_ref_cache(refs->loose);
                refs->loose = NULL;
        }
 }
@@ -932,41 +118,60 @@ static void clear_loose_ref_cache(struct ref_cache *refs)
  * Create a new submodule ref cache and add it to the internal
  * set of caches.
  */
-static struct ref_cache *create_ref_cache(const char *submodule)
-{
-       struct ref_cache *refs;
-       if (!submodule)
-               submodule = "";
-       FLEX_ALLOC_STR(refs, name, submodule);
-       refs->next = submodule_ref_caches;
-       submodule_ref_caches = refs;
-       return refs;
+static struct ref_store *files_ref_store_create(const char *gitdir,
+                                               unsigned int flags)
+{
+       struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
+       struct ref_store *ref_store = (struct ref_store *)refs;
+       struct strbuf sb = STRBUF_INIT;
+
+       base_ref_store_init(ref_store, &refs_be_files);
+       refs->store_flags = flags;
+
+       refs->gitdir = xstrdup(gitdir);
+       get_common_dir_noenv(&sb, gitdir);
+       refs->gitcommondir = strbuf_detach(&sb, NULL);
+       strbuf_addf(&sb, "%s/packed-refs", refs->gitcommondir);
+       refs->packed_refs_path = strbuf_detach(&sb, NULL);
+
+       return ref_store;
 }
 
-static struct ref_cache *lookup_ref_cache(const char *submodule)
+/*
+ * Die if refs is not the main ref store. caller is used in any
+ * necessary error messages.
+ */
+static void files_assert_main_repository(struct files_ref_store *refs,
+                                        const char *caller)
 {
-       struct ref_cache *refs;
-
-       if (!submodule || !*submodule)
-               return &ref_cache;
+       if (refs->store_flags & REF_STORE_MAIN)
+               return;
 
-       for (refs = submodule_ref_caches; refs; refs = refs->next)
-               if (!strcmp(submodule, refs->name))
-                       return refs;
-       return NULL;
+       die("BUG: operation %s only allowed for main ref store", caller);
 }
 
 /*
- * Return a pointer to a ref_cache for the specified submodule. For
- * the main repository, use submodule==NULL. The returned structure
- * will be allocated and initialized but not necessarily populated; it
- * should not be freed.
+ * Downcast ref_store to files_ref_store. Die if ref_store is not a
+ * files_ref_store. required_flags is compared with ref_store's
+ * store_flags to ensure the ref_store has all required capabilities.
+ * "caller" is used in any necessary error messages.
  */
-static struct ref_cache *get_ref_cache(const char *submodule)
+static struct files_ref_store *files_downcast(struct ref_store *ref_store,
+                                             unsigned int required_flags,
+                                             const char *caller)
 {
-       struct ref_cache *refs = lookup_ref_cache(submodule);
-       if (!refs)
-               refs = create_ref_cache(submodule);
+       struct files_ref_store *refs;
+
+       if (ref_store->be != &refs_be_files)
+               die("BUG: ref_store is type \"%s\" not \"files\" in %s",
+                   ref_store->be->name, caller);
+
+       refs = (struct files_ref_store *)ref_store;
+
+       if ((refs->store_flags & required_flags) != required_flags)
+               die("BUG: operation %s requires abilities 0x%x, but only have 0x%x",
+                   caller, required_flags, refs->store_flags);
+
        return refs;
 }
 
@@ -985,27 +190,15 @@ static const char PACKED_REFS_HEADER[] =
  * Return a pointer to the refname within the line (null-terminated),
  * or NULL if there was a problem.
  */
-static const char *parse_ref_line(struct strbuf *line, unsigned char *sha1)
+static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
 {
        const char *ref;
 
-       /*
-        * 42: the answer to everything.
-        *
-        * In this case, it happens to be the answer to
-        *  40 (length of sha1 hex representation)
-        *  +1 (space in between hex and name)
-        *  +1 (newline at the end of the line)
-        */
-       if (line->len <= 42)
-               return NULL;
-
-       if (get_sha1_hex(line->buf, sha1) < 0)
+       if (parse_oid_hex(line->buf, oid, &ref) < 0)
                return NULL;
-       if (!isspace(line->buf[40]))
+       if (!isspace(*ref++))
                return NULL;
 
-       ref = line->buf + 41;
        if (isspace(*ref))
                return NULL;
 
@@ -1017,7 +210,9 @@ static const char *parse_ref_line(struct strbuf *line, unsigned char *sha1)
 }
 
 /*
- * Read f, which is a packed-refs file, into dir.
+ * Read from `packed_refs_file` into a newly-allocated
+ * `packed_ref_cache` and return it. The return value will already
+ * have its reference count incremented.
  *
  * A comment line of the form "# pack-refs with: " may contain zero or
  * more traits. We interpret the traits as follows:
@@ -1043,14 +238,38 @@ static const char *parse_ref_line(struct strbuf *line, unsigned char *sha1)
  *      compatibility with older clients, but we do not require it
  *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
  */
-static void read_packed_refs(FILE *f, struct ref_dir *dir)
+static struct packed_ref_cache *read_packed_refs(const char *packed_refs_file)
 {
+       FILE *f;
+       struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
        struct ref_entry *last = NULL;
        struct strbuf line = STRBUF_INIT;
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
+       struct ref_dir *dir;
+
+       acquire_packed_ref_cache(packed_refs);
+       packed_refs->cache = create_ref_cache(NULL, NULL);
+       packed_refs->cache->root->flag &= ~REF_INCOMPLETE;
 
+       f = fopen(packed_refs_file, "r");
+       if (!f) {
+               if (errno == ENOENT) {
+                       /*
+                        * This is OK; it just means that no
+                        * "packed-refs" file has been written yet,
+                        * which is equivalent to it being empty.
+                        */
+                       return packed_refs;
+               } else {
+                       die_errno("couldn't read %s", packed_refs_file);
+               }
+       }
+
+       stat_validity_update(&packed_refs->validity, fileno(f));
+
+       dir = get_ref_dir(packed_refs->cache->root);
        while (strbuf_getwholeline(&line, f, '\n') != EOF) {
-               unsigned char sha1[20];
+               struct object_id oid;
                const char *refname;
                const char *traits;
 
@@ -1063,29 +282,29 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
                        continue;
                }
 
-               refname = parse_ref_line(&line, sha1);
+               refname = parse_ref_line(&line, &oid);
                if (refname) {
                        int flag = REF_ISPACKED;
 
                        if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                                if (!refname_is_safe(refname))
                                        die("packed refname is dangerous: %s", refname);
-                               hashclr(sha1);
+                               oidclr(&oid);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
-                       last = create_ref_entry(refname, sha1, flag, 0);
+                       last = create_ref_entry(refname, &oid, flag);
                        if (peeled == PEELED_FULLY ||
                            (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
                                last->flag |= REF_KNOWS_PEELED;
-                       add_ref(dir, last);
+                       add_ref_entry(dir, last);
                        continue;
                }
                if (last &&
                    line.buf[0] == '^' &&
                    line.len == PEELED_LINE_LENGTH &&
                    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
-                   !get_sha1_hex(line.buf + 1, sha1)) {
-                       hashcpy(last->u.value.peeled.hash, sha1);
+                   !get_oid_hex(line.buf + 1, &oid)) {
+                       oidcpy(&last->u.value.peeled, &oid);
                        /*
                         * Regardless of what the file header said,
                         * we definitely know the value of *this*
@@ -1095,49 +314,101 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
                }
        }
 
+       fclose(f);
        strbuf_release(&line);
+
+       return packed_refs;
 }
 
-/*
- * Get the packed_ref_cache for the specified ref_cache, creating it
- * if necessary.
- */
-static struct packed_ref_cache *get_packed_ref_cache(struct ref_cache *refs)
+static const char *files_packed_refs_path(struct files_ref_store *refs)
 {
-       char *packed_refs_file;
+       return refs->packed_refs_path;
+}
 
-       if (*refs->name)
-               packed_refs_file = git_pathdup_submodule(refs->name, "packed-refs");
-       else
-               packed_refs_file = git_pathdup("packed-refs");
+static void files_reflog_path(struct files_ref_store *refs,
+                             struct strbuf *sb,
+                             const char *refname)
+{
+       if (!refname) {
+               /*
+                * FIXME: of course this is wrong in multi worktree
+                * setting. To be fixed real soon.
+                */
+               strbuf_addf(sb, "%s/logs", refs->gitcommondir);
+               return;
+       }
 
+       switch (ref_type(refname)) {
+       case REF_TYPE_PER_WORKTREE:
+       case REF_TYPE_PSEUDOREF:
+               strbuf_addf(sb, "%s/logs/%s", refs->gitdir, refname);
+               break;
+       case REF_TYPE_NORMAL:
+               strbuf_addf(sb, "%s/logs/%s", refs->gitcommondir, refname);
+               break;
+       default:
+               die("BUG: unknown ref type %d of ref %s",
+                   ref_type(refname), refname);
+       }
+}
+
+static void files_ref_path(struct files_ref_store *refs,
+                          struct strbuf *sb,
+                          const char *refname)
+{
+       switch (ref_type(refname)) {
+       case REF_TYPE_PER_WORKTREE:
+       case REF_TYPE_PSEUDOREF:
+               strbuf_addf(sb, "%s/%s", refs->gitdir, refname);
+               break;
+       case REF_TYPE_NORMAL:
+               strbuf_addf(sb, "%s/%s", refs->gitcommondir, refname);
+               break;
+       default:
+               die("BUG: unknown ref type %d of ref %s",
+                   ref_type(refname), refname);
+       }
+}
+
+/*
+ * Check that the packed refs cache (if any) still reflects the
+ * contents of the file. If not, clear the cache.
+ */
+static void validate_packed_ref_cache(struct files_ref_store *refs)
+{
        if (refs->packed &&
-           !stat_validity_check(&refs->packed->validity, packed_refs_file))
+           !stat_validity_check(&refs->packed->validity,
+                                files_packed_refs_path(refs)))
                clear_packed_ref_cache(refs);
+}
+
+/*
+ * Get the packed_ref_cache for the specified files_ref_store,
+ * creating and populating it if it hasn't been read before or if the
+ * file has been changed (according to its `validity` field) since it
+ * was last read. On the other hand, if we hold the lock, then assume
+ * that the file hasn't been changed out from under us, so skip the
+ * extra `stat()` call in `stat_validity_check()`.
+ */
+static struct packed_ref_cache *get_packed_ref_cache(struct files_ref_store *refs)
+{
+       const char *packed_refs_file = files_packed_refs_path(refs);
+
+       if (!is_lock_file_locked(&refs->packed_refs_lock))
+               validate_packed_ref_cache(refs);
+
+       if (!refs->packed)
+               refs->packed = read_packed_refs(packed_refs_file);
 
-       if (!refs->packed) {
-               FILE *f;
-
-               refs->packed = xcalloc(1, sizeof(*refs->packed));
-               acquire_packed_ref_cache(refs->packed);
-               refs->packed->root = create_dir_entry(refs, "", 0, 0);
-               f = fopen(packed_refs_file, "r");
-               if (f) {
-                       stat_validity_update(&refs->packed->validity, fileno(f));
-                       read_packed_refs(f, get_ref_dir(refs->packed->root));
-                       fclose(f);
-               }
-       }
-       free(packed_refs_file);
        return refs->packed;
 }
 
 static struct ref_dir *get_packed_ref_dir(struct packed_ref_cache *packed_ref_cache)
 {
-       return get_ref_dir(packed_ref_cache->root);
+       return get_ref_dir(packed_ref_cache->cache->root);
 }
 
-static struct ref_dir *get_packed_refs(struct ref_cache *refs)
+static struct ref_dir *get_packed_refs(struct files_ref_store *refs)
 {
        return get_packed_ref_dir(get_packed_ref_cache(refs));
 }
@@ -1148,15 +419,19 @@ static struct ref_dir *get_packed_refs(struct ref_cache *refs)
  * lock_packed_refs()).  To actually write the packed-refs file, call
  * commit_packed_refs().
  */
-static void add_packed_ref(const char *refname, const unsigned char *sha1)
+static void add_packed_ref(struct files_ref_store *refs,
+                          const char *refname, const struct object_id *oid)
 {
-       struct packed_ref_cache *packed_ref_cache =
-               get_packed_ref_cache(&ref_cache);
+       struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
+
+       if (!is_lock_file_locked(&refs->packed_refs_lock))
+               die("BUG: packed refs not locked");
 
-       if (!packed_ref_cache->lock)
-               die("internal error: packed refs not locked");
-       add_ref(get_packed_ref_dir(packed_ref_cache),
-               create_ref_entry(refname, sha1, REF_ISPACKED, 1));
+       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
+               die("Reference has invalid format: '%s'", refname);
+
+       add_ref_entry(get_packed_ref_dir(packed_ref_cache),
+                     create_ref_entry(refname, oid, REF_ISPACKED));
 }
 
 /*
@@ -1164,9 +439,11 @@ static void add_packed_ref(const char *refname, const unsigned char *sha1)
  * (without recursing).  dirname must end with '/'.  dir must be the
  * directory entry corresponding to dirname.
  */
-static void read_loose_refs(const char *dirname, struct ref_dir *dir)
+static void loose_fill_ref_dir(struct ref_store *ref_store,
+                              struct ref_dir *dir, const char *dirname)
 {
-       struct ref_cache *refs = dir->ref_cache;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ, "fill_ref_dir");
        DIR *d;
        struct dirent *de;
        int dirnamelen = strlen(dirname);
@@ -1174,10 +451,7 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
        struct strbuf path = STRBUF_INIT;
        size_t path_baselen;
 
-       if (*refs->name)
-               strbuf_git_path_submodule(&path, refs->name, "%s", dirname);
-       else
-               strbuf_git_path(&path, "%s", dirname);
+       files_ref_path(refs, &path, dirname);
        path_baselen = path.len;
 
        d = opendir(path.buf);
@@ -1190,7 +464,7 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
        strbuf_add(&refname, dirname, dirnamelen);
 
        while ((de = readdir(d)) != NULL) {
-               unsigned char sha1[20];
+               struct object_id oid;
                struct stat st;
                int flag;
 
@@ -1205,26 +479,16 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
                } else if (S_ISDIR(st.st_mode)) {
                        strbuf_addch(&refname, '/');
                        add_entry_to_dir(dir,
-                                        create_dir_entry(refs, refname.buf,
+                                        create_dir_entry(dir->cache, refname.buf,
                                                          refname.len, 1));
                } else {
-                       int read_ok;
-
-                       if (*refs->name) {
-                               hashclr(sha1);
-                               flag = 0;
-                               read_ok = !resolve_gitlink_ref(refs->name,
-                                                              refname.buf, sha1);
-                       } else {
-                               read_ok = !read_ref_full(refname.buf,
-                                                        RESOLVE_REF_READING,
-                                                        sha1, &flag);
-                       }
-
-                       if (!read_ok) {
-                               hashclr(sha1);
+                       if (!refs_resolve_ref_unsafe(&refs->base,
+                                                    refname.buf,
+                                                    RESOLVE_REF_READING,
+                                                    oid.hash, &flag)) {
+                               oidclr(&oid);
                                flag |= REF_ISBROKEN;
-                       } else if (is_null_sha1(sha1)) {
+                       } else if (is_null_oid(&oid)) {
                                /*
                                 * It is so astronomically unlikely
                                 * that NULL_SHA1 is the SHA-1 of an
@@ -1240,11 +504,11 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
                                                 REFNAME_ALLOW_ONELEVEL)) {
                                if (!refname_is_safe(refname.buf))
                                        die("loose refname is dangerous: %s", refname.buf);
-                               hashclr(sha1);
+                               oidclr(&oid);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        add_entry_to_dir(dir,
-                                        create_ref_entry(refname.buf, sha1, flag, 0));
+                                        create_ref_entry(refname.buf, &oid, flag));
                }
                strbuf_setlen(&refname, dirnamelen);
                strbuf_setlen(&path, path_baselen);
@@ -1252,132 +516,62 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
        strbuf_release(&refname);
        strbuf_release(&path);
        closedir(d);
-}
-
-static struct ref_dir *get_loose_refs(struct ref_cache *refs)
-{
-       if (!refs->loose) {
-               /*
-                * Mark the top-level directory complete because we
-                * are about to read the only subdirectory that can
-                * hold references:
-                */
-               refs->loose = create_dir_entry(refs, "", 0, 0);
-               /*
-                * Create an incomplete entry for "refs/":
-                */
-               add_entry_to_dir(get_ref_dir(refs->loose),
-                                create_dir_entry(refs, "refs/", 5, 1));
-       }
-       return get_ref_dir(refs->loose);
-}
-
-/* We allow "recursive" symbolic refs. Only within reason, though */
-#define MAXDEPTH 5
-#define MAXREFLEN (1024)
 
-/*
- * Called by resolve_gitlink_ref_recursive() after it failed to read
- * from the loose refs in ref_cache refs. Find <refname> in the
- * packed-refs file for the submodule.
- */
-static int resolve_gitlink_packed_ref(struct ref_cache *refs,
-                                     const char *refname, unsigned char *sha1)
-{
-       struct ref_entry *ref;
-       struct ref_dir *dir = get_packed_refs(refs);
-
-       ref = find_ref(dir, refname);
-       if (ref == NULL)
-               return -1;
-
-       hashcpy(sha1, ref->u.value.oid.hash);
-       return 0;
-}
-
-static int resolve_gitlink_ref_recursive(struct ref_cache *refs,
-                                        const char *refname, unsigned char *sha1,
-                                        int recursion)
-{
-       int fd, len;
-       char buffer[128], *p;
-       char *path;
-
-       if (recursion > MAXDEPTH || strlen(refname) > MAXREFLEN)
-               return -1;
-       path = *refs->name
-               ? git_pathdup_submodule(refs->name, "%s", refname)
-               : git_pathdup("%s", refname);
-       fd = open(path, O_RDONLY);
-       free(path);
-       if (fd < 0)
-               return resolve_gitlink_packed_ref(refs, refname, sha1);
-
-       len = read(fd, buffer, sizeof(buffer)-1);
-       close(fd);
-       if (len < 0)
-               return -1;
-       while (len && isspace(buffer[len-1]))
-               len--;
-       buffer[len] = 0;
-
-       /* Was it a detached head or an old-fashioned symlink? */
-       if (!get_sha1_hex(buffer, sha1))
-               return 0;
-
-       /* Symref? */
-       if (strncmp(buffer, "ref:", 4))
-               return -1;
-       p = buffer + 4;
-       while (isspace(*p))
-               p++;
+       /*
+        * Manually add refs/bisect, which, being per-worktree, might
+        * not appear in the directory listing for refs/ in the main
+        * repo.
+        */
+       if (!strcmp(dirname, "refs/")) {
+               int pos = search_ref_dir(dir, "refs/bisect/", 12);
 
-       return resolve_gitlink_ref_recursive(refs, p, sha1, recursion+1);
+               if (pos < 0) {
+                       struct ref_entry *child_entry = create_dir_entry(
+                                       dir->cache, "refs/bisect/", 12, 1);
+                       add_entry_to_dir(dir, child_entry);
+               }
+       }
 }
 
-int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *sha1)
+static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
 {
-       int len = strlen(path), retval;
-       struct strbuf submodule = STRBUF_INIT;
-       struct ref_cache *refs;
+       if (!refs->loose) {
+               /*
+                * Mark the top-level directory complete because we
+                * are about to read the only subdirectory that can
+                * hold references:
+                */
+               refs->loose = create_ref_cache(&refs->base, loose_fill_ref_dir);
 
-       while (len && path[len-1] == '/')
-               len--;
-       if (!len)
-               return -1;
+               /* We're going to fill the top level ourselves: */
+               refs->loose->root->flag &= ~REF_INCOMPLETE;
 
-       strbuf_add(&submodule, path, len);
-       refs = lookup_ref_cache(submodule.buf);
-       if (!refs) {
-               if (!is_nonbare_repository_dir(&submodule)) {
-                       strbuf_release(&submodule);
-                       return -1;
-               }
-               refs = create_ref_cache(submodule.buf);
+               /*
+                * Add an incomplete entry for "refs/" (to be filled
+                * lazily):
+                */
+               add_entry_to_dir(get_ref_dir(refs->loose->root),
+                                create_dir_entry(refs->loose, "refs/", 5, 1));
        }
-       strbuf_release(&submodule);
-
-       retval = resolve_gitlink_ref_recursive(refs, refname, sha1, 0);
-       return retval;
+       return refs->loose;
 }
 
 /*
  * Return the ref_entry for the given refname from the packed
  * references.  If it does not exist, return NULL.
  */
-static struct ref_entry *get_packed_ref(const char *refname)
+static struct ref_entry *get_packed_ref(struct files_ref_store *refs,
+                                       const char *refname)
 {
-       return find_ref(get_packed_refs(&ref_cache), refname);
+       return find_ref_entry(get_packed_refs(refs), refname);
 }
 
 /*
- * A loose ref file doesn't exist; check for a packed ref.  The
- * options are forwarded from resolve_safe_unsafe().
+ * A loose ref file doesn't exist; check for a packed ref.
  */
-static int resolve_missing_loose_ref(const char *refname,
-                                    int resolve_flags,
-                                    unsigned char *sha1,
-                                    int *flags)
+static int resolve_packed_ref(struct files_ref_store *refs,
+                             const char *refname,
+                             unsigned char *sha1, unsigned int *flags)
 {
        struct ref_entry *entry;
 
@@ -1385,260 +579,418 @@ static int resolve_missing_loose_ref(const char *refname,
         * The loose reference file does not exist; check for a packed
         * reference.
         */
-       entry = get_packed_ref(refname);
+       entry = get_packed_ref(refs, refname);
        if (entry) {
                hashcpy(sha1, entry->u.value.oid.hash);
-               if (flags)
-                       *flags |= REF_ISPACKED;
-               return 0;
-       }
-       /* The reference is not a packed reference, either. */
-       if (resolve_flags & RESOLVE_REF_READING) {
-               errno = ENOENT;
-               return -1;
-       } else {
-               hashclr(sha1);
+               *flags |= REF_ISPACKED;
                return 0;
        }
+       /* refname is not a packed reference. */
+       return -1;
 }
 
-/* This function needs to return a meaningful errno on failure */
-static const char *resolve_ref_1(const char *refname,
-                                int resolve_flags,
-                                unsigned char *sha1,
-                                int *flags,
-                                struct strbuf *sb_refname,
-                                struct strbuf *sb_path,
-                                struct strbuf *sb_contents)
+static int files_read_raw_ref(struct ref_store *ref_store,
+                             const char *refname, unsigned char *sha1,
+                             struct strbuf *referent, unsigned int *type)
 {
-       int depth = MAXDEPTH;
-       int bad_name = 0;
-
-       if (flags)
-               *flags = 0;
-
-       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
-               if (flags)
-                       *flags |= REF_BAD_NAME;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
+       struct strbuf sb_contents = STRBUF_INIT;
+       struct strbuf sb_path = STRBUF_INIT;
+       const char *path;
+       const char *buf;
+       struct stat st;
+       int fd;
+       int ret = -1;
+       int save_errno;
+       int remaining_retries = 3;
 
-               if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
-                   !refname_is_safe(refname)) {
-                       errno = EINVAL;
-                       return NULL;
-               }
-               /*
-                * dwim_ref() uses REF_ISBROKEN to distinguish between
-                * missing refs and refs that were present but invalid,
-                * to complain about the latter to stderr.
-                *
-                * We don't know whether the ref exists, so don't set
-                * REF_ISBROKEN yet.
-                */
-               bad_name = 1;
-       }
-       for (;;) {
-               const char *path;
-               struct stat st;
-               char *buf;
-               int fd;
+       *type = 0;
+       strbuf_reset(&sb_path);
 
-               if (--depth < 0) {
-                       errno = ELOOP;
-                       return NULL;
-               }
+       files_ref_path(refs, &sb_path, refname);
 
-               strbuf_reset(sb_path);
-               strbuf_git_path(sb_path, "%s", refname);
-               path = sb_path->buf;
+       path = sb_path.buf;
 
-               /*
-                * We might have to loop back here to avoid a race
-                * condition: first we lstat() the file, then we try
-                * to read it as a link or as a file.  But if somebody
-                * changes the type of the file (file <-> directory
-                * <-> symlink) between the lstat() and reading, then
-                * we don't want to report that as an error but rather
-                * try again starting with the lstat().
-                */
-       stat_ref:
-               if (lstat(path, &st) < 0) {
-                       if (errno != ENOENT)
-                               return NULL;
-                       if (resolve_missing_loose_ref(refname, resolve_flags,
-                                                     sha1, flags))
-                               return NULL;
-                       if (bad_name) {
-                               hashclr(sha1);
-                               if (flags)
-                                       *flags |= REF_ISBROKEN;
-                       }
-                       return refname;
-               }
+stat_ref:
+       /*
+        * We might have to loop back here to avoid a race
+        * condition: first we lstat() the file, then we try
+        * to read it as a link or as a file.  But if somebody
+        * changes the type of the file (file <-> directory
+        * <-> symlink) between the lstat() and reading, then
+        * we don't want to report that as an error but rather
+        * try again starting with the lstat().
+        *
+        * We'll keep a count of the retries, though, just to avoid
+        * any confusing situation sending us into an infinite loop.
+        */
 
-               /* Follow "normalized" - ie "refs/.." symlinks by hand */
-               if (S_ISLNK(st.st_mode)) {
-                       strbuf_reset(sb_contents);
-                       if (strbuf_readlink(sb_contents, path, 0) < 0) {
-                               if (errno == ENOENT || errno == EINVAL)
-                                       /* inconsistent with lstat; retry */
-                                       goto stat_ref;
-                               else
-                                       return NULL;
-                       }
-                       if (starts_with(sb_contents->buf, "refs/") &&
-                           !check_refname_format(sb_contents->buf, 0)) {
-                               strbuf_swap(sb_refname, sb_contents);
-                               refname = sb_refname->buf;
-                               if (flags)
-                                       *flags |= REF_ISSYMREF;
-                               if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
-                                       hashclr(sha1);
-                                       return refname;
-                               }
-                               continue;
-                       }
-               }
+       if (remaining_retries-- <= 0)
+               goto out;
 
-               /* Is it a directory? */
-               if (S_ISDIR(st.st_mode)) {
-                       errno = EISDIR;
-                       return NULL;
+       if (lstat(path, &st) < 0) {
+               if (errno != ENOENT)
+                       goto out;
+               if (resolve_packed_ref(refs, refname, sha1, type)) {
+                       errno = ENOENT;
+                       goto out;
                }
+               ret = 0;
+               goto out;
+       }
 
-               /*
-                * Anything else, just open it and try to use it as
-                * a ref
-                */
-               fd = open(path, O_RDONLY);
-               if (fd < 0) {
-                       if (errno == ENOENT)
+       /* Follow "normalized" - ie "refs/.." symlinks by hand */
+       if (S_ISLNK(st.st_mode)) {
+               strbuf_reset(&sb_contents);
+               if (strbuf_readlink(&sb_contents, path, 0) < 0) {
+                       if (errno == ENOENT || errno == EINVAL)
                                /* inconsistent with lstat; retry */
                                goto stat_ref;
                        else
-                               return NULL;
+                               goto out;
                }
-               strbuf_reset(sb_contents);
-               if (strbuf_read(sb_contents, fd, 256) < 0) {
-                       int save_errno = errno;
-                       close(fd);
-                       errno = save_errno;
-                       return NULL;
+               if (starts_with(sb_contents.buf, "refs/") &&
+                   !check_refname_format(sb_contents.buf, 0)) {
+                       strbuf_swap(&sb_contents, referent);
+                       *type |= REF_ISSYMREF;
+                       ret = 0;
+                       goto out;
                }
-               close(fd);
-               strbuf_rtrim(sb_contents);
+               /*
+                * It doesn't look like a refname; fall through to just
+                * treating it like a non-symlink, and reading whatever it
+                * points to.
+                */
+       }
 
+       /* Is it a directory? */
+       if (S_ISDIR(st.st_mode)) {
                /*
-                * Is it a symbolic ref?
+                * Even though there is a directory where the loose
+                * ref is supposed to be, there could still be a
+                * packed ref:
                 */
-               if (!starts_with(sb_contents->buf, "ref:")) {
-                       /*
-                        * Please note that FETCH_HEAD has a second
-                        * line containing other data.
-                        */
-                       if (get_sha1_hex(sb_contents->buf, sha1) ||
-                           (sb_contents->buf[40] != '\0' && !isspace(sb_contents->buf[40]))) {
-                               if (flags)
-                                       *flags |= REF_ISBROKEN;
-                               errno = EINVAL;
-                               return NULL;
-                       }
-                       if (bad_name) {
-                               hashclr(sha1);
-                               if (flags)
-                                       *flags |= REF_ISBROKEN;
-                       }
-                       return refname;
+               if (resolve_packed_ref(refs, refname, sha1, type)) {
+                       errno = EISDIR;
+                       goto out;
                }
-               if (flags)
-                       *flags |= REF_ISSYMREF;
-               buf = sb_contents->buf + 4;
+               ret = 0;
+               goto out;
+       }
+
+       /*
+        * Anything else, just open it and try to use it as
+        * a ref
+        */
+       fd = open(path, O_RDONLY);
+       if (fd < 0) {
+               if (errno == ENOENT && !S_ISLNK(st.st_mode))
+                       /* inconsistent with lstat; retry */
+                       goto stat_ref;
+               else
+                       goto out;
+       }
+       strbuf_reset(&sb_contents);
+       if (strbuf_read(&sb_contents, fd, 256) < 0) {
+               int save_errno = errno;
+               close(fd);
+               errno = save_errno;
+               goto out;
+       }
+       close(fd);
+       strbuf_rtrim(&sb_contents);
+       buf = sb_contents.buf;
+       if (starts_with(buf, "ref:")) {
+               buf += 4;
                while (isspace(*buf))
                        buf++;
-               strbuf_reset(sb_refname);
-               strbuf_addstr(sb_refname, buf);
-               refname = sb_refname->buf;
-               if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
-                       hashclr(sha1);
-                       return refname;
-               }
-               if (check_refname_format(buf, REFNAME_ALLOW_ONELEVEL)) {
-                       if (flags)
-                               *flags |= REF_ISBROKEN;
-
-                       if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
-                           !refname_is_safe(buf)) {
-                               errno = EINVAL;
-                               return NULL;
-                       }
-                       bad_name = 1;
-               }
+
+               strbuf_reset(referent);
+               strbuf_addstr(referent, buf);
+               *type |= REF_ISSYMREF;
+               ret = 0;
+               goto out;
        }
-}
 
-const char *resolve_ref_unsafe(const char *refname, int resolve_flags,
-                              unsigned char *sha1, int *flags)
-{
-       static struct strbuf sb_refname = STRBUF_INIT;
-       struct strbuf sb_contents = STRBUF_INIT;
-       struct strbuf sb_path = STRBUF_INIT;
-       const char *ret;
+       /*
+        * Please note that FETCH_HEAD has additional
+        * data after the sha.
+        */
+       if (get_sha1_hex(buf, sha1) ||
+           (buf[40] != '\0' && !isspace(buf[40]))) {
+               *type |= REF_ISBROKEN;
+               errno = EINVAL;
+               goto out;
+       }
+
+       ret = 0;
 
-       ret = resolve_ref_1(refname, resolve_flags, sha1, flags,
-                           &sb_refname, &sb_path, &sb_contents);
+out:
+       save_errno = errno;
        strbuf_release(&sb_path);
        strbuf_release(&sb_contents);
+       errno = save_errno;
        return ret;
 }
 
+static void unlock_ref(struct ref_lock *lock)
+{
+       /* Do not free lock->lk -- atexit() still looks at them */
+       if (lock->lk)
+               rollback_lock_file(lock->lk);
+       free(lock->ref_name);
+       free(lock);
+}
+
 /*
- * Peel the entry (if possible) and return its new peel_status.  If
- * repeel is true, re-peel the entry even if there is an old peeled
- * value that is already stored in it.
+ * Lock refname, without following symrefs, and set *lock_p to point
+ * at a newly-allocated lock object. Fill in lock->old_oid, referent,
+ * and type similarly to read_raw_ref().
+ *
+ * The caller must verify that refname is a "safe" reference name (in
+ * the sense of refname_is_safe()) before calling this function.
+ *
+ * If the reference doesn't already exist, verify that refname doesn't
+ * have a D/F conflict with any existing references. extras and skip
+ * are passed to refs_verify_refname_available() for this check.
+ *
+ * If mustexist is not set and the reference is not found or is
+ * broken, lock the reference anyway but clear sha1.
+ *
+ * Return 0 on success. On failure, write an error message to err and
+ * return TRANSACTION_NAME_CONFLICT or TRANSACTION_GENERIC_ERROR.
+ *
+ * Implementation note: This function is basically
+ *
+ *     lock reference
+ *     read_raw_ref()
  *
- * It is OK to call this function with a packed reference entry that
- * might be stale and might even refer to an object that has since
- * been garbage-collected.  In such a case, if the entry has
- * REF_KNOWS_PEELED then leave the status unchanged and return
- * PEEL_PEELED or PEEL_NON_TAG; otherwise, return PEEL_INVALID.
+ * but it includes a lot more code to
+ * - Deal with possible races with other processes
+ * - Avoid calling refs_verify_refname_available() when it can be
+ *   avoided, namely if we were successfully able to read the ref
+ * - Generate informative error messages in the case of failure
  */
-static enum peel_status peel_entry(struct ref_entry *entry, int repeel)
+static int lock_raw_ref(struct files_ref_store *refs,
+                       const char *refname, int mustexist,
+                       const struct string_list *extras,
+                       const struct string_list *skip,
+                       struct ref_lock **lock_p,
+                       struct strbuf *referent,
+                       unsigned int *type,
+                       struct strbuf *err)
 {
-       enum peel_status status;
+       struct ref_lock *lock;
+       struct strbuf ref_file = STRBUF_INIT;
+       int attempts_remaining = 3;
+       int ret = TRANSACTION_GENERIC_ERROR;
+
+       assert(err);
+       files_assert_main_repository(refs, "lock_raw_ref");
+
+       *type = 0;
+
+       /* First lock the file so it can't change out from under us. */
+
+       *lock_p = lock = xcalloc(1, sizeof(*lock));
+
+       lock->ref_name = xstrdup(refname);
+       files_ref_path(refs, &ref_file, refname);
+
+retry:
+       switch (safe_create_leading_directories(ref_file.buf)) {
+       case SCLD_OK:
+               break; /* success */
+       case SCLD_EXISTS:
+               /*
+                * Suppose refname is "refs/foo/bar". We just failed
+                * to create the containing directory, "refs/foo",
+                * because there was a non-directory in the way. This
+                * indicates a D/F conflict, probably because of
+                * another reference such as "refs/foo". There is no
+                * reason to expect this error to be transitory.
+                */
+               if (refs_verify_refname_available(&refs->base, refname,
+                                                 extras, skip, err)) {
+                       if (mustexist) {
+                               /*
+                                * To the user the relevant error is
+                                * that the "mustexist" reference is
+                                * missing:
+                                */
+                               strbuf_reset(err);
+                               strbuf_addf(err, "unable to resolve reference '%s'",
+                                           refname);
+                       } else {
+                               /*
+                                * The error message set by
+                                * refs_verify_refname_available() is
+                                * OK.
+                                */
+                               ret = TRANSACTION_NAME_CONFLICT;
+                       }
+               } else {
+                       /*
+                        * The file that is in the way isn't a loose
+                        * reference. Report it as a low-level
+                        * failure.
+                        */
+                       strbuf_addf(err, "unable to create lock file %s.lock; "
+                                   "non-directory in the way",
+                                   ref_file.buf);
+               }
+               goto error_return;
+       case SCLD_VANISHED:
+               /* Maybe another process was tidying up. Try again. */
+               if (--attempts_remaining > 0)
+                       goto retry;
+               /* fall through */
+       default:
+               strbuf_addf(err, "unable to create directory for %s",
+                           ref_file.buf);
+               goto error_return;
+       }
+
+       if (!lock->lk)
+               lock->lk = xcalloc(1, sizeof(struct lock_file));
 
-       if (entry->flag & REF_KNOWS_PEELED) {
-               if (repeel) {
-                       entry->flag &= ~REF_KNOWS_PEELED;
-                       oidclr(&entry->u.value.peeled);
+       if (hold_lock_file_for_update(lock->lk, ref_file.buf, LOCK_NO_DEREF) < 0) {
+               if (errno == ENOENT && --attempts_remaining > 0) {
+                       /*
+                        * Maybe somebody just deleted one of the
+                        * directories leading to ref_file.  Try
+                        * again:
+                        */
+                       goto retry;
                } else {
-                       return is_null_oid(&entry->u.value.peeled) ?
-                               PEEL_NON_TAG : PEEL_PEELED;
+                       unable_to_lock_message(ref_file.buf, errno, err);
+                       goto error_return;
                }
        }
-       if (entry->flag & REF_ISBROKEN)
-               return PEEL_BROKEN;
-       if (entry->flag & REF_ISSYMREF)
-               return PEEL_IS_SYMREF;
 
-       status = peel_object(entry->u.value.oid.hash, entry->u.value.peeled.hash);
-       if (status == PEEL_PEELED || status == PEEL_NON_TAG)
-               entry->flag |= REF_KNOWS_PEELED;
-       return status;
+       /*
+        * Now we hold the lock and can read the reference without
+        * fear that its value will change.
+        */
+
+       if (files_read_raw_ref(&refs->base, refname,
+                              lock->old_oid.hash, referent, type)) {
+               if (errno == ENOENT) {
+                       if (mustexist) {
+                               /* Garden variety missing reference. */
+                               strbuf_addf(err, "unable to resolve reference '%s'",
+                                           refname);
+                               goto error_return;
+                       } else {
+                               /*
+                                * Reference is missing, but that's OK. We
+                                * know that there is not a conflict with
+                                * another loose reference because
+                                * (supposing that we are trying to lock
+                                * reference "refs/foo/bar"):
+                                *
+                                * - We were successfully able to create
+                                *   the lockfile refs/foo/bar.lock, so we
+                                *   know there cannot be a loose reference
+                                *   named "refs/foo".
+                                *
+                                * - We got ENOENT and not EISDIR, so we
+                                *   know that there cannot be a loose
+                                *   reference named "refs/foo/bar/baz".
+                                */
+                       }
+               } else if (errno == EISDIR) {
+                       /*
+                        * There is a directory in the way. It might have
+                        * contained references that have been deleted. If
+                        * we don't require that the reference already
+                        * exists, try to remove the directory so that it
+                        * doesn't cause trouble when we want to rename the
+                        * lockfile into place later.
+                        */
+                       if (mustexist) {
+                               /* Garden variety missing reference. */
+                               strbuf_addf(err, "unable to resolve reference '%s'",
+                                           refname);
+                               goto error_return;
+                       } else if (remove_dir_recursively(&ref_file,
+                                                         REMOVE_DIR_EMPTY_ONLY)) {
+                               if (refs_verify_refname_available(
+                                                   &refs->base, refname,
+                                                   extras, skip, err)) {
+                                       /*
+                                        * The error message set by
+                                        * verify_refname_available() is OK.
+                                        */
+                                       ret = TRANSACTION_NAME_CONFLICT;
+                                       goto error_return;
+                               } else {
+                                       /*
+                                        * We can't delete the directory,
+                                        * but we also don't know of any
+                                        * references that it should
+                                        * contain.
+                                        */
+                                       strbuf_addf(err, "there is a non-empty directory '%s' "
+                                                   "blocking reference '%s'",
+                                                   ref_file.buf, refname);
+                                       goto error_return;
+                               }
+                       }
+               } else if (errno == EINVAL && (*type & REF_ISBROKEN)) {
+                       strbuf_addf(err, "unable to resolve reference '%s': "
+                                   "reference broken", refname);
+                       goto error_return;
+               } else {
+                       strbuf_addf(err, "unable to resolve reference '%s': %s",
+                                   refname, strerror(errno));
+                       goto error_return;
+               }
+
+               /*
+                * If the ref did not exist and we are creating it,
+                * make sure there is no existing ref that conflicts
+                * with refname:
+                */
+               if (refs_verify_refname_available(
+                                   &refs->base, refname,
+                                   extras, skip, err))
+                       goto error_return;
+       }
+
+       ret = 0;
+       goto out;
+
+error_return:
+       unlock_ref(lock);
+       *lock_p = NULL;
+
+out:
+       strbuf_release(&ref_file);
+       return ret;
 }
 
-int peel_ref(const char *refname, unsigned char *sha1)
+static int files_peel_ref(struct ref_store *ref_store,
+                         const char *refname, unsigned char *sha1)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ | REF_STORE_ODB,
+                              "peel_ref");
        int flag;
        unsigned char base[20];
 
-       if (current_ref && (current_ref->name == refname
-                           || !strcmp(current_ref->name, refname))) {
-               if (peel_entry(current_ref, 0))
+       if (current_ref_iter && current_ref_iter->refname == refname) {
+               struct object_id peeled;
+
+               if (ref_iterator_peel(current_ref_iter, &peeled))
                        return -1;
-               hashcpy(sha1, current_ref->u.value.peeled.hash);
+               hashcpy(sha1, peeled.hash);
                return 0;
        }
 
-       if (read_ref_full(refname, RESOLVE_REF_READING, base, &flag))
+       if (refs_read_ref_full(ref_store, refname,
+                              RESOLVE_REF_READING, base, &flag))
                return -1;
 
        /*
@@ -1650,7 +1002,7 @@ int peel_ref(const char *refname, unsigned char *sha1)
         * have REF_KNOWS_PEELED.
         */
        if (flag & REF_ISPACKED) {
-               struct ref_entry *r = get_packed_ref(refname);
+               struct ref_entry *r = get_packed_ref(refs, refname);
                if (r) {
                        if (peel_entry(r, 0))
                                return -1;
@@ -1662,177 +1014,121 @@ int peel_ref(const char *refname, unsigned char *sha1)
        return peel_object(base, sha1);
 }
 
-/*
- * Call fn for each reference in the specified ref_cache, omitting
- * references not in the containing_dir of base.  fn is called for all
- * references, including broken ones.  If fn ever returns a non-zero
- * value, stop the iteration and return that value; otherwise, return
- * 0.
- */
-static int do_for_each_entry(struct ref_cache *refs, const char *base,
-                            each_ref_entry_fn fn, void *cb_data)
-{
-       struct packed_ref_cache *packed_ref_cache;
-       struct ref_dir *loose_dir;
-       struct ref_dir *packed_dir;
-       int retval = 0;
-
-       /*
-        * We must make sure that all loose refs are read before accessing the
-        * packed-refs file; this avoids a race condition in which loose refs
-        * are migrated to the packed-refs file by a simultaneous process, but
-        * our in-memory view is from before the migration. get_packed_ref_cache()
-        * takes care of making sure our view is up to date with what is on
-        * disk.
-        */
-       loose_dir = get_loose_refs(refs);
-       if (base && *base) {
-               loose_dir = find_containing_dir(loose_dir, base, 0);
-       }
-       if (loose_dir)
-               prime_ref_dir(loose_dir);
-
-       packed_ref_cache = get_packed_ref_cache(refs);
-       acquire_packed_ref_cache(packed_ref_cache);
-       packed_dir = get_packed_ref_dir(packed_ref_cache);
-       if (base && *base) {
-               packed_dir = find_containing_dir(packed_dir, base, 0);
-       }
-
-       if (packed_dir && loose_dir) {
-               sort_ref_dir(packed_dir);
-               sort_ref_dir(loose_dir);
-               retval = do_for_each_entry_in_dirs(
-                               packed_dir, loose_dir, fn, cb_data);
-       } else if (packed_dir) {
-               sort_ref_dir(packed_dir);
-               retval = do_for_each_entry_in_dir(
-                               packed_dir, 0, fn, cb_data);
-       } else if (loose_dir) {
-               sort_ref_dir(loose_dir);
-               retval = do_for_each_entry_in_dir(
-                               loose_dir, 0, fn, cb_data);
-       }
+struct files_ref_iterator {
+       struct ref_iterator base;
 
-       release_packed_ref_cache(packed_ref_cache);
-       return retval;
-}
+       struct packed_ref_cache *packed_ref_cache;
+       struct ref_iterator *iter0;
+       unsigned int flags;
+};
 
-/*
- * Call fn for each reference in the specified ref_cache for which the
- * refname begins with base.  If trim is non-zero, then trim that many
- * characters off the beginning of each refname before passing the
- * refname to fn.  flags can be DO_FOR_EACH_INCLUDE_BROKEN to include
- * broken references in the iteration.  If fn ever returns a non-zero
- * value, stop the iteration and return that value; otherwise, return
- * 0.
- */
-static int do_for_each_ref(struct ref_cache *refs, const char *base,
-                          each_ref_fn fn, int trim, int flags, void *cb_data)
+static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
 {
-       struct ref_entry_cb data;
-       data.base = base;
-       data.trim = trim;
-       data.flags = flags;
-       data.fn = fn;
-       data.cb_data = cb_data;
+       struct files_ref_iterator *iter =
+               (struct files_ref_iterator *)ref_iterator;
+       int ok;
 
-       if (ref_paranoia < 0)
-               ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
-       if (ref_paranoia)
-               data.flags |= DO_FOR_EACH_INCLUDE_BROKEN;
-
-       return do_for_each_entry(refs, base, do_one_ref, &data);
-}
-
-static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
-{
-       struct object_id oid;
-       int flag;
+       while ((ok = ref_iterator_advance(iter->iter0)) == ITER_OK) {
+               if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
+                   ref_type(iter->iter0->refname) != REF_TYPE_PER_WORKTREE)
+                       continue;
 
-       if (submodule) {
-               if (resolve_gitlink_ref(submodule, "HEAD", oid.hash) == 0)
-                       return fn("HEAD", &oid, 0, cb_data);
+               if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+                   !ref_resolves_to_object(iter->iter0->refname,
+                                           iter->iter0->oid,
+                                           iter->iter0->flags))
+                       continue;
 
-               return 0;
+               iter->base.refname = iter->iter0->refname;
+               iter->base.oid = iter->iter0->oid;
+               iter->base.flags = iter->iter0->flags;
+               return ITER_OK;
        }
 
-       if (!read_ref_full("HEAD", RESOLVE_REF_READING, oid.hash, &flag))
-               return fn("HEAD", &oid, flag, cb_data);
+       iter->iter0 = NULL;
+       if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+               ok = ITER_ERROR;
 
-       return 0;
+       return ok;
 }
 
-int head_ref(each_ref_fn fn, void *cb_data)
+static int files_ref_iterator_peel(struct ref_iterator *ref_iterator,
+                                  struct object_id *peeled)
 {
-       return do_head_ref(NULL, fn, cb_data);
-}
+       struct files_ref_iterator *iter =
+               (struct files_ref_iterator *)ref_iterator;
 
-int head_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
-{
-       return do_head_ref(submodule, fn, cb_data);
+       return ref_iterator_peel(iter->iter0, peeled);
 }
 
-int for_each_ref(each_ref_fn fn, void *cb_data)
+static int files_ref_iterator_abort(struct ref_iterator *ref_iterator)
 {
-       return do_for_each_ref(&ref_cache, "", fn, 0, 0, cb_data);
-}
+       struct files_ref_iterator *iter =
+               (struct files_ref_iterator *)ref_iterator;
+       int ok = ITER_DONE;
 
-int for_each_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
-{
-       return do_for_each_ref(get_ref_cache(submodule), "", fn, 0, 0, cb_data);
-}
+       if (iter->iter0)
+               ok = ref_iterator_abort(iter->iter0);
 
-int for_each_ref_in(const char *prefix, each_ref_fn fn, void *cb_data)
-{
-       return do_for_each_ref(&ref_cache, prefix, fn, strlen(prefix), 0, cb_data);
+       release_packed_ref_cache(iter->packed_ref_cache);
+       base_ref_iterator_free(ref_iterator);
+       return ok;
 }
 
-int for_each_fullref_in(const char *prefix, each_ref_fn fn, void *cb_data, unsigned int broken)
+static struct ref_iterator_vtable files_ref_iterator_vtable = {
+       files_ref_iterator_advance,
+       files_ref_iterator_peel,
+       files_ref_iterator_abort
+};
+
+static struct ref_iterator *files_ref_iterator_begin(
+               struct ref_store *ref_store,
+               const char *prefix, unsigned int flags)
 {
-       unsigned int flag = 0;
+       struct files_ref_store *refs;
+       struct ref_iterator *loose_iter, *packed_iter;
+       struct files_ref_iterator *iter;
+       struct ref_iterator *ref_iterator;
+       unsigned int required_flags = REF_STORE_READ;
 
-       if (broken)
-               flag = DO_FOR_EACH_INCLUDE_BROKEN;
-       return do_for_each_ref(&ref_cache, prefix, fn, 0, flag, cb_data);
-}
+       if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
+               required_flags |= REF_STORE_ODB;
 
-int for_each_ref_in_submodule(const char *submodule, const char *prefix,
-               each_ref_fn fn, void *cb_data)
-{
-       return do_for_each_ref(get_ref_cache(submodule), prefix, fn, strlen(prefix), 0, cb_data);
-}
+       refs = files_downcast(ref_store, required_flags, "ref_iterator_begin");
 
-int for_each_replace_ref(each_ref_fn fn, void *cb_data)
-{
-       return do_for_each_ref(&ref_cache, git_replace_ref_base, fn,
-                              strlen(git_replace_ref_base), 0, cb_data);
-}
+       iter = xcalloc(1, sizeof(*iter));
+       ref_iterator = &iter->base;
+       base_ref_iterator_init(ref_iterator, &files_ref_iterator_vtable);
 
-int for_each_namespaced_ref(each_ref_fn fn, void *cb_data)
-{
-       struct strbuf buf = STRBUF_INIT;
-       int ret;
-       strbuf_addf(&buf, "%srefs/", get_git_namespace());
-       ret = do_for_each_ref(&ref_cache, buf.buf, fn, 0, 0, cb_data);
-       strbuf_release(&buf);
-       return ret;
-}
+       /*
+        * We must make sure that all loose refs are read before
+        * accessing the packed-refs file; this avoids a race
+        * condition if loose refs are migrated to the packed-refs
+        * file by a simultaneous process, but our in-memory view is
+        * from before the migration. We ensure this as follows:
+        * First, we call start the loose refs iteration with its
+        * `prime_ref` argument set to true. This causes the loose
+        * references in the subtree to be pre-read into the cache.
+        * (If they've already been read, that's OK; we only need to
+        * guarantee that they're read before the packed refs, not
+        * *how much* before.) After that, we call
+        * get_packed_ref_cache(), which internally checks whether the
+        * packed-ref cache is up to date with what is on disk, and
+        * re-reads it if not.
+        */
 
-int for_each_rawref(each_ref_fn fn, void *cb_data)
-{
-       return do_for_each_ref(&ref_cache, "", fn, 0,
-                              DO_FOR_EACH_INCLUDE_BROKEN, cb_data);
-}
+       loose_iter = cache_ref_iterator_begin(get_loose_ref_cache(refs),
+                                             prefix, 1);
 
-static void unlock_ref(struct ref_lock *lock)
-{
-       /* Do not free lock->lk -- atexit() still looks at them */
-       if (lock->lk)
-               rollback_lock_file(lock->lk);
-       free(lock->ref_name);
-       free(lock->orig_ref_name);
-       free(lock);
+       iter->packed_ref_cache = get_packed_ref_cache(refs);
+       acquire_packed_ref_cache(iter->packed_ref_cache);
+       packed_iter = cache_ref_iterator_begin(iter->packed_ref_cache->cache,
+                                              prefix, 0);
+
+       iter->iter0 = overlay_ref_iterator_begin(loose_iter, packed_iter);
+       iter->flags = flags;
+
+       return ref_iterator;
 }
 
 /*
@@ -1841,29 +1137,29 @@ static void unlock_ref(struct ref_lock *lock)
  * on success. On error, write an error message to err, set errno, and
  * return a negative value.
  */
-static int verify_lock(struct ref_lock *lock,
+static int verify_lock(struct ref_store *ref_store, struct ref_lock *lock,
                       const unsigned char *old_sha1, int mustexist,
                       struct strbuf *err)
 {
        assert(err);
 
-       if (read_ref_full(lock->ref_name,
-                         mustexist ? RESOLVE_REF_READING : 0,
-                         lock->old_oid.hash, NULL)) {
+       if (refs_read_ref_full(ref_store, lock->ref_name,
+                              mustexist ? RESOLVE_REF_READING : 0,
+                              lock->old_oid.hash, NULL)) {
                if (old_sha1) {
                        int save_errno = errno;
-                       strbuf_addf(err, "can't verify ref %s", lock->ref_name);
+                       strbuf_addf(err, "can't verify ref '%s'", lock->ref_name);
                        errno = save_errno;
                        return -1;
                } else {
-                       hashclr(lock->old_oid.hash);
+                       oidclr(&lock->old_oid);
                        return 0;
                }
        }
        if (old_sha1 && hashcmp(lock->old_oid.hash, old_sha1)) {
-               strbuf_addf(err, "ref %s is at %s but expected %s",
+               strbuf_addf(err, "ref '%s' is at %s but expected %s",
                            lock->ref_name,
-                           sha1_to_hex(lock->old_oid.hash),
+                           oid_to_hex(&lock->old_oid),
                            sha1_to_hex(old_sha1));
                errno = EBUSY;
                return -1;
@@ -1881,28 +1177,33 @@ static int remove_empty_directories(struct strbuf *path)
        return remove_dir_recursively(path, REMOVE_DIR_EMPTY_ONLY);
 }
 
+static int create_reflock(const char *path, void *cb)
+{
+       struct lock_file *lk = cb;
+
+       return hold_lock_file_for_update(lk, path, LOCK_NO_DEREF) < 0 ? -1 : 0;
+}
+
 /*
  * Locks a ref returning the lock on success and NULL on failure.
  * On failure errno is set to something meaningful.
  */
-static struct ref_lock *lock_ref_sha1_basic(const char *refname,
+static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
+                                           const char *refname,
                                            const unsigned char *old_sha1,
                                            const struct string_list *extras,
                                            const struct string_list *skip,
-                                           unsigned int flags, int *type_p,
+                                           unsigned int flags, int *type,
                                            struct strbuf *err)
 {
        struct strbuf ref_file = STRBUF_INIT;
-       struct strbuf orig_ref_file = STRBUF_INIT;
-       const char *orig_refname = refname;
        struct ref_lock *lock;
        int last_errno = 0;
-       int type;
-       int lflags = 0;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
-       int resolve_flags = 0;
-       int attempts_remaining = 3;
+       int resolve_flags = RESOLVE_REF_NO_RECURSE;
+       int resolved;
 
+       files_assert_main_repository(refs, "lock_ref_sha1_basic");
        assert(err);
 
        lock = xcalloc(1, sizeof(struct ref_lock));
@@ -1911,48 +1212,42 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
                resolve_flags |= RESOLVE_REF_READING;
        if (flags & REF_DELETING)
                resolve_flags |= RESOLVE_REF_ALLOW_BAD_NAME;
-       if (flags & REF_NODEREF) {
-               resolve_flags |= RESOLVE_REF_NO_RECURSE;
-               lflags |= LOCK_NO_DEREF;
-       }
 
-       refname = resolve_ref_unsafe(refname, resolve_flags,
-                                    lock->old_oid.hash, &type);
-       if (!refname && errno == EISDIR) {
+       files_ref_path(refs, &ref_file, refname);
+       resolved = !!refs_resolve_ref_unsafe(&refs->base,
+                                            refname, resolve_flags,
+                                            lock->old_oid.hash, type);
+       if (!resolved && errno == EISDIR) {
                /*
                 * we are trying to lock foo but we used to
                 * have foo/bar which now does not exist;
                 * it is normal for the empty directory 'foo'
                 * to remain.
                 */
-               strbuf_git_path(&orig_ref_file, "%s", orig_refname);
-               if (remove_empty_directories(&orig_ref_file)) {
+               if (remove_empty_directories(&ref_file)) {
                        last_errno = errno;
-                       if (!verify_refname_available_dir(orig_refname, extras, skip,
-                                                         get_loose_refs(&ref_cache), err))
+                       if (!refs_verify_refname_available(
+                                           &refs->base,
+                                           refname, extras, skip, err))
                                strbuf_addf(err, "there are still refs under '%s'",
-                                           orig_refname);
+                                           refname);
                        goto error_return;
                }
-               refname = resolve_ref_unsafe(orig_refname, resolve_flags,
-                                            lock->old_oid.hash, &type);
+               resolved = !!refs_resolve_ref_unsafe(&refs->base,
+                                                    refname, resolve_flags,
+                                                    lock->old_oid.hash, type);
        }
-       if (type_p)
-           *type_p = type;
-       if (!refname) {
+       if (!resolved) {
                last_errno = errno;
                if (last_errno != ENOTDIR ||
-                   !verify_refname_available_dir(orig_refname, extras, skip,
-                                                 get_loose_refs(&ref_cache), err))
-                       strbuf_addf(err, "unable to resolve reference %s: %s",
-                                   orig_refname, strerror(last_errno));
+                   !refs_verify_refname_available(&refs->base, refname,
+                                                  extras, skip, err))
+                       strbuf_addf(err, "unable to resolve reference '%s': %s",
+                                   refname, strerror(last_errno));
 
                goto error_return;
        }
 
-       if (flags & REF_NODEREF)
-               refname = orig_refname;
-
        /*
         * If the ref did not exist and we are creating it, make sure
         * there is no existing packed ref whose name begins with our
@@ -1960,8 +1255,8 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
         * our refname.
         */
        if (is_null_oid(&lock->old_oid) &&
-           verify_refname_available_dir(refname, extras, skip,
-                                        get_packed_refs(&ref_cache), err)) {
+           refs_verify_refname_available(&refs->base, refname,
+                                         extras, skip, err)) {
                last_errno = ENOTDIR;
                goto error_return;
        }
@@ -1969,39 +1264,14 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
        lock->lk = xcalloc(1, sizeof(struct lock_file));
 
        lock->ref_name = xstrdup(refname);
-       lock->orig_ref_name = xstrdup(orig_refname);
-       strbuf_git_path(&ref_file, "%s", refname);
 
- retry:
-       switch (safe_create_leading_directories_const(ref_file.buf)) {
-       case SCLD_OK:
-               break; /* success */
-       case SCLD_VANISHED:
-               if (--attempts_remaining > 0)
-                       goto retry;
-               /* fall through */
-       default:
+       if (raceproof_create_file(ref_file.buf, create_reflock, lock->lk)) {
                last_errno = errno;
-               strbuf_addf(err, "unable to create directory for %s",
-                           ref_file.buf);
+               unable_to_lock_message(ref_file.buf, errno, err);
                goto error_return;
        }
 
-       if (hold_lock_file_for_update(lock->lk, ref_file.buf, lflags) < 0) {
-               last_errno = errno;
-               if (errno == ENOENT && --attempts_remaining > 0)
-                       /*
-                        * Maybe somebody just deleted one of the
-                        * directories leading to ref_file.  Try
-                        * again:
-                        */
-                       goto retry;
-               else {
-                       unable_to_lock_message(ref_file.buf, errno, err);
-                       goto error_return;
-               }
-       }
-       if (verify_lock(lock, old_sha1, mustexist, err)) {
+       if (verify_lock(&refs->base, lock, old_sha1, mustexist, err)) {
                last_errno = errno;
                goto error_return;
        }
@@ -2013,7 +1283,6 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 
  out:
        strbuf_release(&ref_file);
-       strbuf_release(&orig_ref_file);
        errno = last_errno;
        return lock;
 }
@@ -2022,59 +1291,49 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
  * Write an entry to the packed-refs file for the specified refname.
  * If peeled is non-NULL, write it as the entry's peeled value.
  */
-static void write_packed_entry(FILE *fh, char *refname, unsigned char *sha1,
-                              unsigned char *peeled)
+static void write_packed_entry(FILE *fh, const char *refname,
+                              const unsigned char *sha1,
+                              const unsigned char *peeled)
 {
        fprintf_or_die(fh, "%s %s\n", sha1_to_hex(sha1), refname);
        if (peeled)
                fprintf_or_die(fh, "^%s\n", sha1_to_hex(peeled));
 }
 
-/*
- * An each_ref_entry_fn that writes the entry to a packed-refs file.
- */
-static int write_packed_entry_fn(struct ref_entry *entry, void *cb_data)
-{
-       enum peel_status peel_status = peel_entry(entry, 0);
-
-       if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
-               error("internal error: %s is not a valid packed reference!",
-                     entry->name);
-       write_packed_entry(cb_data, entry->name, entry->u.value.oid.hash,
-                          peel_status == PEEL_PEELED ?
-                          entry->u.value.peeled.hash : NULL);
-       return 0;
-}
-
 /*
  * Lock the packed-refs file for writing. Flags is passed to
  * hold_lock_file_for_update(). Return 0 on success. On errors, set
  * errno appropriately and return a nonzero value.
  */
-static int lock_packed_refs(int flags)
+static int lock_packed_refs(struct files_ref_store *refs, int flags)
 {
        static int timeout_configured = 0;
        static int timeout_value = 1000;
-
        struct packed_ref_cache *packed_ref_cache;
 
+       files_assert_main_repository(refs, "lock_packed_refs");
+
        if (!timeout_configured) {
                git_config_get_int("core.packedrefstimeout", &timeout_value);
                timeout_configured = 1;
        }
 
        if (hold_lock_file_for_update_timeout(
-                           &packlock, git_path("packed-refs"),
+                           &refs->packed_refs_lock, files_packed_refs_path(refs),
                            flags, timeout_value) < 0)
                return -1;
+
        /*
-        * Get the current packed-refs while holding the lock.  If the
-        * packed-refs file has been modified since we last read it,
-        * this will automatically invalidate the cache and re-read
-        * the packed-refs file.
+        * Now that we hold the `packed-refs` lock, make sure that our
+        * cache matches the current version of the file. Normally
+        * `get_packed_ref_cache()` does that for us, but that
+        * function assumes that when the file is locked, any existing
+        * cache is still valid. We've just locked the file, but it
+        * might have changed the moment *before* we locked it.
         */
-       packed_ref_cache = get_packed_ref_cache(&ref_cache);
-       packed_ref_cache->lock = &packlock;
+       validate_packed_ref_cache(refs);
+
+       packed_ref_cache = get_packed_ref_cache(refs);
        /* Increment the reference count to prevent it from being freed: */
        acquire_packed_ref_cache(packed_ref_cache);
        return 0;
@@ -2086,30 +1345,42 @@ static int lock_packed_refs(int flags)
  * lock_packed_refs()). Return zero on success. On errors, set errno
  * and return a nonzero value
  */
-static int commit_packed_refs(void)
+static int commit_packed_refs(struct files_ref_store *refs)
 {
        struct packed_ref_cache *packed_ref_cache =
-               get_packed_ref_cache(&ref_cache);
-       int error = 0;
+               get_packed_ref_cache(refs);
+       int ok, error = 0;
        int save_errno = 0;
        FILE *out;
+       struct ref_iterator *iter;
+
+       files_assert_main_repository(refs, "commit_packed_refs");
 
-       if (!packed_ref_cache->lock)
-               die("internal error: packed-refs not locked");
+       if (!is_lock_file_locked(&refs->packed_refs_lock))
+               die("BUG: packed-refs not locked");
 
-       out = fdopen_lock_file(packed_ref_cache->lock, "w");
+       out = fdopen_lock_file(&refs->packed_refs_lock, "w");
        if (!out)
                die_errno("unable to fdopen packed-refs descriptor");
 
        fprintf_or_die(out, "%s", PACKED_REFS_HEADER);
-       do_for_each_entry_in_dir(get_packed_ref_dir(packed_ref_cache),
-                                0, write_packed_entry_fn, out);
 
-       if (commit_lock_file(packed_ref_cache->lock)) {
+       iter = cache_ref_iterator_begin(packed_ref_cache->cache, NULL, 0);
+       while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+               struct object_id peeled;
+               int peel_error = ref_iterator_peel(iter, &peeled);
+
+               write_packed_entry(out, iter->refname, iter->oid->hash,
+                                  peel_error ? NULL : peeled.hash);
+       }
+
+       if (ok != ITER_DONE)
+               die("error while iterating over references");
+
+       if (commit_lock_file(&refs->packed_refs_lock)) {
                save_errno = errno;
                error = -1;
        }
-       packed_ref_cache->lock = NULL;
        release_packed_ref_cache(packed_ref_cache);
        errno = save_errno;
        return error;
@@ -2120,17 +1391,18 @@ static int commit_packed_refs(void)
  * in-memory packed reference cache.  (The packed-refs file will be
  * read anew if it is needed again after this function is called.)
  */
-static void rollback_packed_refs(void)
+static void rollback_packed_refs(struct files_ref_store *refs)
 {
        struct packed_ref_cache *packed_ref_cache =
-               get_packed_ref_cache(&ref_cache);
+               get_packed_ref_cache(refs);
 
-       if (!packed_ref_cache->lock)
-               die("internal error: packed-refs not locked");
-       rollback_lock_file(packed_ref_cache->lock);
-       packed_ref_cache->lock = NULL;
+       files_assert_main_repository(refs, "rollback_packed_refs");
+
+       if (!is_lock_file_locked(&refs->packed_refs_lock))
+               die("BUG: packed-refs not locked");
+       rollback_lock_file(&refs->packed_refs_lock);
        release_packed_ref_cache(packed_ref_cache);
-       clear_packed_ref_cache(&ref_cache);
+       clear_packed_ref_cache(refs);
 }
 
 struct ref_to_prune {
@@ -2139,74 +1411,28 @@ struct ref_to_prune {
        char name[FLEX_ARRAY];
 };
 
-struct pack_refs_cb_data {
-       unsigned int flags;
-       struct ref_dir *packed_refs;
-       struct ref_to_prune *ref_to_prune;
+enum {
+       REMOVE_EMPTY_PARENTS_REF = 0x01,
+       REMOVE_EMPTY_PARENTS_REFLOG = 0x02
 };
 
 /*
- * An each_ref_entry_fn that is run over loose references only.  If
- * the loose reference can be packed, add an entry in the packed ref
- * cache.  If the reference should be pruned, also add it to
- * ref_to_prune in the pack_refs_cb_data.
- */
-static int pack_if_possible_fn(struct ref_entry *entry, void *cb_data)
-{
-       struct pack_refs_cb_data *cb = cb_data;
-       enum peel_status peel_status;
-       struct ref_entry *packed_entry;
-       int is_tag_ref = starts_with(entry->name, "refs/tags/");
-
-       /* Do not pack per-worktree refs: */
-       if (ref_type(entry->name) != REF_TYPE_NORMAL)
-               return 0;
-
-       /* ALWAYS pack tags */
-       if (!(cb->flags & PACK_REFS_ALL) && !is_tag_ref)
-               return 0;
-
-       /* Do not pack symbolic or broken refs: */
-       if ((entry->flag & REF_ISSYMREF) || !ref_resolves_to_object(entry))
-               return 0;
-
-       /* Add a packed ref cache entry equivalent to the loose entry. */
-       peel_status = peel_entry(entry, 1);
-       if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
-               die("internal error peeling reference %s (%s)",
-                   entry->name, oid_to_hex(&entry->u.value.oid));
-       packed_entry = find_ref(cb->packed_refs, entry->name);
-       if (packed_entry) {
-               /* Overwrite existing packed entry with info from loose entry */
-               packed_entry->flag = REF_ISPACKED | REF_KNOWS_PEELED;
-               oidcpy(&packed_entry->u.value.oid, &entry->u.value.oid);
-       } else {
-               packed_entry = create_ref_entry(entry->name, entry->u.value.oid.hash,
-                                               REF_ISPACKED | REF_KNOWS_PEELED, 0);
-               add_ref(cb->packed_refs, packed_entry);
-       }
-       oidcpy(&packed_entry->u.value.peeled, &entry->u.value.peeled);
-
-       /* Schedule the loose reference for pruning if requested. */
-       if ((cb->flags & PACK_REFS_PRUNE)) {
-               struct ref_to_prune *n;
-               FLEX_ALLOC_STR(n, name, entry->name);
-               hashcpy(n->sha1, entry->u.value.oid.hash);
-               n->next = cb->ref_to_prune;
-               cb->ref_to_prune = n;
-       }
-       return 0;
-}
-
-/*
- * Remove empty parents, but spare refs/ and immediate subdirs.
- * Note: munges *name.
+ * Remove empty parent directories associated with the specified
+ * reference and/or its reflog, but spare [logs/]refs/ and immediate
+ * subdirs. flags is a combination of REMOVE_EMPTY_PARENTS_REF and/or
+ * REMOVE_EMPTY_PARENTS_REFLOG.
  */
-static void try_remove_empty_parents(char *name)
+static void try_remove_empty_parents(struct files_ref_store *refs,
+                                    const char *refname,
+                                    unsigned int flags)
 {
+       struct strbuf buf = STRBUF_INIT;
+       struct strbuf sb = STRBUF_INIT;
        char *p, *q;
        int i;
-       p = name;
+
+       strbuf_addstr(&buf, refname);
+       p = buf.buf;
        for (i = 0; i < 2; i++) { /* refs/{heads,tags,...}/ */
                while (*p && *p != '/')
                        p++;
@@ -2214,23 +1440,32 @@ static void try_remove_empty_parents(char *name)
                while (*p == '/')
                        p++;
        }
-       for (q = p; *q; q++)
-               ;
-       while (1) {
+       q = buf.buf + buf.len;
+       while (flags & (REMOVE_EMPTY_PARENTS_REF | REMOVE_EMPTY_PARENTS_REFLOG)) {
                while (q > p && *q != '/')
                        q--;
                while (q > p && *(q-1) == '/')
                        q--;
                if (q == p)
                        break;
-               *q = '\0';
-               if (rmdir(git_path("%s", name)))
-                       break;
+               strbuf_setlen(&buf, q - buf.buf);
+
+               strbuf_reset(&sb);
+               files_ref_path(refs, &sb, buf.buf);
+               if ((flags & REMOVE_EMPTY_PARENTS_REF) && rmdir(sb.buf))
+                       flags &= ~REMOVE_EMPTY_PARENTS_REF;
+
+               strbuf_reset(&sb);
+               files_reflog_path(refs, &sb, buf.buf);
+               if ((flags & REMOVE_EMPTY_PARENTS_REFLOG) && rmdir(sb.buf))
+                       flags &= ~REMOVE_EMPTY_PARENTS_REFLOG;
        }
+       strbuf_release(&buf);
+       strbuf_release(&sb);
 }
 
 /* make sure nobody touched the ref, and unlink */
-static void prune_ref(struct ref_to_prune *r)
+static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
 {
        struct ref_transaction *transaction;
        struct strbuf err = STRBUF_INIT;
@@ -2238,10 +1473,10 @@ static void prune_ref(struct ref_to_prune *r)
        if (check_refname_format(r->name, 0))
                return;
 
-       transaction = ref_transaction_begin(&err);
+       transaction = ref_store_transaction_begin(&refs->base, &err);
        if (!transaction ||
            ref_transaction_delete(transaction, r->name, r->sha1,
-                                  REF_ISPRUNING, NULL, &err) ||
+                                  REF_ISPRUNING | REF_NODEREF, NULL, &err) ||
            ref_transaction_commit(transaction, &err)) {
                ref_transaction_free(transaction);
                error("%s", err.buf);
@@ -2250,34 +1485,102 @@ static void prune_ref(struct ref_to_prune *r)
        }
        ref_transaction_free(transaction);
        strbuf_release(&err);
-       try_remove_empty_parents(r->name);
 }
 
-static void prune_refs(struct ref_to_prune *r)
+static void prune_refs(struct files_ref_store *refs, struct ref_to_prune *r)
 {
        while (r) {
-               prune_ref(r);
+               prune_ref(refs, r);
                r = r->next;
        }
 }
 
-int pack_refs(unsigned int flags)
+/*
+ * Return true if the specified reference should be packed.
+ */
+static int should_pack_ref(const char *refname,
+                          const struct object_id *oid, unsigned int ref_flags,
+                          unsigned int pack_flags)
+{
+       /* Do not pack per-worktree refs: */
+       if (ref_type(refname) != REF_TYPE_NORMAL)
+               return 0;
+
+       /* Do not pack non-tags unless PACK_REFS_ALL is set: */
+       if (!(pack_flags & PACK_REFS_ALL) && !starts_with(refname, "refs/tags/"))
+               return 0;
+
+       /* Do not pack symbolic refs: */
+       if (ref_flags & REF_ISSYMREF)
+               return 0;
+
+       /* Do not pack broken refs: */
+       if (!ref_resolves_to_object(refname, oid, ref_flags))
+               return 0;
+
+       return 1;
+}
+
+static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 {
-       struct pack_refs_cb_data cbdata;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE | REF_STORE_ODB,
+                              "pack_refs");
+       struct ref_iterator *iter;
+       struct ref_dir *packed_refs;
+       int ok;
+       struct ref_to_prune *refs_to_prune = NULL;
+
+       lock_packed_refs(refs, LOCK_DIE_ON_ERROR);
+       packed_refs = get_packed_refs(refs);
 
-       memset(&cbdata, 0, sizeof(cbdata));
-       cbdata.flags = flags;
+       iter = cache_ref_iterator_begin(get_loose_ref_cache(refs), NULL, 0);
+       while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+               /*
+                * If the loose reference can be packed, add an entry
+                * in the packed ref cache. If the reference should be
+                * pruned, also add it to refs_to_prune.
+                */
+               struct ref_entry *packed_entry;
 
-       lock_packed_refs(LOCK_DIE_ON_ERROR);
-       cbdata.packed_refs = get_packed_refs(&ref_cache);
+               if (!should_pack_ref(iter->refname, iter->oid, iter->flags,
+                                    flags))
+                       continue;
 
-       do_for_each_entry_in_dir(get_loose_refs(&ref_cache), 0,
-                                pack_if_possible_fn, &cbdata);
+               /*
+                * Create an entry in the packed-refs cache equivalent
+                * to the one from the loose ref cache, except that
+                * we don't copy the peeled status, because we want it
+                * to be re-peeled.
+                */
+               packed_entry = find_ref_entry(packed_refs, iter->refname);
+               if (packed_entry) {
+                       /* Overwrite existing packed entry with info from loose entry */
+                       packed_entry->flag = REF_ISPACKED;
+                       oidcpy(&packed_entry->u.value.oid, iter->oid);
+               } else {
+                       packed_entry = create_ref_entry(iter->refname, iter->oid,
+                                                       REF_ISPACKED);
+                       add_ref_entry(packed_refs, packed_entry);
+               }
+               oidclr(&packed_entry->u.value.peeled);
+
+               /* Schedule the loose reference for pruning if requested. */
+               if ((flags & PACK_REFS_PRUNE)) {
+                       struct ref_to_prune *n;
+                       FLEX_ALLOC_STR(n, name, iter->refname);
+                       hashcpy(n->sha1, iter->oid->hash);
+                       n->next = refs_to_prune;
+                       refs_to_prune = n;
+               }
+       }
+       if (ok != ITER_DONE)
+               die("error while iterating over references");
 
-       if (commit_packed_refs())
+       if (commit_packed_refs(refs))
                die_errno("unable to overwrite old ref-pack file");
 
-       prune_refs(cbdata.ref_to_prune);
+       prune_refs(refs, refs_to_prune);
        return 0;
 }
 
@@ -2288,17 +1591,19 @@ int pack_refs(unsigned int flags)
  *
  * The refs in 'refnames' needn't be sorted. `err` must not be NULL.
  */
-static int repack_without_refs(struct string_list *refnames, struct strbuf *err)
+static int repack_without_refs(struct files_ref_store *refs,
+                              struct string_list *refnames, struct strbuf *err)
 {
        struct ref_dir *packed;
        struct string_list_item *refname;
        int ret, needs_repacking = 0, removed = 0;
 
+       files_assert_main_repository(refs, "repack_without_refs");
        assert(err);
 
        /* Look for a packed ref */
        for_each_string_list_item(refname, refnames) {
-               if (get_packed_ref(refname->string)) {
+               if (get_packed_ref(refs, refname->string)) {
                        needs_repacking = 1;
                        break;
                }
@@ -2308,60 +1613,45 @@ static int repack_without_refs(struct string_list *refnames, struct strbuf *err)
        if (!needs_repacking)
                return 0; /* no refname exists in packed refs */
 
-       if (lock_packed_refs(0)) {
-               unable_to_lock_message(git_path("packed-refs"), errno, err);
+       if (lock_packed_refs(refs, 0)) {
+               unable_to_lock_message(files_packed_refs_path(refs), errno, err);
                return -1;
        }
-       packed = get_packed_refs(&ref_cache);
+       packed = get_packed_refs(refs);
 
        /* Remove refnames from the cache */
        for_each_string_list_item(refname, refnames)
-               if (remove_entry(packed, refname->string) != -1)
+               if (remove_entry_from_dir(packed, refname->string) != -1)
                        removed = 1;
        if (!removed) {
                /*
                 * All packed entries disappeared while we were
                 * acquiring the lock.
                 */
-               rollback_packed_refs();
+               rollback_packed_refs(refs);
                return 0;
        }
 
        /* Write what remains */
-       ret = commit_packed_refs();
+       ret = commit_packed_refs(refs);
        if (ret)
                strbuf_addf(err, "unable to overwrite old ref-pack file: %s",
                            strerror(errno));
        return ret;
 }
 
-static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
-{
-       assert(err);
-
-       if (!(flag & REF_ISPACKED) || flag & REF_ISSYMREF) {
-               /*
-                * loose.  The loose file name is the same as the
-                * lockfile name, minus ".lock":
-                */
-               char *loose_filename = get_locked_file_path(lock->lk);
-               int res = unlink_or_msg(loose_filename, err);
-               free(loose_filename);
-               if (res)
-                       return 1;
-       }
-       return 0;
-}
-
-int delete_refs(struct string_list *refnames)
+static int files_delete_refs(struct ref_store *ref_store, const char *msg,
+                            struct string_list *refnames, unsigned int flags)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
        struct strbuf err = STRBUF_INIT;
        int i, result = 0;
 
        if (!refnames->nr)
                return 0;
 
-       result = repack_without_refs(refnames, &err);
+       result = repack_without_refs(refs, refnames, &err);
        if (result) {
                /*
                 * If we failed to rewrite the packed-refs file, then
@@ -2382,7 +1672,7 @@ int delete_refs(struct string_list *refnames)
        for (i = 0; i < refnames->nr; i++) {
                const char *refname = refnames->items[i].string;
 
-               if (delete_ref(refname, NULL, 0))
+               if (refs_delete_ref(&refs->base, msg, refname, NULL, flags))
                        result |= error(_("could not remove reference %s"), refname);
        }
 
@@ -2398,123 +1688,138 @@ int delete_refs(struct string_list *refnames)
  * IOW, to avoid cross device rename errors, the temporary renamed log must
  * live into logs/refs.
  */
-#define TMP_RENAMED_LOG  "logs/refs/.tmp-renamed-log"
+#define TMP_RENAMED_LOG  "refs/.tmp-renamed-log"
 
-static int rename_tmp_log(const char *newrefname)
-{
-       int attempts_remaining = 4;
-       struct strbuf path = STRBUF_INIT;
-       int ret = -1;
+struct rename_cb {
+       const char *tmp_renamed_log;
+       int true_errno;
+};
 
- retry:
-       strbuf_reset(&path);
-       strbuf_git_path(&path, "logs/%s", newrefname);
-       switch (safe_create_leading_directories_const(path.buf)) {
-       case SCLD_OK:
-               break; /* success */
-       case SCLD_VANISHED:
-               if (--attempts_remaining > 0)
-                       goto retry;
-               /* fall through */
-       default:
-               error("unable to create directory for %s", newrefname);
-               goto out;
-       }
+static int rename_tmp_log_callback(const char *path, void *cb_data)
+{
+       struct rename_cb *cb = cb_data;
 
-       if (rename(git_path(TMP_RENAMED_LOG), path.buf)) {
-               if ((errno==EISDIR || errno==ENOTDIR) && --attempts_remaining > 0) {
-                       /*
-                        * rename(a, b) when b is an existing
-                        * directory ought to result in ISDIR, but
-                        * Solaris 5.8 gives ENOTDIR.  Sheesh.
-                        */
-                       if (remove_empty_directories(&path)) {
-                               error("Directory not empty: logs/%s", newrefname);
-                               goto out;
-                       }
-                       goto retry;
-               } else if (errno == ENOENT && --attempts_remaining > 0) {
-                       /*
-                        * Maybe another process just deleted one of
-                        * the directories in the path to newrefname.
-                        * Try again from the beginning.
-                        */
-                       goto retry;
-               } else {
-                       error("unable to move logfile "TMP_RENAMED_LOG" to logs/%s: %s",
-                               newrefname, strerror(errno));
-                       goto out;
-               }
+       if (rename(cb->tmp_renamed_log, path)) {
+               /*
+                * rename(a, b) when b is an existing directory ought
+                * to result in ISDIR, but Solaris 5.8 gives ENOTDIR.
+                * Sheesh. Record the true errno for error reporting,
+                * but report EISDIR to raceproof_create_file() so
+                * that it knows to retry.
+                */
+               cb->true_errno = errno;
+               if (errno == ENOTDIR)
+                       errno = EISDIR;
+               return -1;
+       } else {
+               return 0;
        }
-       ret = 0;
-out:
-       strbuf_release(&path);
-       return ret;
 }
 
-int verify_refname_available(const char *newname,
-                            struct string_list *extras,
-                            struct string_list *skip,
-                            struct strbuf *err)
+static int rename_tmp_log(struct files_ref_store *refs, const char *newrefname)
 {
-       struct ref_dir *packed_refs = get_packed_refs(&ref_cache);
-       struct ref_dir *loose_refs = get_loose_refs(&ref_cache);
+       struct strbuf path = STRBUF_INIT;
+       struct strbuf tmp = STRBUF_INIT;
+       struct rename_cb cb;
+       int ret;
 
-       if (verify_refname_available_dir(newname, extras, skip,
-                                        packed_refs, err) ||
-           verify_refname_available_dir(newname, extras, skip,
-                                        loose_refs, err))
-               return -1;
+       files_reflog_path(refs, &path, newrefname);
+       files_reflog_path(refs, &tmp, TMP_RENAMED_LOG);
+       cb.tmp_renamed_log = tmp.buf;
+       ret = raceproof_create_file(path.buf, rename_tmp_log_callback, &cb);
+       if (ret) {
+               if (errno == EISDIR)
+                       error("directory not empty: %s", path.buf);
+               else
+                       error("unable to move logfile %s to %s: %s",
+                             tmp.buf, path.buf,
+                             strerror(cb.true_errno));
+       }
 
-       return 0;
+       strbuf_release(&path);
+       strbuf_release(&tmp);
+       return ret;
 }
 
 static int write_ref_to_lockfile(struct ref_lock *lock,
-                                const unsigned char *sha1, struct strbuf *err);
-static int commit_ref_update(struct ref_lock *lock,
-                            const unsigned char *sha1, const char *logmsg,
-                            int flags, struct strbuf *err);
-
-int rename_ref(const char *oldrefname, const char *newrefname, const char *logmsg)
-{
-       unsigned char sha1[20], orig_sha1[20];
+                                const struct object_id *oid, struct strbuf *err);
+static int commit_ref_update(struct files_ref_store *refs,
+                            struct ref_lock *lock,
+                            const struct object_id *oid, const char *logmsg,
+                            struct strbuf *err);
+
+static int files_rename_ref(struct ref_store *ref_store,
+                           const char *oldrefname, const char *newrefname,
+                           const char *logmsg)
+{
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "rename_ref");
+       struct object_id oid, orig_oid;
        int flag = 0, logmoved = 0;
        struct ref_lock *lock;
        struct stat loginfo;
-       int log = !lstat(git_path("logs/%s", oldrefname), &loginfo);
-       const char *symref = NULL;
+       struct strbuf sb_oldref = STRBUF_INIT;
+       struct strbuf sb_newref = STRBUF_INIT;
+       struct strbuf tmp_renamed_log = STRBUF_INIT;
+       int log, ret;
        struct strbuf err = STRBUF_INIT;
 
-       if (log && S_ISLNK(loginfo.st_mode))
-               return error("reflog for %s is a symlink", oldrefname);
+       files_reflog_path(refs, &sb_oldref, oldrefname);
+       files_reflog_path(refs, &sb_newref, newrefname);
+       files_reflog_path(refs, &tmp_renamed_log, TMP_RENAMED_LOG);
 
-       symref = resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING,
-                                   orig_sha1, &flag);
-       if (flag & REF_ISSYMREF)
-               return error("refname %s is a symbolic ref, renaming it is not supported",
-                       oldrefname);
-       if (!symref)
-               return error("refname %s not found", oldrefname);
+       log = !lstat(sb_oldref.buf, &loginfo);
+       if (log && S_ISLNK(loginfo.st_mode)) {
+               ret = error("reflog for %s is a symlink", oldrefname);
+               goto out;
+       }
 
-       if (!rename_ref_available(oldrefname, newrefname))
-               return 1;
+       if (!refs_resolve_ref_unsafe(&refs->base, oldrefname,
+                                    RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+                               orig_oid.hash, &flag)) {
+               ret = error("refname %s not found", oldrefname);
+               goto out;
+       }
 
-       if (log && rename(git_path("logs/%s", oldrefname), git_path(TMP_RENAMED_LOG)))
-               return error("unable to move logfile logs/%s to "TMP_RENAMED_LOG": %s",
-                       oldrefname, strerror(errno));
+       if (flag & REF_ISSYMREF) {
+               ret = error("refname %s is a symbolic ref, renaming it is not supported",
+                           oldrefname);
+               goto out;
+       }
+       if (!refs_rename_ref_available(&refs->base, oldrefname, newrefname)) {
+               ret = 1;
+               goto out;
+       }
 
-       if (delete_ref(oldrefname, orig_sha1, REF_NODEREF)) {
+       if (log && rename(sb_oldref.buf, tmp_renamed_log.buf)) {
+               ret = error("unable to move logfile logs/%s to logs/"TMP_RENAMED_LOG": %s",
+                           oldrefname, strerror(errno));
+               goto out;
+       }
+
+       if (refs_delete_ref(&refs->base, logmsg, oldrefname,
+                           orig_oid.hash, REF_NODEREF)) {
                error("unable to delete old %s", oldrefname);
                goto rollback;
        }
 
-       if (!read_ref_full(newrefname, RESOLVE_REF_READING, sha1, NULL) &&
-           delete_ref(newrefname, sha1, REF_NODEREF)) {
-               if (errno==EISDIR) {
+       /*
+        * Since we are doing a shallow lookup, oid is not the
+        * correct value to pass to delete_ref as old_oid. But that
+        * doesn't matter, because an old_oid check wouldn't add to
+        * the safety anyway; we want to delete the reference whatever
+        * its current value.
+        */
+       if (!refs_read_ref_full(&refs->base, newrefname,
+                               RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+                               oid.hash, NULL) &&
+           refs_delete_ref(&refs->base, NULL, newrefname,
+                           NULL, REF_NODEREF)) {
+               if (errno == EISDIR) {
                        struct strbuf path = STRBUF_INIT;
                        int result;
 
-                       strbuf_git_path(&path, "%s", newrefname);
+                       files_ref_path(refs, &path, newrefname);
                        result = remove_empty_directories(&path);
                        strbuf_release(&path);
 
@@ -2528,30 +1833,33 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
                }
        }
 
-       if (log && rename_tmp_log(newrefname))
+       if (log && rename_tmp_log(refs, newrefname))
                goto rollback;
 
        logmoved = log;
 
-       lock = lock_ref_sha1_basic(newrefname, NULL, NULL, NULL, 0, NULL, &err);
+       lock = lock_ref_sha1_basic(refs, newrefname, NULL, NULL, NULL,
+                                  REF_NODEREF, NULL, &err);
        if (!lock) {
                error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
                strbuf_release(&err);
                goto rollback;
        }
-       hashcpy(lock->old_oid.hash, orig_sha1);
+       oidcpy(&lock->old_oid, &orig_oid);
 
-       if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
-           commit_ref_update(lock, orig_sha1, logmsg, 0, &err)) {
+       if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
+           commit_ref_update(refs, lock, &orig_oid, logmsg, &err)) {
                error("unable to write current sha1 into %s: %s", newrefname, err.buf);
                strbuf_release(&err);
                goto rollback;
        }
 
-       return 0;
+       ret = 0;
+       goto out;
 
  rollback:
-       lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, NULL, 0, NULL, &err);
+       lock = lock_ref_sha1_basic(refs, oldrefname, NULL, NULL, NULL,
+                                  REF_NODEREF, NULL, &err);
        if (!lock) {
                error("unable to lock %s for rollback: %s", oldrefname, err.buf);
                strbuf_release(&err);
@@ -2559,24 +1867,29 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
        }
 
        flag = log_all_ref_updates;
-       log_all_ref_updates = 0;
-       if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
-           commit_ref_update(lock, orig_sha1, NULL, 0, &err)) {
+       log_all_ref_updates = LOG_REFS_NONE;
+       if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
+           commit_ref_update(refs, lock, &orig_oid, NULL, &err)) {
                error("unable to write current sha1 into %s: %s", oldrefname, err.buf);
                strbuf_release(&err);
        }
        log_all_ref_updates = flag;
 
  rollbacklog:
-       if (logmoved && rename(git_path("logs/%s", newrefname), git_path("logs/%s", oldrefname)))
+       if (logmoved && rename(sb_newref.buf, sb_oldref.buf))
                error("unable to restore logfile %s from %s: %s",
                        oldrefname, newrefname, strerror(errno));
        if (!logmoved && log &&
-           rename(git_path(TMP_RENAMED_LOG), git_path("logs/%s", oldrefname)))
-               error("unable to restore logfile %s from "TMP_RENAMED_LOG": %s",
+           rename(tmp_renamed_log.buf, sb_oldref.buf))
+               error("unable to restore logfile %s from logs/"TMP_RENAMED_LOG": %s",
                        oldrefname, strerror(errno));
+       ret = 1;
+ out:
+       strbuf_release(&sb_newref);
+       strbuf_release(&sb_oldref);
+       strbuf_release(&tmp_renamed_log);
 
-       return 1;
+       return ret;
 }
 
 static int close_ref(struct ref_lock *lock)
@@ -2588,70 +1901,126 @@ static int close_ref(struct ref_lock *lock)
 
 static int commit_ref(struct ref_lock *lock)
 {
+       char *path = get_locked_file_path(lock->lk);
+       struct stat st;
+
+       if (!lstat(path, &st) && S_ISDIR(st.st_mode)) {
+               /*
+                * There is a directory at the path we want to rename
+                * the lockfile to. Hopefully it is empty; try to
+                * delete it.
+                */
+               size_t len = strlen(path);
+               struct strbuf sb_path = STRBUF_INIT;
+
+               strbuf_attach(&sb_path, path, len, len);
+
+               /*
+                * If this fails, commit_lock_file() will also fail
+                * and will report the problem.
+                */
+               remove_empty_directories(&sb_path);
+               strbuf_release(&sb_path);
+       } else {
+               free(path);
+       }
+
        if (commit_lock_file(lock->lk))
                return -1;
        return 0;
 }
 
+static int open_or_create_logfile(const char *path, void *cb)
+{
+       int *fd = cb;
+
+       *fd = open(path, O_APPEND | O_WRONLY | O_CREAT, 0666);
+       return (*fd < 0) ? -1 : 0;
+}
+
 /*
- * Create a reflog for a ref.  If force_create = 0, the reflog will
- * only be created for certain refs (those for which
- * should_autocreate_reflog returns non-zero.  Otherwise, create it
- * regardless of the ref name.  Fill in *err and return -1 on failure.
+ * Create a reflog for a ref. If force_create = 0, only create the
+ * reflog for certain refs (those for which should_autocreate_reflog
+ * returns non-zero). Otherwise, create it regardless of the reference
+ * name. If the logfile already existed or was created, return 0 and
+ * set *logfd to the file descriptor opened for appending to the file.
+ * If no logfile exists and we decided not to create one, return 0 and
+ * set *logfd to -1. On failure, fill in *err, set *logfd to -1, and
+ * return -1.
  */
-static int log_ref_setup(const char *refname, struct strbuf *logfile, struct strbuf *err, int force_create)
+static int log_ref_setup(struct files_ref_store *refs,
+                        const char *refname, int force_create,
+                        int *logfd, struct strbuf *err)
 {
-       int logfd, oflags = O_APPEND | O_WRONLY;
+       struct strbuf logfile_sb = STRBUF_INIT;
+       char *logfile;
 
-       strbuf_git_path(logfile, "logs/%s", refname);
-       if (force_create || should_autocreate_reflog(refname)) {
-               if (safe_create_leading_directories(logfile->buf) < 0) {
-                       strbuf_addf(err, "unable to create directory for %s: "
-                                   "%s", logfile->buf, strerror(errno));
-                       return -1;
-               }
-               oflags |= O_CREAT;
-       }
+       files_reflog_path(refs, &logfile_sb, refname);
+       logfile = strbuf_detach(&logfile_sb, NULL);
 
-       logfd = open(logfile->buf, oflags, 0666);
-       if (logfd < 0) {
-               if (!(oflags & O_CREAT) && (errno == ENOENT || errno == EISDIR))
-                       return 0;
+       if (force_create || should_autocreate_reflog(refname)) {
+               if (raceproof_create_file(logfile, open_or_create_logfile, logfd)) {
+                       if (errno == ENOENT)
+                               strbuf_addf(err, "unable to create directory for '%s': "
+                                           "%s", logfile, strerror(errno));
+                       else if (errno == EISDIR)
+                               strbuf_addf(err, "there are still logs under '%s'",
+                                           logfile);
+                       else
+                               strbuf_addf(err, "unable to append to '%s': %s",
+                                           logfile, strerror(errno));
 
-               if (errno == EISDIR) {
-                       if (remove_empty_directories(logfile)) {
-                               strbuf_addf(err, "There are still logs under "
-                                           "'%s'", logfile->buf);
-                               return -1;
-                       }
-                       logfd = open(logfile->buf, oflags, 0666);
+                       goto error;
                }
-
-               if (logfd < 0) {
-                       strbuf_addf(err, "unable to append to %s: %s",
-                                   logfile->buf, strerror(errno));
-                       return -1;
+       } else {
+               *logfd = open(logfile, O_APPEND | O_WRONLY, 0666);
+               if (*logfd < 0) {
+                       if (errno == ENOENT || errno == EISDIR) {
+                               /*
+                                * The logfile doesn't already exist,
+                                * but that is not an error; it only
+                                * means that we won't write log
+                                * entries to it.
+                                */
+                               ;
+                       } else {
+                               strbuf_addf(err, "unable to append to '%s': %s",
+                                           logfile, strerror(errno));
+                               goto error;
+                       }
                }
        }
 
-       adjust_shared_perm(logfile->buf);
-       close(logfd);
+       if (*logfd >= 0)
+               adjust_shared_perm(logfile);
+
+       free(logfile);
        return 0;
-}
 
+error:
+       free(logfile);
+       return -1;
+}
 
-int safe_create_reflog(const char *refname, int force_create, struct strbuf *err)
+static int files_create_reflog(struct ref_store *ref_store,
+                              const char *refname, int force_create,
+                              struct strbuf *err)
 {
-       int ret;
-       struct strbuf sb = STRBUF_INIT;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "create_reflog");
+       int fd;
 
-       ret = log_ref_setup(refname, &sb, err, force_create);
-       strbuf_release(&sb);
-       return ret;
+       if (log_ref_setup(refs, refname, force_create, &fd, err))
+               return -1;
+
+       if (fd >= 0)
+               close(fd);
+
+       return 0;
 }
 
-static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
-                           const unsigned char *new_sha1,
+static int log_ref_write_fd(int fd, const struct object_id *old_oid,
+                           const struct object_id *new_oid,
                            const char *committer, const char *msg)
 {
        int msglen, written;
@@ -2662,8 +2031,8 @@ static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
        maxlen = strlen(committer) + msglen + 100;
        logrec = xmalloc(maxlen);
        len = xsnprintf(logrec, maxlen, "%s %s %s\n",
-                       sha1_to_hex(old_sha1),
-                       sha1_to_hex(new_sha1),
+                       oid_to_hex(old_oid),
+                       oid_to_hex(new_oid),
                        committer);
        if (msglen)
                len += copy_reflog_msg(logrec + len - 1, msg) - 1;
@@ -2676,92 +2045,84 @@ static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
        return 0;
 }
 
-static int log_ref_write_1(const char *refname, const unsigned char *old_sha1,
-                          const unsigned char *new_sha1, const char *msg,
-                          struct strbuf *logfile, int flags,
-                          struct strbuf *err)
+static int files_log_ref_write(struct files_ref_store *refs,
+                              const char *refname, const struct object_id *old_oid,
+                              const struct object_id *new_oid, const char *msg,
+                              int flags, struct strbuf *err)
 {
-       int logfd, result, oflags = O_APPEND | O_WRONLY;
+       int logfd, result;
 
-       if (log_all_ref_updates < 0)
-               log_all_ref_updates = !is_bare_repository();
+       if (log_all_ref_updates == LOG_REFS_UNSET)
+               log_all_ref_updates = is_bare_repository() ? LOG_REFS_NONE : LOG_REFS_NORMAL;
 
-       result = log_ref_setup(refname, logfile, err, flags & REF_FORCE_CREATE_REFLOG);
+       result = log_ref_setup(refs, refname,
+                              flags & REF_FORCE_CREATE_REFLOG,
+                              &logfd, err);
 
        if (result)
                return result;
 
-       logfd = open(logfile->buf, oflags);
        if (logfd < 0)
                return 0;
-       result = log_ref_write_fd(logfd, old_sha1, new_sha1,
+       result = log_ref_write_fd(logfd, old_oid, new_oid,
                                  git_committer_info(0), msg);
        if (result) {
-               strbuf_addf(err, "unable to append to %s: %s", logfile->buf,
-                           strerror(errno));
+               struct strbuf sb = STRBUF_INIT;
+               int save_errno = errno;
+
+               files_reflog_path(refs, &sb, refname);
+               strbuf_addf(err, "unable to append to '%s': %s",
+                           sb.buf, strerror(save_errno));
+               strbuf_release(&sb);
                close(logfd);
                return -1;
        }
        if (close(logfd)) {
-               strbuf_addf(err, "unable to append to %s: %s", logfile->buf,
-                           strerror(errno));
+               struct strbuf sb = STRBUF_INIT;
+               int save_errno = errno;
+
+               files_reflog_path(refs, &sb, refname);
+               strbuf_addf(err, "unable to append to '%s': %s",
+                           sb.buf, strerror(save_errno));
+               strbuf_release(&sb);
                return -1;
        }
        return 0;
 }
 
-static int log_ref_write(const char *refname, const unsigned char *old_sha1,
-                        const unsigned char *new_sha1, const char *msg,
-                        int flags, struct strbuf *err)
-{
-       return files_log_ref_write(refname, old_sha1, new_sha1, msg, flags,
-                                  err);
-}
-
-int files_log_ref_write(const char *refname, const unsigned char *old_sha1,
-                       const unsigned char *new_sha1, const char *msg,
-                       int flags, struct strbuf *err)
-{
-       struct strbuf sb = STRBUF_INIT;
-       int ret = log_ref_write_1(refname, old_sha1, new_sha1, msg, &sb, flags,
-                                 err);
-       strbuf_release(&sb);
-       return ret;
-}
-
 /*
  * Write sha1 into the open lockfile, then close the lockfile. On
  * errors, rollback the lockfile, fill in *err and
  * return -1.
  */
 static int write_ref_to_lockfile(struct ref_lock *lock,
-                                const unsigned char *sha1, struct strbuf *err)
+                                const struct object_id *oid, struct strbuf *err)
 {
        static char term = '\n';
        struct object *o;
        int fd;
 
-       o = parse_object(sha1);
+       o = parse_object(oid);
        if (!o) {
                strbuf_addf(err,
-                           "Trying to write ref %s with nonexistent object %s",
-                           lock->ref_name, sha1_to_hex(sha1));
+                           "trying to write ref '%s' with nonexistent object %s",
+                           lock->ref_name, oid_to_hex(oid));
                unlock_ref(lock);
                return -1;
        }
        if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
                strbuf_addf(err,
-                           "Trying to write non-commit object %s to branch %s",
-                           sha1_to_hex(sha1), lock->ref_name);
+                           "trying to write non-commit object %s to branch '%s'",
+                           oid_to_hex(oid), lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
        fd = get_lock_file_fd(lock->lk);
-       if (write_in_full(fd, sha1_to_hex(sha1), 40) != 40 ||
+       if (write_in_full(fd, oid_to_hex(oid), GIT_SHA1_HEXSZ) != GIT_SHA1_HEXSZ ||
            write_in_full(fd, &term, 1) != 1 ||
            close_ref(lock) < 0) {
                strbuf_addf(err,
-                           "Couldn't write %s", get_lock_file_path(lock->lk));
+                           "couldn't write '%s'", get_lock_file_path(lock->lk));
                unlock_ref(lock);
                return -1;
        }
@@ -2773,22 +2134,26 @@ static int write_ref_to_lockfile(struct ref_lock *lock,
  * to the loose reference lockfile. Also update the reflogs if
  * necessary, using the specified lockmsg (which can be NULL).
  */
-static int commit_ref_update(struct ref_lock *lock,
-                            const unsigned char *sha1, const char *logmsg,
-                            int flags, struct strbuf *err)
-{
-       clear_loose_ref_cache(&ref_cache);
-       if (log_ref_write(lock->ref_name, lock->old_oid.hash, sha1, logmsg, flags, err) < 0 ||
-           (strcmp(lock->ref_name, lock->orig_ref_name) &&
-            log_ref_write(lock->orig_ref_name, lock->old_oid.hash, sha1, logmsg, flags, err) < 0)) {
+static int commit_ref_update(struct files_ref_store *refs,
+                            struct ref_lock *lock,
+                            const struct object_id *oid, const char *logmsg,
+                            struct strbuf *err)
+{
+       files_assert_main_repository(refs, "commit_ref_update");
+
+       clear_loose_ref_cache(refs);
+       if (files_log_ref_write(refs, lock->ref_name,
+                               &lock->old_oid, oid,
+                               logmsg, 0, err)) {
                char *old_msg = strbuf_detach(err, NULL);
-               strbuf_addf(err, "Cannot update the ref '%s': %s",
+               strbuf_addf(err, "cannot update the ref '%s': %s",
                            lock->ref_name, old_msg);
                free(old_msg);
                unlock_ref(lock);
                return -1;
        }
-       if (strcmp(lock->orig_ref_name, "HEAD") != 0) {
+
+       if (strcmp(lock->ref_name, "HEAD") != 0) {
                /*
                 * Special hack: If a branch is updated directly and HEAD
                 * points to it (may happen on the remote side of a push
@@ -2801,23 +2166,27 @@ static int commit_ref_update(struct ref_lock *lock,
                 * check with HEAD only which should cover 99% of all usage
                 * scenarios (even 100% of the default ones).
                 */
-               unsigned char head_sha1[20];
+               struct object_id head_oid;
                int head_flag;
                const char *head_ref;
-               head_ref = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
-                                             head_sha1, &head_flag);
+
+               head_ref = refs_resolve_ref_unsafe(&refs->base, "HEAD",
+                                                  RESOLVE_REF_READING,
+                                                  head_oid.hash, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
                    !strcmp(head_ref, lock->ref_name)) {
                        struct strbuf log_err = STRBUF_INIT;
-                       if (log_ref_write("HEAD", lock->old_oid.hash, sha1,
-                                         logmsg, 0, &log_err)) {
+                       if (files_log_ref_write(refs, "HEAD",
+                                               &lock->old_oid, oid,
+                                               logmsg, 0, &log_err)) {
                                error("%s", log_err.buf);
                                strbuf_release(&log_err);
                        }
                }
        }
+
        if (commit_ref(lock)) {
-               error("Couldn't set %s", lock->ref_name);
+               strbuf_addf(err, "couldn't set '%s'", lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
@@ -2841,23 +2210,28 @@ static int create_ref_symlink(struct ref_lock *lock, const char *target)
        return ret;
 }
 
-static void update_symref_reflog(struct ref_lock *lock, const char *refname,
+static void update_symref_reflog(struct files_ref_store *refs,
+                                struct ref_lock *lock, const char *refname,
                                 const char *target, const char *logmsg)
 {
        struct strbuf err = STRBUF_INIT;
-       unsigned char new_sha1[20];
-       if (logmsg && !read_ref(target, new_sha1) &&
-           log_ref_write(refname, lock->old_oid.hash, new_sha1, logmsg, 0, &err)) {
+       struct object_id new_oid;
+       if (logmsg &&
+           !refs_read_ref_full(&refs->base, target,
+                               RESOLVE_REF_READING, new_oid.hash, NULL) &&
+           files_log_ref_write(refs, refname, &lock->old_oid,
+                               &new_oid, logmsg, 0, &err)) {
                error("%s", err.buf);
                strbuf_release(&err);
        }
 }
 
-static int create_symref_locked(struct ref_lock *lock, const char *refname,
+static int create_symref_locked(struct files_ref_store *refs,
+                               struct ref_lock *lock, const char *refname,
                                const char *target, const char *logmsg)
 {
        if (prefer_symlink_refs && !create_ref_symlink(lock, target)) {
-               update_symref_reflog(lock, refname, target, logmsg);
+               update_symref_reflog(refs, lock, refname, target, logmsg);
                return 0;
        }
 
@@ -2865,7 +2239,7 @@ static int create_symref_locked(struct ref_lock *lock, const char *refname,
                return error("unable to fdopen %s: %s",
                             lock->lk->tempfile.filename.buf, strerror(errno));
 
-       update_symref_reflog(lock, refname, target, logmsg);
+       update_symref_reflog(refs, lock, refname, target, logmsg);
 
        /* no error check; commit_ref will check ferror */
        fprintf(lock->lk->tempfile.fp, "ref: %s\n", target);
@@ -2875,13 +2249,18 @@ static int create_symref_locked(struct ref_lock *lock, const char *refname,
        return 0;
 }
 
-int create_symref(const char *refname, const char *target, const char *logmsg)
+static int files_create_symref(struct ref_store *ref_store,
+                              const char *refname, const char *target,
+                              const char *logmsg)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "create_symref");
        struct strbuf err = STRBUF_INIT;
        struct ref_lock *lock;
        int ret;
 
-       lock = lock_ref_sha1_basic(refname, NULL, NULL, NULL, REF_NODEREF, NULL,
+       lock = lock_ref_sha1_basic(refs, refname, NULL,
+                                  NULL, NULL, REF_NODEREF, NULL,
                                   &err);
        if (!lock) {
                error("%s", err.buf);
@@ -2889,74 +2268,55 @@ int create_symref(const char *refname, const char *target, const char *logmsg)
                return -1;
        }
 
-       ret = create_symref_locked(lock, refname, target, logmsg);
+       ret = create_symref_locked(refs, lock, refname, target, logmsg);
        unlock_ref(lock);
        return ret;
 }
 
-int set_worktree_head_symref(const char *gitdir, const char *target)
+static int files_reflog_exists(struct ref_store *ref_store,
+                              const char *refname)
 {
-       static struct lock_file head_lock;
-       struct ref_lock *lock;
-       struct strbuf head_path = STRBUF_INIT;
-       const char *head_rel;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ, "reflog_exists");
+       struct strbuf sb = STRBUF_INIT;
+       struct stat st;
        int ret;
 
-       strbuf_addf(&head_path, "%s/HEAD", absolute_path(gitdir));
-       if (hold_lock_file_for_update(&head_lock, head_path.buf,
-                                     LOCK_NO_DEREF) < 0) {
-               struct strbuf err = STRBUF_INIT;
-               unable_to_lock_message(head_path.buf, errno, &err);
-               error("%s", err.buf);
-               strbuf_release(&err);
-               strbuf_release(&head_path);
-               return -1;
-       }
-
-       /* head_rel will be "HEAD" for the main tree, "worktrees/wt/HEAD" for
-          linked trees */
-       head_rel = remove_leading_path(head_path.buf,
-                                      absolute_path(get_git_common_dir()));
-       /* to make use of create_symref_locked(), initialize ref_lock */
-       lock = xcalloc(1, sizeof(struct ref_lock));
-       lock->lk = &head_lock;
-       lock->ref_name = xstrdup(head_rel);
-       lock->orig_ref_name = xstrdup(head_rel);
-
-       ret = create_symref_locked(lock, head_rel, target, NULL);
-
-       unlock_ref(lock); /* will free lock */
-       strbuf_release(&head_path);
+       files_reflog_path(refs, &sb, refname);
+       ret = !lstat(sb.buf, &st) && S_ISREG(st.st_mode);
+       strbuf_release(&sb);
        return ret;
 }
 
-int reflog_exists(const char *refname)
+static int files_delete_reflog(struct ref_store *ref_store,
+                              const char *refname)
 {
-       struct stat st;
-
-       return !lstat(git_path("logs/%s", refname), &st) &&
-               S_ISREG(st.st_mode);
-}
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "delete_reflog");
+       struct strbuf sb = STRBUF_INIT;
+       int ret;
 
-int delete_reflog(const char *refname)
-{
-       return remove_path(git_path("logs/%s", refname));
+       files_reflog_path(refs, &sb, refname);
+       ret = remove_path(sb.buf);
+       strbuf_release(&sb);
+       return ret;
 }
 
 static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *cb_data)
 {
-       unsigned char osha1[20], nsha1[20];
+       struct object_id ooid, noid;
        char *email_end, *message;
-       unsigned long timestamp;
+       timestamp_t timestamp;
        int tz;
+       const char *p = sb->buf;
 
        /* old SP new SP name <email> SP time TAB msg LF */
-       if (sb->len < 83 || sb->buf[sb->len - 1] != '\n' ||
-           get_sha1_hex(sb->buf, osha1) || sb->buf[40] != ' ' ||
-           get_sha1_hex(sb->buf + 41, nsha1) || sb->buf[81] != ' ' ||
-           !(email_end = strchr(sb->buf + 82, '>')) ||
+       if (!sb->len || sb->buf[sb->len - 1] != '\n' ||
+           parse_oid_hex(p, &ooid, &p) || *p++ != ' ' ||
+           parse_oid_hex(p, &noid, &p) || *p++ != ' ' ||
+           !(email_end = strchr(p, '>')) ||
            email_end[1] != ' ' ||
-           !(timestamp = strtoul(email_end + 2, &message, 10)) ||
+           !(timestamp = parse_timestamp(email_end + 2, &message, 10)) ||
            !message || message[0] != ' ' ||
            (message[1] != '+' && message[1] != '-') ||
            !isdigit(message[2]) || !isdigit(message[3]) ||
@@ -2968,7 +2328,7 @@ static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *c
                message += 6;
        else
                message += 7;
-       return fn(osha1, nsha1, sb->buf + 82, timestamp, tz, message, cb_data);
+       return fn(&ooid, &noid, p, timestamp, tz, message, cb_data);
 }
 
 static char *find_beginning_of_line(char *bob, char *scan)
@@ -2982,21 +2342,29 @@ static char *find_beginning_of_line(char *bob, char *scan)
        return scan;
 }
 
-int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void *cb_data)
+static int files_for_each_reflog_ent_reverse(struct ref_store *ref_store,
+                                            const char *refname,
+                                            each_reflog_ent_fn fn,
+                                            void *cb_data)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ,
+                              "for_each_reflog_ent_reverse");
        struct strbuf sb = STRBUF_INIT;
        FILE *logfp;
        long pos;
        int ret = 0, at_tail = 1;
 
-       logfp = fopen(git_path("logs/%s", refname), "r");
+       files_reflog_path(refs, &sb, refname);
+       logfp = fopen(sb.buf, "r");
+       strbuf_release(&sb);
        if (!logfp)
                return -1;
 
        /* Jump to the end */
        if (fseek(logfp, 0, SEEK_END) < 0)
-               return error("cannot seek back reflog for %s: %s",
-                            refname, strerror(errno));
+               ret = error("cannot seek back reflog for %s: %s",
+                           refname, strerror(errno));
        pos = ftell(logfp);
        while (!ret && 0 < pos) {
                int cnt;
@@ -3006,13 +2374,17 @@ int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void
 
                /* Fill next block from the end */
                cnt = (sizeof(buf) < pos) ? sizeof(buf) : pos;
-               if (fseek(logfp, pos - cnt, SEEK_SET))
-                       return error("cannot seek back reflog for %s: %s",
-                                    refname, strerror(errno));
+               if (fseek(logfp, pos - cnt, SEEK_SET)) {
+                       ret = error("cannot seek back reflog for %s: %s",
+                                   refname, strerror(errno));
+                       break;
+               }
                nread = fread(buf, cnt, 1, logfp);
-               if (nread != 1)
-                       return error("cannot read %d bytes from reflog for %s: %s",
-                                    cnt, refname, strerror(errno));
+               if (nread != 1) {
+                       ret = error("cannot read %d bytes from reflog for %s: %s",
+                                   cnt, refname, strerror(errno));
+                       break;
+               }
                pos -= cnt;
 
                scanp = endp = buf + cnt;
@@ -3084,13 +2456,20 @@ int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void
        return ret;
 }
 
-int for_each_reflog_ent(const char *refname, each_reflog_ent_fn fn, void *cb_data)
+static int files_for_each_reflog_ent(struct ref_store *ref_store,
+                                    const char *refname,
+                                    each_reflog_ent_fn fn, void *cb_data)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ,
+                              "for_each_reflog_ent");
        FILE *logfp;
        struct strbuf sb = STRBUF_INIT;
        int ret = 0;
 
-       logfp = fopen(git_path("logs/%s", refname), "r");
+       files_reflog_path(refs, &sb, refname);
+       logfp = fopen(sb.buf, "r");
+       strbuf_release(&sb);
        if (!logfp)
                return -1;
 
@@ -3100,238 +2479,644 @@ int for_each_reflog_ent(const char *refname, each_reflog_ent_fn fn, void *cb_dat
        strbuf_release(&sb);
        return ret;
 }
-/*
- * Call fn for each reflog in the namespace indicated by name.  name
- * must be empty or end with '/'.  Name will be used as a scratch
- * space, but its contents will be restored before return.
- */
-static int do_for_each_reflog(struct strbuf *name, each_ref_fn fn, void *cb_data)
-{
-       DIR *d = opendir(git_path("logs/%s", name->buf));
-       int retval = 0;
-       struct dirent *de;
-       int oldlen = name->len;
 
-       if (!d)
-               return name->len ? errno : 0;
+struct files_reflog_iterator {
+       struct ref_iterator base;
 
-       while ((de = readdir(d)) != NULL) {
-               struct stat st;
+       struct ref_store *ref_store;
+       struct dir_iterator *dir_iterator;
+       struct object_id oid;
+};
 
-               if (de->d_name[0] == '.')
+static int files_reflog_iterator_advance(struct ref_iterator *ref_iterator)
+{
+       struct files_reflog_iterator *iter =
+               (struct files_reflog_iterator *)ref_iterator;
+       struct dir_iterator *diter = iter->dir_iterator;
+       int ok;
+
+       while ((ok = dir_iterator_advance(diter)) == ITER_OK) {
+               int flags;
+
+               if (!S_ISREG(diter->st.st_mode))
                        continue;
-               if (ends_with(de->d_name, ".lock"))
+               if (diter->basename[0] == '.')
+                       continue;
+               if (ends_with(diter->basename, ".lock"))
                        continue;
-               strbuf_addstr(name, de->d_name);
-               if (stat(git_path("logs/%s", name->buf), &st) < 0) {
-                       ; /* silently ignore */
-               } else {
-                       if (S_ISDIR(st.st_mode)) {
-                               strbuf_addch(name, '/');
-                               retval = do_for_each_reflog(name, fn, cb_data);
-                       } else {
-                               struct object_id oid;
 
-                               if (read_ref_full(name->buf, 0, oid.hash, NULL))
-                                       retval = error("bad ref for %s", name->buf);
-                               else
-                                       retval = fn(name->buf, &oid, 0, cb_data);
-                       }
-                       if (retval)
-                               break;
+               if (refs_read_ref_full(iter->ref_store,
+                                      diter->relative_path, 0,
+                                      iter->oid.hash, &flags)) {
+                       error("bad ref for %s", diter->path.buf);
+                       continue;
                }
-               strbuf_setlen(name, oldlen);
+
+               iter->base.refname = diter->relative_path;
+               iter->base.oid = &iter->oid;
+               iter->base.flags = flags;
+               return ITER_OK;
        }
-       closedir(d);
-       return retval;
+
+       iter->dir_iterator = NULL;
+       if (ref_iterator_abort(ref_iterator) == ITER_ERROR)
+               ok = ITER_ERROR;
+       return ok;
 }
 
-int for_each_reflog(each_ref_fn fn, void *cb_data)
+static int files_reflog_iterator_peel(struct ref_iterator *ref_iterator,
+                                  struct object_id *peeled)
 {
-       int retval;
-       struct strbuf name;
-       strbuf_init(&name, PATH_MAX);
-       retval = do_for_each_reflog(&name, fn, cb_data);
-       strbuf_release(&name);
-       return retval;
+       die("BUG: ref_iterator_peel() called for reflog_iterator");
 }
 
-static int ref_update_reject_duplicates(struct string_list *refnames,
-                                       struct strbuf *err)
+static int files_reflog_iterator_abort(struct ref_iterator *ref_iterator)
 {
-       int i, n = refnames->nr;
+       struct files_reflog_iterator *iter =
+               (struct files_reflog_iterator *)ref_iterator;
+       int ok = ITER_DONE;
 
-       assert(err);
+       if (iter->dir_iterator)
+               ok = dir_iterator_abort(iter->dir_iterator);
+
+       base_ref_iterator_free(ref_iterator);
+       return ok;
+}
+
+static struct ref_iterator_vtable files_reflog_iterator_vtable = {
+       files_reflog_iterator_advance,
+       files_reflog_iterator_peel,
+       files_reflog_iterator_abort
+};
+
+static struct ref_iterator *files_reflog_iterator_begin(struct ref_store *ref_store)
+{
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_READ,
+                              "reflog_iterator_begin");
+       struct files_reflog_iterator *iter = xcalloc(1, sizeof(*iter));
+       struct ref_iterator *ref_iterator = &iter->base;
+       struct strbuf sb = STRBUF_INIT;
+
+       base_ref_iterator_init(ref_iterator, &files_reflog_iterator_vtable);
+       files_reflog_path(refs, &sb, NULL);
+       iter->dir_iterator = dir_iterator_begin(sb.buf);
+       iter->ref_store = ref_store;
+       strbuf_release(&sb);
+       return ref_iterator;
+}
+
+/*
+ * If update is a direct update of head_ref (the reference pointed to
+ * by HEAD), then add an extra REF_LOG_ONLY update for HEAD.
+ */
+static int split_head_update(struct ref_update *update,
+                            struct ref_transaction *transaction,
+                            const char *head_ref,
+                            struct string_list *affected_refnames,
+                            struct strbuf *err)
+{
+       struct string_list_item *item;
+       struct ref_update *new_update;
+
+       if ((update->flags & REF_LOG_ONLY) ||
+           (update->flags & REF_ISPRUNING) ||
+           (update->flags & REF_UPDATE_VIA_HEAD))
+               return 0;
+
+       if (strcmp(update->refname, head_ref))
+               return 0;
+
+       /*
+        * First make sure that HEAD is not already in the
+        * transaction. This insertion is O(N) in the transaction
+        * size, but it happens at most once per transaction.
+        */
+       item = string_list_insert(affected_refnames, "HEAD");
+       if (item->util) {
+               /* An entry already existed */
+               strbuf_addf(err,
+                           "multiple updates for 'HEAD' (including one "
+                           "via its referent '%s') are not allowed",
+                           update->refname);
+               return TRANSACTION_NAME_CONFLICT;
+       }
+
+       new_update = ref_transaction_add_update(
+                       transaction, "HEAD",
+                       update->flags | REF_LOG_ONLY | REF_NODEREF,
+                       update->new_oid.hash, update->old_oid.hash,
+                       update->msg);
+
+       item->util = new_update;
+
+       return 0;
+}
+
+/*
+ * update is for a symref that points at referent and doesn't have
+ * REF_NODEREF set. Split it into two updates:
+ * - The original update, but with REF_LOG_ONLY and REF_NODEREF set
+ * - A new, separate update for the referent reference
+ * Note that the new update will itself be subject to splitting when
+ * the iteration gets to it.
+ */
+static int split_symref_update(struct files_ref_store *refs,
+                              struct ref_update *update,
+                              const char *referent,
+                              struct ref_transaction *transaction,
+                              struct string_list *affected_refnames,
+                              struct strbuf *err)
+{
+       struct string_list_item *item;
+       struct ref_update *new_update;
+       unsigned int new_flags;
+
+       /*
+        * First make sure that referent is not already in the
+        * transaction. This insertion is O(N) in the transaction
+        * size, but it happens at most once per symref in a
+        * transaction.
+        */
+       item = string_list_insert(affected_refnames, referent);
+       if (item->util) {
+               /* An entry already existed */
+               strbuf_addf(err,
+                           "multiple updates for '%s' (including one "
+                           "via symref '%s') are not allowed",
+                           referent, update->refname);
+               return TRANSACTION_NAME_CONFLICT;
+       }
+
+       new_flags = update->flags;
+       if (!strcmp(update->refname, "HEAD")) {
+               /*
+                * Record that the new update came via HEAD, so that
+                * when we process it, split_head_update() doesn't try
+                * to add another reflog update for HEAD. Note that
+                * this bit will be propagated if the new_update
+                * itself needs to be split.
+                */
+               new_flags |= REF_UPDATE_VIA_HEAD;
+       }
+
+       new_update = ref_transaction_add_update(
+                       transaction, referent, new_flags,
+                       update->new_oid.hash, update->old_oid.hash,
+                       update->msg);
+
+       new_update->parent_update = update;
+
+       /*
+        * Change the symbolic ref update to log only. Also, it
+        * doesn't need to check its old SHA-1 value, as that will be
+        * done when new_update is processed.
+        */
+       update->flags |= REF_LOG_ONLY | REF_NODEREF;
+       update->flags &= ~REF_HAVE_OLD;
+
+       item->util = new_update;
+
+       return 0;
+}
+
+/*
+ * Return the refname under which update was originally requested.
+ */
+static const char *original_update_refname(struct ref_update *update)
+{
+       while (update->parent_update)
+               update = update->parent_update;
+
+       return update->refname;
+}
+
+/*
+ * Check whether the REF_HAVE_OLD and old_oid values stored in update
+ * are consistent with oid, which is the reference's current value. If
+ * everything is OK, return 0; otherwise, write an error message to
+ * err and return -1.
+ */
+static int check_old_oid(struct ref_update *update, struct object_id *oid,
+                        struct strbuf *err)
+{
+       if (!(update->flags & REF_HAVE_OLD) ||
+                  !oidcmp(oid, &update->old_oid))
+               return 0;
+
+       if (is_null_oid(&update->old_oid))
+               strbuf_addf(err, "cannot lock ref '%s': "
+                           "reference already exists",
+                           original_update_refname(update));
+       else if (is_null_oid(oid))
+               strbuf_addf(err, "cannot lock ref '%s': "
+                           "reference is missing but expected %s",
+                           original_update_refname(update),
+                           oid_to_hex(&update->old_oid));
+       else
+               strbuf_addf(err, "cannot lock ref '%s': "
+                           "is at %s but expected %s",
+                           original_update_refname(update),
+                           oid_to_hex(oid),
+                           oid_to_hex(&update->old_oid));
+
+       return -1;
+}
+
+/*
+ * Prepare for carrying out update:
+ * - Lock the reference referred to by update.
+ * - Read the reference under lock.
+ * - Check that its old SHA-1 value (if specified) is correct, and in
+ *   any case record it in update->lock->old_oid for later use when
+ *   writing the reflog.
+ * - If it is a symref update without REF_NODEREF, split it up into a
+ *   REF_LOG_ONLY update of the symref and add a separate update for
+ *   the referent to transaction.
+ * - If it is an update of head_ref, add a corresponding REF_LOG_ONLY
+ *   update of HEAD.
+ */
+static int lock_ref_for_update(struct files_ref_store *refs,
+                              struct ref_update *update,
+                              struct ref_transaction *transaction,
+                              const char *head_ref,
+                              struct string_list *affected_refnames,
+                              struct strbuf *err)
+{
+       struct strbuf referent = STRBUF_INIT;
+       int mustexist = (update->flags & REF_HAVE_OLD) &&
+               !is_null_oid(&update->old_oid);
+       int ret;
+       struct ref_lock *lock;
+
+       files_assert_main_repository(refs, "lock_ref_for_update");
+
+       if ((update->flags & REF_HAVE_NEW) && is_null_oid(&update->new_oid))
+               update->flags |= REF_DELETING;
+
+       if (head_ref) {
+               ret = split_head_update(update, transaction, head_ref,
+                                       affected_refnames, err);
+               if (ret)
+                       return ret;
+       }
+
+       ret = lock_raw_ref(refs, update->refname, mustexist,
+                          affected_refnames, NULL,
+                          &lock, &referent,
+                          &update->type, err);
+       if (ret) {
+               char *reason;
+
+               reason = strbuf_detach(err, NULL);
+               strbuf_addf(err, "cannot lock ref '%s': %s",
+                           original_update_refname(update), reason);
+               free(reason);
+               return ret;
+       }
+
+       update->backend_data = lock;
+
+       if (update->type & REF_ISSYMREF) {
+               if (update->flags & REF_NODEREF) {
+                       /*
+                        * We won't be reading the referent as part of
+                        * the transaction, so we have to read it here
+                        * to record and possibly check old_sha1:
+                        */
+                       if (refs_read_ref_full(&refs->base,
+                                              referent.buf, 0,
+                                              lock->old_oid.hash, NULL)) {
+                               if (update->flags & REF_HAVE_OLD) {
+                                       strbuf_addf(err, "cannot lock ref '%s': "
+                                                   "error reading reference",
+                                                   original_update_refname(update));
+                                       return -1;
+                               }
+                       } else if (check_old_oid(update, &lock->old_oid, err)) {
+                               return TRANSACTION_GENERIC_ERROR;
+                       }
+               } else {
+                       /*
+                        * Create a new update for the reference this
+                        * symref is pointing at. Also, we will record
+                        * and verify old_sha1 for this update as part
+                        * of processing the split-off update, so we
+                        * don't have to do it here.
+                        */
+                       ret = split_symref_update(refs, update,
+                                                 referent.buf, transaction,
+                                                 affected_refnames, err);
+                       if (ret)
+                               return ret;
+               }
+       } else {
+               struct ref_update *parent_update;
+
+               if (check_old_oid(update, &lock->old_oid, err))
+                       return TRANSACTION_GENERIC_ERROR;
+
+               /*
+                * If this update is happening indirectly because of a
+                * symref update, record the old SHA-1 in the parent
+                * update:
+                */
+               for (parent_update = update->parent_update;
+                    parent_update;
+                    parent_update = parent_update->parent_update) {
+                       struct ref_lock *parent_lock = parent_update->backend_data;
+                       oidcpy(&parent_lock->old_oid, &lock->old_oid);
+               }
+       }
+
+       if ((update->flags & REF_HAVE_NEW) &&
+           !(update->flags & REF_DELETING) &&
+           !(update->flags & REF_LOG_ONLY)) {
+               if (!(update->type & REF_ISSYMREF) &&
+                   !oidcmp(&lock->old_oid, &update->new_oid)) {
+                       /*
+                        * The reference already has the desired
+                        * value, so we don't need to write it.
+                        */
+               } else if (write_ref_to_lockfile(lock, &update->new_oid,
+                                                err)) {
+                       char *write_err = strbuf_detach(err, NULL);
 
-       for (i = 1; i < n; i++)
-               if (!strcmp(refnames->items[i - 1].string, refnames->items[i].string)) {
+                       /*
+                        * The lock was freed upon failure of
+                        * write_ref_to_lockfile():
+                        */
+                       update->backend_data = NULL;
                        strbuf_addf(err,
-                                   "Multiple updates for ref '%s' not allowed.",
-                                   refnames->items[i].string);
-                       return 1;
+                                   "cannot update ref '%s': %s",
+                                   update->refname, write_err);
+                       free(write_err);
+                       return TRANSACTION_GENERIC_ERROR;
+               } else {
+                       update->flags |= REF_NEEDS_COMMIT;
+               }
+       }
+       if (!(update->flags & REF_NEEDS_COMMIT)) {
+               /*
+                * We didn't call write_ref_to_lockfile(), so
+                * the lockfile is still open. Close it to
+                * free up the file descriptor:
+                */
+               if (close_ref(lock)) {
+                       strbuf_addf(err, "couldn't close '%s.lock'",
+                                   update->refname);
+                       return TRANSACTION_GENERIC_ERROR;
                }
+       }
        return 0;
 }
 
-int ref_transaction_commit(struct ref_transaction *transaction,
-                          struct strbuf *err)
+/*
+ * Unlock any references in `transaction` that are still locked, and
+ * mark the transaction closed.
+ */
+static void files_transaction_cleanup(struct ref_transaction *transaction)
 {
-       int ret = 0, i;
-       int n = transaction->nr;
-       struct ref_update **updates = transaction->updates;
-       struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
-       struct string_list_item *ref_to_delete;
+       size_t i;
+
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+               struct ref_lock *lock = update->backend_data;
+
+               if (lock) {
+                       unlock_ref(lock);
+                       update->backend_data = NULL;
+               }
+       }
+
+       transaction->state = REF_TRANSACTION_CLOSED;
+}
+
+static int files_transaction_prepare(struct ref_store *ref_store,
+                                    struct ref_transaction *transaction,
+                                    struct strbuf *err)
+{
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE,
+                              "ref_transaction_prepare");
+       size_t i;
+       int ret = 0;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
+       char *head_ref = NULL;
+       int head_type;
+       struct object_id head_oid;
 
        assert(err);
 
-       if (transaction->state != REF_TRANSACTION_OPEN)
-               die("BUG: commit called for transaction that is not open");
+       if (!transaction->nr)
+               goto cleanup;
 
-       if (!n) {
-               transaction->state = REF_TRANSACTION_CLOSED;
-               return 0;
-       }
+       /*
+        * Fail if a refname appears more than once in the
+        * transaction. (If we end up splitting up any updates using
+        * split_symref_update() or split_head_update(), those
+        * functions will check that the new updates don't have the
+        * same refname as any existing ones.)
+        */
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+               struct string_list_item *item =
+                       string_list_append(&affected_refnames, update->refname);
 
-       /* Fail if a refname appears more than once in the transaction: */
-       for (i = 0; i < n; i++)
-               string_list_append(&affected_refnames, updates[i]->refname);
+               /*
+                * We store a pointer to update in item->util, but at
+                * the moment we never use the value of this field
+                * except to check whether it is non-NULL.
+                */
+               item->util = update;
+       }
        string_list_sort(&affected_refnames);
        if (ref_update_reject_duplicates(&affected_refnames, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }
 
+       /*
+        * Special hack: If a branch is updated directly and HEAD
+        * points to it (may happen on the remote side of a push
+        * for example) then logically the HEAD reflog should be
+        * updated too.
+        *
+        * A generic solution would require reverse symref lookups,
+        * but finding all symrefs pointing to a given branch would be
+        * rather costly for this rare event (the direct update of a
+        * branch) to be worth it. So let's cheat and check with HEAD
+        * only, which should cover 99% of all usage scenarios (even
+        * 100% of the default ones).
+        *
+        * So if HEAD is a symbolic reference, then record the name of
+        * the reference that it points to. If we see an update of
+        * head_ref within the transaction, then split_head_update()
+        * arranges for the reflog of HEAD to be updated, too.
+        */
+       head_ref = refs_resolve_refdup(ref_store, "HEAD",
+                                      RESOLVE_REF_NO_RECURSE,
+                                      head_oid.hash, &head_type);
+
+       if (head_ref && !(head_type & REF_ISSYMREF)) {
+               FREE_AND_NULL(head_ref);
+       }
+
        /*
         * Acquire all locks, verify old values if provided, check
         * that new values are valid, and write new values to the
         * lockfiles, ready to be activated. Only keep one lockfile
         * open at a time to avoid running out of file descriptors.
+        * Note that lock_ref_for_update() might append more updates
+        * to the transaction.
         */
-       for (i = 0; i < n; i++) {
-               struct ref_update *update = updates[i];
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
 
-               if ((update->flags & REF_HAVE_NEW) &&
-                   is_null_sha1(update->new_sha1))
-                       update->flags |= REF_DELETING;
-               update->lock = lock_ref_sha1_basic(
-                               update->refname,
-                               ((update->flags & REF_HAVE_OLD) ?
-                                update->old_sha1 : NULL),
-                               &affected_refnames, NULL,
-                               update->flags,
-                               &update->type,
-                               err);
-               if (!update->lock) {
-                       char *reason;
-
-                       ret = (errno == ENOTDIR)
-                               ? TRANSACTION_NAME_CONFLICT
-                               : TRANSACTION_GENERIC_ERROR;
-                       reason = strbuf_detach(err, NULL);
-                       strbuf_addf(err, "cannot lock ref '%s': %s",
-                                   update->refname, reason);
-                       free(reason);
-                       goto cleanup;
-               }
-               if ((update->flags & REF_HAVE_NEW) &&
-                   !(update->flags & REF_DELETING)) {
-                       int overwriting_symref = ((update->type & REF_ISSYMREF) &&
-                                                 (update->flags & REF_NODEREF));
+               ret = lock_ref_for_update(refs, update, transaction,
+                                         head_ref, &affected_refnames, err);
+               if (ret)
+                       break;
+       }
 
-                       if (!overwriting_symref &&
-                           !hashcmp(update->lock->old_oid.hash, update->new_sha1)) {
-                               /*
-                                * The reference already has the desired
-                                * value, so we don't need to write it.
-                                */
-                       } else if (write_ref_to_lockfile(update->lock,
-                                                        update->new_sha1,
-                                                        err)) {
-                               char *write_err = strbuf_detach(err, NULL);
+cleanup:
+       free(head_ref);
+       string_list_clear(&affected_refnames, 0);
 
-                               /*
-                                * The lock was freed upon failure of
-                                * write_ref_to_lockfile():
-                                */
-                               update->lock = NULL;
-                               strbuf_addf(err,
-                                           "cannot update the ref '%s': %s",
-                                           update->refname, write_err);
-                               free(write_err);
+       if (ret)
+               files_transaction_cleanup(transaction);
+       else
+               transaction->state = REF_TRANSACTION_PREPARED;
+
+       return ret;
+}
+
+static int files_transaction_finish(struct ref_store *ref_store,
+                                   struct ref_transaction *transaction,
+                                   struct strbuf *err)
+{
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 0, "ref_transaction_finish");
+       size_t i;
+       int ret = 0;
+       struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
+       struct string_list_item *ref_to_delete;
+       struct strbuf sb = STRBUF_INIT;
+
+       assert(err);
+
+       if (!transaction->nr) {
+               transaction->state = REF_TRANSACTION_CLOSED;
+               return 0;
+       }
+
+       /* Perform updates first so live commits remain referenced */
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+               struct ref_lock *lock = update->backend_data;
+
+               if (update->flags & REF_NEEDS_COMMIT ||
+                   update->flags & REF_LOG_ONLY) {
+                       if (files_log_ref_write(refs,
+                                               lock->ref_name,
+                                               &lock->old_oid,
+                                               &update->new_oid,
+                                               update->msg, update->flags,
+                                               err)) {
+                               char *old_msg = strbuf_detach(err, NULL);
+
+                               strbuf_addf(err, "cannot update the ref '%s': %s",
+                                           lock->ref_name, old_msg);
+                               free(old_msg);
+                               unlock_ref(lock);
+                               update->backend_data = NULL;
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
-                       } else {
-                               update->flags |= REF_NEEDS_COMMIT;
-                       }
-               }
-               if (!(update->flags & REF_NEEDS_COMMIT)) {
-                       /*
-                        * We didn't have to write anything to the lockfile.
-                        * Close it to free up the file descriptor:
-                        */
-                       if (close_ref(update->lock)) {
-                               strbuf_addf(err, "Couldn't close %s.lock",
-                                           update->refname);
-                               goto cleanup;
                        }
                }
-       }
-
-       /* Perform updates first so live commits remain referenced */
-       for (i = 0; i < n; i++) {
-               struct ref_update *update = updates[i];
-
                if (update->flags & REF_NEEDS_COMMIT) {
-                       if (commit_ref_update(update->lock,
-                                             update->new_sha1, update->msg,
-                                             update->flags, err)) {
-                               /* freed by commit_ref_update(): */
-                               update->lock = NULL;
+                       clear_loose_ref_cache(refs);
+                       if (commit_ref(lock)) {
+                               strbuf_addf(err, "couldn't set '%s'", lock->ref_name);
+                               unlock_ref(lock);
+                               update->backend_data = NULL;
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
-                       } else {
-                               /* freed by commit_ref_update(): */
-                               update->lock = NULL;
                        }
                }
        }
-
        /* Perform deletes now that updates are safely completed */
-       for (i = 0; i < n; i++) {
-               struct ref_update *update = updates[i];
-
-               if (update->flags & REF_DELETING) {
-                       if (delete_ref_loose(update->lock, update->type, err)) {
-                               ret = TRANSACTION_GENERIC_ERROR;
-                               goto cleanup;
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+               struct ref_lock *lock = update->backend_data;
+
+               if (update->flags & REF_DELETING &&
+                   !(update->flags & REF_LOG_ONLY)) {
+                       if (!(update->type & REF_ISPACKED) ||
+                           update->type & REF_ISSYMREF) {
+                               /* It is a loose reference. */
+                               strbuf_reset(&sb);
+                               files_ref_path(refs, &sb, lock->ref_name);
+                               if (unlink_or_msg(sb.buf, err)) {
+                                       ret = TRANSACTION_GENERIC_ERROR;
+                                       goto cleanup;
+                               }
+                               update->flags |= REF_DELETED_LOOSE;
                        }
 
                        if (!(update->flags & REF_ISPRUNING))
                                string_list_append(&refs_to_delete,
-                                                  update->lock->ref_name);
+                                                  lock->ref_name);
                }
        }
 
-       if (repack_without_refs(&refs_to_delete, err)) {
+       if (repack_without_refs(refs, &refs_to_delete, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }
-       for_each_string_list_item(ref_to_delete, &refs_to_delete)
-               unlink_or_warn(git_path("logs/%s", ref_to_delete->string));
-       clear_loose_ref_cache(&ref_cache);
+
+       /* Delete the reflogs of any references that were deleted: */
+       for_each_string_list_item(ref_to_delete, &refs_to_delete) {
+               strbuf_reset(&sb);
+               files_reflog_path(refs, &sb, ref_to_delete->string);
+               if (!unlink_or_warn(sb.buf))
+                       try_remove_empty_parents(refs, ref_to_delete->string,
+                                                REMOVE_EMPTY_PARENTS_REFLOG);
+       }
+
+       clear_loose_ref_cache(refs);
 
 cleanup:
-       transaction->state = REF_TRANSACTION_CLOSED;
+       files_transaction_cleanup(transaction);
+
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
+
+               if (update->flags & REF_DELETED_LOOSE) {
+                       /*
+                        * The loose reference was deleted. Delete any
+                        * empty parent directories. (Note that this
+                        * can only work because we have already
+                        * removed the lockfile.)
+                        */
+                       try_remove_empty_parents(refs, update->refname,
+                                                REMOVE_EMPTY_PARENTS_REF);
+               }
+       }
 
-       for (i = 0; i < n; i++)
-               if (updates[i]->lock)
-                       unlock_ref(updates[i]->lock);
+       strbuf_release(&sb);
        string_list_clear(&refs_to_delete, 0);
-       string_list_clear(&affected_refnames, 0);
        return ret;
 }
 
+static int files_transaction_abort(struct ref_store *ref_store,
+                                  struct ref_transaction *transaction,
+                                  struct strbuf *err)
+{
+       files_transaction_cleanup(transaction);
+       return 0;
+}
+
 static int ref_present(const char *refname,
                       const struct object_id *oid, int flags, void *cb_data)
 {
@@ -3340,12 +3125,15 @@ static int ref_present(const char *refname,
        return string_list_has_string(affected_refnames, refname);
 }
 
-int initial_ref_transaction_commit(struct ref_transaction *transaction,
-                                  struct strbuf *err)
+static int files_initial_transaction_commit(struct ref_store *ref_store,
+                                           struct ref_transaction *transaction,
+                                           struct strbuf *err)
 {
-       int ret = 0, i;
-       int n = transaction->nr;
-       struct ref_update **updates = transaction->updates;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE,
+                              "initial_ref_transaction_commit");
+       size_t i;
+       int ret = 0;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
 
        assert(err);
@@ -3354,8 +3142,9 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
                die("BUG: commit called for transaction that is not open");
 
        /* Fail if a refname appears more than once in the transaction: */
-       for (i = 0; i < n; i++)
-               string_list_append(&affected_refnames, updates[i]->refname);
+       for (i = 0; i < transaction->nr; i++)
+               string_list_append(&affected_refnames,
+                                  transaction->updates[i]->refname);
        string_list_sort(&affected_refnames);
        if (ref_update_reject_duplicates(&affected_refnames, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
@@ -3374,39 +3163,41 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
         * so here we really only check that none of the references
         * that we are creating already exists.
         */
-       if (for_each_rawref(ref_present, &affected_refnames))
+       if (refs_for_each_rawref(&refs->base, ref_present,
+                                &affected_refnames))
                die("BUG: initial ref transaction called with existing refs");
 
-       for (i = 0; i < n; i++) {
-               struct ref_update *update = updates[i];
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
 
                if ((update->flags & REF_HAVE_OLD) &&
-                   !is_null_sha1(update->old_sha1))
+                   !is_null_oid(&update->old_oid))
                        die("BUG: initial ref transaction with old_sha1 set");
-               if (verify_refname_available(update->refname,
-                                            &affected_refnames, NULL,
-                                            err)) {
+               if (refs_verify_refname_available(&refs->base, update->refname,
+                                                 &affected_refnames, NULL,
+                                                 err)) {
                        ret = TRANSACTION_NAME_CONFLICT;
                        goto cleanup;
                }
        }
 
-       if (lock_packed_refs(0)) {
+       if (lock_packed_refs(refs, 0)) {
                strbuf_addf(err, "unable to lock packed-refs file: %s",
                            strerror(errno));
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }
 
-       for (i = 0; i < n; i++) {
-               struct ref_update *update = updates[i];
+       for (i = 0; i < transaction->nr; i++) {
+               struct ref_update *update = transaction->updates[i];
 
                if ((update->flags & REF_HAVE_NEW) &&
-                   !is_null_sha1(update->new_sha1))
-                       add_packed_ref(update->refname, update->new_sha1);
+                   !is_null_oid(&update->new_oid))
+                       add_packed_ref(refs, update->refname,
+                                      &update->new_oid);
        }
 
-       if (commit_packed_refs()) {
+       if (commit_packed_refs(refs)) {
                strbuf_addf(err, "unable to commit packed-refs file: %s",
                            strerror(errno));
                ret = TRANSACTION_GENERIC_ERROR;
@@ -3424,20 +3215,20 @@ struct expire_reflog_cb {
        reflog_expiry_should_prune_fn *should_prune_fn;
        void *policy_cb;
        FILE *newlog;
-       unsigned char last_kept_sha1[20];
+       struct object_id last_kept_oid;
 };
 
-static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
-                            const char *email, unsigned long timestamp, int tz,
+static int expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
+                            const char *email, timestamp_t timestamp, int tz,
                             const char *message, void *cb_data)
 {
        struct expire_reflog_cb *cb = cb_data;
        struct expire_reflog_policy_cb *policy_cb = cb->policy_cb;
 
        if (cb->flags & EXPIRE_REFLOGS_REWRITE)
-               osha1 = cb->last_kept_sha1;
+               ooid = &cb->last_kept_oid;
 
-       if ((*cb->should_prune_fn)(osha1, nsha1, email, timestamp, tz,
+       if ((*cb->should_prune_fn)(ooid, noid, email, timestamp, tz,
                                   message, policy_cb)) {
                if (!cb->newlog)
                        printf("would prune %s", message);
@@ -3445,10 +3236,10 @@ static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
                        printf("prune %s", message);
        } else {
                if (cb->newlog) {
-                       fprintf(cb->newlog, "%s %s %s %lu %+05d\t%s",
-                               sha1_to_hex(osha1), sha1_to_hex(nsha1),
+                       fprintf(cb->newlog, "%s %s %s %"PRItime" %+05d\t%s",
+                               oid_to_hex(ooid), oid_to_hex(noid),
                                email, timestamp, tz, message);
-                       hashcpy(cb->last_kept_sha1, nsha1);
+                       oidcpy(&cb->last_kept_oid, noid);
                }
                if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
                        printf("keep %s", message);
@@ -3456,20 +3247,25 @@ static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
        return 0;
 }
 
-int reflog_expire(const char *refname, const unsigned char *sha1,
-                unsigned int flags,
-                reflog_expiry_prepare_fn prepare_fn,
-                reflog_expiry_should_prune_fn should_prune_fn,
-                reflog_expiry_cleanup_fn cleanup_fn,
-                void *policy_cb_data)
+static int files_reflog_expire(struct ref_store *ref_store,
+                              const char *refname, const unsigned char *sha1,
+                              unsigned int flags,
+                              reflog_expiry_prepare_fn prepare_fn,
+                              reflog_expiry_should_prune_fn should_prune_fn,
+                              reflog_expiry_cleanup_fn cleanup_fn,
+                              void *policy_cb_data)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "reflog_expire");
        static struct lock_file reflog_lock;
        struct expire_reflog_cb cb;
        struct ref_lock *lock;
+       struct strbuf log_file_sb = STRBUF_INIT;
        char *log_file;
        int status = 0;
        int type;
        struct strbuf err = STRBUF_INIT;
+       struct object_id oid;
 
        memset(&cb, 0, sizeof(cb));
        cb.flags = flags;
@@ -3481,18 +3277,21 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
         * reference itself, plus we might need to update the
         * reference if --updateref was specified:
         */
-       lock = lock_ref_sha1_basic(refname, sha1, NULL, NULL, 0, &type, &err);
+       lock = lock_ref_sha1_basic(refs, refname, sha1,
+                                  NULL, NULL, REF_NODEREF,
+                                  &type, &err);
        if (!lock) {
                error("cannot lock ref '%s': %s", refname, err.buf);
                strbuf_release(&err);
                return -1;
        }
-       if (!reflog_exists(refname)) {
+       if (!refs_reflog_exists(ref_store, refname)) {
                unlock_ref(lock);
                return 0;
        }
 
-       log_file = git_pathdup("logs/%s", refname);
+       files_reflog_path(refs, &log_file_sb, refname);
+       log_file = strbuf_detach(&log_file_sb, NULL);
        if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
                /*
                 * Even though holding $GIT_DIR/logs/$reflog.lock has
@@ -3516,8 +3315,10 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
                }
        }
 
-       (*prepare_fn)(refname, sha1, cb.policy_cb);
-       for_each_reflog_ent(refname, expire_reflog_ent, &cb);
+       hashcpy(oid.hash, sha1);
+
+       (*prepare_fn)(refname, &oid, cb.policy_cb);
+       refs_for_each_reflog_ent(ref_store, refname, expire_reflog_ent, &cb);
        (*cleanup_fn)(cb.policy_cb);
 
        if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
@@ -3530,14 +3331,14 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
                 */
                int update = (flags & EXPIRE_REFLOGS_UPDATE_REF) &&
                        !(type & REF_ISSYMREF) &&
-                       !is_null_sha1(cb.last_kept_sha1);
+                       !is_null_oid(&cb.last_kept_oid);
 
                if (close_lock_file(&reflog_lock)) {
                        status |= error("couldn't write %s: %s", log_file,
                                        strerror(errno));
                } else if (update &&
                           (write_in_full(get_lock_file_fd(lock->lk),
-                               sha1_to_hex(cb.last_kept_sha1), 40) != 40 ||
+                               oid_to_hex(&cb.last_kept_oid), GIT_SHA1_HEXSZ) != GIT_SHA1_HEXSZ ||
                            write_str_in_full(get_lock_file_fd(lock->lk), "\n") != 1 ||
                            close_ref(lock) < 0)) {
                        status |= error("couldn't write %s",
@@ -3560,3 +3361,51 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
        unlock_ref(lock);
        return -1;
 }
+
+static int files_init_db(struct ref_store *ref_store, struct strbuf *err)
+{
+       struct files_ref_store *refs =
+               files_downcast(ref_store, REF_STORE_WRITE, "init_db");
+       struct strbuf sb = STRBUF_INIT;
+
+       /*
+        * Create .git/refs/{heads,tags}
+        */
+       files_ref_path(refs, &sb, "refs/heads");
+       safe_create_dir(sb.buf, 1);
+
+       strbuf_reset(&sb);
+       files_ref_path(refs, &sb, "refs/tags");
+       safe_create_dir(sb.buf, 1);
+
+       strbuf_release(&sb);
+       return 0;
+}
+
+struct ref_storage_be refs_be_files = {
+       NULL,
+       "files",
+       files_ref_store_create,
+       files_init_db,
+       files_transaction_prepare,
+       files_transaction_finish,
+       files_transaction_abort,
+       files_initial_transaction_commit,
+
+       files_pack_refs,
+       files_peel_ref,
+       files_create_symref,
+       files_delete_refs,
+       files_rename_ref,
+
+       files_ref_iterator_begin,
+       files_read_raw_ref,
+
+       files_reflog_iterator_begin,
+       files_for_each_reflog_ent,
+       files_for_each_reflog_ent_reverse,
+       files_reflog_exists,
+       files_create_reflog,
+       files_delete_reflog,
+       files_reflog_expire
+};