refs: implement iteration over only per-worktree refs
[gitweb.git] / refs / files-backend.c
index ea78ce9d90f0bb02fcd471c1c113e22053a3ba94..47710fcf286283871c0949e888e4b319a7e83616 100644 (file)
@@ -1,13 +1,14 @@
 #include "../cache.h"
 #include "../refs.h"
 #include "refs-internal.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;
 };
@@ -38,7 +39,7 @@ struct ref_value {
        struct object_id peeled;
 };
 
-struct ref_cache;
+struct files_ref_store;
 
 /*
  * Information used (along with the information in ref_entry) to
@@ -77,8 +78,8 @@ struct ref_dir {
         */
        int sorted;
 
-       /* A pointer to the ref_cache that contains this ref_dir. */
-       struct ref_cache *ref_cache;
+       /* A pointer to the files_ref_store that contains this ref_dir. */
+       struct files_ref_store *ref_store;
 
        struct ref_entry **entries;
 };
@@ -160,7 +161,7 @@ struct ref_entry {
 
 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,
+static struct ref_entry *create_dir_entry(struct files_ref_store *ref_store,
                                          const char *dirname, size_t len,
                                          int incomplete);
 static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry);
@@ -182,7 +183,7 @@ static struct ref_dir *get_ref_dir(struct ref_entry *entry)
                        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,
+                               child_entry = create_dir_entry(dir->ref_store,
                                                               "refs/bisect/",
                                                               12, 1);
                                add_entry_to_dir(dir, child_entry);
@@ -260,13 +261,13 @@ static void clear_ref_dir(struct ref_dir *dir)
  * 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,
+static struct ref_entry *create_dir_entry(struct files_ref_store *ref_store,
                                          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->u.subdir.ref_store = ref_store;
        direntry->flag = REF_DIR | (incomplete ? REF_INCOMPLETE : 0);
        return direntry;
 }
@@ -342,7 +343,7 @@ static struct ref_dir *search_for_subdir(struct ref_dir *dir,
                 * therefore, create an empty record for it but mark
                 * the record complete.
                 */
-               entry = create_dir_entry(dir->ref_cache, subdirname, len, 0);
+               entry = create_dir_entry(dir->ref_store, subdirname, len, 0);
                add_entry_to_dir(dir, entry);
        } else {
                entry = dir->entries[entry_index];
@@ -513,72 +514,37 @@ static void sort_ref_dir(struct ref_dir *dir)
        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.
+ * 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 int ref_resolves_to_object(struct ref_entry *entry)
+static int ref_resolves_to_object(const char *refname,
+                                 const struct object_id *oid,
+                                 unsigned int flags)
 {
-       if (entry->flag & REF_ISBROKEN)
+       if (flags & REF_ISBROKEN)
                return 0;
-       if (!has_sha1_file(entry->u.value.oid.hash)) {
-               error("%s does not point to a valid object!", entry->name);
+       if (!has_sha1_file(oid->hash)) {
+               error("%s does not point to a valid object!", refname);
                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.
+ * Return true if the reference described by entry can be resolved to
+ * an object in the database; otherwise, emit a warning and return
+ * false.
  */
-static int do_one_ref(struct ref_entry *entry, void *cb_data)
+static int entry_resolves_to_object(struct ref_entry *entry)
 {
-       struct ref_entry_cb *data = cb_data;
-       struct ref_entry *old_current_ref;
-       int retval;
-
-       if (!starts_with(entry->name, data->base))
-               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;
+       return ref_resolves_to_object(entry->name,
+                                     &entry->u.value.oid, entry->flag);
 }
 
+typedef int each_ref_entry_fn(struct ref_entry *entry, void *cb_data);
+
 /*
  * Call fn for each reference in dir that has index in the range
  * offset <= index < dir->nr.  Recurse into subdirectories that are in
@@ -607,78 +573,6 @@ static int do_for_each_entry_in_dir(struct ref_dir *dir, int offset,
        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
@@ -695,6 +589,153 @@ static void prime_ref_dir(struct ref_dir *dir)
        }
 }
 
+/*
+ * A level in the reference hierarchy that is currently being iterated
+ * through.
+ */
+struct cache_ref_iterator_level {
+       /*
+        * The ref_dir being iterated over at this level. The ref_dir
+        * is sorted before being stored here.
+        */
+       struct ref_dir *dir;
+
+       /*
+        * The index of the current entry within dir (which might
+        * itself be a directory). If index == -1, then the iteration
+        * hasn't yet begun. If index == dir->nr, then the iteration
+        * through this level is over.
+        */
+       int index;
+};
+
+/*
+ * Represent an iteration through a ref_dir in the memory cache. The
+ * iteration recurses through subdirectories.
+ */
+struct cache_ref_iterator {
+       struct ref_iterator base;
+
+       /*
+        * The number of levels currently on the stack. This is always
+        * at least 1, because when it becomes zero the iteration is
+        * ended and this struct is freed.
+        */
+       size_t levels_nr;
+
+       /* The number of levels that have been allocated on the stack */
+       size_t levels_alloc;
+
+       /*
+        * A stack of levels. levels[0] is the uppermost level that is
+        * being iterated over in this iteration. (This is not
+        * necessary the top level in the references hierarchy. If we
+        * are iterating through a subtree, then levels[0] will hold
+        * the ref_dir for that subtree, and subsequent levels will go
+        * on from there.)
+        */
+       struct cache_ref_iterator_level *levels;
+};
+
+static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+       struct cache_ref_iterator *iter =
+               (struct cache_ref_iterator *)ref_iterator;
+
+       while (1) {
+               struct cache_ref_iterator_level *level =
+                       &iter->levels[iter->levels_nr - 1];
+               struct ref_dir *dir = level->dir;
+               struct ref_entry *entry;
+
+               if (level->index == -1)
+                       sort_ref_dir(dir);
+
+               if (++level->index == level->dir->nr) {
+                       /* This level is exhausted; pop up a level */
+                       if (--iter->levels_nr == 0)
+                               return ref_iterator_abort(ref_iterator);
+
+                       continue;
+               }
+
+               entry = dir->entries[level->index];
+
+               if (entry->flag & REF_DIR) {
+                       /* push down a level */
+                       ALLOC_GROW(iter->levels, iter->levels_nr + 1,
+                                  iter->levels_alloc);
+
+                       level = &iter->levels[iter->levels_nr++];
+                       level->dir = get_ref_dir(entry);
+                       level->index = -1;
+               } else {
+                       iter->base.refname = entry->name;
+                       iter->base.oid = &entry->u.value.oid;
+                       iter->base.flags = entry->flag;
+                       return ITER_OK;
+               }
+       }
+}
+
+static enum peel_status peel_entry(struct ref_entry *entry, int repeel);
+
+static int cache_ref_iterator_peel(struct ref_iterator *ref_iterator,
+                                  struct object_id *peeled)
+{
+       struct cache_ref_iterator *iter =
+               (struct cache_ref_iterator *)ref_iterator;
+       struct cache_ref_iterator_level *level;
+       struct ref_entry *entry;
+
+       level = &iter->levels[iter->levels_nr - 1];
+
+       if (level->index == -1)
+               die("BUG: peel called before advance for cache iterator");
+
+       entry = level->dir->entries[level->index];
+
+       if (peel_entry(entry, 0))
+               return -1;
+       hashcpy(peeled->hash, entry->u.value.peeled.hash);
+       return 0;
+}
+
+static int cache_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+       struct cache_ref_iterator *iter =
+               (struct cache_ref_iterator *)ref_iterator;
+
+       free(iter->levels);
+       base_ref_iterator_free(ref_iterator);
+       return ITER_DONE;
+}
+
+static struct ref_iterator_vtable cache_ref_iterator_vtable = {
+       cache_ref_iterator_advance,
+       cache_ref_iterator_peel,
+       cache_ref_iterator_abort
+};
+
+static struct ref_iterator *cache_ref_iterator_begin(struct ref_dir *dir)
+{
+       struct cache_ref_iterator *iter;
+       struct ref_iterator *ref_iterator;
+       struct cache_ref_iterator_level *level;
+
+       iter = xcalloc(1, sizeof(*iter));
+       ref_iterator = &iter->base;
+       base_ref_iterator_init(ref_iterator, &cache_ref_iterator_vtable);
+       ALLOC_GROW(iter->levels, 10, iter->levels_alloc);
+
+       iter->levels_nr = 1;
+       level = &iter->levels[0];
+       level->index = -1;
+       level->dir = dir;
+
+       return ref_iterator;
+}
+
 struct nonmatching_ref_data {
        const struct string_list *skip;
        const char *conflicting_refname;
@@ -846,9 +887,9 @@ struct packed_ref_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;
 
@@ -869,17 +910,11 @@ 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 files_ref_store {
+       struct ref_store base;
        struct ref_entry *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.
-        */
-       char name[1];
-} ref_cache, *submodule_ref_caches;
+};
 
 /* Lock used for the main packed-refs file: */
 static struct lock_file packlock;
@@ -908,7 +943,7 @@ 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;
@@ -920,7 +955,7 @@ static void clear_packed_ref_cache(struct ref_cache *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);
@@ -932,42 +967,34 @@ 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_cache *lookup_ref_cache(const char *submodule)
+static struct ref_store *files_ref_store_create(const char *submodule)
 {
-       struct ref_cache *refs;
+       struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
+       struct ref_store *ref_store = (struct ref_store *)refs;
 
-       if (!submodule || !*submodule)
-               return &ref_cache;
+       base_ref_store_init(ref_store, &refs_be_files, submodule);
 
-       for (refs = submodule_ref_caches; refs; refs = refs->next)
-               if (!strcmp(submodule, refs->name))
-                       return refs;
-       return NULL;
+       return ref_store;
 }
 
 /*
- * 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. If submodule_allowed is not true, then also die if
+ * files_ref_store is for a submodule (i.e., not for the main
+ * repository). 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, int submodule_allowed,
+               const char *caller)
 {
-       struct ref_cache *refs = lookup_ref_cache(submodule);
-       if (!refs)
-               refs = create_ref_cache(submodule);
-       return refs;
+       if (ref_store->be != &refs_be_files)
+               die("BUG: ref_store is type \"%s\" not \"files\" in %s",
+                   ref_store->be->name, caller);
+
+       if (!submodule_allowed)
+               assert_main_repository(ref_store, caller);
+
+       return (struct files_ref_store *)ref_store;
 }
 
 /* The length of a peeled reference line in packed-refs, including EOL: */
@@ -1099,15 +1126,16 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 }
 
 /*
- * Get the packed_ref_cache for the specified ref_cache, creating it
- * if necessary.
+ * Get the packed_ref_cache for the specified files_ref_store,
+ * creating it if necessary.
  */
-static struct packed_ref_cache *get_packed_ref_cache(struct ref_cache *refs)
+static struct packed_ref_cache *get_packed_ref_cache(struct files_ref_store *refs)
 {
        char *packed_refs_file;
 
-       if (*refs->name)
-               packed_refs_file = git_pathdup_submodule(refs->name, "packed-refs");
+       if (*refs->base.submodule)
+               packed_refs_file = git_pathdup_submodule(refs->base.submodule,
+                                                        "packed-refs");
        else
                packed_refs_file = git_pathdup("packed-refs");
 
@@ -1137,7 +1165,7 @@ static struct ref_dir *get_packed_ref_dir(struct packed_ref_cache *packed_ref_ca
        return get_ref_dir(packed_ref_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));
 }
@@ -1148,10 +1176,10 @@ 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 unsigned char *sha1)
 {
-       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 (!packed_ref_cache->lock)
                die("internal error: packed refs not locked");
@@ -1166,7 +1194,7 @@ static void add_packed_ref(const char *refname, const unsigned char *sha1)
  */
 static void read_loose_refs(const char *dirname, struct ref_dir *dir)
 {
-       struct ref_cache *refs = dir->ref_cache;
+       struct files_ref_store *refs = dir->ref_store;
        DIR *d;
        struct dirent *de;
        int dirnamelen = strlen(dirname);
@@ -1174,8 +1202,8 @@ 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);
+       if (*refs->base.submodule)
+               strbuf_git_path_submodule(&path, refs->base.submodule, "%s", dirname);
        else
                strbuf_git_path(&path, "%s", dirname);
        path_baselen = path.len;
@@ -1210,10 +1238,10 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
                } else {
                        int read_ok;
 
-                       if (*refs->name) {
+                       if (*refs->base.submodule) {
                                hashclr(sha1);
                                flag = 0;
-                               read_ok = !resolve_gitlink_ref(refs->name,
+                               read_ok = !resolve_gitlink_ref(refs->base.submodule,
                                                               refname.buf, sha1);
                        } else {
                                read_ok = !read_ref_full(refname.buf,
@@ -1254,7 +1282,7 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
        closedir(d);
 }
 
-static struct ref_dir *get_loose_refs(struct ref_cache *refs)
+static struct ref_dir *get_loose_refs(struct files_ref_store *refs)
 {
        if (!refs->loose) {
                /*
@@ -1272,112 +1300,22 @@ static struct ref_dir *get_loose_refs(struct ref_cache *refs)
        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++;
-
-       return resolve_gitlink_ref_recursive(refs, p, sha1, recursion+1);
-}
-
-int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *sha1)
-{
-       int len = strlen(path), retval;
-       struct strbuf submodule = STRBUF_INIT;
-       struct ref_cache *refs;
-
-       while (len && path[len-1] == '/')
-               len--;
-       if (!len)
-               return -1;
-
-       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);
-       }
-       strbuf_release(&submodule);
-
-       retval = resolve_gitlink_ref_recursive(refs, refname, sha1, 0);
-       return retval;
-}
-
 /*
  * 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(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;
 
@@ -1385,208 +1323,388 @@ 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;
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 1, "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;
 
-       if (flags)
-               *flags = 0;
+       *type = 0;
+       strbuf_reset(&sb_path);
 
-       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
-               if (flags)
-                       *flags |= REF_BAD_NAME;
+       if (*refs->base.submodule)
+               strbuf_git_path_submodule(&sb_path, refs->base.submodule, "%s", refname);
+       else
+               strbuf_git_path(&sb_path, "%s", refname);
 
-               if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
-                   !refname_is_safe(refname)) {
-                       errno = EINVAL;
-                       return NULL;
+       path = sb_path.buf;
+
+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().
+        */
+
+       if (lstat(path, &st) < 0) {
+               if (errno != ENOENT)
+                       goto out;
+               if (resolve_packed_ref(refs, refname, sha1, type)) {
+                       errno = ENOENT;
+                       goto out;
                }
-               /*
-                * 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;
+               ret = 0;
+               goto out;
        }
-       for (;;) {
-               const char *path;
-               struct stat st;
-               char *buf;
-               int fd;
 
-               if (--depth < 0) {
-                       errno = ELOOP;
-                       return NULL;
+       /* 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
+                               goto out;
                }
+               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;
+               }
+       }
 
-               strbuf_reset(sb_path);
-               strbuf_git_path(sb_path, "%s", refname);
-               path = sb_path->buf;
-
+       /* Is it a directory? */
+       if (S_ISDIR(st.st_mode)) {
                /*
-                * 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().
+                * Even though there is a directory where the loose
+                * ref is supposed to be, there could still be a
+                * packed ref:
                 */
-       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;
+               if (resolve_packed_ref(refs, refname, sha1, type)) {
+                       errno = EISDIR;
+                       goto out;
                }
+               ret = 0;
+               goto out;
+       }
 
-               /* 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;
-                       }
-               }
+       /*
+        * Anything else, just open it and try to use it as
+        * a ref
+        */
+       fd = open(path, O_RDONLY);
+       if (fd < 0) {
+               if (errno == ENOENT)
+                       /* 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++;
 
-               /* Is it a directory? */
-               if (S_ISDIR(st.st_mode)) {
-                       errno = EISDIR;
-                       return NULL;
-               }
+               strbuf_reset(referent);
+               strbuf_addstr(referent, buf);
+               *type |= REF_ISSYMREF;
+               ret = 0;
+               goto out;
+       }
+
+       /*
+        * 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;
+
+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);
+}
+
+/*
+ * 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 verify_refname_available_dir() 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()
+ *
+ * but it includes a lot more code to
+ * - Deal with possible races with other processes
+ * - Avoid calling verify_refname_available_dir() 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 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)
+{
+       struct ref_lock *lock;
+       struct strbuf ref_file = STRBUF_INIT;
+       int attempts_remaining = 3;
+       int ret = TRANSACTION_GENERIC_ERROR;
 
+       assert(err);
+       assert_main_repository(&refs->base, "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);
+       strbuf_git_path(&ref_file, "%s", refname);
+
+retry:
+       switch (safe_create_leading_directories(ref_file.buf)) {
+       case SCLD_OK:
+               break; /* success */
+       case SCLD_EXISTS:
                /*
-                * Anything else, just open it and try to use it as
-                * a ref
+                * 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.
                 */
-               fd = open(path, O_RDONLY);
-               if (fd < 0) {
-                       if (errno == ENOENT)
-                               /* inconsistent with lstat; retry */
-                               goto stat_ref;
-                       else
-                               return NULL;
-               }
-               strbuf_reset(sb_contents);
-               if (strbuf_read(sb_contents, fd, 256) < 0) {
-                       int save_errno = errno;
-                       close(fd);
-                       errno = save_errno;
-                       return NULL;
+               if (verify_refname_available(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
+                                * verify_refname_available_dir() 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);
                }
-               close(fd);
-               strbuf_rtrim(sb_contents);
+               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;
+       }
 
-               /*
-                * Is it a symbolic ref?
-                */
-               if (!starts_with(sb_contents->buf, "ref:")) {
+       if (!lock->lk)
+               lock->lk = xcalloc(1, sizeof(struct lock_file));
+
+       if (hold_lock_file_for_update(lock->lk, ref_file.buf, LOCK_NO_DEREF) < 0) {
+               if (errno == ENOENT && --attempts_remaining > 0) {
                        /*
-                        * Please note that FETCH_HEAD has a second
-                        * line containing other data.
+                        * Maybe somebody just deleted one of the
+                        * directories leading to ref_file.  Try
+                        * again:
                         */
-                       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;
+                       goto retry;
+               } else {
+                       unable_to_lock_message(ref_file.buf, errno, err);
+                       goto error_return;
+               }
+       }
+
+       /*
+        * 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".
+                                */
                        }
-                       if (bad_name) {
-                               hashclr(sha1);
-                               if (flags)
-                                       *flags |= REF_ISBROKEN;
+               } 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 (verify_refname_available_dir(
+                                                   refname, extras, skip,
+                                                   get_loose_refs(refs),
+                                                   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;
+                               }
                        }
-                       return refname;
-               }
-               if (flags)
-                       *flags |= REF_ISSYMREF;
-               buf = sb_contents->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;
+               } 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 (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;
+
+               /*
+                * If the ref did not exist and we are creating it,
+                * make sure there is no existing packed ref whose
+                * name begins with our refname, nor a packed ref
+                * whose name is a proper prefix of our refname.
+                */
+               if (verify_refname_available_dir(
+                                   refname, extras, skip,
+                                   get_packed_refs(refs),
+                                   err)) {
+                       goto error_return;
                }
        }
-}
 
-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;
+       ret = 0;
+       goto out;
 
-       ret = resolve_ref_1(refname, resolve_flags, sha1, flags,
-                           &sb_refname, &sb_path, &sb_contents);
-       strbuf_release(&sb_path);
-       strbuf_release(&sb_contents);
+error_return:
+       unlock_ref(lock);
+       *lock_p = NULL;
+
+out:
+       strbuf_release(&ref_file);
        return ret;
 }
 
@@ -1625,16 +1743,19 @@ static enum peel_status peel_entry(struct ref_entry *entry, int repeel)
        return status;
 }
 
-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, 0, "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;
        }
 
@@ -1650,7 +1771,7 @@ 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 +1783,142 @@ 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 files_ref_iterator {
+       struct ref_iterator base;
+
        struct packed_ref_cache *packed_ref_cache;
-       struct ref_dir *loose_dir;
-       struct ref_dir *packed_dir;
-       int retval = 0;
+       struct ref_iterator *iter0;
+       unsigned int flags;
+};
 
-       /*
-        * 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);
+static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+       struct files_ref_iterator *iter =
+               (struct files_ref_iterator *)ref_iterator;
+       int ok;
 
-       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);
-       }
+       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;
 
-       release_packed_ref_cache(packed_ref_cache);
-       return retval;
-}
+               if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+                   !ref_resolves_to_object(iter->iter0->refname,
+                                           iter->iter0->oid,
+                                           iter->iter0->flags))
+                       continue;
 
-/*
- * 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)
-{
-       struct ref_entry_cb data;
-       data.base = base;
-       data.trim = trim;
-       data.flags = flags;
-       data.fn = fn;
-       data.cb_data = cb_data;
+               iter->base.refname = iter->iter0->refname;
+               iter->base.oid = iter->iter0->oid;
+               iter->base.flags = iter->iter0->flags;
+               return ITER_OK;
+       }
 
-       if (ref_paranoia < 0)
-               ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
-       if (ref_paranoia)
-               data.flags |= DO_FOR_EACH_INCLUDE_BROKEN;
+       iter->iter0 = NULL;
+       if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+               ok = ITER_ERROR;
 
-       return do_for_each_entry(refs, base, do_one_ref, &data);
+       return ok;
 }
 
-static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
+static int files_ref_iterator_peel(struct ref_iterator *ref_iterator,
+                                  struct object_id *peeled)
 {
-       struct object_id oid;
-       int flag;
+       struct files_ref_iterator *iter =
+               (struct files_ref_iterator *)ref_iterator;
 
-       if (submodule) {
-               if (resolve_gitlink_ref(submodule, "HEAD", oid.hash) == 0)
-                       return fn("HEAD", &oid, 0, cb_data);
+       return ref_iterator_peel(iter->iter0, peeled);
+}
 
-               return 0;
-       }
+static int files_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+       struct files_ref_iterator *iter =
+               (struct files_ref_iterator *)ref_iterator;
+       int ok = ITER_DONE;
 
-       if (!read_ref_full("HEAD", RESOLVE_REF_READING, oid.hash, &flag))
-               return fn("HEAD", &oid, flag, cb_data);
+       if (iter->iter0)
+               ok = ref_iterator_abort(iter->iter0);
 
-       return 0;
+       release_packed_ref_cache(iter->packed_ref_cache);
+       base_ref_iterator_free(ref_iterator);
+       return ok;
 }
 
-int head_ref(each_ref_fn fn, void *cb_data)
-{
-       return do_head_ref(NULL, fn, cb_data);
-}
+static struct ref_iterator_vtable files_ref_iterator_vtable = {
+       files_ref_iterator_advance,
+       files_ref_iterator_peel,
+       files_ref_iterator_abort
+};
 
-int head_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
+static struct ref_iterator *files_ref_iterator_begin(
+               struct ref_store *ref_store,
+               const char *prefix, unsigned int flags)
 {
-       return do_head_ref(submodule, fn, cb_data);
-}
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 1, "ref_iterator_begin");
+       struct ref_dir *loose_dir, *packed_dir;
+       struct ref_iterator *loose_iter, *packed_iter;
+       struct files_ref_iterator *iter;
+       struct ref_iterator *ref_iterator;
+
+       if (!refs)
+               return empty_ref_iterator_begin();
+
+       if (ref_paranoia < 0)
+               ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
+       if (ref_paranoia)
+               flags |= DO_FOR_EACH_INCLUDE_BROKEN;
 
-int for_each_ref(each_ref_fn fn, void *cb_data)
-{
-       return do_for_each_ref(&ref_cache, "", fn, 0, 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_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);
-}
+       /*
+        * 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 prime_ref_dir(), which pre-reads the loose
+        * references for the subtree 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_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);
-}
+       loose_dir = get_loose_refs(refs);
 
-int for_each_fullref_in(const char *prefix, each_ref_fn fn, void *cb_data, unsigned int broken)
-{
-       unsigned int flag = 0;
+       if (prefix && *prefix)
+               loose_dir = find_containing_dir(loose_dir, prefix, 0);
 
-       if (broken)
-               flag = DO_FOR_EACH_INCLUDE_BROKEN;
-       return do_for_each_ref(&ref_cache, prefix, fn, 0, flag, cb_data);
-}
+       if (loose_dir) {
+               prime_ref_dir(loose_dir);
+               loose_iter = cache_ref_iterator_begin(loose_dir);
+       } else {
+               /* There's nothing to iterate over. */
+               loose_iter = empty_ref_iterator_begin();
+       }
 
-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);
-}
+       iter->packed_ref_cache = get_packed_ref_cache(refs);
+       acquire_packed_ref_cache(iter->packed_ref_cache);
+       packed_dir = get_packed_ref_dir(iter->packed_ref_cache);
 
-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);
-}
+       if (prefix && *prefix)
+               packed_dir = find_containing_dir(packed_dir, prefix, 0);
 
-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;
-}
+       if (packed_dir) {
+               packed_iter = cache_ref_iterator_begin(packed_dir);
+       } else {
+               /* There's nothing to iterate over. */
+               packed_iter = empty_ref_iterator_begin();
+       }
 
-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);
-}
+       iter->iter0 = overlay_ref_iterator_begin(loose_iter, packed_iter);
+       iter->flags = flags;
 
-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);
+       return ref_iterator;
 }
 
 /*
@@ -1852,7 +1938,7 @@ static int verify_lock(struct ref_lock *lock,
                          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 {
@@ -1861,7 +1947,7 @@ static int verify_lock(struct ref_lock *lock,
                }
        }
        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),
                            sha1_to_hex(old_sha1));
@@ -1885,24 +1971,24 @@ static int remove_empty_directories(struct strbuf *path)
  * 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 lflags = LOCK_NO_DEREF;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
-       int resolve_flags = 0;
+       int resolve_flags = RESOLVE_REF_NO_RECURSE;
        int attempts_remaining = 3;
+       int resolved;
 
+       assert_main_repository(&refs->base, "lock_ref_sha1_basic");
        assert(err);
 
        lock = xcalloc(1, sizeof(struct ref_lock));
@@ -1911,48 +1997,41 @@ 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) {
+       strbuf_git_path(&ref_file, "%s", refname);
+       resolved = !!resolve_ref_unsafe(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 (!verify_refname_available_dir(
+                                           refname, extras, skip,
+                                           get_loose_refs(refs), 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 = !!resolve_ref_unsafe(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));
+                   !verify_refname_available_dir(
+                                   refname, extras, skip,
+                                   get_loose_refs(refs), 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
@@ -1961,7 +2040,8 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
         */
        if (is_null_oid(&lock->old_oid) &&
            verify_refname_available_dir(refname, extras, skip,
-                                        get_packed_refs(&ref_cache), err)) {
+                                        get_packed_refs(refs),
+                                        err)) {
                last_errno = ENOTDIR;
                goto error_return;
        }
@@ -1969,8 +2049,6 @@ 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)) {
@@ -1982,7 +2060,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
                /* fall through */
        default:
                last_errno = errno;
-               strbuf_addf(err, "unable to create directory for %s",
+               strbuf_addf(err, "unable to create directory for '%s'",
                            ref_file.buf);
                goto error_return;
        }
@@ -2013,7 +2091,6 @@ 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;
 }
@@ -2051,13 +2128,14 @@ static int write_packed_entry_fn(struct ref_entry *entry, void *cb_data)
  * 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;
 
+       assert_main_repository(&refs->base, "lock_packed_refs");
+
        if (!timeout_configured) {
                git_config_get_int("core.packedrefstimeout", &timeout_value);
                timeout_configured = 1;
@@ -2073,7 +2151,7 @@ static int lock_packed_refs(int flags)
         * this will automatically invalidate the cache and re-read
         * the packed-refs file.
         */
-       packed_ref_cache = get_packed_ref_cache(&ref_cache);
+       packed_ref_cache = get_packed_ref_cache(refs);
        packed_ref_cache->lock = &packlock;
        /* Increment the reference count to prevent it from being freed: */
        acquire_packed_ref_cache(packed_ref_cache);
@@ -2086,14 +2164,16 @@ 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);
+               get_packed_ref_cache(refs);
        int error = 0;
        int save_errno = 0;
        FILE *out;
 
+       assert_main_repository(&refs->base, "commit_packed_refs");
+
        if (!packed_ref_cache->lock)
                die("internal error: packed-refs not locked");
 
@@ -2120,17 +2200,19 @@ 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);
+
+       assert_main_repository(&refs->base, "rollback_packed_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;
        release_packed_ref_cache(packed_ref_cache);
-       clear_packed_ref_cache(&ref_cache);
+       clear_packed_ref_cache(refs);
 }
 
 struct ref_to_prune {
@@ -2167,7 +2249,7 @@ static int pack_if_possible_fn(struct ref_entry *entry, void *cb_data)
                return 0;
 
        /* Do not pack symbolic or broken refs: */
-       if ((entry->flag & REF_ISSYMREF) || !ref_resolves_to_object(entry))
+       if ((entry->flag & REF_ISSYMREF) || !entry_resolves_to_object(entry))
                return 0;
 
        /* Add a packed ref cache entry equivalent to the loose entry. */
@@ -2241,7 +2323,7 @@ static void prune_ref(struct ref_to_prune *r)
        transaction = ref_transaction_begin(&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);
@@ -2261,20 +2343,22 @@ static void prune_refs(struct ref_to_prune *r)
        }
 }
 
-int pack_refs(unsigned int flags)
+static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 0, "pack_refs");
        struct pack_refs_cb_data cbdata;
 
        memset(&cbdata, 0, sizeof(cbdata));
        cbdata.flags = flags;
 
-       lock_packed_refs(LOCK_DIE_ON_ERROR);
-       cbdata.packed_refs = get_packed_refs(&ref_cache);
+       lock_packed_refs(refs, LOCK_DIE_ON_ERROR);
+       cbdata.packed_refs = get_packed_refs(refs);
 
-       do_for_each_entry_in_dir(get_loose_refs(&ref_cache), 0,
+       do_for_each_entry_in_dir(get_loose_refs(refs), 0,
                                 pack_if_possible_fn, &cbdata);
 
-       if (commit_packed_refs())
+       if (commit_packed_refs(refs))
                die_errno("unable to overwrite old ref-pack file");
 
        prune_refs(cbdata.ref_to_prune);
@@ -2288,17 +2372,19 @@ 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;
 
+       assert_main_repository(&refs->base, "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;
                }
@@ -2308,11 +2394,11 @@ 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)) {
+       if (lock_packed_refs(refs, 0)) {
                unable_to_lock_message(git_path("packed-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)
@@ -2323,12 +2409,12 @@ static int repack_without_refs(struct string_list *refnames, struct strbuf *err)
                 * 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));
@@ -2353,15 +2439,18 @@ static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
        return 0;
 }
 
-int delete_refs(struct string_list *refnames)
+static int files_delete_refs(struct ref_store *ref_store,
+                            struct string_list *refnames, unsigned int flags)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 0, "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 +2471,7 @@ 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 (delete_ref(refname, NULL, flags))
                        result |= error(_("could not remove reference %s"), refname);
        }
 
@@ -2452,13 +2541,16 @@ static int rename_tmp_log(const char *newrefname)
        return ret;
 }
 
-int verify_refname_available(const char *newname,
-                            struct string_list *extras,
-                            struct string_list *skip,
-                            struct strbuf *err)
+static int files_verify_refname_available(struct ref_store *ref_store,
+                                         const char *newname,
+                                         const struct string_list *extras,
+                                         const struct string_list *skip,
+                                         struct strbuf *err)
 {
-       struct ref_dir *packed_refs = get_packed_refs(&ref_cache);
-       struct ref_dir *loose_refs = get_loose_refs(&ref_cache);
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 1, "verify_refname_available");
+       struct ref_dir *packed_refs = get_packed_refs(refs);
+       struct ref_dir *loose_refs = get_loose_refs(refs);
 
        if (verify_refname_available_dir(newname, extras, skip,
                                         packed_refs, err) ||
@@ -2471,31 +2563,34 @@ int verify_refname_available(const char *newname,
 
 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,
+static int commit_ref_update(struct files_ref_store *refs,
+                            struct ref_lock *lock,
                             const unsigned char *sha1, const char *logmsg,
-                            int flags, struct strbuf *err);
+                            struct strbuf *err);
 
-int rename_ref(const char *oldrefname, const char *newrefname, const char *logmsg)
+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, 0, "rename_ref");
        unsigned char sha1[20], orig_sha1[20];
        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 err = STRBUF_INIT;
 
        if (log && S_ISLNK(loginfo.st_mode))
                return error("reflog for %s is a symlink", oldrefname);
 
-       symref = resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING,
-                                   orig_sha1, &flag);
+       if (!resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+                               orig_sha1, &flag))
+               return error("refname %s not found", oldrefname);
+
        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);
-
        if (!rename_ref_available(oldrefname, newrefname))
                return 1;
 
@@ -2508,8 +2603,16 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
                goto rollback;
        }
 
-       if (!read_ref_full(newrefname, RESOLVE_REF_READING, sha1, NULL) &&
-           delete_ref(newrefname, sha1, REF_NODEREF)) {
+       /*
+        * Since we are doing a shallow lookup, sha1 is not the
+        * correct value to pass to delete_ref as old_sha1. But that
+        * doesn't matter, because an old_sha1 check wouldn't add to
+        * the safety anyway; we want to delete the reference whatever
+        * its current value.
+        */
+       if (!read_ref_full(newrefname, RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+                          sha1, NULL) &&
+           delete_ref(newrefname, NULL, REF_NODEREF)) {
                if (errno==EISDIR) {
                        struct strbuf path = STRBUF_INIT;
                        int result;
@@ -2533,7 +2636,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 
        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);
@@ -2542,7 +2646,7 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
        hashcpy(lock->old_oid.hash, orig_sha1);
 
        if (write_ref_to_lockfile(lock, orig_sha1, &err) ||
-           commit_ref_update(lock, orig_sha1, logmsg, 0, &err)) {
+           commit_ref_update(refs, lock, orig_sha1, logmsg, &err)) {
                error("unable to write current sha1 into %s: %s", newrefname, err.buf);
                strbuf_release(&err);
                goto rollback;
@@ -2551,7 +2655,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
        return 0;
 
  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);
@@ -2561,7 +2666,7 @@ 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)) {
+           commit_ref_update(refs, lock, orig_sha1, NULL, &err)) {
                error("unable to write current sha1 into %s: %s", oldrefname, err.buf);
                strbuf_release(&err);
        }
@@ -2588,6 +2693,30 @@ 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;
@@ -2606,7 +2735,7 @@ static int log_ref_setup(const char *refname, struct strbuf *logfile, struct str
        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: "
+                       strbuf_addf(err, "unable to create directory for '%s': "
                                    "%s", logfile->buf, strerror(errno));
                        return -1;
                }
@@ -2620,7 +2749,7 @@ static int log_ref_setup(const char *refname, struct strbuf *logfile, struct str
 
                if (errno == EISDIR) {
                        if (remove_empty_directories(logfile)) {
-                               strbuf_addf(err, "There are still logs under "
+                               strbuf_addf(err, "there are still logs under "
                                            "'%s'", logfile->buf);
                                return -1;
                        }
@@ -2628,7 +2757,7 @@ static int log_ref_setup(const char *refname, struct strbuf *logfile, struct str
                }
 
                if (logfd < 0) {
-                       strbuf_addf(err, "unable to append to %s: %s",
+                       strbuf_addf(err, "unable to append to '%s': %s",
                                    logfile->buf, strerror(errno));
                        return -1;
                }
@@ -2640,11 +2769,16 @@ static int log_ref_setup(const char *refname, struct strbuf *logfile, struct str
 }
 
 
-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;
 
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "create_reflog");
+
        ret = log_ref_setup(refname, &sb, err, force_create);
        strbuf_release(&sb);
        return ret;
@@ -2697,13 +2831,13 @@ static int log_ref_write_1(const char *refname, const unsigned char *old_sha1,
        result = log_ref_write_fd(logfd, old_sha1, new_sha1,
                                  git_committer_info(0), msg);
        if (result) {
-               strbuf_addf(err, "unable to append to %s: %s", logfile->buf,
+               strbuf_addf(err, "unable to append to '%s': %s", logfile->buf,
                            strerror(errno));
                close(logfd);
                return -1;
        }
        if (close(logfd)) {
-               strbuf_addf(err, "unable to append to %s: %s", logfile->buf,
+               strbuf_addf(err, "unable to append to '%s': %s", logfile->buf,
                            strerror(errno));
                return -1;
        }
@@ -2744,14 +2878,14 @@ static int write_ref_to_lockfile(struct ref_lock *lock,
        o = parse_object(sha1);
        if (!o) {
                strbuf_addf(err,
-                           "Trying to write ref %s with nonexistent object %s",
+                           "trying to write ref '%s' with nonexistent object %s",
                            lock->ref_name, sha1_to_hex(sha1));
                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",
+                           "trying to write non-commit object %s to branch '%s'",
                            sha1_to_hex(sha1), lock->ref_name);
                unlock_ref(lock);
                return -1;
@@ -2761,7 +2895,7 @@ static int write_ref_to_lockfile(struct ref_lock *lock,
            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 +2907,24 @@ 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,
+static int commit_ref_update(struct files_ref_store *refs,
+                            struct ref_lock *lock,
                             const unsigned char *sha1, const char *logmsg,
-                            int flags, struct strbuf *err)
+                            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)) {
+       assert_main_repository(&refs->base, "commit_ref_update");
+
+       clear_loose_ref_cache(refs);
+       if (log_ref_write(lock->ref_name, lock->old_oid.hash, sha1, 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
@@ -2804,6 +2940,7 @@ static int commit_ref_update(struct ref_lock *lock,
                unsigned char head_sha1[20];
                int head_flag;
                const char *head_ref;
+
                head_ref = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
                                              head_sha1, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
@@ -2816,8 +2953,9 @@ static int commit_ref_update(struct ref_lock *lock,
                        }
                }
        }
+
        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;
        }
@@ -2875,13 +3013,18 @@ 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, 0, "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);
@@ -2921,7 +3064,6 @@ int set_worktree_head_symref(const char *gitdir, const char *target)
        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);
 
@@ -2930,16 +3072,24 @@ int set_worktree_head_symref(const char *gitdir, const char *target)
        return ret;
 }
 
-int reflog_exists(const char *refname)
+static int files_reflog_exists(struct ref_store *ref_store,
+                              const char *refname)
 {
        struct stat st;
 
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "reflog_exists");
+
        return !lstat(git_path("logs/%s", refname), &st) &&
                S_ISREG(st.st_mode);
 }
 
-int delete_reflog(const char *refname)
+static int files_delete_reflog(struct ref_store *ref_store,
+                              const char *refname)
 {
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "delete_reflog");
+
        return remove_path(git_path("logs/%s", refname));
 }
 
@@ -2982,13 +3132,19 @@ 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 strbuf sb = STRBUF_INIT;
        FILE *logfp;
        long pos;
        int ret = 0, at_tail = 1;
 
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "for_each_reflog_ent_reverse");
+
        logfp = fopen(git_path("logs/%s", refname), "r");
        if (!logfp)
                return -1;
@@ -3084,12 +3240,17 @@ 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)
 {
        FILE *logfp;
        struct strbuf sb = STRBUF_INIT;
        int ret = 0;
 
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "for_each_reflog_ent");
+
        logfp = fopen(git_path("logs/%s", refname), "r");
        if (!logfp)
                return -1;
@@ -3100,60 +3261,85 @@ 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 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 (read_ref_full(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;
+}
+
+static int files_reflog_iterator_peel(struct ref_iterator *ref_iterator,
+                                  struct object_id *peeled)
+{
+       die("BUG: ref_iterator_peel() called for reflog_iterator");
+}
+
+static int files_reflog_iterator_abort(struct ref_iterator *ref_iterator)
+{
+       struct files_reflog_iterator *iter =
+               (struct files_reflog_iterator *)ref_iterator;
+       int ok = ITER_DONE;
+
+       if (iter->dir_iterator)
+               ok = dir_iterator_abort(iter->dir_iterator);
+
+       base_ref_iterator_free(ref_iterator);
+       return ok;
 }
 
-int for_each_reflog(each_ref_fn fn, void *cb_data)
+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)
 {
-       int retval;
-       struct strbuf name;
-       strbuf_init(&name, PATH_MAX);
-       retval = do_for_each_reflog(&name, fn, cb_data);
-       strbuf_release(&name);
-       return retval;
+       struct files_reflog_iterator *iter = xcalloc(1, sizeof(*iter));
+       struct ref_iterator *ref_iterator = &iter->base;
+
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "reflog_iterator_begin");
+
+       base_ref_iterator_init(ref_iterator, &files_reflog_iterator_vtable);
+       iter->dir_iterator = dir_iterator_begin(git_path("logs"));
+       return ref_iterator;
 }
 
 static int ref_update_reject_duplicates(struct string_list *refnames,
@@ -3166,169 +3352,465 @@ static int ref_update_reject_duplicates(struct string_list *refnames,
        for (i = 1; i < n; i++)
                if (!strcmp(refnames->items[i - 1].string, refnames->items[i].string)) {
                        strbuf_addf(err,
-                                   "Multiple updates for ref '%s' not allowed.",
+                                   "multiple updates for ref '%s' not allowed.",
                                    refnames->items[i].string);
                        return 1;
                }
        return 0;
 }
 
-int ref_transaction_commit(struct ref_transaction *transaction,
-                          struct strbuf *err)
+/*
+ * 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_sha1, update->old_sha1,
+                       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_sha1, update->old_sha1,
+                       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;
+}
+
+/*
+ * 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_sha1(update->old_sha1);
+       int ret;
+       struct ref_lock *lock;
+
+       assert_main_repository(&refs->base, "lock_ref_for_update");
+
+       if ((update->flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1))
+               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",
+                           update->refname, 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 (read_ref_full(update->refname,
+                                         mustexist ? RESOLVE_REF_READING : 0,
+                                         lock->old_oid.hash, NULL)) {
+                               if (update->flags & REF_HAVE_OLD) {
+                                       strbuf_addf(err, "cannot lock ref '%s': "
+                                                   "can't resolve old value",
+                                                   update->refname);
+                                       return TRANSACTION_GENERIC_ERROR;
+                               } else {
+                                       hashclr(lock->old_oid.hash);
+                               }
+                       }
+                       if ((update->flags & REF_HAVE_OLD) &&
+                           hashcmp(lock->old_oid.hash, update->old_sha1)) {
+                               strbuf_addf(err, "cannot lock ref '%s': "
+                                           "is at %s but expected %s",
+                                           update->refname,
+                                           sha1_to_hex(lock->old_oid.hash),
+                                           sha1_to_hex(update->old_sha1));
+                               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 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_OLD) &&
+                   hashcmp(lock->old_oid.hash, update->old_sha1)) {
+                       if (is_null_sha1(update->old_sha1))
+                               strbuf_addf(err, "cannot lock ref '%s': reference already exists",
+                                           original_update_refname(update));
+                       else
+                               strbuf_addf(err, "cannot lock ref '%s': is at %s but expected %s",
+                                           original_update_refname(update),
+                                           sha1_to_hex(lock->old_oid.hash),
+                                           sha1_to_hex(update->old_sha1));
+
+                       return TRANSACTION_GENERIC_ERROR;
+               }
+       }
+
+       if ((update->flags & REF_HAVE_NEW) &&
+           !(update->flags & REF_DELETING) &&
+           !(update->flags & REF_LOG_ONLY)) {
+               if (!(update->type & REF_ISSYMREF) &&
+                   !hashcmp(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(lock, update->new_sha1,
+                                                err)) {
+                       char *write_err = strbuf_detach(err, NULL);
+
+                       /*
+                        * The lock was freed upon failure of
+                        * write_ref_to_lockfile():
+                        */
+                       update->backend_data = NULL;
+                       strbuf_addf(err,
+                                   "cannot update the 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;
+}
+
+static int files_transaction_commit(struct ref_store *ref_store,
+                                   struct ref_transaction *transaction,
+                                   struct strbuf *err)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 0, "ref_transaction_commit");
        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;
        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 (!n) {
+       if (!transaction->nr) {
                transaction->state = REF_TRANSACTION_CLOSED;
                return 0;
        }
 
-       /* 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);
+       /*
+        * 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);
+
+               /*
+                * 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 = resolve_refdup("HEAD", RESOLVE_REF_NO_RECURSE,
+                                 head_oid.hash, &head_type);
+
+       if (head_ref && !(head_type & REF_ISSYMREF)) {
+               free(head_ref);
+               head_ref = NULL;
+       }
+
        /*
         * 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.
         */
-       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);
+               ret = lock_ref_for_update(refs, update, transaction,
+                                         head_ref, &affected_refnames, err);
+               if (ret)
                        goto cleanup;
-               }
-               if ((update->flags & REF_HAVE_NEW) &&
-                   !(update->flags & REF_DELETING)) {
-                       int overwriting_symref = ((update->type & REF_ISSYMREF) &&
-                                                 (update->flags & REF_NODEREF));
-
-                       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);
+       }
 
-                               /*
-                                * 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);
+       /* 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 (log_ref_write(lock->ref_name, lock->old_oid.hash,
+                                         update->new_sha1,
+                                         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];
+       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) {
-                       if (delete_ref_loose(update->lock, update->type, err)) {
+               if (update->flags & REF_DELETING &&
+                   !(update->flags & REF_LOG_ONLY)) {
+                       if (delete_ref_loose(lock, update->type, err)) {
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
                        }
 
                        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);
+       clear_loose_ref_cache(refs);
 
 cleanup:
        transaction->state = REF_TRANSACTION_CLOSED;
 
-       for (i = 0; i < n; i++)
-               if (updates[i]->lock)
-                       unlock_ref(updates[i]->lock);
+       for (i = 0; i < transaction->nr; i++)
+               if (transaction->updates[i]->backend_data)
+                       unlock_ref(transaction->updates[i]->backend_data);
        string_list_clear(&refs_to_delete, 0);
+       free(head_ref);
        string_list_clear(&affected_refnames, 0);
+
        return ret;
 }
 
@@ -3340,12 +3822,13 @@ 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)
 {
+       struct files_ref_store *refs =
+               files_downcast(ref_store, 0, "initial_ref_transaction_commit");
        int ret = 0, i;
-       int n = transaction->nr;
-       struct ref_update **updates = transaction->updates;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
 
        assert(err);
@@ -3354,8 +3837,9 @@ 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;
@@ -3377,8 +3861,8 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
        if (for_each_rawref(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))
@@ -3391,22 +3875,22 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
                }
        }
 
-       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);
+                       add_packed_ref(refs, update->refname, update->new_sha1);
        }
 
-       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;
@@ -3456,13 +3940,16 @@ 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, 0, "reflog_expire");
        static struct lock_file reflog_lock;
        struct expire_reflog_cb cb;
        struct ref_lock *lock;
@@ -3481,7 +3968,9 @@ 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);
@@ -3560,3 +4049,47 @@ 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)
+{
+       /* Check validity (but we don't need the result): */
+       files_downcast(ref_store, 0, "init_db");
+
+       /*
+        * Create .git/refs/{heads,tags}
+        */
+       safe_create_dir(git_path("refs/heads"), 1);
+       safe_create_dir(git_path("refs/tags"), 1);
+       if (get_shared_repository()) {
+               adjust_shared_perm(git_path("refs/heads"));
+               adjust_shared_perm(git_path("refs/tags"));
+       }
+       return 0;
+}
+
+struct ref_storage_be refs_be_files = {
+       NULL,
+       "files",
+       files_ref_store_create,
+       files_init_db,
+       files_transaction_commit,
+       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_verify_refname_available,
+
+       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
+};