transport: simplify duplicating a substring in transport_get() using xmemdupz()
[gitweb.git] / refs.c
diff --git a/refs.c b/refs.c
index e9eda881d0d17fa5cfaac56e619059054e6036cd..5ff457ebfce7aba9cd470f3e2e1f3f5dbe9d9741 100644 (file)
--- a/refs.c
+++ b/refs.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "lockfile.h"
 #include "refs.h"
 #include "object.h"
 #include "tag.h"
@@ -6,8 +7,34 @@
 #include "string-list.h"
 
 /*
- * Make sure "ref" is something reasonable to have under ".git/refs/";
- * We do not like it if:
+ * How to handle various characters in refnames:
+ * 0: An acceptable character for refs
+ * 1: End-of-component
+ * 2: ., look for a preceding . to reject .. in refs
+ * 3: {, look for a preceding @ to reject @{ in refs
+ * 4: A bad character: ASCII control characters, "~", "^", ":" or SP
+ */
+static unsigned char refname_disposition[256] = {
+       1, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+       4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
+       4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 2, 1,
+       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 4,
+       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 4, 0, 4, 0,
+       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 4, 4
+};
+
+/*
+ * Used as a flag to ref_transaction_delete when a loose ref is being
+ * pruned.
+ */
+#define REF_ISPRUNING  0x0100
+/*
+ * Try to read one refname component from the front of refname.
+ * Return the length of the component found, or -1 if the component is
+ * not legal.  It is legal if it is something reasonable to have under
+ * ".git/refs/"; We do not like it if:
  *
  * - any path component of it begins with ".", or
  * - it has double dots "..", or
  * - it ends with ".lock"
  * - it contains a "\" (backslash)
  */
-
-/* Return true iff ch is not allowed in reference names. */
-static inline int bad_ref_char(int ch)
-{
-       if (((unsigned) ch) <= ' ' || ch == 0x7f ||
-           ch == '~' || ch == '^' || ch == ':' || ch == '\\')
-               return 1;
-       /* 2.13 Pattern Matching Notation */
-       if (ch == '*' || ch == '?' || ch == '[') /* Unsupported */
-               return 1;
-       return 0;
-}
-
-/*
- * Try to read one refname component from the front of refname.  Return
- * the length of the component found, or -1 if the component is not
- * legal.
- */
 static int check_refname_component(const char *refname, int flags)
 {
        const char *cp;
        char last = '\0';
 
        for (cp = refname; ; cp++) {
-               char ch = *cp;
-               if (ch == '\0' || ch == '/')
+               int ch = *cp & 255;
+               unsigned char disp = refname_disposition[ch];
+               switch (disp) {
+               case 1:
+                       goto out;
+               case 2:
+                       if (last == '.')
+                               return -1; /* Refname contains "..". */
+                       break;
+               case 3:
+                       if (last == '@')
+                               return -1; /* Refname contains "@{". */
                        break;
-               if (bad_ref_char(ch))
-                       return -1; /* Illegal character in refname. */
-               if (last == '.' && ch == '.')
-                       return -1; /* Refname contains "..". */
-               if (last == '@' && ch == '{')
-                       return -1; /* Refname contains "@{". */
+               case 4:
+                       return -1;
+               }
                last = ch;
        }
+out:
        if (cp == refname)
                return 0; /* Component has zero length. */
-       if (refname[0] == '.') {
-               if (!(flags & REFNAME_DOT_COMPONENT))
-                       return -1; /* Component starts with '.'. */
-               /*
-                * Even if leading dots are allowed, don't allow "."
-                * as a component (".." is prevented by a rule above).
-                */
-               if (refname[1] == '\0')
-                       return -1; /* Component equals ".". */
-       }
-       if (cp - refname >= 5 && !memcmp(cp - 5, ".lock", 5))
+       if (refname[0] == '.')
+               return -1; /* Component starts with '.'. */
+       if (cp - refname >= LOCK_SUFFIX_LEN &&
+           !memcmp(cp - LOCK_SUFFIX_LEN, LOCK_SUFFIX, LOCK_SUFFIX_LEN))
                return -1; /* Refname ends with ".lock". */
        return cp - refname;
 }
@@ -177,8 +187,8 @@ struct ref_dir {
 
 /*
  * Bit values for ref_entry::flag.  REF_ISSYMREF=0x01,
- * REF_ISPACKED=0x02, and REF_ISBROKEN=0x04 are public values; see
- * refs.h.
+ * REF_ISPACKED=0x02, REF_ISBROKEN=0x04 and REF_BAD_NAME=0x08 are
+ * public values; see refs.h.
  */
 
 /*
@@ -186,16 +196,16 @@ struct ref_dir {
  * the correct peeled value for the reference, which might be
  * null_sha1 if the reference is not a tag or if it is broken.
  */
-#define REF_KNOWS_PEELED 0x08
+#define REF_KNOWS_PEELED 0x10
 
 /* ref_entry represents a directory of references */
-#define REF_DIR 0x10
+#define REF_DIR 0x20
 
 /*
  * Entry has not yet been read from disk (used only for REF_DIR
  * entries representing loose references)
  */
-#define REF_INCOMPLETE 0x20
+#define REF_INCOMPLETE 0x40
 
 /*
  * A ref_entry represents either a reference or a "subdirectory" of
@@ -264,6 +274,39 @@ static struct ref_dir *get_ref_dir(struct ref_entry *entry)
        return dir;
 }
 
+/*
+ * Check if a refname is safe.
+ * For refs that start with "refs/" we consider it safe as long they do
+ * not try to resolve to outside of refs/.
+ *
+ * For all other refs we only consider them safe iff they only contain
+ * upper case characters and '_' (like "HEAD" AND "MERGE_HEAD", and not like
+ * "config").
+ */
+static int refname_is_safe(const char *refname)
+{
+       if (starts_with(refname, "refs/")) {
+               char *buf;
+               int result;
+
+               buf = xmalloc(strlen(refname) + 1);
+               /*
+                * Does the refname try to escape refs/?
+                * For example: refs/foo/../bar is safe but refs/foo/../../bar
+                * is not.
+                */
+               result = !normalize_path_copy(buf, refname + strlen("refs/"));
+               free(buf);
+               return result;
+       }
+       while (*refname) {
+               if (!isupper(*refname) && *refname != '_')
+                       return 0;
+               refname++;
+       }
+       return 1;
+}
+
 static struct ref_entry *create_ref_entry(const char *refname,
                                          const unsigned char *sha1, int flag,
                                          int check_name)
@@ -272,8 +315,10 @@ static struct ref_entry *create_ref_entry(const char *refname,
        struct ref_entry *ref;
 
        if (check_name &&
-           check_refname_format(refname, REFNAME_ALLOW_ONELEVEL|REFNAME_DOT_COMPONENT))
+           check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
                die("Reference has invalid format: '%s'", refname);
+       if (!check_name && !refname_is_safe(refname))
+               die("Reference has invalid name: '%s'", refname);
        len = strlen(refname) + 1;
        ref = xmalloc(sizeof(struct ref_entry) + len);
        hashcpy(ref->u.value.sha1, sha1);
@@ -768,60 +813,121 @@ static void prime_ref_dir(struct ref_dir *dir)
                        prime_ref_dir(get_ref_dir(entry));
        }
 }
-/*
- * Return true iff refname1 and refname2 conflict with each other.
- * Two reference names conflict if one of them exactly matches the
- * leading components of the other; e.g., "foo/bar" conflicts with
- * both "foo" and with "foo/bar/baz" but not with "foo/bar" or
- * "foo/barbados".
- */
-static int names_conflict(const char *refname1, const char *refname2)
+
+static int entry_matches(struct ref_entry *entry, const struct string_list *list)
 {
-       for (; *refname1 && *refname1 == *refname2; refname1++, refname2++)
-               ;
-       return (*refname1 == '\0' && *refname2 == '/')
-               || (*refname1 == '/' && *refname2 == '\0');
+       return list && string_list_has_string(list, entry->name);
 }
 
-struct name_conflict_cb {
-       const char *refname;
-       const char *oldrefname;
-       const char *conflicting_refname;
+struct nonmatching_ref_data {
+       const struct string_list *skip;
+       struct ref_entry *found;
 };
 
-static int name_conflict_fn(struct ref_entry *entry, void *cb_data)
+static int nonmatching_ref_fn(struct ref_entry *entry, void *vdata)
 {
-       struct name_conflict_cb *data = (struct name_conflict_cb *)cb_data;
-       if (data->oldrefname && !strcmp(data->oldrefname, entry->name))
+       struct nonmatching_ref_data *data = vdata;
+
+       if (entry_matches(entry, data->skip))
                return 0;
-       if (names_conflict(data->refname, entry->name)) {
-               data->conflicting_refname = entry->name;
-               return 1;
-       }
-       return 0;
+
+       data->found = entry;
+       return 1;
+}
+
+static void report_refname_conflict(struct ref_entry *entry,
+                                   const char *refname)
+{
+       error("'%s' exists; cannot create '%s'", entry->name, refname);
 }
 
 /*
  * Return true iff a reference named refname could be created without
  * conflicting with the name of an existing reference in dir.  If
- * oldrefname is non-NULL, ignore potential conflicts with oldrefname
- * (e.g., because oldrefname is scheduled for deletion in the same
+ * skip is non-NULL, ignore potential conflicts with refs in skip
+ * (e.g., because they are scheduled for deletion in the same
  * operation).
+ *
+ * Two reference names conflict if one of them exactly matches the
+ * leading components of the other; e.g., "foo/bar" conflicts with
+ * both "foo" and with "foo/bar/baz" but not with "foo/bar" or
+ * "foo/barbados".
+ *
+ * skip must be sorted.
  */
-static int is_refname_available(const char *refname, const char *oldrefname,
+static int is_refname_available(const char *refname,
+                               const struct string_list *skip,
                                struct ref_dir *dir)
 {
-       struct name_conflict_cb data;
-       data.refname = refname;
-       data.oldrefname = oldrefname;
-       data.conflicting_refname = NULL;
+       const char *slash;
+       size_t len;
+       int pos;
+       char *dirname;
 
-       sort_ref_dir(dir);
-       if (do_for_each_entry_in_dir(dir, 0, name_conflict_fn, &data)) {
-               error("'%s' exists; cannot create '%s'",
-                     data.conflicting_refname, refname);
+       for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
+               /*
+                * We are still at a leading dir of the refname; we are
+                * looking for a conflict with a leaf entry.
+                *
+                * If we find one, we still must make sure it is
+                * not in "skip".
+                */
+               pos = search_ref_dir(dir, refname, slash - refname);
+               if (pos >= 0) {
+                       struct ref_entry *entry = dir->entries[pos];
+                       if (entry_matches(entry, skip))
+                               return 1;
+                       report_refname_conflict(entry, refname);
+                       return 0;
+               }
+
+
+               /*
+                * Otherwise, we can try to continue our search with
+                * the next component; if we come up empty, we know
+                * there is nothing under this whole prefix.
+                */
+               pos = search_ref_dir(dir, refname, slash + 1 - refname);
+               if (pos < 0)
+                       return 1;
+
+               dir = get_ref_dir(dir->entries[pos]);
+       }
+
+       /*
+        * We are at the leaf of our refname; we want to
+        * make sure there are no directories which match it.
+        */
+       len = strlen(refname);
+       dirname = xmallocz(len + 1);
+       sprintf(dirname, "%s/", refname);
+       pos = search_ref_dir(dir, dirname, len + 1);
+       free(dirname);
+
+       if (pos >= 0) {
+               /*
+                * We found a directory named "refname". It is a
+                * problem iff it contains any ref that is not
+                * in "skip".
+                */
+               struct ref_entry *entry = dir->entries[pos];
+               struct ref_dir *dir = get_ref_dir(entry);
+               struct nonmatching_ref_data data;
+
+               data.skip = skip;
+               sort_ref_dir(dir);
+               if (!do_for_each_entry_in_dir(dir, 0, nonmatching_ref_fn, &data))
+                       return 1;
+
+               report_refname_conflict(data.found, refname);
                return 0;
        }
+
+       /*
+        * There is no point in searching for another leaf
+        * node which matches it; such an entry would be the
+        * ref we are looking for, not a conflict.
+        */
        return 1;
 }
 
@@ -1040,7 +1146,13 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 
                refname = parse_ref_line(refline, sha1);
                if (refname) {
-                       last = create_ref_entry(refname, sha1, REF_ISPACKED, 1);
+                       int flag = REF_ISPACKED;
+
+                       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+                               hashclr(sha1);
+                               flag |= REF_BAD_NAME | REF_ISBROKEN;
+                       }
+                       last = create_ref_entry(refname, sha1, flag, 0);
                        if (peeled == PEELED_FULLY ||
                            (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
                                last->flag |= REF_KNOWS_PEELED;
@@ -1151,7 +1263,7 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
 
                if (de->d_name[0] == '.')
                        continue;
-               if (has_extension(de->d_name, ".lock"))
+               if (ends_with(de->d_name, ".lock"))
                        continue;
                strbuf_addstr(&refname, de->d_name);
                refdir = *refs->name
@@ -1172,12 +1284,19 @@ static void read_loose_refs(const char *dirname, struct ref_dir *dir)
                                        hashclr(sha1);
                                        flag |= REF_ISBROKEN;
                                }
-                       } else if (read_ref_full(refname.buf, sha1, 1, &flag)) {
+                       } else if (read_ref_full(refname.buf,
+                                                RESOLVE_REF_READING,
+                                                sha1, &flag)) {
                                hashclr(sha1);
                                flag |= REF_ISBROKEN;
                        }
+                       if (check_refname_format(refname.buf,
+                                                REFNAME_ALLOW_ONELEVEL)) {
+                               hashclr(sha1);
+                               flag |= REF_BAD_NAME | REF_ISBROKEN;
+                       }
                        add_entry_to_dir(dir,
-                                        create_ref_entry(refname.buf, sha1, flag, 1));
+                                        create_ref_entry(refname.buf, sha1, flag, 0));
                }
                strbuf_setlen(&refname, dirnamelen);
        }
@@ -1296,10 +1415,10 @@ static struct ref_entry *get_packed_ref(const char *refname)
  * A loose ref file doesn't exist; check for a packed ref.  The
  * options are forwarded from resolve_safe_unsafe().
  */
-static const char *handle_missing_loose_ref(const char *refname,
-                                           unsigned char *sha1,
-                                           int reading,
-                                           int *flag)
+static int resolve_missing_loose_ref(const char *refname,
+                                    int resolve_flags,
+                                    unsigned char *sha1,
+                                    int *flags)
 {
        struct ref_entry *entry;
 
@@ -1310,40 +1429,61 @@ static const char *handle_missing_loose_ref(const char *refname,
        entry = get_packed_ref(refname);
        if (entry) {
                hashcpy(sha1, entry->u.value.sha1);
-               if (flag)
-                       *flag |= REF_ISPACKED;
-               return refname;
+               if (flags)
+                       *flags |= REF_ISPACKED;
+               return 0;
        }
        /* The reference is not a packed reference, either. */
-       if (reading) {
-               return NULL;
+       if (resolve_flags & RESOLVE_REF_READING) {
+               errno = ENOENT;
+               return -1;
        } else {
                hashclr(sha1);
-               return refname;
+               return 0;
        }
 }
 
-const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int reading, int *flag)
+/* This function needs to return a meaningful errno on failure */
+const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned char *sha1, int *flags)
 {
        int depth = MAXDEPTH;
        ssize_t len;
        char buffer[256];
        static char refname_buffer[256];
+       int bad_name = 0;
 
-       if (flag)
-               *flag = 0;
+       if (flags)
+               *flags = 0;
 
-       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
-               return NULL;
+       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+               if (flags)
+                       *flags |= REF_BAD_NAME;
 
+               if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
+                   !refname_is_safe(refname)) {
+                       errno = EINVAL;
+                       return NULL;
+               }
+               /*
+                * dwim_ref() uses REF_ISBROKEN to distinguish between
+                * missing refs and refs that were present but invalid,
+                * to complain about the latter to stderr.
+                *
+                * We don't know whether the ref exists, so don't set
+                * REF_ISBROKEN yet.
+                */
+               bad_name = 1;
+       }
        for (;;) {
                char path[PATH_MAX];
                struct stat st;
                char *buf;
                int fd;
 
-               if (--depth < 0)
+               if (--depth < 0) {
+                       errno = ELOOP;
                        return NULL;
+               }
 
                git_snpath(path, sizeof(path), "%s", refname);
 
@@ -1358,11 +1498,17 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
                 */
        stat_ref:
                if (lstat(path, &st) < 0) {
-                       if (errno == ENOENT)
-                               return handle_missing_loose_ref(refname, sha1,
-                                                               reading, flag);
-                       else
+                       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;
                }
 
                /* Follow "normalized" - ie "refs/.." symlinks by hand */
@@ -1380,8 +1526,12 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
                                        !check_refname_format(buffer, 0)) {
                                strcpy(refname_buffer, buffer);
                                refname = refname_buffer;
-                               if (flag)
-                                       *flag |= REF_ISSYMREF;
+                               if (flags)
+                                       *flags |= REF_ISSYMREF;
+                               if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
+                                       hashclr(sha1);
+                                       return refname;
+                               }
                                continue;
                        }
                }
@@ -1405,9 +1555,13 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
                                return NULL;
                }
                len = read_in_full(fd, buffer, sizeof(buffer)-1);
-               close(fd);
-               if (len < 0)
+               if (len < 0) {
+                       int save_errno = errno;
+                       close(fd);
+                       errno = save_errno;
                        return NULL;
+               }
+               close(fd);
                while (len && isspace(buffer[len-1]))
                        len--;
                buffer[len] = '\0';
@@ -1422,29 +1576,45 @@ const char *resolve_ref_unsafe(const char *refname, unsigned char *sha1, int rea
                         */
                        if (get_sha1_hex(buffer, sha1) ||
                            (buffer[40] != '\0' && !isspace(buffer[40]))) {
-                               if (flag)
-                                       *flag |= REF_ISBROKEN;
+                               if (flags)
+                                       *flags |= REF_ISBROKEN;
+                               errno = EINVAL;
                                return NULL;
                        }
+                       if (bad_name) {
+                               hashclr(sha1);
+                               if (flags)
+                                       *flags |= REF_ISBROKEN;
+                       }
                        return refname;
                }
-               if (flag)
-                       *flag |= REF_ISSYMREF;
+               if (flags)
+                       *flags |= REF_ISSYMREF;
                buf = buffer + 4;
                while (isspace(*buf))
                        buf++;
+               refname = strcpy(refname_buffer, buf);
+               if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
+                       hashclr(sha1);
+                       return refname;
+               }
                if (check_refname_format(buf, REFNAME_ALLOW_ONELEVEL)) {
-                       if (flag)
-                               *flag |= REF_ISBROKEN;
-                       return NULL;
+                       if (flags)
+                               *flags |= REF_ISBROKEN;
+
+                       if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
+                           !refname_is_safe(buf)) {
+                               errno = EINVAL;
+                               return NULL;
+                       }
+                       bad_name = 1;
                }
-               refname = strcpy(refname_buffer, buf);
        }
 }
 
-char *resolve_refdup(const char *ref, unsigned char *sha1, int reading, int *flag)
+char *resolve_refdup(const char *ref, int resolve_flags, unsigned char *sha1, int *flags)
 {
-       const char *ret = resolve_ref_unsafe(ref, sha1, reading, flag);
+       const char *ret = resolve_ref_unsafe(ref, resolve_flags, sha1, flags);
        return ret ? xstrdup(ret) : NULL;
 }
 
@@ -1455,22 +1625,22 @@ struct ref_filter {
        void *cb_data;
 };
 
-int read_ref_full(const char *refname, unsigned char *sha1, int reading, int *flags)
+int read_ref_full(const char *refname, int resolve_flags, unsigned char *sha1, int *flags)
 {
-       if (resolve_ref_unsafe(refname, sha1, reading, flags))
+       if (resolve_ref_unsafe(refname, resolve_flags, sha1, flags))
                return 0;
        return -1;
 }
 
 int read_ref(const char *refname, unsigned char *sha1)
 {
-       return read_ref_full(refname, sha1, 1, NULL);
+       return read_ref_full(refname, RESOLVE_REF_READING, sha1, NULL);
 }
 
 int ref_exists(const char *refname)
 {
        unsigned char sha1[20];
-       return !!resolve_ref_unsafe(refname, sha1, 1, NULL);
+       return !!resolve_ref_unsafe(refname, RESOLVE_REF_READING, sha1, NULL);
 }
 
 static int filter_refs(const char *refname, const unsigned char *sha1, int flags,
@@ -1583,7 +1753,7 @@ int peel_ref(const char *refname, unsigned char *sha1)
                return 0;
        }
 
-       if (read_ref_full(refname, base, 1, &flag))
+       if (read_ref_full(refname, RESOLVE_REF_READING, base, &flag))
                return -1;
 
        /*
@@ -1624,7 +1794,7 @@ static int warn_if_dangling_symref(const char *refname, const unsigned char *sha
        if (!(flags & REF_ISSYMREF))
                return 0;
 
-       resolves_to = resolve_ref_unsafe(refname, junk, 0, NULL);
+       resolves_to = resolve_ref_unsafe(refname, 0, junk, NULL);
        if (!resolves_to
            || (d->refname
                ? strcmp(resolves_to, d->refname)
@@ -1749,7 +1919,7 @@ static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
                return 0;
        }
 
-       if (!read_ref_full("HEAD", sha1, 1, &flag))
+       if (!read_ref_full("HEAD", RESOLVE_REF_READING, sha1, &flag))
                return fn("HEAD", sha1, flag, cb_data);
 
        return 0;
@@ -1829,7 +1999,7 @@ int head_ref_namespaced(each_ref_fn fn, void *cb_data)
        int flag;
 
        strbuf_addf(&buf, "%sHEAD", get_git_namespace());
-       if (!read_ref_full(buf.buf, sha1, 1, &flag))
+       if (!read_ref_full(buf.buf, RESOLVE_REF_READING, sha1, &flag))
                ret = fn(buf.buf, sha1, flag, cb_data);
        strbuf_release(&buf);
 
@@ -1920,18 +2090,24 @@ int refname_match(const char *abbrev_name, const char *full_name)
        return 0;
 }
 
+/* This function should make sure errno is meaningful on error */
 static struct ref_lock *verify_lock(struct ref_lock *lock,
        const unsigned char *old_sha1, int mustexist)
 {
-       if (read_ref_full(lock->ref_name, lock->old_sha1, mustexist, NULL)) {
+       if (read_ref_full(lock->ref_name,
+                         mustexist ? RESOLVE_REF_READING : 0,
+                         lock->old_sha1, NULL)) {
+               int save_errno = errno;
                error("Can't verify ref %s", lock->ref_name);
                unlock_ref(lock);
+               errno = save_errno;
                return NULL;
        }
        if (hashcmp(lock->old_sha1, old_sha1)) {
                error("Ref %s is at %s but expected %s", lock->ref_name,
                        sha1_to_hex(lock->old_sha1), sha1_to_hex(old_sha1));
                unlock_ref(lock);
+               errno = EBUSY;
                return NULL;
        }
        return lock;
@@ -1944,14 +2120,16 @@ static int remove_empty_directories(const char *file)
         * only empty directories), remove them.
         */
        struct strbuf path;
-       int result;
+       int result, save_errno;
 
        strbuf_init(&path, 20);
        strbuf_addstr(&path, file);
 
        result = remove_dir_recursively(&path, REMOVE_DIR_EMPTY_ONLY);
+       save_errno = errno;
 
        strbuf_release(&path);
+       errno = save_errno;
 
        return result;
 }
@@ -1991,7 +2169,8 @@ int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref)
 
                this_result = refs_found ? sha1_from_ref : sha1;
                mksnpath(fullref, sizeof(fullref), *p, len, str);
-               r = resolve_ref_unsafe(fullref, this_result, 1, &flag);
+               r = resolve_ref_unsafe(fullref, RESOLVE_REF_READING,
+                                      this_result, &flag);
                if (r) {
                        if (!refs_found++)
                                *ref = xstrdup(r);
@@ -2015,21 +2194,18 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
 
        *log = NULL;
        for (p = ref_rev_parse_rules; *p; p++) {
-               struct stat st;
                unsigned char hash[20];
                char path[PATH_MAX];
                const char *ref, *it;
 
                mksnpath(path, sizeof(path), *p, len, str);
-               ref = resolve_ref_unsafe(path, hash, 1, NULL);
+               ref = resolve_ref_unsafe(path, RESOLVE_REF_READING,
+                                        hash, NULL);
                if (!ref)
                        continue;
-               if (!stat(git_path("logs/%s", path), &st) &&
-                   S_ISREG(st.st_mode))
+               if (reflog_exists(path))
                        it = path;
-               else if (strcmp(ref, path) &&
-                        !stat(git_path("logs/%s", ref), &st) &&
-                        S_ISREG(st.st_mode))
+               else if (strcmp(ref, path) && reflog_exists(ref))
                        it = ref;
                else
                        continue;
@@ -2044,8 +2220,13 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
        return logs_found;
 }
 
+/*
+ * 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,
                                            const unsigned char *old_sha1,
+                                           const struct string_list *skip,
                                            int flags, int *type_p)
 {
        char *ref_file;
@@ -2054,13 +2235,23 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
        int last_errno = 0;
        int type, lflags;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
+       int resolve_flags = 0;
        int missing = 0;
        int attempts_remaining = 3;
 
        lock = xcalloc(1, sizeof(struct ref_lock));
        lock->lock_fd = -1;
 
-       refname = resolve_ref_unsafe(refname, lock->old_sha1, mustexist, &type);
+       if (mustexist)
+               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;
+       }
+
+       refname = resolve_ref_unsafe(refname, resolve_flags,
+                                    lock->old_sha1, &type);
        if (!refname && errno == EISDIR) {
                /* we are trying to lock foo but we used to
                 * have foo/bar which now does not exist;
@@ -2073,7 +2264,8 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
                        error("there are still refs under '%s'", orig_refname);
                        goto error_return;
                }
-               refname = resolve_ref_unsafe(orig_refname, lock->old_sha1, mustexist, &type);
+               refname = resolve_ref_unsafe(orig_refname, resolve_flags,
+                                            lock->old_sha1, &type);
        }
        if (type_p)
            *type_p = type;
@@ -2090,7 +2282,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
         * name is a proper prefix of our refname.
         */
        if (missing &&
-            !is_refname_available(refname, NULL, get_packed_refs(&ref_cache))) {
+            !is_refname_available(refname, skip, get_packed_refs(&ref_cache))) {
                last_errno = ENOTDIR;
                goto error_return;
        }
@@ -2100,7 +2292,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
        lflags = 0;
        if (flags & REF_NODEREF) {
                refname = orig_refname;
-               lflags |= LOCK_NODEREF;
+               lflags |= LOCK_NO_DEREF;
        }
        lock->ref_name = xstrdup(refname);
        lock->orig_ref_name = xstrdup(orig_refname);
@@ -2134,7 +2326,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
                         */
                        goto retry;
                else
-                       unable_to_lock_index_die(ref_file, errno);
+                       unable_to_lock_die(ref_file, errno);
        }
        return old_sha1 ? verify_lock(lock, old_sha1, mustexist) : lock;
 
@@ -2144,47 +2336,23 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
        return NULL;
 }
 
-struct ref_lock *lock_ref_sha1(const char *refname, const unsigned char *old_sha1)
-{
-       char refpath[PATH_MAX];
-       if (check_refname_format(refname, 0))
-               return NULL;
-       strcpy(refpath, mkpath("refs/%s", refname));
-       return lock_ref_sha1_basic(refpath, old_sha1, 0, NULL);
-}
-
 struct ref_lock *lock_any_ref_for_update(const char *refname,
                                         const unsigned char *old_sha1,
                                         int flags, int *type_p)
 {
-       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
-               return NULL;
-       return lock_ref_sha1_basic(refname, old_sha1, flags, type_p);
+       return lock_ref_sha1_basic(refname, old_sha1, NULL, flags, type_p);
 }
 
 /*
  * Write an entry to the packed-refs file for the specified refname.
  * If peeled is non-NULL, write it as the entry's peeled value.
  */
-static void write_packed_entry(int fd, char *refname, unsigned char *sha1,
+static void write_packed_entry(FILE *fh, char *refname, unsigned char *sha1,
                               unsigned char *peeled)
 {
-       char line[PATH_MAX + 100];
-       int len;
-
-       len = snprintf(line, sizeof(line), "%s %s\n",
-                      sha1_to_hex(sha1), refname);
-       /* this should not happen but just being defensive */
-       if (len > sizeof(line))
-               die("too long a refname '%s'", refname);
-       write_or_die(fd, line, len);
-
-       if (peeled) {
-               if (snprintf(line, sizeof(line), "^%s\n",
-                            sha1_to_hex(peeled)) != PEELED_LINE_LENGTH)
-                       die("internal error");
-               write_or_die(fd, line, PEELED_LINE_LENGTH);
-       }
+       fprintf_or_die(fh, "%s %s\n", sha1_to_hex(sha1), refname);
+       if (peeled)
+               fprintf_or_die(fh, "^%s\n", sha1_to_hex(peeled));
 }
 
 /*
@@ -2192,18 +2360,18 @@ static void write_packed_entry(int fd, char *refname, unsigned char *sha1,
  */
 static int write_packed_entry_fn(struct ref_entry *entry, void *cb_data)
 {
-       int *fd = cb_data;
        enum peel_status peel_status = peel_entry(entry, 0);
 
        if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
                error("internal error: %s is not a valid packed reference!",
                      entry->name);
-       write_packed_entry(*fd, entry->name, entry->u.value.sha1,
+       write_packed_entry(cb_data, entry->name, entry->u.value.sha1,
                           peel_status == PEEL_PEELED ?
                           entry->u.value.peeled : NULL);
        return 0;
 }
 
+/* This should return a meaningful errno on failure */
 int lock_packed_refs(int flags)
 {
        struct packed_ref_cache *packed_ref_cache;
@@ -2223,24 +2391,36 @@ int lock_packed_refs(int flags)
        return 0;
 }
 
+/*
+ * Commit the packed refs changes.
+ * On error we must make sure that errno contains a meaningful value.
+ */
 int commit_packed_refs(void)
 {
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);
        int error = 0;
+       int save_errno = 0;
+       FILE *out;
 
        if (!packed_ref_cache->lock)
                die("internal error: packed-refs not locked");
-       write_or_die(packed_ref_cache->lock->fd,
-                    PACKED_REFS_HEADER, strlen(PACKED_REFS_HEADER));
 
+       out = fdopen_lock_file(packed_ref_cache->lock, "w");
+       if (!out)
+               die_errno("unable to fdopen packed-refs descriptor");
+
+       fprintf_or_die(out, "%s", PACKED_REFS_HEADER);
        do_for_each_entry_in_dir(get_packed_ref_dir(packed_ref_cache),
-                                0, write_packed_entry_fn,
-                                &packed_ref_cache->lock->fd);
-       if (commit_lock_file(packed_ref_cache->lock))
+                                0, write_packed_entry_fn, out);
+
+       if (commit_lock_file(packed_ref_cache->lock)) {
+               save_errno = errno;
                error = -1;
+       }
        packed_ref_cache->lock = NULL;
        release_packed_ref_cache(packed_ref_cache);
+       errno = save_errno;
        return error;
 }
 
@@ -2353,13 +2533,25 @@ static void try_remove_empty_parents(char *name)
 /* make sure nobody touched the ref, and unlink */
 static void prune_ref(struct ref_to_prune *r)
 {
-       struct ref_lock *lock = lock_ref_sha1(r->name + 5, r->sha1);
+       struct ref_transaction *transaction;
+       struct strbuf err = STRBUF_INIT;
 
-       if (lock) {
-               unlink_or_warn(git_path("%s", r->name));
-               unlock_ref(lock);
-               try_remove_empty_parents(r->name);
+       if (check_refname_format(r->name, 0))
+               return;
+
+       transaction = ref_transaction_begin(&err);
+       if (!transaction ||
+           ref_transaction_delete(transaction, r->name, r->sha1,
+                                  REF_ISPRUNING, 1, NULL, &err) ||
+           ref_transaction_commit(transaction, &err)) {
+               ref_transaction_free(transaction);
+               error("%s", err.buf);
+               strbuf_release(&err);
+               return;
        }
+       ref_transaction_free(transaction);
+       strbuf_release(&err);
+       try_remove_empty_parents(r->name);
 }
 
 static void prune_refs(struct ref_to_prune *r)
@@ -2415,7 +2607,7 @@ static int curate_packed_ref_fn(struct ref_entry *entry, void *cb_data)
                unsigned char sha1[20];
                int flags;
 
-               if (read_ref_full(entry->name, sha1, 0, &flags))
+               if (read_ref_full(entry->name, 0, sha1, &flags))
                        /* We should at least have found the packed ref. */
                        die("Internal error");
                if ((flags & REF_ISSYMREF) || !(flags & REF_ISPACKED)) {
@@ -2447,12 +2639,14 @@ static int curate_packed_ref_fn(struct ref_entry *entry, void *cb_data)
        return 0;
 }
 
-int repack_without_refs(const char **refnames, int n)
+int repack_without_refs(const char **refnames, int n, struct strbuf *err)
 {
        struct ref_dir *packed;
        struct string_list refs_to_delete = STRING_LIST_INIT_DUP;
        struct string_list_item *ref_to_delete;
-       int i, removed = 0;
+       int i, ret, removed = 0;
+
+       assert(err);
 
        /* Look for a packed ref */
        for (i = 0; i < n; i++)
@@ -2464,8 +2658,8 @@ int repack_without_refs(const char **refnames, int n)
                return 0; /* no refname exists in packed refs */
 
        if (lock_packed_refs(0)) {
-               unable_to_lock_error(git_path("packed-refs"), errno);
-               return error("cannot delete '%s' from packed refs", refnames[i]);
+               unable_to_lock_message(git_path("packed-refs"), errno, err);
+               return -1;
        }
        packed = get_packed_refs(&ref_cache);
 
@@ -2490,24 +2684,26 @@ int repack_without_refs(const char **refnames, int n)
        }
 
        /* Write what remains */
-       return commit_packed_refs();
+       ret = commit_packed_refs();
+       if (ret)
+               strbuf_addf(err, "unable to overwrite old ref-pack file: %s",
+                           strerror(errno));
+       return ret;
 }
 
-static int repack_without_ref(const char *refname)
+static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
 {
-       return repack_without_refs(&refname, 1);
-}
+       assert(err);
 
-static int delete_ref_loose(struct ref_lock *lock, int flag)
-{
        if (!(flag & REF_ISPACKED) || flag & REF_ISSYMREF) {
-               /* loose */
-               int err, i = strlen(lock->lk->filename) - 5; /* .lock */
-
-               lock->lk->filename[i] = 0;
-               err = unlink_or_warn(lock->lk->filename);
-               lock->lk->filename[i] = '.';
-               if (err && errno != ENOENT)
+               /*
+                * loose.  The loose file name is the same as the
+                * lockfile name, minus ".lock":
+                */
+               char *loose_filename = get_locked_file_path(lock->lk);
+               int res = unlink_or_msg(loose_filename, err);
+               free(loose_filename);
+               if (res)
                        return 1;
        }
        return 0;
@@ -2515,24 +2711,22 @@ static int delete_ref_loose(struct ref_lock *lock, int flag)
 
 int delete_ref(const char *refname, const unsigned char *sha1, int delopt)
 {
-       struct ref_lock *lock;
-       int ret = 0, flag = 0;
-
-       lock = lock_ref_sha1_basic(refname, sha1, delopt, &flag);
-       if (!lock)
+       struct ref_transaction *transaction;
+       struct strbuf err = STRBUF_INIT;
+
+       transaction = ref_transaction_begin(&err);
+       if (!transaction ||
+           ref_transaction_delete(transaction, refname, sha1, delopt,
+                                  sha1 && !is_null_sha1(sha1), NULL, &err) ||
+           ref_transaction_commit(transaction, &err)) {
+               error("%s", err.buf);
+               ref_transaction_free(transaction);
+               strbuf_release(&err);
                return 1;
-       ret |= delete_ref_loose(lock, flag);
-
-       /* removing the loose one could have resurrected an earlier
-        * packed one.  Also, if it was not loose we need to repack
-        * without it.
-        */
-       ret |= repack_without_ref(lock->ref_name);
-
-       unlink_or_warn(git_path("logs/%s", lock->ref_name));
-       clear_loose_ref_cache(&ref_cache);
-       unlock_ref(lock);
-       return ret;
+       }
+       ref_transaction_free(transaction);
+       strbuf_release(&err);
+       return 0;
 }
 
 /*
@@ -2589,6 +2783,21 @@ static int rename_tmp_log(const char *newrefname)
        return 0;
 }
 
+static int rename_ref_available(const char *oldname, const char *newname)
+{
+       struct string_list skip = STRING_LIST_INIT_NODUP;
+       int ret;
+
+       string_list_insert(&skip, oldname);
+       ret = is_refname_available(newname, &skip, get_packed_refs(&ref_cache))
+           && is_refname_available(newname, &skip, get_loose_refs(&ref_cache));
+       string_list_clear(&skip, 0);
+       return ret;
+}
+
+static int write_ref_sha1(struct ref_lock *lock, const unsigned char *sha1,
+                         const char *logmsg);
+
 int rename_ref(const char *oldrefname, const char *newrefname, const char *logmsg)
 {
        unsigned char sha1[20], orig_sha1[20];
@@ -2601,17 +2810,15 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
        if (log && S_ISLNK(loginfo.st_mode))
                return error("reflog for %s is a symlink", oldrefname);
 
-       symref = resolve_ref_unsafe(oldrefname, orig_sha1, 1, &flag);
+       symref = resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING,
+                                   orig_sha1, &flag);
        if (flag & REF_ISSYMREF)
                return error("refname %s is a symbolic ref, renaming it is not supported",
                        oldrefname);
        if (!symref)
                return error("refname %s not found", oldrefname);
 
-       if (!is_refname_available(newrefname, oldrefname, get_packed_refs(&ref_cache)))
-               return 1;
-
-       if (!is_refname_available(newrefname, oldrefname, get_loose_refs(&ref_cache)))
+       if (!rename_ref_available(oldrefname, newrefname))
                return 1;
 
        if (log && rename(git_path("logs/%s", oldrefname), git_path(TMP_RENAMED_LOG)))
@@ -2623,7 +2830,7 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
                goto rollback;
        }
 
-       if (!read_ref_full(newrefname, sha1, 1, &flag) &&
+       if (!read_ref_full(newrefname, RESOLVE_REF_READING, sha1, NULL) &&
            delete_ref(newrefname, sha1, REF_NODEREF)) {
                if (errno==EISDIR) {
                        if (remove_empty_directories(git_path("%s", newrefname))) {
@@ -2641,7 +2848,7 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 
        logmoved = log;
 
-       lock = lock_ref_sha1_basic(newrefname, NULL, 0, NULL);
+       lock = lock_ref_sha1_basic(newrefname, NULL, NULL, 0, NULL);
        if (!lock) {
                error("unable to lock %s for update", newrefname);
                goto rollback;
@@ -2656,7 +2863,7 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
        return 0;
 
  rollback:
-       lock = lock_ref_sha1_basic(oldrefname, NULL, 0, NULL);
+       lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, 0, NULL);
        if (!lock) {
                error("unable to lock %s for rollback", oldrefname);
                goto rollbacklog;
@@ -2733,6 +2940,7 @@ static int copy_msg(char *buf, const char *msg)
        return cp - buf;
 }
 
+/* This function must set a meaningful errno on failure */
 int log_ref_setup(const char *refname, char *logfile, int bufsize)
 {
        int logfd, oflags = O_APPEND | O_WRONLY;
@@ -2743,9 +2951,12 @@ int log_ref_setup(const char *refname, char *logfile, int bufsize)
             starts_with(refname, "refs/remotes/") ||
             starts_with(refname, "refs/notes/") ||
             !strcmp(refname, "HEAD"))) {
-               if (safe_create_leading_directories(logfile) < 0)
-                       return error("unable to create directory for %s",
-                                    logfile);
+               if (safe_create_leading_directories(logfile) < 0) {
+                       int save_errno = errno;
+                       error("unable to create directory for %s", logfile);
+                       errno = save_errno;
+                       return -1;
+               }
                oflags |= O_CREAT;
        }
 
@@ -2756,15 +2967,22 @@ int log_ref_setup(const char *refname, char *logfile, int bufsize)
 
                if (errno == EISDIR) {
                        if (remove_empty_directories(logfile)) {
-                               return error("There are still logs under '%s'",
-                                            logfile);
+                               int save_errno = errno;
+                               error("There are still logs under '%s'",
+                                     logfile);
+                               errno = save_errno;
+                               return -1;
                        }
                        logfd = open(logfile, oflags, 0666);
                }
 
-               if (logfd < 0)
-                       return error("Unable to append to %s: %s",
-                                    logfile, strerror(errno));
+               if (logfd < 0) {
+                       int save_errno = errno;
+                       error("Unable to append to %s: %s", logfile,
+                             strerror(errno));
+                       errno = save_errno;
+                       return -1;
+               }
        }
 
        adjust_shared_perm(logfile);
@@ -2804,24 +3022,41 @@ static int log_ref_write(const char *refname, const unsigned char *old_sha1,
                len += copy_msg(logrec + len - 1, msg) - 1;
        written = len <= maxlen ? write_in_full(logfd, logrec, len) : -1;
        free(logrec);
-       if (close(logfd) != 0 || written != len)
-               return error("Unable to append to %s", log_file);
+       if (written != len) {
+               int save_errno = errno;
+               close(logfd);
+               error("Unable to append to %s", log_file);
+               errno = save_errno;
+               return -1;
+       }
+       if (close(logfd)) {
+               int save_errno = errno;
+               error("Unable to append to %s", log_file);
+               errno = save_errno;
+               return -1;
+       }
        return 0;
 }
 
-static int is_branch(const char *refname)
+int is_branch(const char *refname)
 {
        return !strcmp(refname, "HEAD") || starts_with(refname, "refs/heads/");
 }
 
-int write_ref_sha1(struct ref_lock *lock,
+/*
+ * Write sha1 into the ref specified by the lock. Make sure that errno
+ * is sane on error.
+ */
+static int write_ref_sha1(struct ref_lock *lock,
        const unsigned char *sha1, const char *logmsg)
 {
        static char term = '\n';
        struct object *o;
 
-       if (!lock)
+       if (!lock) {
+               errno = EINVAL;
                return -1;
+       }
        if (!lock->force_write && !hashcmp(lock->old_sha1, sha1)) {
                unlock_ref(lock);
                return 0;
@@ -2831,19 +3066,23 @@ int write_ref_sha1(struct ref_lock *lock,
                error("Trying to write ref %s with nonexistent object %s",
                        lock->ref_name, sha1_to_hex(sha1));
                unlock_ref(lock);
+               errno = EINVAL;
                return -1;
        }
        if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
                error("Trying to write non-commit object %s to branch %s",
                        sha1_to_hex(sha1), lock->ref_name);
                unlock_ref(lock);
+               errno = EINVAL;
                return -1;
        }
        if (write_in_full(lock->lock_fd, sha1_to_hex(sha1), 40) != 40 ||
-           write_in_full(lock->lock_fd, &term, 1) != 1
-               || close_ref(lock) < 0) {
-               error("Couldn't write %s", lock->lk->filename);
+           write_in_full(lock->lock_fd, &term, 1) != 1 ||
+           close_ref(lock) < 0) {
+               int save_errno = errno;
+               error("Couldn't write %s", lock->lk->filename.buf);
                unlock_ref(lock);
+               errno = save_errno;
                return -1;
        }
        clear_loose_ref_cache(&ref_cache);
@@ -2869,7 +3108,8 @@ int write_ref_sha1(struct ref_lock *lock,
                unsigned char head_sha1[20];
                int head_flag;
                const char *head_ref;
-               head_ref = resolve_ref_unsafe("HEAD", head_sha1, 1, &head_flag);
+               head_ref = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
+                                             head_sha1, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
                    !strcmp(head_ref, lock->ref_name))
                        log_ref_write("HEAD", lock->old_sha1, sha1, logmsg);
@@ -2946,122 +3186,137 @@ int create_symref(const char *ref_target, const char *refs_heads_master,
        return 0;
 }
 
-static char *ref_msg(const char *line, const char *endp)
-{
-       const char *ep;
-       line += 82;
-       ep = memchr(line, '\n', endp - line);
-       if (!ep)
-               ep = endp;
-       return xmemdupz(line, ep - line);
+struct read_ref_at_cb {
+       const char *refname;
+       unsigned long at_time;
+       int cnt;
+       int reccnt;
+       unsigned char *sha1;
+       int found_it;
+
+       unsigned char osha1[20];
+       unsigned char nsha1[20];
+       int tz;
+       unsigned long date;
+       char **msg;
+       unsigned long *cutoff_time;
+       int *cutoff_tz;
+       int *cutoff_cnt;
+};
+
+static int read_ref_at_ent(unsigned char *osha1, unsigned char *nsha1,
+               const char *email, unsigned long timestamp, int tz,
+               const char *message, void *cb_data)
+{
+       struct read_ref_at_cb *cb = cb_data;
+
+       cb->reccnt++;
+       cb->tz = tz;
+       cb->date = timestamp;
+
+       if (timestamp <= cb->at_time || cb->cnt == 0) {
+               if (cb->msg)
+                       *cb->msg = xstrdup(message);
+               if (cb->cutoff_time)
+                       *cb->cutoff_time = timestamp;
+               if (cb->cutoff_tz)
+                       *cb->cutoff_tz = tz;
+               if (cb->cutoff_cnt)
+                       *cb->cutoff_cnt = cb->reccnt - 1;
+               /*
+                * we have not yet updated cb->[n|o]sha1 so they still
+                * hold the values for the previous record.
+                */
+               if (!is_null_sha1(cb->osha1)) {
+                       hashcpy(cb->sha1, nsha1);
+                       if (hashcmp(cb->osha1, nsha1))
+                               warning("Log for ref %s has gap after %s.",
+                                       cb->refname, show_date(cb->date, cb->tz, DATE_RFC2822));
+               }
+               else if (cb->date == cb->at_time)
+                       hashcpy(cb->sha1, nsha1);
+               else if (hashcmp(nsha1, cb->sha1))
+                       warning("Log for ref %s unexpectedly ended on %s.",
+                               cb->refname, show_date(cb->date, cb->tz,
+                                                  DATE_RFC2822));
+               hashcpy(cb->osha1, osha1);
+               hashcpy(cb->nsha1, nsha1);
+               cb->found_it = 1;
+               return 1;
+       }
+       hashcpy(cb->osha1, osha1);
+       hashcpy(cb->nsha1, nsha1);
+       if (cb->cnt > 0)
+               cb->cnt--;
+       return 0;
 }
 
-int read_ref_at(const char *refname, unsigned long at_time, int cnt,
+static int read_ref_at_ent_oldest(unsigned char *osha1, unsigned char *nsha1,
+                                 const char *email, unsigned long timestamp,
+                                 int tz, const char *message, void *cb_data)
+{
+       struct read_ref_at_cb *cb = cb_data;
+
+       if (cb->msg)
+               *cb->msg = xstrdup(message);
+       if (cb->cutoff_time)
+               *cb->cutoff_time = timestamp;
+       if (cb->cutoff_tz)
+               *cb->cutoff_tz = tz;
+       if (cb->cutoff_cnt)
+               *cb->cutoff_cnt = cb->reccnt;
+       hashcpy(cb->sha1, osha1);
+       if (is_null_sha1(cb->sha1))
+               hashcpy(cb->sha1, nsha1);
+       /* We just want the first entry */
+       return 1;
+}
+
+int read_ref_at(const char *refname, unsigned int flags, unsigned long at_time, int cnt,
                unsigned char *sha1, char **msg,
                unsigned long *cutoff_time, int *cutoff_tz, int *cutoff_cnt)
 {
-       const char *logfile, *logdata, *logend, *rec, *lastgt, *lastrec;
-       char *tz_c;
-       int logfd, tz, reccnt = 0;
-       struct stat st;
-       unsigned long date;
-       unsigned char logged_sha1[20];
-       void *log_mapped;
-       size_t mapsz;
+       struct read_ref_at_cb cb;
 
-       logfile = git_path("logs/%s", refname);
-       logfd = open(logfile, O_RDONLY, 0);
-       if (logfd < 0)
-               die_errno("Unable to read log '%s'", logfile);
-       fstat(logfd, &st);
-       if (!st.st_size)
-               die("Log %s is empty.", logfile);
-       mapsz = xsize_t(st.st_size);
-       log_mapped = xmmap(NULL, mapsz, PROT_READ, MAP_PRIVATE, logfd, 0);
-       logdata = log_mapped;
-       close(logfd);
+       memset(&cb, 0, sizeof(cb));
+       cb.refname = refname;
+       cb.at_time = at_time;
+       cb.cnt = cnt;
+       cb.msg = msg;
+       cb.cutoff_time = cutoff_time;
+       cb.cutoff_tz = cutoff_tz;
+       cb.cutoff_cnt = cutoff_cnt;
+       cb.sha1 = sha1;
 
-       lastrec = NULL;
-       rec = logend = logdata + st.st_size;
-       while (logdata < rec) {
-               reccnt++;
-               if (logdata < rec && *(rec-1) == '\n')
-                       rec--;
-               lastgt = NULL;
-               while (logdata < rec && *(rec-1) != '\n') {
-                       rec--;
-                       if (*rec == '>')
-                               lastgt = rec;
-               }
-               if (!lastgt)
-                       die("Log %s is corrupt.", logfile);
-               date = strtoul(lastgt + 1, &tz_c, 10);
-               if (date <= at_time || cnt == 0) {
-                       tz = strtoul(tz_c, NULL, 10);
-                       if (msg)
-                               *msg = ref_msg(rec, logend);
-                       if (cutoff_time)
-                               *cutoff_time = date;
-                       if (cutoff_tz)
-                               *cutoff_tz = tz;
-                       if (cutoff_cnt)
-                               *cutoff_cnt = reccnt - 1;
-                       if (lastrec) {
-                               if (get_sha1_hex(lastrec, logged_sha1))
-                                       die("Log %s is corrupt.", logfile);
-                               if (get_sha1_hex(rec + 41, sha1))
-                                       die("Log %s is corrupt.", logfile);
-                               if (hashcmp(logged_sha1, sha1)) {
-                                       warning("Log %s has gap after %s.",
-                                               logfile, show_date(date, tz, DATE_RFC2822));
-                               }
-                       }
-                       else if (date == at_time) {
-                               if (get_sha1_hex(rec + 41, sha1))
-                                       die("Log %s is corrupt.", logfile);
-                       }
-                       else {
-                               if (get_sha1_hex(rec + 41, logged_sha1))
-                                       die("Log %s is corrupt.", logfile);
-                               if (hashcmp(logged_sha1, sha1)) {
-                                       warning("Log %s unexpectedly ended on %s.",
-                                               logfile, show_date(date, tz, DATE_RFC2822));
-                               }
-                       }
-                       munmap(log_mapped, mapsz);
-                       return 0;
-               }
-               lastrec = rec;
-               if (cnt > 0)
-                       cnt--;
-       }
-
-       rec = logdata;
-       while (rec < logend && *rec != '>' && *rec != '\n')
-               rec++;
-       if (rec == logend || *rec == '\n')
-               die("Log %s is corrupt.", logfile);
-       date = strtoul(rec + 1, &tz_c, 10);
-       tz = strtoul(tz_c, NULL, 10);
-       if (get_sha1_hex(logdata, sha1))
-               die("Log %s is corrupt.", logfile);
-       if (is_null_sha1(sha1)) {
-               if (get_sha1_hex(logdata + 41, sha1))
-                       die("Log %s is corrupt.", logfile);
+       for_each_reflog_ent_reverse(refname, read_ref_at_ent, &cb);
+
+       if (!cb.reccnt) {
+               if (flags & GET_SHA1_QUIETLY)
+                       exit(128);
+               else
+                       die("Log for %s is empty.", refname);
        }
-       if (msg)
-               *msg = ref_msg(logdata, logend);
-       munmap(log_mapped, mapsz);
-
-       if (cutoff_time)
-               *cutoff_time = date;
-       if (cutoff_tz)
-               *cutoff_tz = tz;
-       if (cutoff_cnt)
-               *cutoff_cnt = reccnt;
+       if (cb.found_it)
+               return 0;
+
+       for_each_reflog_ent(refname, read_ref_at_ent_oldest, &cb);
+
        return 1;
 }
 
+int reflog_exists(const char *refname)
+{
+       struct stat st;
+
+       return !lstat(git_path("logs/%s", refname), &st) &&
+               S_ISREG(st.st_mode);
+}
+
+int delete_reflog(const char *refname)
+{
+       return remove_path(git_path("logs/%s", refname));
+}
+
 static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *cb_data)
 {
        unsigned char osha1[20], nsha1[20];
@@ -3214,7 +3469,7 @@ static int do_for_each_reflog(struct strbuf *name, each_ref_fn fn, void *cb_data
 
                if (de->d_name[0] == '.')
                        continue;
-               if (has_extension(de->d_name, ".lock"))
+               if (ends_with(de->d_name, ".lock"))
                        continue;
                strbuf_addstr(name, de->d_name);
                if (stat(git_path("logs/%s", name->buf), &st) < 0) {
@@ -3225,7 +3480,7 @@ static int do_for_each_reflog(struct strbuf *name, each_ref_fn fn, void *cb_data
                                retval = do_for_each_reflog(name, fn, cb_data);
                        } else {
                                unsigned char sha1[20];
-                               if (read_ref_full(name->buf, sha1, 0, NULL))
+                               if (read_ref_full(name->buf, 0, sha1, NULL))
                                        retval = error("bad ref for %s", name->buf);
                                else
                                        retval = fn(name->buf, sha1, 0, cb_data);
@@ -3249,37 +3504,177 @@ int for_each_reflog(each_ref_fn fn, void *cb_data)
        return retval;
 }
 
-static struct ref_lock *update_ref_lock(const char *refname,
-                                       const unsigned char *oldval,
-                                       int flags, int *type_p,
-                                       enum action_on_err onerr)
-{
+/**
+ * Information needed for a single ref update.  Set new_sha1 to the
+ * new value or to zero to delete the ref.  To check the old value
+ * while locking the ref, set have_old to 1 and set old_sha1 to the
+ * value or to zero to ensure the ref does not exist before update.
+ */
+struct ref_update {
+       unsigned char new_sha1[20];
+       unsigned char old_sha1[20];
+       int flags; /* REF_NODEREF? */
+       int have_old; /* 1 if old_sha1 is valid, 0 otherwise */
        struct ref_lock *lock;
-       lock = lock_any_ref_for_update(refname, oldval, flags, type_p);
-       if (!lock) {
-               const char *str = "Cannot lock the ref '%s'.";
-               switch (onerr) {
-               case MSG_ON_ERR: error(str, refname); break;
-               case DIE_ON_ERR: die(str, refname); break;
-               case QUIET_ON_ERR: break;
-               }
+       int type;
+       char *msg;
+       const char refname[FLEX_ARRAY];
+};
+
+/*
+ * Transaction states.
+ * OPEN:   The transaction is in a valid state and can accept new updates.
+ *         An OPEN transaction can be committed.
+ * CLOSED: A closed transaction is no longer active and no other operations
+ *         than free can be used on it in this state.
+ *         A transaction can either become closed by successfully committing
+ *         an active transaction or if there is a failure while building
+ *         the transaction thus rendering it failed/inactive.
+ */
+enum ref_transaction_state {
+       REF_TRANSACTION_OPEN   = 0,
+       REF_TRANSACTION_CLOSED = 1
+};
+
+/*
+ * Data structure for holding a reference transaction, which can
+ * consist of checks and updates to multiple references, carried out
+ * as atomically as possible.  This structure is opaque to callers.
+ */
+struct ref_transaction {
+       struct ref_update **updates;
+       size_t alloc;
+       size_t nr;
+       enum ref_transaction_state state;
+};
+
+struct ref_transaction *ref_transaction_begin(struct strbuf *err)
+{
+       assert(err);
+
+       return xcalloc(1, sizeof(struct ref_transaction));
+}
+
+void ref_transaction_free(struct ref_transaction *transaction)
+{
+       int i;
+
+       if (!transaction)
+               return;
+
+       for (i = 0; i < transaction->nr; i++) {
+               free(transaction->updates[i]->msg);
+               free(transaction->updates[i]);
        }
-       return lock;
+       free(transaction->updates);
+       free(transaction);
 }
 
-static int update_ref_write(const char *action, const char *refname,
-                           const unsigned char *sha1, struct ref_lock *lock,
-                           enum action_on_err onerr)
+static struct ref_update *add_update(struct ref_transaction *transaction,
+                                    const char *refname)
 {
-       if (write_ref_sha1(lock, sha1, action) < 0) {
-               const char *str = "Cannot update the ref '%s'.";
-               switch (onerr) {
-               case MSG_ON_ERR: error(str, refname); break;
-               case DIE_ON_ERR: die(str, refname); break;
-               case QUIET_ON_ERR: break;
-               }
-               return 1;
+       size_t len = strlen(refname);
+       struct ref_update *update = xcalloc(1, sizeof(*update) + len + 1);
+
+       strcpy((char *)update->refname, refname);
+       ALLOC_GROW(transaction->updates, transaction->nr + 1, transaction->alloc);
+       transaction->updates[transaction->nr++] = update;
+       return update;
+}
+
+int ref_transaction_update(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *new_sha1,
+                          const unsigned char *old_sha1,
+                          int flags, int have_old, const char *msg,
+                          struct strbuf *err)
+{
+       struct ref_update *update;
+
+       assert(err);
+
+       if (transaction->state != REF_TRANSACTION_OPEN)
+               die("BUG: update called for transaction that is not open");
+
+       if (have_old && !old_sha1)
+               die("BUG: have_old is true but old_sha1 is NULL");
+
+       if (!is_null_sha1(new_sha1) &&
+           check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+               strbuf_addf(err, "refusing to update ref with bad name %s",
+                           refname);
+               return -1;
+       }
+
+       update = add_update(transaction, refname);
+       hashcpy(update->new_sha1, new_sha1);
+       update->flags = flags;
+       update->have_old = have_old;
+       if (have_old)
+               hashcpy(update->old_sha1, old_sha1);
+       if (msg)
+               update->msg = xstrdup(msg);
+       return 0;
+}
+
+int ref_transaction_create(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *new_sha1,
+                          int flags, const char *msg,
+                          struct strbuf *err)
+{
+       struct ref_update *update;
+
+       assert(err);
+
+       if (transaction->state != REF_TRANSACTION_OPEN)
+               die("BUG: create called for transaction that is not open");
+
+       if (!new_sha1 || is_null_sha1(new_sha1))
+               die("BUG: create ref with null new_sha1");
+
+       if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+               strbuf_addf(err, "refusing to create ref with bad name %s",
+                           refname);
+               return -1;
+       }
+
+       update = add_update(transaction, refname);
+
+       hashcpy(update->new_sha1, new_sha1);
+       hashclr(update->old_sha1);
+       update->flags = flags;
+       update->have_old = 1;
+       if (msg)
+               update->msg = xstrdup(msg);
+       return 0;
+}
+
+int ref_transaction_delete(struct ref_transaction *transaction,
+                          const char *refname,
+                          const unsigned char *old_sha1,
+                          int flags, int have_old, const char *msg,
+                          struct strbuf *err)
+{
+       struct ref_update *update;
+
+       assert(err);
+
+       if (transaction->state != REF_TRANSACTION_OPEN)
+               die("BUG: delete called for transaction that is not open");
+
+       if (have_old && !old_sha1)
+               die("BUG: have_old is true but old_sha1 is NULL");
+
+       update = add_update(transaction, refname);
+       update->flags = flags;
+       update->have_old = have_old;
+       if (have_old) {
+               assert(!is_null_sha1(old_sha1));
+               hashcpy(update->old_sha1, old_sha1);
        }
+       if (msg)
+               update->msg = xstrdup(msg);
        return 0;
 }
 
@@ -3287,109 +3682,157 @@ int update_ref(const char *action, const char *refname,
               const unsigned char *sha1, const unsigned char *oldval,
               int flags, enum action_on_err onerr)
 {
-       struct ref_lock *lock;
-       lock = update_ref_lock(refname, oldval, flags, NULL, onerr);
-       if (!lock)
+       struct ref_transaction *t;
+       struct strbuf err = STRBUF_INIT;
+
+       t = ref_transaction_begin(&err);
+       if (!t ||
+           ref_transaction_update(t, refname, sha1, oldval, flags,
+                                  !!oldval, action, &err) ||
+           ref_transaction_commit(t, &err)) {
+               const char *str = "update_ref failed for ref '%s': %s";
+
+               ref_transaction_free(t);
+               switch (onerr) {
+               case UPDATE_REFS_MSG_ON_ERR:
+                       error(str, refname, err.buf);
+                       break;
+               case UPDATE_REFS_DIE_ON_ERR:
+                       die(str, refname, err.buf);
+                       break;
+               case UPDATE_REFS_QUIET_ON_ERR:
+                       break;
+               }
+               strbuf_release(&err);
                return 1;
-       return update_ref_write(action, refname, sha1, lock, onerr);
+       }
+       strbuf_release(&err);
+       ref_transaction_free(t);
+       return 0;
 }
 
 static int ref_update_compare(const void *r1, const void *r2)
 {
        const struct ref_update * const *u1 = r1;
        const struct ref_update * const *u2 = r2;
-       return strcmp((*u1)->ref_name, (*u2)->ref_name);
+       return strcmp((*u1)->refname, (*u2)->refname);
 }
 
 static int ref_update_reject_duplicates(struct ref_update **updates, int n,
-                                       enum action_on_err onerr)
+                                       struct strbuf *err)
 {
        int i;
+
+       assert(err);
+
        for (i = 1; i < n; i++)
-               if (!strcmp(updates[i - 1]->ref_name, updates[i]->ref_name)) {
-                       const char *str =
-                               "Multiple updates for ref '%s' not allowed.";
-                       switch (onerr) {
-                       case MSG_ON_ERR:
-                               error(str, updates[i]->ref_name); break;
-                       case DIE_ON_ERR:
-                               die(str, updates[i]->ref_name); break;
-                       case QUIET_ON_ERR:
-                               break;
-                       }
+               if (!strcmp(updates[i - 1]->refname, updates[i]->refname)) {
+                       strbuf_addf(err,
+                                   "Multiple updates for ref '%s' not allowed.",
+                                   updates[i]->refname);
                        return 1;
                }
        return 0;
 }
 
-int update_refs(const char *action, const struct ref_update **updates_orig,
-               int n, enum action_on_err onerr)
+int ref_transaction_commit(struct ref_transaction *transaction,
+                          struct strbuf *err)
 {
        int ret = 0, delnum = 0, i;
-       struct ref_update **updates;
-       int *types;
-       struct ref_lock **locks;
        const char **delnames;
+       int n = transaction->nr;
+       struct ref_update **updates = transaction->updates;
+
+       assert(err);
 
-       if (!updates_orig || !n)
+       if (transaction->state != REF_TRANSACTION_OPEN)
+               die("BUG: commit called for transaction that is not open");
+
+       if (!n) {
+               transaction->state = REF_TRANSACTION_CLOSED;
                return 0;
+       }
 
        /* Allocate work space */
-       updates = xmalloc(sizeof(*updates) * n);
-       types = xmalloc(sizeof(*types) * n);
-       locks = xcalloc(n, sizeof(*locks));
        delnames = xmalloc(sizeof(*delnames) * n);
 
        /* Copy, sort, and reject duplicate refs */
-       memcpy(updates, updates_orig, sizeof(*updates) * n);
        qsort(updates, n, sizeof(*updates), ref_update_compare);
-       ret = ref_update_reject_duplicates(updates, n, onerr);
-       if (ret)
+       if (ref_update_reject_duplicates(updates, n, err)) {
+               ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
+       }
 
        /* Acquire all locks while verifying old values */
        for (i = 0; i < n; i++) {
-               locks[i] = update_ref_lock(updates[i]->ref_name,
-                                          (updates[i]->have_old ?
-                                           updates[i]->old_sha1 : NULL),
-                                          updates[i]->flags,
-                                          &types[i], onerr);
-               if (!locks[i]) {
-                       ret = 1;
+               struct ref_update *update = updates[i];
+               int flags = update->flags;
+
+               if (is_null_sha1(update->new_sha1))
+                       flags |= REF_DELETING;
+               update->lock = lock_ref_sha1_basic(update->refname,
+                                                  (update->have_old ?
+                                                   update->old_sha1 :
+                                                   NULL),
+                                                  NULL,
+                                                  flags,
+                                                  &update->type);
+               if (!update->lock) {
+                       ret = (errno == ENOTDIR)
+                               ? TRANSACTION_NAME_CONFLICT
+                               : TRANSACTION_GENERIC_ERROR;
+                       strbuf_addf(err, "Cannot lock the ref '%s'.",
+                                   update->refname);
                        goto cleanup;
                }
        }
 
        /* Perform updates first so live commits remain referenced */
-       for (i = 0; i < n; i++)
-               if (!is_null_sha1(updates[i]->new_sha1)) {
-                       ret = update_ref_write(action,
-                                              updates[i]->ref_name,
-                                              updates[i]->new_sha1,
-                                              locks[i], onerr);
-                       locks[i] = NULL; /* freed by update_ref_write */
-                       if (ret)
+       for (i = 0; i < n; i++) {
+               struct ref_update *update = updates[i];
+
+               if (!is_null_sha1(update->new_sha1)) {
+                       if (write_ref_sha1(update->lock, update->new_sha1,
+                                          update->msg)) {
+                               update->lock = NULL; /* freed by write_ref_sha1 */
+                               strbuf_addf(err, "Cannot update the ref '%s'.",
+                                           update->refname);
+                               ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
+                       }
+                       update->lock = NULL; /* freed by write_ref_sha1 */
                }
+       }
 
        /* Perform deletes now that updates are safely completed */
-       for (i = 0; i < n; i++)
-               if (locks[i]) {
-                       delnames[delnum++] = locks[i]->ref_name;
-                       ret |= delete_ref_loose(locks[i], types[i]);
+       for (i = 0; i < n; i++) {
+               struct ref_update *update = updates[i];
+
+               if (update->lock) {
+                       if (delete_ref_loose(update->lock, update->type, err)) {
+                               ret = TRANSACTION_GENERIC_ERROR;
+                               goto cleanup;
+                       }
+
+                       if (!(update->flags & REF_ISPRUNING))
+                               delnames[delnum++] = update->lock->ref_name;
                }
-       ret |= repack_without_refs(delnames, delnum);
+       }
+
+       if (repack_without_refs(delnames, delnum, err)) {
+               ret = TRANSACTION_GENERIC_ERROR;
+               goto cleanup;
+       }
        for (i = 0; i < delnum; i++)
                unlink_or_warn(git_path("logs/%s", delnames[i]));
        clear_loose_ref_cache(&ref_cache);
 
 cleanup:
+       transaction->state = REF_TRANSACTION_CLOSED;
+
        for (i = 0; i < n; i++)
-               if (locks[i])
-                       unlock_ref(locks[i]);
-       free(updates);
-       free(types);
-       free(locks);
+               if (updates[i]->lock)
+                       unlock_ref(updates[i]->lock);
        free(delnames);
        return ret;
 }