Merge branch 'rs/code-cleaning'
[gitweb.git] / refs.c
diff --git a/refs.c b/refs.c
index 28d5eca8eaff7be48a4b89e6217886b8c96eb3b2..061d233e4b6404c57ff8d62715758b900d02e051 100644 (file)
--- a/refs.c
+++ b/refs.c
@@ -6,51 +6,71 @@
 #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:
+ * This table is used by both the SIMD and non-SIMD code.  It has
+ * some cases that are only useful for the SIMD; these are handled
+ * equivalently to the listed disposition in the non-SIMD code.
+ * 0: An acceptable character for refs
+ * 1: @, look for a following { to reject @{ in refs (SIMD or = 0)
+ * 2: \0: End-of-component and string
+ * 3: /: End-of-component (SIMD or = 2)
+ * 4: ., look for a preceding . to reject .. in refs
+ * 5: {, look for a preceding @ to reject @{ in refs
+ * 6: *, usually a bad character except, once as a wildcard (SIMD or = 7)
+ * 7: A bad character except * (see check_refname_component below)
+ */
+static unsigned char refname_disposition[256] = {
+       2, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+       7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+       7, 0, 0, 0, 0, 0, 0, 0, 0, 0, 6, 0, 0, 0, 4, 3,
+       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 7,
+       1, 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, 7, 7, 0, 7, 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, 5, 0, 0, 7, 7
+};
+
+/*
+ * 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 has ASCII control character, "~", "^", ":" or SP, anywhere, or
- * - it ends with a "/".
- * - it ends with ".lock"
+ * - it has pattern-matching notation "*", "?", "[", anywhere, or
+ * - it ends with a "/", or
+ * - it ends with ".lock", or
  * - 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 2: /* fall-through */
+               case 3:
+                       goto out;
+               case 4:
+                       if (last == '.')
+                               return -1; /* Refname contains "..". */
+                       break;
+               case 5:
+                       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 6: /* fall-through */
+               case 7:
+                       return -1;
+               }
                last = ch;
        }
+out:
        if (cp == refname)
                return 0; /* Component has zero length. */
        if (refname[0] == '.') {
@@ -68,7 +88,7 @@ static int check_refname_component(const char *refname, int flags)
        return cp - refname;
 }
 
-int check_refname_format(const char *refname, int flags)
+static int check_refname_format_bytewise(const char *refname, int flags)
 {
        int component_len, component_count = 0;
 
@@ -104,6 +124,196 @@ int check_refname_format(const char *refname, int flags)
        return 0;
 }
 
+#if defined(__GNUC__) && defined(__x86_64__)
+#define SSE_VECTOR_BYTES 16
+
+/* Vectorized version of check_refname_format. */
+int check_refname_format(const char *refname, int flags)
+{
+       const char *cp = refname;
+
+       const __m128i dot = _mm_set1_epi8('.');
+       const __m128i at = _mm_set1_epi8('@');
+       const __m128i curly = _mm_set1_epi8('{');
+       const __m128i slash = _mm_set1_epi8('/');
+       const __m128i zero = _mm_set1_epi8('\000');
+       const __m128i el = _mm_set1_epi8('l');
+
+       /* below '*', all characters are forbidden or rare */
+       const __m128i star_ub = _mm_set1_epi8('*' + 1);
+
+       const __m128i colon = _mm_set1_epi8(':');
+       const __m128i question = _mm_set1_epi8('?');
+
+       /* '['..'^' contains 4 characters: 3 forbidden and 1 rare */
+       const __m128i bracket_lb = _mm_set1_epi8('[' - 1);
+       const __m128i caret_ub = _mm_set1_epi8('^' + 1);
+
+       /* '~' and above are forbidden */
+       const __m128i tilde_lb = _mm_set1_epi8('~' - 1);
+
+       int component_count = 0;
+       int orig_flags = flags;
+
+       if (refname[0] == 0 || refname[0] == '/') {
+               /* entirely empty ref or initial ref component */
+               return -1;
+       }
+
+       /*
+        * Initial ref component of '.'; below we look for /. so we'll
+        * miss this.
+        */
+       if (refname[0] == '.') {
+               if (refname[1] == '/' || refname[1] == '\0')
+                       return -1;
+               if (!(flags & REFNAME_DOT_COMPONENT))
+                       return -1;
+       }
+       while(1) {
+               __m128i tmp, tmp1, result;
+               uint64_t mask;
+
+               if ((uintptr_t) cp % PAGE_SIZE > PAGE_SIZE - SSE_VECTOR_BYTES  - 1)
+                       /*
+                        * End-of-page; fall back to slow method for
+                        * this entire ref.
+                        */
+                       return check_refname_format_bytewise(refname, orig_flags);
+
+               tmp = _mm_loadu_si128((__m128i *)cp);
+               tmp1 = _mm_loadu_si128((__m128i *)(cp + 1));
+
+               /*
+                * This range (note the lt) contains some
+                * permissible-but-rare characters (including all
+                * characters >= 128), which we handle later.  It also
+                * includes \000.
+                */
+               result = _mm_cmplt_epi8(tmp, star_ub);
+
+               result = _mm_or_si128(result, _mm_cmpeq_epi8(tmp, question));
+               result = _mm_or_si128(result, _mm_cmpeq_epi8(tmp, colon));
+
+               /* This range contains the permissible ] as bycatch */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpgt_epi8(tmp, bracket_lb),
+                                             _mm_cmplt_epi8(tmp, caret_ub)));
+
+               result = _mm_or_si128(result, _mm_cmpgt_epi8(tmp, tilde_lb));
+
+               /* .. */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpeq_epi8(tmp, dot),
+                                             _mm_cmpeq_epi8(tmp1, dot)));
+               /* @{ */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpeq_epi8(tmp, at),
+                                             _mm_cmpeq_epi8(tmp1, curly)));
+               /* // */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpeq_epi8(tmp, slash),
+                                             _mm_cmpeq_epi8(tmp1, slash)));
+               /* trailing / */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpeq_epi8(tmp, slash),
+                                             _mm_cmpeq_epi8(tmp1, zero)));
+               /* .l, beginning of .lock */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpeq_epi8(tmp, dot),
+                                             _mm_cmpeq_epi8(tmp1, el)));
+               /*
+                * Even though /. is not necessarily an error, we flag
+                * it anyway. If we find it, we'll check if it's valid
+                * and if so we'll advance just past it.
+                */
+               result = _mm_or_si128(result, _mm_and_si128(
+                                             _mm_cmpeq_epi8(tmp, slash),
+                                             _mm_cmpeq_epi8(tmp1, dot)));
+
+               mask = _mm_movemask_epi8(result);
+               if (mask) {
+                       /*
+                        * We've found either end-of-string, or some
+                        * probably-bad character or substring.
+                        */
+                       int i = __builtin_ctz(mask);
+                       switch (refname_disposition[cp[i] & 255]) {
+                       case 0: /* fall-through */
+                       case 5:
+                               /*
+                                * bycatch: a good character that's in
+                                * one of the ranges of mostly-forbidden
+                                * characters
+                                */
+                               cp += i + 1;
+                               break;
+                       case 1:
+                               if (cp[i + 1] == '{')
+                                       return -1;
+                               cp += i + 1;
+                               break;
+                       case 2:
+                               if (!(flags & REFNAME_ALLOW_ONELEVEL)
+                                   && !component_count && !strchr(refname, '/'))
+                                       /* Refname has only one component. */
+                                       return -1;
+                               return 0;
+                       case 3:
+                               component_count ++;
+                               /*
+                                * Even if leading dots are allowed, don't
+                                * allow "." as a component (".." is
+                                * prevented by case 4 below).
+                                */
+                               if (cp[i + 1] == '.') {
+                                       if (cp[i + 2] == '\0')
+                                               return -1;
+                                       if (flags & REFNAME_DOT_COMPONENT) {
+                                               /* skip to just after the /. */
+                                               cp += i + 2;
+                                               break;
+                                       }
+                                       return -1;
+                               } else if (cp[i + 1] == '/' || cp[i + 1] == '\0')
+                                       return -1;
+                               break;
+                       case 4:
+                               if (cp[i + 1] == '.' || cp[i + 1] == '\0')
+                                       return -1;
+                               /* .lock as end-of-component or end-of-string */
+                               if ((!strncmp(cp + i, ".lock", 5))
+                                   && (cp[i + 5] == '/' || cp[i + 5] == 0))
+                                       return -1;
+                               cp += 1;
+                               break;
+                       case 6:
+                               if (((cp == refname + i) || cp[i - 1] == '/')
+                                   && (cp[i + 1] == '/' || cp[i + 1] == 0))
+                                       if (flags & REFNAME_REFSPEC_PATTERN) {
+                                               flags &= ~REFNAME_REFSPEC_PATTERN;
+                                               /* restart after the * */
+                                               cp += i + 1;
+                                               continue;
+                                       }
+                               /* fall-through */
+                       case 7:
+                               return -1;
+                       }
+               } else
+                       cp += SSE_VECTOR_BYTES;
+       }
+}
+
+#else
+
+int check_refname_format (const char *refname, int flags)
+{
+       return check_refname_format_bytewise(refname, flags);
+}
+
+#endif
+
 struct ref_entry;
 
 /*
@@ -1151,7 +1361,7 @@ 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
@@ -1611,6 +1821,7 @@ int peel_ref(const char *refname, unsigned char *sha1)
 struct warn_if_dangling_data {
        FILE *fp;
        const char *refname;
+       const struct string_list *refnames;
        const char *msg_fmt;
 };
 
@@ -1625,8 +1836,12 @@ static int warn_if_dangling_symref(const char *refname, const unsigned char *sha
                return 0;
 
        resolves_to = resolve_ref_unsafe(refname, junk, 0, NULL);
-       if (!resolves_to || strcmp(resolves_to, d->refname))
+       if (!resolves_to
+           || (d->refname
+               ? strcmp(resolves_to, d->refname)
+               : !string_list_has_string(d->refnames, resolves_to))) {
                return 0;
+       }
 
        fprintf(d->fp, d->msg_fmt, refname);
        fputc('\n', d->fp);
@@ -1639,6 +1854,18 @@ void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refname)
 
        data.fp = fp;
        data.refname = refname;
+       data.refnames = NULL;
+       data.msg_fmt = msg_fmt;
+       for_each_rawref(warn_if_dangling_symref, &data);
+}
+
+void warn_dangling_symrefs(FILE *fp, const char *msg_fmt, const struct string_list *refnames)
+{
+       struct warn_if_dangling_data data;
+
+       data.fp = fp;
+       data.refname = NULL;
+       data.refnames = refnames;
        data.msg_fmt = msg_fmt;
        for_each_rawref(warn_if_dangling_symref, &data);
 }
@@ -1999,7 +2226,6 @@ 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;
@@ -2008,12 +2234,9 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
                ref = resolve_ref_unsafe(path, hash, 1, 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;
@@ -2431,7 +2654,7 @@ static int curate_packed_ref_fn(struct ref_entry *entry, void *cb_data)
        return 0;
 }
 
-static int repack_without_refs(const char **refnames, int n)
+int repack_without_refs(const char **refnames, int n)
 {
        struct ref_dir *packed;
        struct string_list refs_to_delete = STRING_LIST_INIT_DUP;
@@ -2930,122 +3153,133 @@ 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;
+}
+
+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 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;
+
+       for_each_reflog_ent_reverse(refname, read_ref_at_ent, &cb);
+
+       if (!cb.reccnt)
+               die("Log for %s is empty.", refname);
+       if (cb.found_it)
+               return 0;
+
+       for_each_reflog_ent(refname, read_ref_at_ent_oldest, &cb);
 
-       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);
-       }
-       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;
        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];
@@ -3198,7 +3432,7 @@ 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) {
@@ -3243,9 +3477,9 @@ static struct ref_lock *update_ref_lock(const char *refname,
        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;
+               case UPDATE_REFS_MSG_ON_ERR: error(str, refname); break;
+               case UPDATE_REFS_DIE_ON_ERR: die(str, refname); break;
+               case UPDATE_REFS_QUIET_ON_ERR: break;
                }
        }
        return lock;
@@ -3258,15 +3492,118 @@ static int update_ref_write(const char *action, 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;
+               case UPDATE_REFS_MSG_ON_ERR: error(str, refname); break;
+               case UPDATE_REFS_DIE_ON_ERR: die(str, refname); break;
+               case UPDATE_REFS_QUIET_ON_ERR: break;
                }
                return 1;
        }
        return 0;
 }
 
+/**
+ * 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;
+       int type;
+       const char refname[FLEX_ARRAY];
+};
+
+/*
+ * 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;
+};
+
+struct ref_transaction *ref_transaction_begin(void)
+{
+       return xcalloc(1, sizeof(struct ref_transaction));
+}
+
+static void ref_transaction_free(struct ref_transaction *transaction)
+{
+       int i;
+
+       for (i = 0; i < transaction->nr; i++)
+               free(transaction->updates[i]);
+
+       free(transaction->updates);
+       free(transaction);
+}
+
+void ref_transaction_rollback(struct ref_transaction *transaction)
+{
+       ref_transaction_free(transaction);
+}
+
+static struct ref_update *add_update(struct ref_transaction *transaction,
+                                    const char *refname)
+{
+       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;
+}
+
+void ref_transaction_update(struct ref_transaction *transaction,
+                           const char *refname,
+                           unsigned char *new_sha1, unsigned char *old_sha1,
+                           int flags, int have_old)
+{
+       struct ref_update *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);
+}
+
+void ref_transaction_create(struct ref_transaction *transaction,
+                           const char *refname,
+                           unsigned char *new_sha1,
+                           int flags)
+{
+       struct ref_update *update = add_update(transaction, refname);
+
+       assert(!is_null_sha1(new_sha1));
+       hashcpy(update->new_sha1, new_sha1);
+       hashclr(update->old_sha1);
+       update->flags = flags;
+       update->have_old = 1;
+}
+
+void ref_transaction_delete(struct ref_transaction *transaction,
+                           const char *refname,
+                           unsigned char *old_sha1,
+                           int flags, int have_old)
+{
+       struct ref_update *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);
+       }
+}
+
 int update_ref(const char *action, const char *refname,
               const unsigned char *sha1, const unsigned char *oldval,
               int flags, enum action_on_err onerr)
@@ -3282,7 +3619,7 @@ 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,
@@ -3290,15 +3627,15 @@ static int ref_update_reject_duplicates(struct ref_update **updates, int n,
 {
        int i;
        for (i = 1; i < n; i++)
-               if (!strcmp(updates[i - 1]->ref_name, updates[i]->ref_name)) {
+               if (!strcmp(updates[i - 1]->refname, updates[i]->refname)) {
                        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:
+                       case UPDATE_REFS_MSG_ON_ERR:
+                               error(str, updates[i]->refname); break;
+                       case UPDATE_REFS_DIE_ON_ERR:
+                               die(str, updates[i]->refname); break;
+                       case UPDATE_REFS_QUIET_ON_ERR:
                                break;
                        }
                        return 1;
@@ -3306,26 +3643,21 @@ static int ref_update_reject_duplicates(struct ref_update **updates, int n,
        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,
+                          const char *msg, enum action_on_err onerr)
 {
        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;
 
-       if (!updates_orig || !n)
+       if (!n)
                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)
@@ -3333,35 +3665,44 @@ int update_refs(const char *action, const struct ref_update **updates_orig,
 
        /* 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]) {
+               struct ref_update *update = updates[i];
+
+               update->lock = update_ref_lock(update->refname,
+                                              (update->have_old ?
+                                               update->old_sha1 : NULL),
+                                              update->flags,
+                                              &update->type, onerr);
+               if (!update->lock) {
                        ret = 1;
                        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 */
+       for (i = 0; i < n; i++) {
+               struct ref_update *update = updates[i];
+
+               if (!is_null_sha1(update->new_sha1)) {
+                       ret = update_ref_write(msg,
+                                              update->refname,
+                                              update->new_sha1,
+                                              update->lock, onerr);
+                       update->lock = NULL; /* freed by update_ref_write */
                        if (ret)
                                goto cleanup;
                }
+       }
 
        /* 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) {
+                       delnames[delnum++] = update->lock->ref_name;
+                       ret |= delete_ref_loose(update->lock, update->type);
                }
+       }
+
        ret |= repack_without_refs(delnames, delnum);
        for (i = 0; i < delnum; i++)
                unlink_or_warn(git_path("logs/%s", delnames[i]));
@@ -3369,12 +3710,10 @@ int update_refs(const char *action, const struct ref_update **updates_orig,
 
 cleanup:
        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);
+       ref_transaction_free(transaction);
        return ret;
 }