find_unique_abbrev: use 4-buffer ring

Some code paths want to format multiple abbreviated sha1s in
the same output line. Because we use a single static buffer
for our return value, they have to either break their output
into several calls or allocate their own arrays and use
find_unique_abbrev_r().

Intead, let's mimic sha1_to_hex() and use a ring of several
buffers, so that the return value stays valid through
multiple calls. This shortens some of the callers, and makes
it harder to for them to make a silly mistake.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1374,12 +1374,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		struct commit *commit;
 
 		if (verbosity >= 0) {
-			char from[GIT_SHA1_HEXSZ + 1], to[GIT_SHA1_HEXSZ + 1];
-			find_unique_abbrev_r(from, head_commit->object.oid.hash,
-					      DEFAULT_ABBREV);
-			find_unique_abbrev_r(to, remoteheads->item->object.oid.hash,
-					      DEFAULT_ABBREV);
-			printf(_("Updating %s..%s\n"), from, to);
+			printf(_("Updating %s..%s\n"),
+			       find_unique_abbrev(head_commit->object.oid.hash,
+						  DEFAULT_ABBREV),
+			       find_unique_abbrev(remoteheads->item->object.oid.hash,
+						  DEFAULT_ABBREV));
 		}
 		strbuf_addstr(&msg, "Fast-forward");
 		if (have_message)

builtin/receive-pack.c

@@ -1163,10 +1163,6 @@ static void check_aliased_update(struct command *cmd, struct string_list *list)
 	struct string_list_item *item;
 	struct command *dst_cmd;
 	unsigned char sha1[GIT_SHA1_RAWSZ];
-	char cmd_oldh[GIT_SHA1_HEXSZ + 1],
-	     cmd_newh[GIT_SHA1_HEXSZ + 1],
-	     dst_oldh[GIT_SHA1_HEXSZ + 1],
-	     dst_newh[GIT_SHA1_HEXSZ + 1];
 	int flag;
 
 	strbuf_addf(&buf, "%s%s", get_git_namespace(), cmd->ref_name);
@@ -1197,14 +1193,14 @@ static void check_aliased_update(struct command *cmd, struct string_list *list)
 
 	dst_cmd->skip_update = 1;
 
-	find_unique_abbrev_r(cmd_oldh, cmd->old_sha1, DEFAULT_ABBREV);
-	find_unique_abbrev_r(cmd_newh, cmd->new_sha1, DEFAULT_ABBREV);
-	find_unique_abbrev_r(dst_oldh, dst_cmd->old_sha1, DEFAULT_ABBREV);
-	find_unique_abbrev_r(dst_newh, dst_cmd->new_sha1, DEFAULT_ABBREV);
 	rp_error("refusing inconsistent update between symref '%s' (%s..%s) and"
 		 " its target '%s' (%s..%s)",
-		 cmd->ref_name, cmd_oldh, cmd_newh,
-		 dst_cmd->ref_name, dst_oldh, dst_newh);
+		 cmd->ref_name,
+		 find_unique_abbrev(cmd->old_sha1, DEFAULT_ABBREV),
+		 find_unique_abbrev(cmd->new_sha1, DEFAULT_ABBREV),
+		 dst_cmd->ref_name,
+		 find_unique_abbrev(dst_cmd->old_sha1, DEFAULT_ABBREV),
+		 find_unique_abbrev(dst_cmd->new_sha1, DEFAULT_ABBREV));
 
 	cmd->error_string = dst_cmd->error_string =
 		"inconsistent aliased update";

cache.h

@@ -903,8 +903,8 @@ extern char *sha1_pack_index_name(const unsigned char *sha1);
  * The result will be at least `len` characters long, and will be NUL
  * terminated.
  *
- * The non-`_r` version returns a static buffer which will be overwritten by
- * subsequent calls.
+ * The non-`_r` version returns a static buffer which remains valid until 4
+ * more calls to find_unique_abbrev are made.
  *
  * The `_r` variant writes to a buffer supplied by the caller, which must be at
  * least `GIT_SHA1_HEXSZ + 1` bytes. The return value is the number of bytes

sha1_name.c

@@ -472,7 +472,9 @@ int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
 
 const char *find_unique_abbrev(const unsigned char *sha1, int len)
 {
-	static char hex[GIT_SHA1_HEXSZ + 1];
+	static int bufno;
+	static char hexbuffer[4][GIT_SHA1_HEXSZ + 1];
+	char *hex = hexbuffer[3 & ++bufno];
 	find_unique_abbrev_r(hex, sha1, len);
 	return hex;
 }