gc --auto: do not lock refs in the background

9f673f9 (gc: config option for running --auto in background -
2014-02-08) puts "gc --auto" in background to reduce user's wait
time. Part of the garbage collecting is pack-refs and pruning
reflogs. These require locking some refs and may abort other processes
trying to lock the same ref. If gc --auto is fired in the middle of a
script, gc's holding locks in the background could fail the script,
which could never happen before 9f673f9.

Keep running pack-refs and "reflog --prune" in foreground to stop
parallel ref updates. The remaining background operations (repack,
prune and rerere) should not impact running git processes.

Reported-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/gc.c

@@ -26,6 +26,7 @@ static const char * const builtin_gc_usage[] = {
 };
 
 static int pack_refs = 1;
+static int prune_reflogs = 1;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
 static int gc_auto_pack_limit = 50;
@@ -252,6 +253,19 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 	return NULL;
 }
 
+static int gc_before_repack(void)
+{
+	if (pack_refs && run_command_v_opt(pack_refs_cmd.argv, RUN_GIT_CMD))
+		return error(FAILED_RUN, pack_refs_cmd.argv[0]);
+
+	if (prune_reflogs && run_command_v_opt(reflog.argv, RUN_GIT_CMD))
+		return error(FAILED_RUN, reflog.argv[0]);
+
+	pack_refs = 0;
+	prune_reflogs = 0;
+	return 0;
+}
+
 int cmd_gc(int argc, const char **argv, const char *prefix)
 {
 	int aggressive = 0;
@@ -313,12 +327,15 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 				fprintf(stderr, _("Auto packing the repository for optimum performance.\n"));
 			fprintf(stderr, _("See \"git help gc\" for manual housekeeping.\n"));
 		}
-		if (detach_auto)
+		if (detach_auto) {
+			if (gc_before_repack())
+				return -1;
 			/*
 			 * failure to daemonize is ok, we'll continue
 			 * in foreground
 			 */
 			daemonize();
+		}
 	} else
 		add_repack_all_option();
 
@@ -330,11 +347,8 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		    name, (uintmax_t)pid);
 	}
 
-	if (pack_refs && run_command_v_opt(pack_refs_cmd.argv, RUN_GIT_CMD))
-		return error(FAILED_RUN, pack_refs_cmd.argv[0]);
-
-	if (run_command_v_opt(reflog.argv, RUN_GIT_CMD))
-		return error(FAILED_RUN, reflog.argv[0]);
+	if (gc_before_repack())
+		return -1;
 
 	if (run_command_v_opt(repack.argv, RUN_GIT_CMD))
 		return error(FAILED_RUN, repack.argv[0]);