[PATCH v6 2/3] rerere: add "gc --skip-locked" for auto maintenance

@@ -17,4 +17,6 @@ rerere.lockTimeout::
 	`git rerere gc`.  Value 0 means not to wait at all; -1 means
 	to wait indefinitely.  Default is 1000 (i.e., wait for 1
 	second).  When the time is up, the command fails as it does
-	for any other lock it cannot take.
+	for any other lock it cannot take.  A `git rerere gc` run by
+	`git maintenance run --auto` or `git gc --auto` does not wait
+	and does nothing while the lock is held.

@@ -385,12 +385,14 @@ out:
 	return should_prune;
 }
 
-static int maintenance_task_rerere_gc(struct maintenance_run_opts *opts UNUSED,
+static int maintenance_task_rerere_gc(struct maintenance_run_opts *opts,
 				      struct gc_config *cfg UNUSED)
 {
 	struct child_process rerere_cmd = CHILD_PROCESS_INIT;
 	rerere_cmd.git_cmd = 1;
 	strvec_pushl(&rerere_cmd.args, "rerere", "gc", NULL);
+	if (opts->auto_flag)
+		strvec_push(&rerere_cmd.args, "--skip-locked");
 	return run_command(&rerere_cmd);
 }
 

@@ -56,16 +56,21 @@ int cmd_rerere(int argc,
 	       struct repository *repo UNUSED)
 {
 	struct string_list merge_rr = STRING_LIST_INIT_DUP;
-	int autoupdate = -1, flags = 0;
+	int autoupdate = -1, skip_locked = 0, flags = 0;
 
 	struct option options[] = {
 		OPT_SET_INT(0, "rerere-autoupdate", &autoupdate,
 			N_("register clean resolutions in index"), 1),
+		OPT_HIDDEN_BOOL(0, "skip-locked", &skip_locked,
+			N_("skip gc while another process holds the lock")),
 		OPT_END(),
 	};
 
 	argc = parse_options(argc, argv, prefix, options, rerere_usage, 0);
 
+	if (skip_locked && (argc < 1 || strcmp(argv[0], "gc")))
+		die(_("the option '%s' requires '%s'"), "--skip-locked", "gc");
+
 	repo_config(the_repository, git_xmerge_config, NULL);
 
 	if (autoupdate == 1)

@@ -94,7 +99,8 @@ int cmd_rerere(int argc,
 	if (!strcmp(argv[0], "clear")) {
 		rerere_clear(the_repository, &merge_rr);
 	} else if (!strcmp(argv[0], "gc"))
-		rerere_gc(the_repository, &merge_rr);
+		rerere_gc(the_repository, &merge_rr,
+			  skip_locked ? RERERE_NOWAIT : 0);
 	else if (!strcmp(argv[0], "status")) {
 		if (setup_rerere(the_repository, &merge_rr,
 				 flags | RERERE_READONLY) < 0)

@@ -887,18 +887,30 @@ int setup_rerere(struct repository *r, struct string_list *merge_rr, int flags)
 
 	if (flags & (RERERE_AUTOUPDATE|RERERE_NOAUTOUPDATE))
 		rerere_autoupdate = !!(flags & RERERE_AUTOUPDATE);
+	if ((flags & RERERE_READONLY) && (flags & RERERE_NOWAIT))
+		BUG("RERERE_NOWAIT does not apply with RERERE_READONLY");
 	if (flags & RERERE_READONLY) {
 		fd = 0;
 	} else {
+		int lock_flags = LOCK_DIE_ON_ERROR;
+		int timeout_ms = rerere_lock_timeout_ms;
+
 		/*
 		 * Another process may hold the lock for a while, e.g.
 		 * "git rerere gc" while it prunes rr-cache, so wait for
-		 * it instead of dying right away.
+		 * it instead of dying right away.  The gc of an automatic
+		 * maintenance run does not wait, since skipping one of
+		 * its runs costs nothing.
 		 */
+		if (flags & RERERE_NOWAIT) {
+			lock_flags = 0;
+			timeout_ms = 0;
+		}
 		fd = repo_hold_lock_file_for_update_timeout(r, &write_lock,
 							    git_path_merge_rr(r),
-							    LOCK_DIE_ON_ERROR,
-							    rerere_lock_timeout_ms);
+							    lock_flags, timeout_ms);
+		if (fd < 0)
+			return -1;
 	}
 	read_rr(r, merge_rr);
 	return fd;

@@ -1284,7 +1296,7 @@ out:
 	return needed;
 }
 
-void rerere_gc(struct repository *r, struct string_list *rr)
+void rerere_gc(struct repository *r, struct string_list *rr, int flags)
 {
 	struct string_list to_remove = STRING_LIST_INIT_DUP;
 	DIR *dir;

@@ -1294,7 +1306,7 @@ void rerere_gc(struct repository *r, struct string_list *rr)
 	timestamp_t cutoff_resolve;
 	struct strbuf buf = STRBUF_INIT;
 
-	if (setup_rerere(r, rr, 0) < 0)
+	if (setup_rerere(r, rr, flags) < 0)
 		return;
 
 	rerere_gc_cutoffs(r, &cutoff_resolve, &cutoff_noresolve);

@@ -10,6 +10,8 @@ struct repository;
 #define RERERE_AUTOUPDATE   01
 #define RERERE_NOAUTOUPDATE 02
 #define RERERE_READONLY     04
+/* Take MERGE_RR.lock only if it is free, and return quietly otherwise */
+#define RERERE_NOWAIT       010
 
 /*
  * Marks paths that have been hand-resolved and added to the

@@ -37,7 +39,7 @@ const char *rerere_path(struct strbuf *buf, const struct rerere_id *,
 int rerere_forget(struct repository *, struct pathspec *);
 int rerere_remaining(struct repository *, struct string_list *);
 void rerere_clear(struct repository *, struct string_list *);
-void rerere_gc(struct repository *, struct string_list *);
+void rerere_gc(struct repository *, struct string_list *, int);
 
 /*
  * Check whether garbage collection for rerere entries is needed, which is

@@ -242,6 +242,27 @@ test_expect_success 'old records rest in peace' '
 	test_path_is_missing $rr2/preimage
 '
 
+test_expect_success 'gc --skip-locked does nothing while MERGE_RR is locked' '
+	mkdir -p $rr2 &&
+	echo Hello >$rr2/preimage &&
+	test-tool chmtime =$just_over_15_days_ago $rr2/preimage &&
+
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	git rerere gc --skip-locked 2>err &&
+	test_must_be_empty err &&
+	test_path_is_file $rr2/preimage &&
+
+	rm .git/MERGE_RR.lock &&
+	git rerere gc --skip-locked &&
+	test_path_is_missing $rr2/preimage
+'
+
+test_expect_success '--skip-locked is only accepted by gc' '
+	test_must_fail git rerere --skip-locked clear 2>err &&
+	test_grep "option .--skip-locked. requires .gc." err
+'
+
 test_expect_success 'a held lock is waited out within rerere.lockTimeout' '
 	git reset --hard &&
 	rm -rf $rr &&

@@ -1007,9 +1007,17 @@ test_expect_rerere_gc () {
 		shift
 	fi
 
+	# An automatic run passes --skip-locked to "git rerere gc".
+	skip_locked=
+	case " $* " in
+	*" --auto "*)
+		skip_locked=--skip-locked
+		;;
+	esac
+
 	rm -f "rerere-gc.txt" &&
 	GIT_TRACE2_EVENT="$(pwd)/rerere-gc.txt" "$@" &&
-	test_subcommand $negate git rerere gc <rerere-gc.txt
+	test_subcommand $negate git rerere gc $skip_locked <rerere-gc.txt
 }
 
 test_expect_success 'rerere-gc task without --auto always collects garbage' '

@@ -1084,6 +1092,21 @@ test_expect_success 'rerere-gc task with --auto honors maintenance.rerere-gc.aut
 	test_expect_rerere_gc ! git -c maintenance.rerere-gc.auto=0 maintenance run --auto --task=rerere-gc
 '
 
+test_expect_success 'rerere-gc task with --auto succeeds while MERGE_RR is locked' '
+	test_when_finished "rm -rf .git/rr-cache .git/MERGE_RR.lock" &&
+	mkdir .git/rr-cache &&
+	>.git/MERGE_RR.lock &&
+	test_expect_rerere_gc git -c maintenance.rerere-gc.auto=-1 maintenance run --auto --task=rerere-gc
+'
+
+test_expect_success 'rerere-gc task without --auto fails while MERGE_RR is locked' '
+	test_when_finished "rm -rf .git/rr-cache .git/MERGE_RR.lock" &&
+	mkdir .git/rr-cache &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 maintenance run --task=rerere-gc 2>err &&
+	test_grep "Unable to create" err
+'
+
 test_expect_success '--auto and --schedule incompatible' '
 	test_must_fail git maintenance run --auto --schedule=daily 2>err &&
 	test_grep "cannot be used together" err