Merge branch 'tb/rerere-wait-for-merge-rr-lock'

Instead of failing to record conflicts to be resolved immediately,
wait while "rerere gc" is ongoing.

* tb/rerere-wait-for-merge-rr-lock:
  rerere: add "gc --skip-locked" for auto maintenance
  rerere: wait for MERGE_RR.lock before giving up

Documentation/config/rerere.adoc

@@ -10,3 +10,12 @@ rerere.enabled::
 	enabled if there is an `rr-cache` directory under the
 	`$GIT_DIR`, e.g. if "rerere" was previously used in the
 	repository.
+
+rerere.lockTimeout::
+	The length of time, in milliseconds, to wait for the rerere
+	lock when another process holds it, typically a background
+	`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).  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.

builtin/gc.c

@@ -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);
 }
 

builtin/rerere.c

@@ -60,16 +60,21 @@ 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)
@@ -98,7 +103,8 @@ 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)

rerere.c

@@ -33,6 +33,9 @@ static int rerere_enabled = -1;
 /* automatically update cleanly resolved paths to the index */
 static int rerere_autoupdate;
 
+/* how long to wait for MERGE_RR.lock, in milliseconds */
+static int rerere_lock_timeout_ms = 1000;
+
 #define RR_HAS_POSTIMAGE 1
 #define RR_HAS_PREIMAGE 2
 struct rerere_dir {
@@ -872,6 +875,8 @@ static void git_rerere_config(void)
 {
 	repo_config_get_bool(the_repository, "rerere.enabled", &rerere_enabled);
 	repo_config_get_bool(the_repository, "rerere.autoupdate", &rerere_autoupdate);
+	repo_config_get_int(the_repository, "rerere.locktimeout",
+			    &rerere_lock_timeout_ms);
 	repo_config(the_repository, git_default_config, NULL);
 }
 
@@ -904,12 +909,30 @@ 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)
+	if ((flags & RERERE_READONLY) && (flags & RERERE_NOWAIT))
+		BUG("RERERE_NOWAIT does not apply with RERERE_READONLY");
+	if (flags & RERERE_READONLY) {
 		fd = 0;
-	else
-		fd = repo_hold_lock_file_for_update(r, &write_lock,
+	} else {
+		int lock_flags = LOCK_DIE_ON_ERROR;
+		int timeout_ms = rerere_lock_timeout_ms;
+
+		if (flags & RERERE_NOWAIT) {
+			lock_flags = 0;
+			timeout_ms = 0;
+		}
+
+		/*
+		 * Another process may hold the lock for a while, e.g.
+		 * "git rerere gc" while it prunes rr-cache, so by default
+		 * wait for it instead of dying right away.
+		 */
+		fd = repo_hold_lock_file_for_update_timeout(r, &write_lock,
 							    git_path_merge_rr(r),
-						    LOCK_DIE_ON_ERROR);
+							    lock_flags, timeout_ms);
+		if (fd < 0)
+			return -1;
+	}
 	read_rr(r, merge_rr);
 	return fd;
 }
@@ -1293,7 +1316,7 @@ 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;
@@ -1303,7 +1326,7 @@ 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);

rerere.h

@@ -10,6 +10,8 @@ struct repository;
 #define RERERE_AUTOUPDATE   01
 #define RERERE_NOAUTOUPDATE 02
 #define RERERE_READONLY     04
+/* If MERGE_RR.lock is taken, return -1 as if rerere were disabled */
+#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

t/t4200-rerere.sh

@@ -242,6 +242,85 @@ 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
+'
+
+test_expect_success 'gc --skip-locked prunes while MERGE_RR is not locked' '
+	mkdir -p $rr2 &&
+	echo Hello >$rr2/preimage &&
+	test-tool chmtime =$just_over_15_days_ago $rr2/preimage &&
+
+	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 &&
+	test_when_finished "wait && rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	{
+		( sleep 1 && rm -f .git/MERGE_RR.lock ) &
+	} &&
+	test_must_fail git -c rerere.lockTimeout=5000 merge first 2>err &&
+	test_grep ! "Unable to create.*MERGE_RR\.lock" err &&
+	test_grep "^=======\$" $rr/preimage
+'
+
+test_expect_success 'merge fails on a lock it cannot take' '
+	git reset --hard &&
+	rm -rf $rr &&
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 merge first 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_grep "^=======\$" a1 &&
+	test_path_is_missing $rr/preimage
+'
+
+test_expect_success 'rerere, forget, clear and gc fail on a lock they cannot take' '
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere forget a1 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere clear 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_must_fail git -c rerere.lockTimeout=0 rerere gc 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err
+'
+
+test_expect_success 'rebase --abort fails on a lock it cannot take' '
+	git reset --hard &&
+	git checkout -b lock-held-abort third &&
+	test_when_finished "git checkout third && git branch -D lock-held-abort" &&
+	test_must_fail git rebase first &&
+	test_when_finished "test_might_fail git rebase --abort" &&
+	test_when_finished "rm -f .git/MERGE_RR.lock" &&
+	>.git/MERGE_RR.lock &&
+	test_must_fail git -c rerere.lockTimeout=0 rebase --abort 2>err &&
+	test_grep "Unable to create.*MERGE_RR\.lock" err &&
+	test_path_is_dir .git/rebase-merge &&
+	rm .git/MERGE_RR.lock &&
+	git rebase --abort &&
+	test_path_is_missing .git/rebase-merge
+'
+
 rerere_gc_custom_expiry_test () {
 	five_days="$1" right_now="$2"
 	test_expect_success "rerere gc with custom expiry ($five_days, $right_now)" '

t/t7900-maintenance.sh

@@ -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 +1092,22 @@ 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" &&
+	# Without "rr-cache", rerere is disabled and never takes the 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.*MERGE_RR\.lock" 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