builtin/maintenance: fix locking race when handling "gc" task

The "gc" task has a similar locking race as the one that we have fixed
for the "pack-refs" and "reflog-expire" tasks in preceding commits. Fix
this by splitting up the logic of the "gc" task:

  - We execute `gc_before_repack()` in the foreground, which contains
    the logic that git-gc(1) itself would execute in the foreground, as
    well.

  - We spawn git-gc(1) after detaching, but with a new hidden flag that
    suppresses calling `gc_before_repack()`.

Like this we have roughly the same logic as git-gc(1) itself and know to
repack refs and reflogs before detaching, thus fixing the race.

Note that `gc_before_repack()` is renamed to `gc_foreground_tasks()` to
better reflect what this function does.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

builtin/gc.c

@@ -816,8 +816,8 @@ static int report_last_gc_error(void)
 	return ret;
 }
 
-static int gc_before_repack(struct maintenance_run_opts *opts,
-			    struct gc_config *cfg)
+static int gc_foreground_tasks(struct maintenance_run_opts *opts,
+			       struct gc_config *cfg)
 {
 	if (cfg->pack_refs && maintenance_task_pack_refs(opts, cfg))
 		return error(FAILED_RUN, "pack-refs");
@@ -837,6 +837,7 @@ int cmd_gc(int argc,
 	pid_t pid;
 	int daemonized = 0;
 	int keep_largest_pack = -1;
+	int skip_foreground_tasks = 0;
 	timestamp_t dummy;
 	struct maintenance_run_opts opts = MAINTENANCE_RUN_OPTS_INIT;
 	struct gc_config cfg = GC_CONFIG_INIT;
@@ -869,6 +870,8 @@ int cmd_gc(int argc,
 			 N_("repack all other packs except the largest pack")),
 		OPT_STRING(0, "expire-to", &cfg.repack_expire_to, N_("dir"),
 			   N_("pack prefix to store a pack containing pruned objects")),
+		OPT_HIDDEN_BOOL(0, "skip-foreground-tasks", &skip_foreground_tasks,
+			   N_("skip maintenance tasks typically done in the foreground")),
 		OPT_END()
 	};
 
@@ -952,14 +955,16 @@ int cmd_gc(int argc,
 			goto out;
 		}
 
-		if (lock_repo_for_gc(force, &pid)) {
-			ret = 0;
-			goto out;
-		}
+		if (!skip_foreground_tasks) {
+			if (lock_repo_for_gc(force, &pid)) {
+				ret = 0;
+				goto out;
+			}
 
-		if (gc_before_repack(&opts, &cfg) < 0)
-			die(NULL);
-		delete_tempfile(&pidfile);
+			if (gc_foreground_tasks(&opts, &cfg) < 0)
+				die(NULL);
+			delete_tempfile(&pidfile);
+		}
 
 		/*
 		 * failure to daemonize is ok, we'll continue
@@ -988,8 +993,8 @@ int cmd_gc(int argc,
 		free(path);
 	}
 
-	if (opts.detach <= 0)
-		gc_before_repack(&opts, &cfg);
+	if (opts.detach <= 0 && !skip_foreground_tasks)
+		gc_foreground_tasks(&opts, &cfg);
 
 	if (!repository_format_precious_objects) {
 		struct child_process repack_cmd = CHILD_PROCESS_INIT;
@@ -1225,8 +1230,14 @@ static int maintenance_task_prefetch(struct maintenance_run_opts *opts,
 	return 0;
 }
 
-static int maintenance_task_gc(struct maintenance_run_opts *opts,
-			       struct gc_config *cfg UNUSED)
+static int maintenance_task_gc_foreground(struct maintenance_run_opts *opts,
+					  struct gc_config *cfg)
+{
+	return gc_foreground_tasks(opts, cfg);
+}
+
+static int maintenance_task_gc_background(struct maintenance_run_opts *opts,
+					  struct gc_config *cfg UNUSED)
 {
 	struct child_process child = CHILD_PROCESS_INIT;
 
@@ -1240,6 +1251,7 @@ static int maintenance_task_gc(struct maintenance_run_opts *opts,
 	else
 		strvec_push(&child.args, "--no-quiet");
 	strvec_push(&child.args, "--no-detach");
+	strvec_push(&child.args, "--skip-foreground-tasks");
 
 	return run_command(&child);
 }
@@ -1571,7 +1583,8 @@ static const struct maintenance_task tasks[] = {
 	},
 	[TASK_GC] = {
 		.name = "gc",
-		.background = maintenance_task_gc,
+		.foreground = maintenance_task_gc_foreground,
+		.background = maintenance_task_gc_background,
 		.auto_condition = need_to_gc,
 	},
 	[TASK_COMMIT_GRAPH] = {

t/t7900-maintenance.sh

@@ -49,9 +49,9 @@ test_expect_success 'run [--auto|--quiet]' '
 		git maintenance run --auto 2>/dev/null &&
 	GIT_TRACE2_EVENT="$(pwd)/run-no-quiet.txt" \
 		git maintenance run --no-quiet 2>/dev/null &&
-	test_subcommand git gc --quiet --no-detach <run-no-auto.txt &&
-	test_subcommand ! git gc --auto --quiet --no-detach <run-auto.txt &&
-	test_subcommand git gc --no-quiet --no-detach <run-no-quiet.txt
+	test_subcommand git gc --quiet --no-detach --skip-foreground-tasks <run-no-auto.txt &&
+	test_subcommand ! git gc --auto --quiet --no-detach --skip-foreground-tasks <run-auto.txt &&
+	test_subcommand git gc --no-quiet --no-detach --skip-foreground-tasks <run-no-quiet.txt
 '
 
 test_expect_success 'maintenance.auto config option' '
@@ -154,9 +154,9 @@ test_expect_success 'run --task=<task>' '
 		git maintenance run --task=commit-graph 2>/dev/null &&
 	GIT_TRACE2_EVENT="$(pwd)/run-both.txt" \
 		git maintenance run --task=commit-graph --task=gc 2>/dev/null &&
-	test_subcommand ! git gc --quiet --no-detach <run-commit-graph.txt &&
-	test_subcommand git gc --quiet --no-detach <run-gc.txt &&
-	test_subcommand git gc --quiet --no-detach <run-both.txt &&
+	test_subcommand ! git gc --quiet --no-detach --skip-foreground-tasks <run-commit-graph.txt &&
+	test_subcommand git gc --quiet --no-detach --skip-foreground-tasks <run-gc.txt &&
+	test_subcommand git gc --quiet --no-detach --skip-foreground-tasks <run-both.txt &&
 	test_subcommand git commit-graph write --split --reachable --no-progress <run-commit-graph.txt &&
 	test_subcommand ! git commit-graph write --split --reachable --no-progress <run-gc.txt &&
 	test_subcommand git commit-graph write --split --reachable --no-progress <run-both.txt