builtin/gc: add a --detach flag

When running `git gc --auto`, the command will by default detach and
continue running in the background. This behaviour can be tweaked via
the `gc.autoDetach` config, but not via a command line switch. We need
that in a subsequent commit though, where git-maintenance(1) will want
to ask its git-gc(1) child process to not detach anymore.

Add a `--[no-]detach` flag that does this for us.

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

Documentation/git-gc.txt

@@ -9,7 +9,7 @@ git-gc - Cleanup unnecessary files and optimize the local repository
 SYNOPSIS
 --------
 [verse]
-'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune] [--force] [--keep-largest-pack]
+'git gc' [--aggressive] [--auto] [--[no-]detach] [--quiet] [--prune=<date> | --no-prune] [--force] [--keep-largest-pack]
 
 DESCRIPTION
 -----------
@@ -53,6 +53,9 @@ configuration options such as `gc.auto` and `gc.autoPackLimit`, all
 other housekeeping tasks (e.g. rerere, working trees, reflog...) will
 be performed as well.
 
+--[no-]detach::
+	Run in the background if the system supports it. This option overrides
+	the `gc.autoDetach` config.
 
 --[no-]cruft::
 	When expiring unreachable objects, pack them separately into a

builtin/gc.c

@@ -242,9 +242,13 @@ static enum schedule_priority parse_schedule(const char *value)
 
 struct maintenance_run_opts {
 	int auto_flag;
+	int detach;
 	int quiet;
 	enum schedule_priority schedule;
 };
+#define MAINTENANCE_RUN_OPTS_INIT { \
+	.detach = -1, \
+}
 
 static int pack_refs_condition(UNUSED struct gc_config *cfg)
 {
@@ -664,7 +668,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	int keep_largest_pack = -1;
 	timestamp_t dummy;
 	struct child_process rerere_cmd = CHILD_PROCESS_INIT;
-	struct maintenance_run_opts opts = {0};
+	struct maintenance_run_opts opts = MAINTENANCE_RUN_OPTS_INIT;
 	struct gc_config cfg = GC_CONFIG_INIT;
 	const char *prune_expire_sentinel = "sentinel";
 	const char *prune_expire_arg = prune_expire_sentinel;
@@ -681,6 +685,8 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		OPT_BOOL(0, "aggressive", &aggressive, N_("be more thorough (increased runtime)")),
 		OPT_BOOL_F(0, "auto", &opts.auto_flag, N_("enable auto-gc mode"),
 			   PARSE_OPT_NOCOMPLETE),
+		OPT_BOOL(0, "detach", &opts.detach,
+			 N_("perform garbage collection in the background")),
 		OPT_BOOL_F(0, "force", &force,
 			   N_("force running gc even if there may be another gc running"),
 			   PARSE_OPT_NOCOMPLETE),
@@ -729,6 +735,9 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		strvec_push(&repack, "-q");
 
 	if (opts.auto_flag) {
+		if (cfg.detach_auto && opts.detach < 0)
+			opts.detach = 1;
+
 		/*
 		 * Auto-gc should be least intrusive as possible.
 		 */
@@ -738,38 +747,12 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		}
 
 		if (!quiet) {
-			if (cfg.detach_auto)
+			if (opts.detach > 0)
 				fprintf(stderr, _("Auto packing the repository in background for optimum performance.\n"));
 			else
 				fprintf(stderr, _("Auto packing the repository for optimum performance.\n"));
 			fprintf(stderr, _("See \"git help gc\" for manual housekeeping.\n"));
 		}
-		if (cfg.detach_auto) {
-			ret = report_last_gc_error();
-			if (ret == 1) {
-				/* Last gc --auto failed. Skip this one. */
-				ret = 0;
-				goto out;
-
-			} else if (ret) {
-				/* an I/O error occurred, already reported */
-				goto out;
-			}
-
-			if (lock_repo_for_gc(force, &pid)) {
-				ret = 0;
-				goto out;
-			}
-
-			gc_before_repack(&opts, &cfg); /* dies on failure */
-			delete_tempfile(&pidfile);
-
-			/*
-			 * failure to daemonize is ok, we'll continue
-			 * in foreground
-			 */
-			daemonized = !daemonize();
-		}
 	} else {
 		struct string_list keep_pack = STRING_LIST_INIT_NODUP;
 
@@ -784,6 +767,33 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		string_list_clear(&keep_pack, 0);
 	}
 
+	if (opts.detach > 0) {
+		ret = report_last_gc_error();
+		if (ret == 1) {
+			/* Last gc --auto failed. Skip this one. */
+			ret = 0;
+			goto out;
+
+		} else if (ret) {
+			/* an I/O error occurred, already reported */
+			goto out;
+		}
+
+		if (lock_repo_for_gc(force, &pid)) {
+			ret = 0;
+			goto out;
+		}
+
+		gc_before_repack(&opts, &cfg); /* dies on failure */
+		delete_tempfile(&pidfile);
+
+		/*
+		 * failure to daemonize is ok, we'll continue
+		 * in foreground
+		 */
+		daemonized = !daemonize();
+	}
+
 	name = lock_repo_for_gc(force, &pid);
 	if (name) {
 		if (opts.auto_flag) {
@@ -1537,7 +1547,7 @@ static int task_option_parse(const struct option *opt UNUSED,
 static int maintenance_run(int argc, const char **argv, const char *prefix)
 {
 	int i;
-	struct maintenance_run_opts opts;
+	struct maintenance_run_opts opts = MAINTENANCE_RUN_OPTS_INIT;
 	struct gc_config cfg = GC_CONFIG_INIT;
 	struct option builtin_maintenance_run_options[] = {
 		OPT_BOOL(0, "auto", &opts.auto_flag,
@@ -1554,8 +1564,6 @@ static int maintenance_run(int argc, const char **argv, const char *prefix)
 	};
 	int ret;
 
-	memset(&opts, 0, sizeof(opts));
-
 	opts.quiet = !isatty(2);
 
 	for (i = 0; i < TASK__COUNT; i++)

t/t6500-gc.sh

@@ -338,14 +338,14 @@ test_expect_success 'gc.maxCruftSize sets appropriate repack options' '
 	test_subcommand $cruft_max_size_opts --max-cruft-size=3145728 <trace2.txt
 '
 
-run_and_wait_for_auto_gc () {
+run_and_wait_for_gc () {
 	# We read stdout from gc for the side effect of waiting until the
 	# background gc process exits, closing its fd 9.  Furthermore, the
 	# variable assignment from a command substitution preserves the
 	# exit status of the main gc process.
 	# Note: this fd trickery doesn't work on Windows, but there is no
 	# need to, because on Win the auto gc always runs in the foreground.
-	doesnt_matter=$(git gc --auto 9>&1)
+	doesnt_matter=$(git gc "$@" 9>&1)
 }
 
 test_expect_success 'background auto gc does not run if gc.log is present and recent but does if it is old' '
@@ -361,7 +361,7 @@ test_expect_success 'background auto gc does not run if gc.log is present and re
 	test-tool chmtime =-345600 .git/gc.log &&
 	git gc --auto &&
 	test_config gc.logexpiry 2.days &&
-	run_and_wait_for_auto_gc &&
+	run_and_wait_for_gc --auto &&
 	ls .git/objects/pack/pack-*.pack >packs &&
 	test_line_count = 1 packs
 '
@@ -391,11 +391,48 @@ test_expect_success 'background auto gc respects lock for all operations' '
 	printf "%d %s" "$shell_pid" "$hostname" >.git/gc.pid &&
 
 	# our gc should exit zero without doing anything
-	run_and_wait_for_auto_gc &&
+	run_and_wait_for_gc --auto &&
 	(ls -1 .git/refs/heads .git/reftable >actual || true) &&
 	test_cmp expect actual
 '
 
+test_expect_success '--detach overrides gc.autoDetach=false' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		# Prepare the repository such that git-gc(1) ends up repacking.
+		test_commit "$(test_oid blob17_1)" &&
+		test_commit "$(test_oid blob17_2)" &&
+		git config gc.autodetach false &&
+		git config gc.auto 2 &&
+
+		# Note that we cannot use `test_cmp` here to compare stderr
+		# because it may contain output from `set -x`.
+		run_and_wait_for_gc --auto --detach 2>actual &&
+		test_grep "Auto packing the repository in background for optimum performance." actual
+	)
+'
+
+test_expect_success '--no-detach overrides gc.autoDetach=true' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		# Prepare the repository such that git-gc(1) ends up repacking.
+		test_commit "$(test_oid blob17_1)" &&
+		test_commit "$(test_oid blob17_2)" &&
+		git config gc.autodetach true &&
+		git config gc.auto 2 &&
+
+		GIT_PROGRESS_DELAY=0 git gc --auto --no-detach 2>output &&
+		test_grep "Auto packing the repository for optimum performance." output &&
+		test_grep "Collecting referenced commits: 2, done." output
+	)
+'
+
 # DO NOT leave a detached auto gc process running near the end of the
 # test script: it can run long enough in the background to racily
 # interfere with the cleanup in 'test_done'.