gc: config option for running --auto in background

`gc --auto` takes time and can block the user temporarily (but not any
less annoyingly). Make it run in background on systems that support
it. The only thing lost with running in background is printouts. But
gc output is not really interesting. You can keep it in foreground by
changing gc.autodetach.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1167,6 +1167,10 @@ gc.autopacklimit::
 	--auto` consolidates them into one larger pack.  The
 	default	value is 50.  Setting this to 0 disables it.
 
+gc.autodetach::
+	Make `git gc --auto` return immediately andrun in background
+	if the system supports it. Default is true.
+
 gc.packrefs::
 	Running `git pack-refs` in a repository renders it
 	unclonable by Git versions prior to 1.5.1.2 over dumb

builtin/gc.c

@@ -29,6 +29,7 @@ static int pack_refs = 1;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
 static int gc_auto_pack_limit = 50;
+static int detach_auto = 1;
 static const char *prune_expire = "2.weeks.ago";
 
 static struct argv_array pack_refs_cmd = ARGV_ARRAY_INIT;
@@ -73,6 +74,10 @@ static int gc_config(const char *var, const char *value, void *cb)
 		gc_auto_pack_limit = git_config_int(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "gc.autodetach")) {
+		detach_auto = git_config_bool(var, value);
+		return 0;
+	}
 	if (!strcmp(var, "gc.pruneexpire")) {
 		if (value && strcmp(value, "now")) {
 			unsigned long now = approxidate("now");
@@ -301,11 +306,19 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		 */
 		if (!need_to_gc())
 			return 0;
-		if (!quiet)
-			fprintf(stderr,
-					_("Auto packing the repository for optimum performance. You may also\n"
-					"run \"git gc\" manually. See "
-					"\"git help gc\" for more information.\n"));
+		if (!quiet) {
+			if (detach_auto)
+				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 (detach_auto)
+			/*
+			 * failure to daemonize is ok, we'll continue
+			 * in foreground
+			 */
+			daemonize();
 	} else
 		add_repack_all_option();
 

t/t5400-send-pack.sh

@@ -164,6 +164,7 @@ test_expect_success 'receive-pack runs auto-gc in remote repo' '
 	    # Set the child to auto-pack if more than one pack exists
 	    cd child &&
 	    git config gc.autopacklimit 1 &&
+	    git config gc.autodetach false &&
 	    git branch test_auto_gc &&
 	    # And create a file that follows the temporary object naming
 	    # convention for the auto-gc to remove