Re: [PATCH v2 2/2] 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 <redacted>
---
 Documentation/config.txt |  4 ++++
 builtin/gc.c             | 23 ++++++++++++++++++-----
 t/t5400-send-pack.sh     |  1 +
 3 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 5f4d793..4781773 100644
--- a/Documentation/config.txt
+++ b/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

diff --git a/builtin/gc.c b/builtin/gc.c
index c19545d..ed5cc3c 100644
--- a/builtin/gc.c
+++ b/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();