gc: do not explode objects which will be immediately pruned

When we pack everything into one big pack with "git repack
-Ad", any unreferenced objects in to-be-deleted packs are
exploded into loose objects, with the intent that they will
be examined and possibly cleaned up by the next run of "git
prune".

Since the exploded objects will receive the mtime of the
pack from which they come, if the source pack is old, those
loose objects will end up pruned immediately. In that case,
it is much more efficient to skip the exploding step
entirely for these objects.

This patch teaches pack-objects to receive the expiration
information and avoid writing these objects out. It also
teaches "git gc" to pass the value of gc.pruneexpire to
repack (which in turn learns to pass it along to
pack-objects) so that this optimization happens
automatically during "git gc" and "git gc --auto".

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 15 insertions, 8 deletions

diff --git a/builtin/gc.c b/builtin/gc.c
index 271376d82b..1bc2fe336e 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -144,6 +144,19 @@ static int too_many_packs(void)
 	return gc_auto_pack_limit <= cnt;
 }
 
+static void add_repack_all_option(void)
+{
+	if (prune_expire && !strcmp(prune_expire, "now"))
+		append_option(argv_repack, "-a", MAX_ADD);
+	else {
+		append_option(argv_repack, "-A", MAX_ADD);
+		if (prune_expire) {
+			append_option(argv_repack, "--unpack-unreachable", MAX_ADD);
+			append_option(argv_repack, prune_expire, MAX_ADD);
+		}
+	}
+}
+
 static int need_to_gc(void)
 {
 	/*
@@ -160,10 +173,7 @@ static int need_to_gc(void)
 	 * there is no need.
 	 */
 	if (too_many_packs())
-		append_option(argv_repack,
-			      prune_expire && !strcmp(prune_expire, "now") ?
-			      "-a" : "-A",
-			      MAX_ADD);
+		add_repack_all_option();
 	else if (!too_many_loose_objects())
 		return 0;
 
@@ -227,10 +237,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 					"run \"git gc\" manually. See "
 					"\"git help gc\" for more information.\n"));
 	} else
-		append_option(argv_repack,
-			      prune_expire && !strcmp(prune_expire, "now")
-			      ? "-a" : "-A",
-			      MAX_ADD);
+		add_repack_all_option();
 
 	if (pack_refs && run_command_v_opt(argv_pack_refs, RUN_GIT_CMD))
 		return error(FAILED_RUN, argv_pack_refs[0]);