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>

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]);

builtin/pack-objects.c

@@ -63,6 +63,7 @@ static uint32_t nr_objects, nr_alloc, nr_result, nr_written;
 static int non_empty;
 static int reuse_delta = 1, reuse_object = 1;
 static int keep_unreachable, unpack_unreachable, include_tag;
+static unsigned long unpack_unreachable_expiration;
 static int local;
 static int incremental;
 static int ignore_packed_keep;
@@ -2249,6 +2250,10 @@ static void loosen_unused_packed_objects(struct rev_info *revs)
 		if (!p->pack_local || p->pack_keep)
 			continue;
 
+		if (unpack_unreachable_expiration &&
+		    p->mtime < unpack_unreachable_expiration)
+			continue;
+
 		if (open_pack_index(p))
 			die("cannot open pack index");
 
@@ -2315,6 +2320,21 @@ static int option_parse_index_version(const struct option *opt,
 	return 0;
 }
 
+static int option_parse_unpack_unreachable(const struct option *opt,
+					   const char *arg, int unset)
+{
+	if (unset) {
+		unpack_unreachable = 0;
+		unpack_unreachable_expiration = 0;
+	}
+	else {
+		unpack_unreachable = 1;
+		if (arg)
+			unpack_unreachable_expiration = approxidate(arg);
+	}
+	return 0;
+}
+
 static int option_parse_ulong(const struct option *opt,
 			      const char *arg, int unset)
 {
@@ -2392,8 +2412,9 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			 "include tag objects that refer to objects to be packed"),
 		OPT_BOOL(0, "keep-unreachable", &keep_unreachable,
 			 "keep unreachable objects"),
-		OPT_BOOL(0, "unpack-unreachable", &unpack_unreachable,
-			 "unpack unreachable objects"),
+		{ OPTION_CALLBACK, 0, "unpack-unreachable", NULL, "time",
+		  "unpack unreachable objects newer than <time>",
+		  PARSE_OPT_OPTARG, option_parse_unpack_unreachable },
 		OPT_BOOL(0, "thin", &thin,
 			 "create thin packs"),
 		OPT_BOOL(0, "honor-pack-keep", &ignore_packed_keep,

git-repack.sh

@@ -15,6 +15,7 @@ F               pass --no-reuse-object to git-pack-objects
 n               do not run git-update-server-info
 q,quiet         be quiet
 l               pass --local to git-pack-objects
+unpack-unreachable=  with -A, do not loosen objects older than this
  Packing constraints
 window=         size of the window used for delta compression
 window-memory=  same as the above, but limit memory size instead of entries count
@@ -33,6 +34,8 @@ do
 	-a)	all_into_one=t ;;
 	-A)	all_into_one=t
 		unpack_unreachable=--unpack-unreachable ;;
+	--unpack-unreachable)
+		unpack_unreachable="--unpack-unreachable=$2"; shift ;;
 	-d)	remove_redundant=t ;;
 	-q)	GIT_QUIET=t ;;
 	-f)	no_reuse=--no-reuse-delta ;;
@@ -76,7 +79,12 @@ case ",$all_into_one," in
 		if test -n "$existing" -a -n "$unpack_unreachable" -a \
 			-n "$remove_redundant"
 		then
-			args="$args $unpack_unreachable"
+			# This may have arbitrary user arguments, so we
+			# have to protect it against whitespace splitting
+			# when it gets run as "pack-objects $args" later.
+			# Fortunately, we know it's an approxidate, so we
+			# can just use dots instead.
+			args="$args $(echo "$unpack_unreachable" | tr ' ' .)"
 		fi
 	fi
 	;;

t/t7701-repack-unpack-unreachable.sh

@@ -95,4 +95,18 @@ test_expect_success 'unpacked objects receive timestamp of pack file' '
 	compare_mtimes < mtimes
 '
 
+test_expect_success 'do not bother loosening old objects' '
+	obj1=$(echo one | git hash-object -w --stdin) &&
+	obj2=$(echo two | git hash-object -w --stdin) &&
+	pack1=$(echo $obj1 | git pack-objects .git/objects/pack/pack) &&
+	pack2=$(echo $obj2 | git pack-objects .git/objects/pack/pack) &&
+	git prune-packed &&
+	git cat-file -p $obj1 &&
+	git cat-file -p $obj2 &&
+	test-chmtime =-86400 .git/objects/pack/pack-$pack2.pack &&
+	git repack -A -d --unpack-unreachable=1.hour.ago &&
+	git cat-file -p $obj1 &&
+	test_must_fail git cat-file -p $obj2
+'
+
 test_done