[PATCH 2/2] repack: avoid loosening promisor pack objects in partial clones

[Rafael Silva <rafaeloliveira.cs@gmail.com>]

When `-A` and `-d` are used together, besides packing all objects (-A)
and removing redundant packs (-d), it also unpack all unreachable
objects and deletes them by calling `git pruned-packed`. For a partial
clone, that contains unreferenced objects, this results in unpacking
all "promisor" objects and deleting them right after, which
unnecessarily increases the `repack` execution time and disk usage
during the unpacking of the objects.

For instance, a partially cloned repository that filters all the blob
objects (e.g. "--filter=blob:none"), `repack` ends up unpacking all
blobs into the filesystem that, depending on the repo size, makes
nearly impossible to repack the operation before running out of disk.

For a partial clone, `git repack` calls `git pack-objects` twice: (1)
for handle the "promisor" objects and (2) for performing the repack
with --exclude-promisor-objects option, that results in unpacking and
deleting of the objects. Given that we actually should keep the
promisor objects, let's teach `repack` to tell `pack-objects` to
--keep the old "promisor" pack file.

The --keep-pack option takes only a packfile name, but we concatenate
both the path and the name in a single string. Instead, let's split
them into separate string in order to easily pass the packfile name
later.

Additionally, add a new perf test to evaluate the performance
impact made by this changes (tested on git.git):

    Test            HEAD^                 HEAD
    ------------------------------------------------------------
    5600.5: gc      137.67(42.48+93.64)   8.08(6.91+1.45) -94.1%

In this particular script, the improvement is big because every
object in the newly-cloned partial repository is a promisor object.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Rafael Silva <rafaeloliveira.cs@gmail.com>
---
 builtin/repack.c              | 9 +++++++--
 t/perf/p5600-partial-clone.sh | 4 ++++
 t/t5616-partial-clone.sh      | 9 +++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index 6baaeb979c..0ecd76b79c 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c
@@ -20,7 +20,7 @@ static int delta_base_offset = 1;
 static int pack_kept_objects = -1;
 static int write_bitmaps = -1;
 static int use_delta_islands;
-static char *packdir, *packtmp;
+static char *packdir, *packtmp_name, *packtmp;
 
 static const char *const git_repack_usage[] = {
 	N_("git repack [<options>]"),
@@ -533,7 +533,8 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 	}
 
 	packdir = mkpathdup("%s/pack", get_object_directory());
-	packtmp = mkpathdup("%s/.tmp-%d-pack", packdir, (int)getpid());
+	packtmp_name = xstrfmt(".tmp-%d-pack", (int)getpid());
+	packtmp = mkpathdup("%s/%s", packdir, packtmp_name);
 
 	sigchain_push_common(remove_pack_on_signal);
 
@@ -576,6 +577,10 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		repack_promisor_objects(&po_args, &names);
 
 		if (existing_packs.nr && delete_redundant) {
+			for_each_string_list_item(item, &names) {
+				strvec_pushf(&cmd.args, "--keep-pack=%s-%s.pack",
+					     packtmp_name, item->string);
+			}
 			if (unpack_unreachable) {
 				strvec_pushf(&cmd.args,
 					     "--unpack-unreachable=%s",
diff --git a/t/perf/p5600-partial-clone.sh b/t/perf/p5600-partial-clone.sh
index ca785a3341..a965f2c4d6 100755
--- a/t/perf/p5600-partial-clone.sh
+++ b/t/perf/p5600-partial-clone.sh
@@ -35,4 +35,8 @@ test_perf 'count non-promisor commits' '
 	git -C bare.git rev-list --all --count --exclude-promisor-objects
 '
 
+test_perf 'gc' '
+	git -C bare.git gc
+'
+
 test_done
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 5cb415386e..de77822735 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -548,6 +548,15 @@ test_expect_success 'fetch from a partial clone, protocol v2' '
 	grep "version 2" trace
 '
 
+test_expect_success 'repack does not loose all objects' '
+	rm -rf client &&
+	git clone --bare --filter=blob:none "file://$(pwd)/srv.bare" client &&
+	test_when_finished "rm -rf client" &&
+	git -C client repack -A -l -d --no-prune-packed &&
+	git -C client count-objects -v >object-count &&
+	grep "^prune-packable: 0" object-count
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd
 
-- 
2.31.0.565.gcc42f43761