[PATCH v2 2/3] midx.c: use `pack-objects --stdin-packs` when repacking

[Taylor Blau <me@ttaylorr.com>]

The `git multi-pack-index repack` command aggregates all objects in
certain packs contained in the MIDX, largely dictated by the value of
its `--batch-size` parameter.

To create the new pack, the MIDX machinery spawns a `pack-objects`
sub-process, which it feeds the object IDs in each of the pack(s) that
it wants to aggregate.

This implementation, which dates back to ce1e4a105b (midx: implement
midx_repack(), 2019-06-10), predates the `--stdin-packs` mode in
`pack-objects`. This mode (which was introduced a couple of years later
in 339bce27f4 (builtin/pack-objects.c: add '--stdin-packs' option,
2021-02-22)) allows `pack-objects` callers to specify the contents of
the output pack by indicating which packs to aggregate.

This patch replaces the pre-`--stdin-packs` invocation (where each
object is given to `pack-objects` one by one) with the more modern
`--stdin-packs` option.

This allows us to avoid some CPU cycles serializing and deserializing
every object ID in all of the packs we're aggregating. It also avoids us
having to send a potentially large amount of data down to
`pack-objects`.

But more importantly, it generates slightly higher quality (read: more
tightly compressed) packs, because of the reachability traversal that
`--stdin-packs` does after the fact in order to gather namehash values
which seed the delta selection process.

In practice, this seems to add a slight amount of overhead (on the order
of a few seconds for git.git broken up into ~100 packs), in exchange for
a modest reduction (on the order of ~3.5%) in the resulting pack size.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 midx.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/midx.c b/midx.c
index db0a70c5af..5fbf964bba 100644
--- a/midx.c
+++ b/midx.c
@@ -1955,6 +1955,7 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	FILE *cmd_in;
 	struct strbuf base_name = STRBUF_INIT;
+	struct strbuf scratch = STRBUF_INIT;
 	struct multi_pack_index *m = lookup_multi_pack_index(r, object_dir);
 	struct repack_info *pack_info = NULL;
 
@@ -1996,7 +1997,7 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 	repo_config_get_bool(r, "repack.usedeltabaseoffset", &delta_base_offset);
 	repo_config_get_bool(r, "repack.usedeltaislands", &use_delta_islands);
 
-	strvec_push(&cmd.args, "pack-objects");
+	strvec_pushl(&cmd.args, "pack-objects", "--stdin-packs", NULL);
 
 	strbuf_addstr(&base_name, object_dir);
 	strbuf_addstr(&base_name, "/pack/pack");
@@ -2025,17 +2026,19 @@ int midx_repack(struct repository *r, const char *object_dir, size_t batch_size,
 
 	cmd_in = xfdopen(cmd.in, "w");
 
-	for (i = 0; i < m->num_objects; i++) {
-		struct object_id oid;
-		uint32_t pack_int_id = nth_midxed_pack_int_id(m, i);
+	for (i = 0; i < m->num_packs; i++) {
+		struct repack_info *info = &pack_info[i];
 
-		if (!include_pack[pack_int_id])
-			continue;
+		strbuf_reset(&scratch);
 
-		nth_midxed_object_oid(&oid, m, i);
-		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
+		strbuf_addstr(&scratch, m->pack_names[info->pack_int_id]);
+		strbuf_strip_suffix(&scratch, ".idx");
+		strbuf_addstr(&scratch, ".pack");
+
+		fprintf(cmd_in, "%s%s\n", include_pack[info->pack_int_id] ? "" : "^", scratch.buf);
 	}
 	fclose(cmd_in);
+	strbuf_release(&scratch);
 
 	if (finish_command(&cmd)) {
 		error(_("could not finish pack-objects"));
-- 
2.37.0.1.g1379af2e9d