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

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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@xxxxxxxxxxxx>
---
Noticed this while working on a semi-related series in:

  https://lore.kernel.org/git/cover.1663638929.git.me@xxxxxxxxxxxx/T/

the savings here are pretty modest, but this is in line with the
strategy we use in the `--geometric` repack mode, which performs a
similar task.

 midx.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/midx.c b/midx.c
index c27d0e5f15..c06f0f00e6 100644
--- a/midx.c
+++ b/midx.c
@@ -1973,6 +1973,7 @@ 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);

 	/*
@@ -1997,7 +1998,7 @@ 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");
@@ -2026,17 +2027,17 @@ 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++) {
+		strbuf_reset(&scratch);

-		if (!include_pack[pack_int_id])
-			continue;
+		strbuf_addstr(&scratch, m->pack_names[i]);
+		strbuf_strip_suffix(&scratch, ".idx");
+		strbuf_addstr(&scratch, ".pack");

-		nth_midxed_object_oid(&oid, m, i);
-		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
+		fprintf(cmd_in, "%s%s\n", include_pack[i] ? "" : "^", scratch.buf);
 	}
 	fclose(cmd_in);
+	strbuf_release(&scratch);

 	if (finish_command(&cmd)) {
 		error(_("could not finish pack-objects"));
--
2.37.0.1.g1379af2e9d



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