midx: add packs to packed_git linked list

The multi-pack-index allows searching for objects across multiple
packs using one object list. The original design gains many of
these performance benefits by keeping the packs in the
multi-pack-index out of the packed_git list.

Unfortunately, this has one major drawback. If the multi-pack-index
covers thousands of packs, and a command loads many of those packs,
then we can hit the limit for open file descriptors. The
close_one_pack() method is used to limit this resource, but it
only looks at the packed_git list, and uses an LRU cache to prevent
thrashing.

Instead of complicating this close_one_pack() logic to include
direct references to the multi-pack-index, simply add the packs
opened by the multi-pack-index to the packed_git list. This
immediately solves the file-descriptor limit problem, but requires
some extra steps to avoid performance issues or other problems:

1. Create a multi_pack_index bit in the packed_git struct that is
   one if and only if the pack was loaded from a multi-pack-index.

2. Skip packs with the multi_pack_index bit when doing object
   lookups and abbreviations. These algorithms already check the
   multi-pack-index before the packed_git struct. This has a very
   small performance hit, as we need to walk more packed_git
   structs. This is acceptable, since these operations run binary
   search on the other packs, so this walk-and-ignore logic is
   very fast by comparison.

3. When closing a multi-pack-index file, do not close its packs,
   as those packs will be closed using close_all_packs(). In some
   cases, such as 'git repack', we run 'close_midx()' without also
   closing the packs, so we need to un-set the multi_pack_index bit
   in those packs. This is necessary, and caught by running
   t6501-freshen-objects.sh with GIT_TEST_MULTI_PACK_INDEX=1.

To manually test this change, I inserted trace2 logging into
close_pack_fd() and set pack_max_fds to 10, then ran 'git rev-list
--all --objects' on a copy of the Git repo with 300+ pack-files and
a multi-pack-index. The logs verified the packs are closed as
we read them beyond the file descriptor limit.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 6 deletions

diff --git a/midx.c b/midx.c
index 8b8faec35a..e7e1fe4d65 100644
--- a/midx.c
+++ b/midx.c
@@ -192,10 +192,8 @@ void close_midx(struct multi_pack_index *m)
 	m->fd = -1;
 
 	for (i = 0; i < m->num_packs; i++) {
-		if (m->packs[i]) {
-			close_pack(m->packs[i]);
-			free(m->packs[i]);
-		}
+		if (m->packs[i])
+			m->packs[i]->multi_pack_index = 0;
 	}
 	FREE_AND_NULL(m->packs);
 	FREE_AND_NULL(m->pack_names);
@@ -204,6 +202,7 @@ void close_midx(struct multi_pack_index *m)
 int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t pack_int_id)
 {
 	struct strbuf pack_name = STRBUF_INIT;
+	struct packed_git *p;
 
 	if (pack_int_id >= m->num_packs)
 		die(_("bad pack-int-id: %u (%u total packs)"),
@@ -215,9 +214,18 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m, uint32_t
 	strbuf_addf(&pack_name, "%s/pack/%s", m->object_dir,
 		    m->pack_names[pack_int_id]);
 
-	m->packs[pack_int_id] = add_packed_git(pack_name.buf, pack_name.len, m->local);
+	p = add_packed_git(pack_name.buf, pack_name.len, m->local);
 	strbuf_release(&pack_name);
-	return !m->packs[pack_int_id];
+
+	if (!p)
+		return 1;
+
+	p->multi_pack_index = 1;
+	m->packs[pack_int_id] = p;
+	install_packed_git(r, p);
+	list_add_tail(&p->mru, &r->objects->packed_git_mru);
+
+	return 0;
 }
 
 int bsearch_midx(const struct object_id *oid, struct multi_pack_index *m, uint32_t *result)