Merge branch 'ps/midx-negative-packfile-cache' into seen

When a stale .midx file refers to .pack files that no longer exist,
we ended up checking for these non-existent files repeatedly, which
has been optimized by memoizing the non-existence.

* ps/midx-negative-packfile-cache:
  midx: stop repeatedly looking up nonexistent packfiles
  packfile: explain ordering of how we look up auxiliary pack files

Author: gitster

midx.c

@@ -405,7 +405,7 @@ void close_midx(struct multi_pack_index *m)
 	munmap((unsigned char *)m->data, m->data_len);
 
 	for (i = 0; i < m->num_packs; i++) {
-		if (m->packs[i])
+		if (m->packs[i] && m->packs[i] != (void *)(intptr_t)-1)
 			m->packs[i]->multi_pack_index = 0;
 	}
 	FREE_AND_NULL(m->packs);
@@ -458,6 +458,8 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m,
 
 	pack_int_id = midx_for_pack(&m, pack_int_id);
 
+	if (m->packs[pack_int_id] == (void *)(intptr_t)-1)
+		return 1;
 	if (m->packs[pack_int_id])
 		return 0;
 
@@ -482,8 +484,10 @@ int prepare_midx_pack(struct repository *r, struct multi_pack_index *m,
 	strbuf_release(&pack_name);
 	strbuf_release(&key);
 
-	if (!p)
+	if (!p) {
+		m->packs[pack_int_id] = (void *)(intptr_t)-1;
 		return 1;
+	}
 
 	p->multi_pack_index = 1;
 	m->packs[pack_int_id] = p;
@@ -495,6 +499,8 @@ struct packed_git *nth_midxed_pack(struct multi_pack_index *m,
 				   uint32_t pack_int_id)
 {
 	uint32_t local_pack_int_id = midx_for_pack(&m, pack_int_id);
+	if (m->packs[local_pack_int_id] == (void *)(intptr_t)-1)
+		return NULL;
 	return m->packs[local_pack_int_id];
 }
 

packfile.c

@@ -737,6 +737,17 @@ struct packed_git *add_packed_git(struct repository *r, const char *path,
 	p = alloc_packed_git(r, alloc);
 	memcpy(p->pack_name, path, path_len);
 
+	/*
+	 * Note that we have to check auxiliary data structures before we check
+	 * for the ".pack" file to exist to avoid races with a packfile that is
+	 * in the process of being deleted. The ".pack" file is unlinked before
+	 * its auxiliary data structures, so we know that we either get a
+	 * consistent snapshot of all data structures or that we'll fail to
+	 * stat(3p) the packfile itself and thus return `NULL`.
+	 *
+	 * As such, we cannot bail out before the access(3p) calls in case the
+	 * packfile doesn't exist without doing two stat(3p) calls for it.
+	 */
 	xsnprintf(p->pack_name + path_len, alloc - path_len, ".keep");
 	if (!access(p->pack_name, F_OK))
 		p->pack_keep = 1;