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

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

1 files changed, 11 insertions, 0 deletions

diff --git a/packfile.c b/packfile.c
index 80e35f1032..70c7208f02 100644
--- a/packfile.c
+++ b/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;