[PATCH 4/4] archive: ignore blob objects when checking reachability

[Jeff King <peff@peff.net>]

We cannot create an archive from a blob object, so we would
not expect anyone to provide one to us. And if they do, we
will fail anyway just after the reachability check.  We can
therefore optimize our reachability check to ignore blobs
completely, and not even create a "struct blob" for them.

Depending on the repository size and the exact place we find
the reachable object in the traversal, this can save 20-25%,
a we can avoid many lookups in the object hash.

The downside of this is that a blob provided to a remote
archive process will fail with "no such object" rather than
"object is not a tree" (we could organize the code to retain
the old message, but since we no longer know whether the
blob is reachable or not, we would potentially be leaking
information about the existence of unreachable objects).

Signed-off-by: Jeff King <peff@peff.net>
---
 archive.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/archive.c b/archive.c
index 4d77624..98691cd 100644
--- a/archive.c
+++ b/archive.c
@@ -290,6 +290,7 @@ static int object_is_reachable(const unsigned char *sha1)
 	save_commit_buffer = 0;
 	init_revisions(&data.revs, NULL);
 	setup_revisions(ARRAY_SIZE(argv) - 1, argv, &data.revs, NULL);
+	data.revs.blob_objects = 0;
 	if (prepare_revision_walk(&data.revs))
 		return 0;
 
-- 
1.8.3.rc2.14.g7eee6b3