upload-pack: avoid parsing objects during ref advertisement

When we advertise a ref, the first thing we do is parse the
pointed-to object. This gives us two things:

  1. a "struct object" we can use to store flags

  2. the type of the object, so we know whether we need to
     dereference it as a tag

Instead, we can just use lookup_unknown_object to get an
object struct, and then fill in just the type field using
sha1_object_info (which, in the case of packed files, can
find the information without actually inflating the object
data).

This can save time if you have a large number of refs, and
the client isn't actually going to request those refs (e.g.,
because most of them are already up-to-date).

The downside is that we are no longer verifying objects that
we advertise by fully parsing them (however, we do still
know we actually have them, because sha1_object_info must
find them to get the type). While we might fail to detect a
corrupt object here, if the client actually fetches the
object, we will parse (and verify) it then.

On a repository with 120K refs, the advertisement portion of
upload-pack goes from ~3.4s to 3.2s (the failure to speed up
more is largely due to the fact that most of these refs are
tags, which need dereferenced to find the tag destination
anyway).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -720,11 +720,14 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 	static const char *capabilities = "multi_ack thin-pack side-band"
 		" side-band-64k ofs-delta shallow no-progress"
 		" include-tag multi_ack_detailed";
-	struct object *o = parse_object(sha1);
+	struct object *o = lookup_unknown_object(sha1);
 	const char *refname_nons = strip_namespace(refname);
 
-	if (!o)
-		die("git upload-pack: cannot find object %s:", sha1_to_hex(sha1));
+	if (o->type == OBJ_NONE) {
+		o->type = sha1_object_info(sha1, NULL);
+		if (o->type < 0)
+		    die("git upload-pack: cannot find object %s:", sha1_to_hex(sha1));
+	}
 
 	if (capabilities)
 		packet_write(1, "%s %s%c%s%s\n", sha1_to_hex(sha1), refname_nons,
@@ -738,6 +741,7 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo
 		nr_our_refs++;
 	}
 	if (o->type == OBJ_TAG) {
+		o = parse_object(o->sha1);
 		o = deref_tag(o, refname, 0);
 		if (o)
 			packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname_nons);