send-pack: use OBJECT_INFO_QUICK to check negative objects

When pushing, we feed pack-objects a list of both positive and negative
objects. The positive objects are what we want to send, and the negative
objects are what the other side told us they have, which we can use to
limit the size of the push.

Before passing along a negative object, send_pack() will make sure we
actually have it (since we only know about it because the remote
mentioned it, not because it's one of our refs). So it's expected that
some of these objects will be missing on the local side. But looking for
a missing object is more expensive than one that we have: it triggers
reprepare_packed_git() to handle a racy repack, plus it has to explore
every alternate's loose object tree (which can be slow if you have a lot
of them, or have a high-latency filesystem).

This isn't usually a big problem, since repositories you're pushing to
don't generally have a large number of refs that are unrelated to what
the client has. But there's no reason such a setup is wrong, and it
currently performs poorly.

We can fix this by using OBJECT_INFO_QUICK, which tells the lookup
code that we expect objects to be missing. Notably, it will not re-scan
the packs, and it will use the loose cache from 61c7711cfe (sha1-file:
use loose object cache for quick existence check, 2018-11-12).

The downside is that in the rare case that we race with a local repack,
we might fail to feed some objects to pack-objects, making the resulting
push larger. But we'd never produce an invalid or incorrect push, just a
less optimal one. That seems like a reasonable tradeoff, and we already
do similar things on the fetch side (e.g., when marking COMPLETE
commits).

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

send-pack.c

@@ -41,7 +41,9 @@ int option_parse_push_signed(const struct option *opt,
 static void feed_object(const struct object_id *oid, FILE *fh, int negative)
 {
 	if (negative &&
-	    !has_object_file_with_flags(oid, OBJECT_INFO_SKIP_FETCH_OBJECT))
+	    !has_object_file_with_flags(oid,
+					OBJECT_INFO_SKIP_FETCH_OBJECT |
+					OBJECT_INFO_QUICK))
 		return;
 
 	if (negative)