[PATCH v3 4/7] fetch-pack: use ref adv. to prune "have" sent

[Jonathan Tan <jonathantanmy@google.com>]

In negotiation using protocol v2, fetch-pack sometimes does not make
full use of the information obtained in the ref advertisement:
specifically, that if the server advertises a commit that the client
also has, the client never needs to inform the server that it has the
commit's parents, since it can just tell the server that it has the
advertised commit and it knows that the server can and will infer the
rest.

This is because, in do_fetch_pack_v2(), rev_list_insert_ref_oid() is
invoked before mark_complete_and_common_ref(). This means that if we
have a commit that is both our ref and their ref, it would be enqueued
by rev_list_insert_ref_oid() as SEEN, and since it is thus already SEEN,
mark_complete_and_common_ref() would not enqueue it.

If mark_complete_and_common_ref() were invoked first, as it is in
do_fetch_pack() for protocol v0, then mark_complete_and_common_ref()
would enqueue it with COMMON_REF | SEEN. The addition of COMMON_REF
ensures that its parents are not sent as "have" lines.

Change the order in do_fetch_pack_v2() to be consistent with
do_fetch_pack(), and to avoid sending unnecessary "have" lines.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
---
 fetch-pack.c          |  6 +++---
 t/t5500-fetch-pack.sh | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/fetch-pack.c b/fetch-pack.c
index 60adfc073..806c40021 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1392,9 +1392,6 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 				for_each_ref(clear_marks, NULL);
 			marked = 1;
 
-			for_each_ref(rev_list_insert_ref_oid, NULL);
-			for_each_cached_alternate(insert_one_alternate_object);
-
 			/* Filter 'ref' by 'sought' and those that aren't local */
 			mark_complete_and_common_ref(args, &ref);
 			filter_refs(args, &ref, sought, nr_sought);
@@ -1402,6 +1399,9 @@ static struct ref *do_fetch_pack_v2(struct fetch_pack_args *args,
 				state = FETCH_DONE;
 			else
 				state = FETCH_SEND_REQUEST;
+
+			for_each_ref(rev_list_insert_ref_oid, NULL);
+			for_each_cached_alternate(insert_one_alternate_object);
 			break;
 		case FETCH_SEND_REQUEST:
 			if (send_fetch_request(fd[1], args, ref, &common,
diff --git a/t/t5500-fetch-pack.sh b/t/t5500-fetch-pack.sh
index d4f435155..e0cdc295d 100755
--- a/t/t5500-fetch-pack.sh
+++ b/t/t5500-fetch-pack.sh
@@ -755,6 +755,39 @@ test_expect_success 'fetching deepen' '
 	)
 '
 
+test_expect_success 'use ref advertisement to prune "have" lines sent' '
+	rm -rf server client &&
+	git init server &&
+	test_commit -C server both_have_1 &&
+	git -C server tag -d both_have_1 &&
+	test_commit -C server both_have_2 &&
+
+	git clone server client &&
+	test_commit -C server server_has &&
+	test_commit -C client client_has &&
+
+	# In both protocol v0 and v2, ensure that the parent of both_have_2 is
+	# not sent as a "have" line. The client should know that the server has
+	# both_have_2, so it only needs to inform the server that it has
+	# both_have_2, and the server can infer the rest.
+
+	rm -f trace &&
+	cp -r client clientv0 &&
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C clientv0 \
+		fetch origin server_has both_have_2 &&
+	grep "have $(git -C client rev-parse client_has)" trace &&
+	grep "have $(git -C client rev-parse both_have_2)" trace &&
+	! grep "have $(git -C client rev-parse both_have_2^)" trace &&
+
+	rm -f trace &&
+	cp -r client clientv2 &&
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C clientv2 -c protocol.version=2 \
+		fetch origin server_has both_have_2 &&
+	grep "have $(git -C client rev-parse client_has)" trace &&
+	grep "have $(git -C client rev-parse both_have_2)" trace &&
+	! grep "have $(git -C client rev-parse both_have_2^)" trace
+'
+
 test_expect_success 'filtering by size' '
 	rm -rf server client &&
 	test_create_repo server &&
-- 
2.17.0.582.gccdcbd54c4