fetch-pack: in protocol v2, reset in_vain upon ACK

In the function process_acks() in fetch-pack.c, the variable
received_ack is meant to track that an ACK was received, but it was
never set. This results in negotiation terminating prematurely through
the in_vain counter, when the counter should have been reset upon every
ACK.

Therefore, reset the in_vain counter upon every ACK.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -1307,6 +1307,7 @@ static enum common_found process_acks(struct fetch_negotiator *negotiator,
 
 		if (skip_prefix(reader->line, "ACK ", &arg)) {
 			struct object_id oid;
+			received_ack = 1;
 			if (!get_oid_hex(arg, &oid)) {
 				struct commit *commit;
 				oidset_insert(common, &oid);

t/t5500-fetch-pack.sh

@@ -403,6 +403,36 @@ test_expect_success 'in_vain not triggered before first ACK' '
 	test_i18ngrep "Total 3 " trace
 '
 
+test_expect_success 'in_vain resetted upon ACK' '
+	rm -rf myserver myclient trace &&
+	git init myserver &&
+
+	# Linked list of commits on master. The first is common; the rest are
+	# not.
+	test_commit -C myserver first_master_commit &&
+	git clone "file://$(pwd)/myserver" myclient &&
+	test_commit_bulk -C myclient 255 &&
+
+	# Another linked list of commits on anotherbranch with no connection to
+	# master. The first is common; the rest are not.
+	git -C myserver checkout --orphan anotherbranch &&
+	test_commit -C myserver first_anotherbranch_commit &&
+	git -C myclient fetch origin anotherbranch:refs/heads/anotherbranch &&
+	git -C myclient checkout anotherbranch &&
+	test_commit_bulk -C myclient 255 &&
+
+	# The new commit that the client wants to fetch.
+	git -C myserver checkout master &&
+	test_commit -C myserver to_fetch &&
+
+	# The client will send (as "have"s) all 256 commits in anotherbranch
+	# first. The 256th commit is common between the client and the server,
+	# and should reset in_vain. This allows negotiation to continue until
+	# the client reports that first_anotherbranch_commit is common.
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C myclient fetch --progress origin master &&
+	test_i18ngrep "Total 3 " trace
+'
+
 test_expect_success 'fetch in shallow repo unreachable shallow objects' '
 	(
 		git clone --bare --branch B --single-branch "file://$(pwd)/." no-reflog &&