send-pack: support push negotiation

Teach Git the push.negotiate config variable.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/push.txt

@@ -120,3 +120,10 @@ push.useForceIfIncludes::
 	`--force-if-includes` as an option to linkgit:git-push[1]
 	in the command line. Adding `--no-force-if-includes` at the
 	time of push overrides this configuration setting.
+
+push.negotiate::
+	If set to "true", attempt to reduce the size of the packfile
+	sent by rounds of negotiation in which the client and the
+	server attempt to find commits in common. If "false", Git will
+	rely solely on the server's ref advertisement to find commits
+	in common.

send-pack.c

@@ -56,7 +56,9 @@ static void feed_object(const struct object_id *oid, FILE *fh, int negative)
 /*
  * Make a pack stream and spit it out into file descriptor fd
  */
-static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struct send_pack_args *args)
+static int pack_objects(int fd, struct ref *refs, struct oid_array *advertised,
+			struct oid_array *negotiated,
+			struct send_pack_args *args)
 {
 	/*
 	 * The child becomes pack-objects --revs; we feed
@@ -94,8 +96,10 @@ static int pack_objects(int fd, struct ref *refs, struct oid_array *extra, struc
 	 * parameters by writing to the pipe.
 	 */
 	po_in = xfdopen(po.in, "w");
-	for (i = 0; i < extra->nr; i++)
+	for (i = 0; i < advertised->nr; i++)
-		feed_object(&extra->oid[i], po_in, 1);
+		feed_object(&advertised->oid[i], po_in, 1);
+	for (i = 0; i < negotiated->nr; i++)
+		feed_object(&negotiated->oid[i], po_in, 1);
 
 	while (refs) {
 		if (!is_null_oid(&refs->old_oid))
@@ -409,11 +413,55 @@ static void reject_invalid_nonce(const char *nonce, int len)
 	}
 }
 
+static void get_commons_through_negotiation(const char *url,
+					    const struct ref *remote_refs,
+					    struct oid_array *commons)
+{
+	struct child_process child = CHILD_PROCESS_INIT;
+	const struct ref *ref;
+	int len = the_hash_algo->hexsz + 1; /* hash + NL */
+
+	child.git_cmd = 1;
+	child.no_stdin = 1;
+	child.out = -1;
+	strvec_pushl(&child.args, "fetch", "--negotiate-only", NULL);
+	for (ref = remote_refs; ref; ref = ref->next)
+		strvec_pushf(&child.args, "--negotiation-tip=%s", oid_to_hex(&ref->new_oid));
+	strvec_push(&child.args, url);
+
+	if (start_command(&child))
+		die(_("send-pack: unable to fork off fetch subprocess"));
+
+	do {
+		char hex_hash[GIT_MAX_HEXSZ + 1];
+		int read_len = read_in_full(child.out, hex_hash, len);
+		struct object_id oid;
+		const char *end;
+
+		if (!read_len)
+			break;
+		if (read_len != len)
+			die("invalid length read %d", read_len);
+		if (parse_oid_hex(hex_hash, &oid, &end) || *end != '\n')
+			die("invalid hash");
+		oid_array_append(commons, &oid);
+	} while (1);
+
+	if (finish_command(&child)) {
+		/*
+		 * The information that push negotiation provides is useful but
+		 * not mandatory.
+		 */
+		warning(_("push negotiation failed; proceeding anyway with push"));
+	}
+}
+
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
 	      struct oid_array *extra_have)
 {
+	struct oid_array commons = OID_ARRAY_INIT;
 	int in = fd[0];
 	int out = fd[1];
 	struct strbuf req_buf = STRBUF_INIT;
@@ -426,6 +474,7 @@ int send_pack(struct send_pack_args *args,
 	int quiet_supported = 0;
 	int agent_supported = 0;
 	int advertise_sid = 0;
+	int push_negotiate = 0;
 	int use_atomic = 0;
 	int atomic_supported = 0;
 	int use_push_options = 0;
@@ -437,6 +486,10 @@ int send_pack(struct send_pack_args *args,
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
 
+	git_config_get_bool("push.negotiate", &push_negotiate);
+	if (push_negotiate)
+		get_commons_through_negotiation(args->url, remote_refs, &commons);
+
 	git_config_get_bool("transfer.advertisesid", &advertise_sid);
 
 	/* Does the other end support the reporting? */
@@ -625,7 +678,7 @@ int send_pack(struct send_pack_args *args,
 			   PACKET_READ_DIE_ON_ERR_PACKET);
 
 	if (need_pack_data && cmds_sent) {
-		if (pack_objects(out, remote_refs, extra_have, args) < 0) {
+		if (pack_objects(out, remote_refs, extra_have, &commons, args) < 0) {
 			if (args->stateless_rpc)
 				close(out);
 			if (git_connection_is_socket(conn))

t/t5516-fetch-push.sh

@@ -191,6 +191,41 @@ test_expect_success 'fetch with pushInsteadOf (should not rewrite)' '
 	)
 '
 
+grep_wrote () {
+	object_count=$1
+	file_name=$2
+	grep 'write_pack_file/wrote.*"value":"'$1'"' $2
+}
+
+test_expect_success 'push with negotiation' '
+	# Without negotiation
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	echo now pushing without negotiation &&
+	GIT_TRACE2_EVENT="$(pwd)/event" git -c protocol.version=2 push testrepo refs/heads/main:refs/remotes/origin/main &&
+	grep_wrote 5 event && # 2 commits, 2 trees, 1 blob
+
+	# Same commands, but with negotiation
+	rm event &&
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	GIT_TRACE2_EVENT="$(pwd)/event" git -c protocol.version=2 -c push.negotiate=1 push testrepo refs/heads/main:refs/remotes/origin/main &&
+	grep_wrote 2 event # 1 commit, 1 tree
+'
+
+test_expect_success 'push with negotiation proceeds anyway even if negotiation fails' '
+	rm event &&
+	mk_empty testrepo &&
+	git push testrepo $the_first_commit:refs/remotes/origin/first_commit &&
+	git -C testrepo config receive.hideRefs refs/remotes/origin/first_commit &&
+	GIT_TEST_PROTOCOL_VERSION=0 GIT_TRACE2_EVENT="$(pwd)/event" \
+		git -c push.negotiate=1 push testrepo refs/heads/main:refs/remotes/origin/main 2>err &&
+	grep_wrote 5 event && # 2 commits, 2 trees, 1 blob
+	test_i18ngrep "push negotiation failed" err
+'
+
 test_expect_success 'push without wildcard' '
 	mk_empty testrepo &&