fetch-pack: do not take shallow lock unnecessarily

When fetching using protocol v2, the remote may send a "shallow-info"
section if the client is shallow. If so, Git as the client currently
takes the shallow file lock, even if the "shallow-info" section is
empty.

This is not a problem except that Git does not support taking the
shallow file lock after modifying the shallow file, because
is_repository_shallow() stores information that is never cleared. And
this take-after-modify occurs when Git does a tag-following fetch from a
shallow repository on a transport that does not support tag following
(since in this case, 2 fetches are performed).

To solve this issue, take the shallow file lock (and perform all other
shallow processing) only if the "shallow-info" section is non-empty;
otherwise, behave as if it were empty.

A full solution (probably, ensuring that any action of committing
shallow file locks also includes clearing the information stored by
is_repository_shallow()) would solve the issue without need for this
patch, but this patch is independently useful (as an optimization to
prevent writing a file in an unnecessary case), hence why I wrote it. I
have included a NEEDSWORK outlining the full solution.

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

fetch-pack.c

@@ -1232,6 +1232,8 @@ static int process_acks(struct fetch_negotiator *negotiator,
 static void receive_shallow_info(struct fetch_pack_args *args,
 				 struct packet_reader *reader)
 {
+	int line_received = 0;
+
 	process_section_header(reader, "shallow-info", 0);
 	while (packet_reader_read(reader) == PACKET_READ_NORMAL) {
 		const char *arg;
@@ -1241,6 +1243,7 @@ static void receive_shallow_info(struct fetch_pack_args *args,
 			if (get_oid_hex(arg, &oid))
 				die(_("invalid shallow line: %s"), reader->line);
 			register_shallow(the_repository, &oid);
+			line_received = 1;
 			continue;
 		}
 		if (skip_prefix(reader->line, "unshallow ", &arg)) {
@@ -1253,6 +1256,7 @@ static void receive_shallow_info(struct fetch_pack_args *args,
 				die(_("error in object: %s"), reader->line);
 			if (unregister_shallow(&oid))
 				die(_("no shallow found: %s"), reader->line);
+			line_received = 1;
 			continue;
 		}
 		die(_("expected shallow/unshallow, got %s"), reader->line);
@@ -1262,8 +1266,11 @@ static void receive_shallow_info(struct fetch_pack_args *args,
 	    reader->status != PACKET_READ_DELIM)
 		die(_("error processing shallow info: %d"), reader->status);
 
-	setup_alternate_shallow(&shallow_lock, &alternate_shallow_file, NULL);
-	args->deepen = 1;
+	if (line_received) {
+		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file,
+					NULL);
+		args->deepen = 1;
+	}
 }
 
 static void receive_wanted_refs(struct packet_reader *reader,

shallow.c

@@ -43,6 +43,13 @@ int register_shallow(struct repository *r, const struct object_id *oid)
 
 int is_repository_shallow(struct repository *r)
 {
+	/*
+	 * NEEDSWORK: This function updates
+	 * r->parsed_objects->{is_shallow,shallow_stat} as a side effect but
+	 * there is no corresponding function to clear them when the shallow
+	 * file is updated.
+	 */
+
 	FILE *fp;
 	char buf[1024];
 	const char *path = r->parsed_objects->alternate_shallow_file;

t/t5702-protocol-v2.sh

@@ -471,6 +471,24 @@ test_expect_success 'upload-pack respects client shallows' '
 	grep "fetch< version 2" trace
 '
 
+test_expect_success 'ensure that multiple fetches in same process from a shallow repo works' '
+	rm -rf server client trace &&
+
+	test_create_repo server &&
+	test_commit -C server one &&
+	test_commit -C server two &&
+	test_commit -C server three &&
+	git clone --shallow-exclude two "file://$(pwd)/server" client &&
+
+	git -C server tag -a -m "an annotated tag" twotag two &&
+
+	# Triggers tag following (thus, 2 fetches in one process)
+	GIT_TRACE_PACKET="$(pwd)/trace" git -C client -c protocol.version=2 \
+		fetch --shallow-exclude one origin &&
+	# Ensure that protocol v2 is used
+	grep "fetch< version 2" trace
+'
+
 # Test protocol v2 with 'http://' transport
 #
 . "$TEST_DIRECTORY"/lib-httpd.sh