transport: list refs before fetch if necessary

The built-in bundle transport and the transport helper interface do not
work when transport_fetch_refs() is called immediately after transport
creation. This will be needed in a subsequent patch, so fix this.

Evidence: fetch_refs_from_bundle() relies on data->header being
initialized in get_refs_from_bundle(), and fetch() in transport-helper.c
relies on either data->fetch or data->import being set by get_helper(),
but neither transport_helper_init() nor fetch() calls get_helper().

Up until the introduction of the partial clone feature, this has not
been a problem, because transport_fetch_refs() is always called after
transport_get_remote_refs(). With the introduction of the partial clone
feature, which involves calling transport_fetch_refs() (to fetch objects
by their OIDs) without transport_get_remote_refs(), this is still not a
problem, but only coincidentally - we do not support partially cloning a
bundle, and as for cloning using a transport-helper-using protocol, it
so happens that before transport_fetch_refs() is called, fetch_refs() in
fetch-object.c calls transport_set_option(), which means that the
aforementioned get_helper() is invoked through set_helper_option() in
transport-helper.c.

This could be fixed by fixing the transports themselves, but it doesn't
seem like a good idea to me to open up previously untested code paths;
also, there may be transport helpers in the wild that assume that "list"
is always called before "fetch". Instead, fix this by having
transport_fetch_refs() call transport_get_remote_refs() to ensure that
the latter is always called at least once, unless the transport
explicitly states that it supports fetching without listing refs.

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

transport-helper.c

@@ -1105,6 +1105,7 @@ static struct ref *get_refs_list(struct transport *transport, int for_push,
 }
 
 static struct transport_vtable vtable = {
+	0,
 	set_helper_option,
 	get_refs_list,
 	fetch,

transport-internal.h

@@ -6,6 +6,12 @@ struct transport;
 struct argv_array;
 
 struct transport_vtable {
+	/**
+	 * This transport supports the fetch() function being called
+	 * without get_refs_list() first being called.
+	 */
+	unsigned fetch_without_list : 1;
+
 	/**
 	 * Returns 0 if successful, positive if the option is not
 	 * recognized or is inapplicable, and negative if the option

transport.c

@@ -733,6 +733,7 @@ static int disconnect_git(struct transport *transport)
 }
 
 static struct transport_vtable taken_over_vtable = {
+	1,
 	NULL,
 	get_refs_via_connect,
 	fetch_refs_via_pack,
@@ -882,6 +883,7 @@ void transport_check_allowed(const char *type)
 }
 
 static struct transport_vtable bundle_vtable = {
+	0,
 	NULL,
 	get_refs_from_bundle,
 	fetch_refs_from_bundle,
@@ -891,6 +893,7 @@ static struct transport_vtable bundle_vtable = {
 };
 
 static struct transport_vtable builtin_smart_vtable = {
+	1,
 	NULL,
 	get_refs_via_connect,
 	fetch_refs_via_pack,
@@ -1254,6 +1257,15 @@ int transport_fetch_refs(struct transport *transport, struct ref *refs)
 	struct ref **heads = NULL;
 	struct ref *rm;
 
+	if (!transport->vtable->fetch_without_list)
+		/*
+		 * Some transports (e.g. the built-in bundle transport and the
+		 * transport helper interface) do not work when fetching is
+		 * done immediately after transport creation. List the remote
+		 * refs anyway (if not already listed) as a workaround.
+		 */
+		transport_get_remote_refs(transport, NULL);
+
 	for (rm = refs; rm; rm = rm->next) {
 		nr_refs++;
 		if (rm->peer_ref &&