clone: support unusual remote ref configurations

When cloning a branchless and tagless but not refless remote using
protocol v0 or v1, Git calls transport_fetch_refs() with an empty ref
list. This makes the clone fail with the message "remote transport
reported error".

Git should have refrained from calling transport_fetch_refs(), just like
it does in the case that the remote is refless. Therefore, teach Git to
do this.

In protocol v2, this does not happen because the client passes
ref-prefix arguments that filter out non-branches and non-tags in the
ref advertisement, making the remote appear empty.

Note that this bug concerns logic in builtin/clone.c and only affects
cloning, not fetching.

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

builtin/clone.c

@@ -862,7 +862,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	const struct ref *refs, *remote_head;
 	struct ref *remote_head_points_at = NULL;
 	const struct ref *our_head_points_at;
-	struct ref *mapped_refs;
+	struct ref *mapped_refs = NULL;
 	const struct ref *ref;
 	struct strbuf key = STRBUF_INIT;
 	struct strbuf branch_top = STRBUF_INIT, reflog_msg = STRBUF_INIT;
@@ -1184,7 +1184,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 
 	refs = transport_get_remote_refs(transport, &transport_ls_refs_options);
 
-	if (refs) {
+	if (refs)
+		mapped_refs = wanted_peer_refs(refs, &remote->fetch);
+
+	if (mapped_refs) {
 		int hash_algo = hash_algo_by_ptr(transport_get_hash_algo(transport));
 
 		/*
@@ -1193,8 +1196,6 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 		 */
 		initialize_repository_version(hash_algo, 1);
 		repo_set_hash_algo(the_repository, hash_algo);
-
-		mapped_refs = wanted_peer_refs(refs, &remote->fetch);
 		/*
 		 * transport_get_remote_refs() may return refs with null sha-1
 		 * in mapped_refs (see struct transport->get_refs_list
@@ -1240,7 +1241,6 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 					option_branch, remote_name);
 
 		warning(_("You appear to have cloned an empty repository."));
-		mapped_refs = NULL;
 		our_head_points_at = NULL;
 		remote_head_points_at = NULL;
 		remote_head = NULL;
@@ -1271,7 +1271,7 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 
 	if (is_local)
 		clone_local(path, git_dir);
-	else if (refs && complete_refs_before_fetch) {
+	else if (mapped_refs && complete_refs_before_fetch) {
 		if (transport_fetch_refs(transport, mapped_refs))
 			die(_("remote transport reported error"));
 	}

t/t5700-protocol-v1.sh

@@ -149,6 +149,21 @@ test_expect_success 'push with file:// using protocol v1' '
 	grep "push< version 1" log
 '
 
+test_expect_success 'cloning branchless tagless but not refless remote' '
+	rm -rf server client &&
+
+	git -c init.defaultbranch=main init server &&
+	echo foo >server/foo.txt &&
+	git -C server add foo.txt &&
+	git -C server commit -m "message" &&
+	git -C server update-ref refs/notbranch/alsonottag HEAD &&
+	git -C server checkout --detach &&
+	git -C server branch -D main &&
+	git -C server symbolic-ref HEAD refs/heads/nonexistentbranch &&
+
+	git -c protocol.version=1 clone "file://$(pwd)/server" client
+'
+
 # Test protocol v1 with 'ssh://' transport
 #
 test_expect_success 'setup ssh wrapper' '