upload-pack: handle unexpected delim packets

When processing the arguments list for a v2 ls-refs or fetch command, we
loop like this:

  while (packet_reader_read(request) != PACKET_READ_FLUSH) {
          const char *arg = request->line;
	  ...handle arg...
  }

to read and handle packets until we see a flush. The hidden assumption
here is that anything except PACKET_READ_FLUSH will give us valid packet
data to read. But that's not true; PACKET_READ_DELIM or PACKET_READ_EOF
will leave packet->line as NULL, and we'll segfault trying to look at
it.

Instead, we should follow the more careful model demonstrated on the
client side (e.g., in process_capabilities_v2): keep looping as long
as we get normal packets, and then make sure that we broke out of the
loop due to a real flush. That fixes the segfault and correctly
diagnoses any unexpected input from the client.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ls-refs.c

@@ -93,7 +93,7 @@ int ls_refs(struct repository *r, struct argv_array *keys,
 
 	git_config(ls_refs_config, NULL);
 
-	while (packet_reader_read(request) != PACKET_READ_FLUSH) {
+	while (packet_reader_read(request) == PACKET_READ_NORMAL) {
 		const char *arg = request->line;
 		const char *out;
 
@@ -105,6 +105,9 @@ int ls_refs(struct repository *r, struct argv_array *keys,
 			argv_array_push(&data.prefixes, out);
 	}
 
+	if (request->status != PACKET_READ_FLUSH)
+		die(_("expected flush after ls-refs arguments"));
+
 	head_ref_namespaced(send_ref, &data);
 	for_each_namespaced_ref(send_ref, &data);
 	packet_flush(1);