Try to retrieve exit code of a failed service call

When service call fails, the remote (service) side sends EOF + exit code
and closes the vchan immediately. If the client side tries to send
something, it will fail and exit immediately (with code 1). But reading
data that was queued in the vchan before its closing is still possible.
So, on send error check if there is anything interesting to receive
(especially exit code, but potentially also some service output) and, if
yes, don't exit immediately. Since the service exit code is sent last
(after all stdout/stderr data and their EOF), it's okay to check just
for remote_status.
This is relevant only to the service client side, as exit status of the
service-handling process is not relevant.

Fixes QubesOS/qubes-issues#9618

libqrexec/process_io.c

@@ -346,7 +346,16 @@ int qrexec_process_io(const struct process_io_request *req,
                         vchan, stdout_fd, stdout_msg_type,
                         &prefix, &remote_buffer)) {
                 case REMOTE_ERROR:
-                    handle_vchan_error("send(handle_input stdout)");
+                    if (!is_service && remote_status == -1) {
+                        /* Even if sending fails, still try to read remaining
+                         * data, if any - especially the exit code. But don't
+                         * send anything anymore.
+                         */
+                        LOG(ERROR, "Error while vchan send (handle_input stdout), reading remaining data");
+                        close_stdout();
+                    } else {
+                        handle_vchan_error("send(handle_input stdout)");
+                    }
                     break;
                 case REMOTE_EOF:
                     close_stdout();