agent: fix error path on failed service exec

(cherry-picked from fixes by Marek Marczykowski-Górecki)

If service fails to start, exit instead of trying to handle
(non-existing) process.

agent/qrexec-agent-data.c

@@ -221,7 +221,23 @@ static int handle_new_process_common(int type, int connect_domain, int connect_p
         case MSG_EXEC_CMDLINE:
             buffer_init(&stdin_buf);
             if (execute_qubes_rpc_command(cmdline, &pid, &stdin_fd, &stdout_fd, &stderr_fd, !qrexec_is_fork_server, &stdin_buf) < 0) {
+                struct msg_header hdr = {
+                    .type = MSG_DATA_STDOUT,
+                    .len = 0,
+                };
                 fputs("failed to spawn process\n", stderr);
+                /* Send stdout+stderr EOF first, since the service is expected to send
+                 * one before exit code in case of MSG_EXEC_CMDLINE. Ignore
+                 * libvchan_send error if any, as we're going to terminate soon
+                 * anyway.
+                 */
+                libvchan_send(data_vchan, &hdr, sizeof(hdr));
+                hdr.type = MSG_DATA_STDERR;
+                libvchan_send(data_vchan, &hdr, sizeof(hdr));
+                exit_code = 127;
+                send_exit_code(data_vchan, exit_code);
+                libvchan_close(data_vchan);
+                return exit_code;
             }
             fprintf(stderr, "executed %s pid %d\n", cmdline, pid);
             is_service = true;

qrexec/tests/socket/agent.py

@@ -295,7 +295,6 @@ def test_exec_service(self):
             (qrexec.MSG_DATA_EXIT_CODE, b'\0\0\0\0')
         ])
 
-    @pytest.mark.xfail
     def test_exec_service_fail(self):
         target = self.execute_qubesrpc('qubes.Service+arg', 'domX')
         target.send_message(qrexec.MSG_DATA_STDIN, b'')