Send EOF on stdout when exiting due to EOF on stdin

Previously, if a service with exit-on-client-eof=true received EOF on
stdin while stdout was still open, no EOF would be sent on stdout.  This
is a (currently harmless) violation of the qrexec protocol.  Fix this by
sending an empty MSG_DATA_STDOUT to indicate EOF on stdout.

Fixes: QubesOS/qubes-issues#9429

libqrexec/process_io.c

@@ -169,6 +169,14 @@ int qrexec_process_io(const struct process_io_request *req,
     /* Convenience macros that eliminate a ton of error-prone boilerplate */
 #define close_stdin() do {                                      \
     if (exit_on_stdin_eof) {                                    \
+        /* If stdout is still open, send EOF */                 \
+        if (stdout_fd != -1) {                                  \
+            const struct msg_header hdr = {                     \
+                .type = stdout_msg_type,                        \
+                .len = 0,                                       \
+            };                                                  \
+            libvchan_send(vchan, &hdr, sizeof(hdr));            \
+        };                                                      \
         /* Set stdin_fd and stdout_fd to -1.                    \
          * No need to close them as the process                 \
          * will soon exit. */                                   \

qrexec/tests/socket/agent.py

@@ -758,6 +758,7 @@ def test_connect_socket_exit_on_stdin_eof(self):
         self.assertEqual(target.recv_all_messages(),
             [
                 (qrexec.MSG_DATA_STDERR, b""),
+                (qrexec.MSG_DATA_STDOUT, b""),
                 (qrexec.MSG_DATA_EXIT_CODE, b"\0\0\0\0"),
             ])
         self.check_dom0(dom0)