fix(console): novncd that cannot listen ends with a failure status

websockify logs "Openening socket failed" and calls sys.exit() when its
port is taken, so novncd ended with status 0. Under systemd the unit
then counts as stopped, not failed: Restart=on-failure does not start it
again, systemctl --failed does not list it, and VNC consoles stay down
until someone starts it by hand. novncd now ends with status 1 there.

console/novncd

@@ -200,7 +200,7 @@ if __name__ == "__main__":
     options.verbose = options.verbose or options.debug
 
     # nginx ends TLS: novncd speaks plain websockets on the loopback
-    WebSocketProxy(
+    server = WebSocketProxy(
         RequestHandlerClass=ConsoleProxyHandler,
         listen_host=options.host,
         listen_port=options.port,
@@ -212,4 +212,10 @@ if __name__ == "__main__":
         # a ping while the VM's screen stays the same: nginx, and a proxy on
         # the way, end a websocket that sends nothing for 60 seconds
         heartbeat=30,
-    ).start_server()
+    )
+    try:
+        server.start_server()
+    except SystemExit as stop:
+        # websockify ends with status 0 when it cannot listen (the port is
+        # taken): systemd's Restart=on-failure would not start it again
+        raise SystemExit(stop.code or 1) from None

console/tests/test_novncd_exit.py

@@ -0,0 +1,22 @@
+"""novncd that cannot listen ends with a failure status, which systemd restarts."""
+
+import socket
+import subprocess
+import sys
+from pathlib import Path
+
+from django.test import SimpleTestCase
+
+ROOT = Path(__file__).resolve().parent.parent.parent
+
+
+class NovncdExitTestCase(SimpleTestCase):
+    def test_a_taken_port_ends_novncd_with_a_failure_status(self):
+        with socket.socket() as taken:
+            taken.bind(("127.0.0.1", 0))
+            taken.listen()
+            port = taken.getsockname()[1]
+            novncd = subprocess.run([sys.executable, str(ROOT / "console" / "novncd"), "-v", "-H", "127.0.0.1",
+                                     "-p", str(port)], cwd=ROOT, capture_output=True, timeout=60)
+        self.assertNotEqual(novncd.returncode, 0)
+        self.assertIn(b"Openening socket failed", novncd.stderr)