nixos/nginx: Allow empty port for listen directive

When listening on unix sockets, it doesn't make sense to specify a port
for nginx's listen directive.

Since nginx defaults to port 80 when the port isn't specified (but the
address is), we can change the default for the option to null as well
without changing any behaviour.

nixos/lib/test-driver/test_driver/machine.py

@@ -791,6 +791,28 @@ class Machine:
         with self.nested(f"waiting for TCP port {port} on {addr}"):
             retry(port_is_open, timeout)
 
+    def wait_for_open_unix_socket(
+        self, addr: str, is_datagram: bool = False, timeout: int = 900
+    ) -> None:
+        """
+        Wait until a process is listening on the given UNIX-domain socket
+        (default to a UNIX-domain stream socket).
+        """
+
+        nc_flags = [
+            "-z",
+            "-uU" if is_datagram else "-U",
+        ]
+
+        def socket_is_open(_: Any) -> bool:
+            status, _ = self.execute(f"nc {' '.join(nc_flags)} {addr}")
+            return status == 0
+
+        with self.nested(
+            f"waiting for UNIX-domain {'datagram' if is_datagram else 'stream'} on '{addr}'"
+        ):
+            retry(socket_is_open, timeout)
+
     def wait_for_closed_port(
         self, port: int, addr: str = "localhost", timeout: int = 900
     ) -> None: