_dbus_loop_iterate: if the kernel says a fd is bad, stop watching it

Again, this shouldn't happen - modules are responsible for cleaning up
their watches - but the failure mode here is really bad: if we leave an
invalid fd in the set, every poll() call will instantly return, marking
it as POLLNVAL. The result is that dbus-daemon busy-loops on poll()
without responding to I/O, so the bad watch will probably never be
cleared up.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=32992
Bug-NB: NB#200248
Reviewed-by: Colin Walters <walters@verbum.org>

dbus/dbus-mainloop.c

@@ -829,6 +829,15 @@ _dbus_loop_iterate (DBusLoop     *loop,
                   
                   retval = TRUE;
                 }
+
+              if (_DBUS_UNLIKELY (fds[i].revents & _DBUS_POLLNVAL))
+                {
+                  _dbus_warn ("invalid request, socket fd %d not open\n",
+                      fds[i].fd);
+                  _dbus_watch_invalidate (wcb->watch);
+                  _dbus_loop_remove_watch (loop, wcb->watch, wcb->function,
+                      ((Callback *)wcb)->data);
+                }
             }
               
           ++i;