extmod/modlwip: Make socket poll return POLLNVAL in case of bad file.

extmod/modlwip.c

@@ -1477,6 +1477,8 @@ STATIC mp_uint_t lwip_socket_ioctl(mp_obj_t self_in, mp_uint_t request, uintptr_
         } else if (socket->state == ERR_RST) {
             // Socket was reset by peer, a write will return an error
             ret |= flags & (MP_STREAM_POLL_WR | MP_STREAM_POLL_HUP);
+        } else if (socket->state == _ERR_BADF) {
+            ret |= MP_STREAM_POLL_NVAL;
         } else if (socket->state < 0) {
             // Socket in some other error state, use catch-all ERR flag
             // TODO: may need to set other return flags here

tests/extmod/uselect_poll_basic.py

@@ -33,3 +33,9 @@ try:
     poller.modify(s, select.POLLIN)
 except OSError as e:
     assert e.args[0] == errno.ENOENT
+
+# poll after closing the socket, should return POLLNVAL
+poller.register(s)
+s.close()
+p = poller.poll(0)
+print(len(p), p[0][-1])