extmod/modlwip: Fix polling of UDP socket so it doesn't return HUP.

STATE_NEW will return HUP when polled so put active UDP sockets into a new
state which is different to STATE_NEW.

Fixes issue #5758.

extmod/modlwip.c

@@ -308,6 +308,7 @@ typedef struct _lwip_socket_obj_t {
     #define STATE_CONNECTING 2
     #define STATE_CONNECTED 3
     #define STATE_PEER_CLOSED 4
+    #define STATE_ACTIVE_UDP 5
     // Negative value is lwIP error
     int8_t state;
 } lwip_socket_obj_t;
@@ -812,9 +813,13 @@ STATIC mp_obj_t lwip_socket_make_new(const mp_obj_type_t *type, size_t n_args, s
 
     lwip_socket_obj_t *socket = m_new_obj_with_finaliser(lwip_socket_obj_t);
     socket->base.type = &lwip_socket_type;
+    socket->timeout = -1;
+    socket->recv_offset = 0;
     socket->domain = MOD_NETWORK_AF_INET;
     socket->type = MOD_NETWORK_SOCK_STREAM;
     socket->callback = MP_OBJ_NULL;
+    socket->state = STATE_NEW;
+
     if (n_args >= 1) {
         socket->domain = mp_obj_get_int(args[0]);
         if (n_args >= 2) {
@@ -856,6 +861,7 @@ STATIC mp_obj_t lwip_socket_make_new(const mp_obj_type_t *type, size_t n_args, s
             break;
         }
         case MOD_NETWORK_SOCK_DGRAM: {
+            socket->state = STATE_ACTIVE_UDP;
             // Register our receive callback now. Since UDP sockets don't require binding or connection
             // before use, there's no other good time to do it.
             udp_recv(socket->pcb.udp, _lwip_udp_incoming, (void *)socket);
@@ -871,9 +877,6 @@ STATIC mp_obj_t lwip_socket_make_new(const mp_obj_type_t *type, size_t n_args, s
         #endif
     }
 
-    socket->timeout = -1;
-    socket->state = STATE_NEW;
-    socket->recv_offset = 0;
     return MP_OBJ_FROM_PTR(socket);
 }
 

tests/extmod/uselect_poll_udp.py

@@ -0,0 +1,28 @@
+# test select.poll on UDP sockets
+
+try:
+    import usocket as socket, uselect as select
+except ImportError:
+    try:
+        import socket, select
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+    
+
+s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+s.bind(socket.getaddrinfo('127.0.0.1', 8000)[0][-1])
+poll = select.poll()
+
+# UDP socket should not be readable
+poll.register(s, select.POLLIN)
+print(len(poll.poll(0)))
+
+# UDP socket should be writable
+poll.modify(s, select.POLLOUT)
+print(poll.poll(0)[0][1] == select.POLLOUT)
+
+# same test for select.select, but just skip it if the function isn't available
+if hasattr(select, "select"):
+    r, w, e = select.select([s], [], [], 0)
+    assert not r and not w and not e