extmod/modlwip: Properly handle non-blocking and timeout on UDP recv.

Fixes UDP non-blocking recv so it returns EAGAIN instead of ETIMEDOUT.
Timeout waiting for incoming data is also improved by replacing 100ms delay
with poll_sockets(), as is done in other parts of this module.

Fixes issue #5759.

extmod/modlwip.c

@@ -598,21 +598,20 @@ STATIC mp_uint_t lwip_raw_udp_send(lwip_socket_obj_t *socket, const byte *buf, m
 STATIC mp_uint_t lwip_raw_udp_receive(lwip_socket_obj_t *socket, byte *buf, mp_uint_t len, byte *ip, mp_uint_t *port, int *_errno) {
 
     if (socket->incoming.pbuf == NULL) {
-        if (socket->timeout != -1) {
+        if (socket->timeout == 0) {
-            for (mp_uint_t retries = socket->timeout / 100; retries--;) {
+            // Non-blocking socket.
-                mp_hal_delay_ms(100);
+            *_errno = MP_EAGAIN;
-                if (socket->incoming.pbuf != NULL) {
+            return -1;
-                    break;
+        }
-                }
+
-            }
+        // Wait for data to arrive on UDP socket.
-            if (socket->incoming.pbuf == NULL) {
+        mp_uint_t start = mp_hal_ticks_ms();
+        while (socket->incoming.pbuf == NULL) {
+            if (socket->timeout != -1 && mp_hal_ticks_ms() - start > socket->timeout) {
                 *_errno = MP_ETIMEDOUT;
                 return -1;
             }
-        } else {
+            poll_sockets();
-            while (socket->incoming.pbuf == NULL) {
-                poll_sockets();
-            }
         }
     }
 

tests/extmod/usocket_udp_nonblock.py

@@ -0,0 +1,20 @@
+# test non-blocking UDP sockets
+
+try:
+    import usocket as socket, uerrno as errno
+except ImportError:
+    try:
+        import socket, errno
+    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])
+s.settimeout(0)
+
+try:
+    s.recv(1)
+except OSError as er:
+    print('EAGAIN:', er.args[0] == errno.EAGAIN)