Merge pull request #9900 from michalpasztamobica/dns_async_handle_would_block

DNS send returning WOULD_BLOCK forces delayed retry

Author: 0xc0170

features/netsocket/nsapi_dns.cpp

@@ -1021,7 +1021,14 @@ static void nsapi_dns_query_async_send(void *ptr)
         err = query->socket->sendto(dns_addr, packet, len);
 
         if (err < 0) {
-            query->dns_server++;
+            if (err == NSAPI_ERROR_WOULD_BLOCK) {
+                nsapi_dns_call_in(query->call_in_cb, DNS_TIMER_TIMEOUT, mbed::callback(nsapi_dns_query_async_send, ptr));
+                free(packet);
+                dns_mutex->unlock();
+                return; // Timeout handler will retry the connection if possible
+            } else {
+                query->dns_server++;
+            }
         } else {
             break;
         }