[ESP8266] Drop signalling SIGIO artificially if UDP send fails

With TCP it's desirable that SIGIO wakes up the application to check
if there is buffer space space available on the mode. With UDP the
behavior is not acceptable as we don't know if the other endpoint is
there as connection establishment is missing. Hence buffers might
stay full forever.

Author: Veijo Pesonen

components/wifi/esp8266-driver/ESP8266Interface.cpp

@@ -505,9 +505,11 @@ int ESP8266Interface::socket_send(void *handle, const void *data, unsigned size)
     } while ((sendStartTime - rtos::Kernel::get_ms_count() < 50)
             && (status != NSAPI_ERROR_OK));
 
-    if (status == NSAPI_ERROR_WOULD_BLOCK) {
+    if (status == NSAPI_ERROR_WOULD_BLOCK && socket->proto == NSAPI_TCP) {
         debug("Enqueuing the event call");
         _global_event_queue->call_in(100, callback(this, &ESP8266Interface::event));
+    } else if (status == NSAPI_ERROR_WOULD_BLOCK && socket->proto == NSAPI_UDP) {
+        status = NSAPI_ERROR_DEVICE_ERROR;
     }
 
     return status != NSAPI_ERROR_OK ? status : size;