extmod/modlwip: Commit TCP out data to lower layers if buffer gets full.

Dramatically improves TCP sending throughput because without an explicit
call to tcp_output() the data is only sent to the lower layers via the
lwIP slow timer which (by default) ticks every 500ms.

extmod/modlwip.c

@@ -498,6 +498,11 @@ STATIC mp_uint_t lwip_tcp_send(lwip_socket_obj_t *socket, const byte *buf, mp_ui
 
     err_t err = tcp_write(socket->pcb.tcp, buf, write_len, TCP_WRITE_FLAG_COPY);
 
+    // If the output buffer is getting full then send the data to the lower layers
+    if (err == ERR_OK && tcp_sndbuf(socket->pcb.tcp) < TCP_SND_BUF / 4) {
+        err = tcp_output(socket->pcb.tcp);
+    }
+
     if (err != ERR_OK) {
         *_errno = error_lookup_table[-err];
         return MP_STREAM_ERROR;