Fixed bug #20287: tcp_output_nagle sends too early. This fixes the nagle algorithm; nagle now also works for all raw API applications and has to be explicitly disabled with 'tcp_pcb->flags |= TF_NODELAY'

src/api/api_msg.c

@@ -819,6 +819,9 @@ do_writemore(struct netconn *conn)
       API_EVENT(conn, NETCONN_EVT_SENDMINUS, len);
     }
   } else if (err == ERR_MEM) {
+    /* tcp_enqueue returned ERR_MEM, try tcp_output anyway */
+    err = tcp_output(conn->pcb.tcp);
+
 #if LWIP_TCPIP_CORE_LOCKING
     conn->write_delayed = 1;
 #endif