WiFiClient::Flush() guarantees that the data has been delivered

option 1 of #3967 (comment)
10ms max wait according to loaded tcp echo/reply scheme

Author: d-a-v

libraries/ESP8266WiFi/src/WiFiClient.cpp

@@ -259,7 +259,7 @@ size_t WiFiClient::peekBytes(uint8_t *buffer, size_t length) {
 void WiFiClient::flush()
 {
     if (_client)
-        _client->arduinoFlush();
+        _client->wait_until_sent();
 }
 
 void WiFiClient::stop()

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -107,7 +107,7 @@ class ClientContext
         if(this != 0) {
             DEBUGV(":ur %d\r\n", _refcnt);
             if(--_refcnt == 0) {
-                flush();
+                discard_received();
                 close();
                 if(_discard_cb) {
                     _discard_cb(_discard_cb_arg, this);
@@ -277,7 +277,7 @@ class ClientContext
         return copy_size;
     }
 
-    void flush()
+    void discard_received()
     {
         if(!_rx_buf) {
             return;
@@ -290,15 +290,20 @@ class ClientContext
         _rx_buf_offset = 0;
     }
 
-    void arduinoFlush()
+    void wait_until_sent()
     {
-#if 1
-	// this is useless since _write_from_source() always exits with _datasource==NULL
-        while (state() == ESTABLISHED && _datasource && _datasource->available()) {
+        // fix option 1 in
+        // https://github.com/esp8266/Arduino/pull/3967#pullrequestreview-83451496
+        // TODO: option 2
+
+        #define WAIT_TRIES_MS 10	// at most 10ms
+
+        int tries = 1+ WAIT_TRIES_MS;
+
+        while (state() == ESTABLISHED && tcp_sndbuf(_pcb) != TCP_SND_BUF && --tries) {
             _write_some();
             delay(1); // esp_ schedule+yield
         }
-#endif
     }
 
     uint8_t state() const

libraries/Ethernet/src/EthernetUdp.cpp

@@ -118,7 +118,7 @@ size_t EthernetUDP::write(const uint8_t *buffer, size_t size)
 int EthernetUDP::parsePacket()
 {
   // discard any remaining bytes in the last packet
-  clear_incoming();
+  clear_remaining();
 
   if (recvAvailable(_sock) > 0)
   {