fix #1002 ::Flush() wait for empty send buffer (#3967)

* fix #1002 ::Flush() wait for empty send buffer

* WiFiClient::Flush() guarantees that the data has been delivered
option 1 of https://github.com/esp8266/Arduino/pull/3967#discussion_r156901071
10ms max wait according to loaded tcp echo/reply scheme

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->flush();
+        _client->wait_until_sent();
 }
 
 void WiFiClient::stop()

libraries/ESP8266WiFi/src/WiFiUdp.cpp

@@ -243,8 +243,7 @@ int WiFiUDP::peek()
 
 void WiFiUDP::flush()
 {
-    if (_ctx)
-        _ctx->flush();
+    endPacket();
 }
 
 IPAddress WiFiUDP::remoteIP()

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -107,7 +107,7 @@ public:
         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 @@ public:
         return copy_size;
     }
 
-    void flush()
+    void discard_received()
     {
         if(!_rx_buf) {
             return;
@@ -290,6 +290,22 @@ public:
         _rx_buf_offset = 0;
     }
 
+    void wait_until_sent()
+    {
+        // 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
+        }
+    }
+
     uint8_t state() const
     {
         if(!_pcb) {

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
-  flush();
+  clear_remaining();
 
   if (recvAvailable(_sock) > 0)
   {
@@ -204,7 +204,7 @@ int EthernetUDP::peek()
   return b;
 }
 
-void EthernetUDP::flush()
+void EthernetUDP::clear_remaining()
 {
   // could this fail (loop endlessly) if _remaining > 0 and recv in read fails?
   // should only occur if recv fails after telling us the data is there, lets
@@ -216,3 +216,8 @@ void EthernetUDP::flush()
   }
 }
 
+void EthernetUDP::flush()
+{
+  endPacket();  
+}
+

libraries/Ethernet/src/EthernetUdp.h

@@ -49,6 +49,9 @@ private:
   uint16_t _remotePort; // remote port for the incoming packet whilst it's being processed
   uint16_t _offset; // offset into the packet being sent
   uint16_t _remaining; // remaining bytes of incoming packet yet to be processed
+  
+protected:
+  void clear_remaining();
 
 public:
   EthernetUDP();  // Constructor