Add WiFiClient flush to clear all non-read data in RX

fixes: https://github.com/espressif/arduino-esp32/issues/119

libraries/WiFi/src/WiFiClient.cpp

@@ -24,6 +24,7 @@
 
 #define WIFI_CLIENT_MAX_WRITE_RETRY   (10)
 #define WIFI_CLIENT_SELECT_TIMEOUT_US (100000)
+#define WIFI_CLIENT_FLUSH_BUFFER_SIZE (1024)
 
 #undef connect
 #undef write
@@ -251,6 +252,32 @@ int WiFiClient::available()
     return count;
 }
 
+// Though flushing means to send all pending data,
+// seems that in Arduino it also means to clear RX
+void WiFiClient::flush() {
+    size_t a = available(), toRead = 0;
+    if(!a){
+        return;//nothing to flush
+    }
+    uint8_t * buf = (uint8_t *)malloc(WIFI_CLIENT_FLUSH_BUFFER_SIZE);
+    if(!buf){
+        return;//memory error
+    }
+    while(a){
+        toRead = (a>WIFI_CLIENT_FLUSH_BUFFER_SIZE)?WIFI_CLIENT_FLUSH_BUFFER_SIZE:a;
+        if(recv(fd(), buf, toRead, MSG_DONTWAIT) < 0) {
+            if(errno != EWOULDBLOCK){
+                log_e("%d", errno);
+                stop();
+                break;
+            }
+            delay(1);//give some time
+        }
+        a = available();
+    }
+    free(buf);
+}
+
 uint8_t WiFiClient::connected()
 {
     uint8_t dummy = 0;

libraries/WiFi/src/WiFiClient.h

@@ -49,7 +49,7 @@ public:
     {
         return 0;
     }
-    void flush() {}
+    void flush();
     void stop();
     uint8_t connected();