Fixed slowdown with repeated CDC bool operator test

Rebase and squash of arduino/Arduino#3624

Author: majenkotech

cores/arduino/CDC.cpp

@@ -29,6 +29,7 @@ typedef struct
 	u8 	bParityType;
 	u8 	bDataBits;
 	u8	lineState;
+	u8	prevLineState;
 } LineInfo;
 
 static volatile LineInfo _usbLineInfo = { 57600, 0x00, 0x00, 0x00, 0x00 };
@@ -221,7 +222,7 @@ size_t Serial_::write(const uint8_t *buffer, size_t size)
 	// TODO - ZE - check behavior on different OSes and test what happens if an
 	// open connection isn't broken cleanly (cable is yanked out, host dies
 	// or locks up, or host virtual serial port hangs)
-	if (_usbLineInfo.lineState > 0)	{
+	if (this) {
 		int r = USB_Send(CDC_TX,buffer,size);
 		if (r > 0) {
 			return r;
@@ -240,12 +241,18 @@ size_t Serial_::write(const uint8_t *buffer, size_t size)
 // setup() before printing to ensure that an application on the host is
 // actually ready to receive and display the data.
 // We add a short delay before returning to fix a bug observed by Federico
-// where the port is configured (lineState != 0) but not quite opened.
+// where the port is configured (lineState != 0) but not quite opened.  This
+// only happens when the line state transitions from ==0 to >0 otherwise you
+// get a nasty delay when one isn't needed.
 Serial_::operator bool() {
 	bool result = false;
-	if (_usbLineInfo.lineState > 0) 
+	if (_usbLineInfo.lineState > 0) {
 		result = true;
-	delay(10);
+		if (_usbLineInfo.prevLineState == 0) {
+			delay(10);
+		}
+	}
+	_usbLineInfo.prevLineState = _usbLineInfo.lineState;
 	return result;
 }