Merge pull request goburrow#36 from grid-x/wz2b-master

fix: flush read buffer before writing request

Author: guelfey

tcpclient.go

@@ -155,6 +155,15 @@ func (mb *tcpTransporter) Send(aduRequest []byte) (aduResponse []byte, err error
 			return
 		}
 
+		// If an answer to a previously timed-out request is already in teh buffer, this will result
+		// in a transaction ID mismatch from which we will never recover.  To prevent this, just
+		// flush any previous reponses before launching the next poll.  That's throwing away
+		// possibly useful data, but the previous request was already satisfied with a timeout
+		// error so that probably makes the most sense here.
+
+		// Be aware that this call resets the read deadline.
+		mb.flushAll()
+
 		// Set timer to close when idle
 		mb.lastActivity = time.Now()
 		mb.startCloseTimer()
@@ -332,3 +341,27 @@ func (mb *tcpTransporter) closeIdle() {
 		mb.close()
 	}
 }
+
+// flushAll implements a non-blocking read flush.  Be warned it resets
+// the read deadline.
+func (mb *tcpTransporter) flushAll() (int, error) {
+	if err := mb.conn.SetReadDeadline(time.Now()); err != nil {
+		return 0, err
+	}
+
+	count := 0
+	buffer := make([]byte, 1024)
+
+	for {
+		n, err := mb.conn.Read(buffer)
+
+		if err != nil {
+			return count + n, err
+		} else if n > 0 {
+			count = count + n
+		} else {
+			// didn't flush any new bytes, return
+			return count, err
+		}
+	}
+}