Fix agent race condition

When agent write goroutine closed the channel heartbeat and send functions
could still write to the channel causing a panic. send already had a recover
for this but heartbeat didn't have. This commit does not close the channel anymore
and to make so the writers don't block sending when an agent is closed a select is used
reading from die channel that signals that a agent is closed.

The write channel is never closed but it's ok as when a Agent is not used anymore it will be
garbage collected as the agent is the only reference to it.

agent/agent.go

@@ -203,7 +203,11 @@ func (a *Agent) send(pendingMsg pendingMessage) (err error) {
 		pWrite.err = util.GetErrorFromPayload(a.serializer, m.Data)
 	}
 
-	a.chSend <- pWrite
+	// chSend is never closed so we need this to don't block if agent is already closed
+	select {
+	case a.chSend <- pWrite:
+	case <-a.chDie:
+	}
 	return
 }
 
@@ -326,10 +330,7 @@ func (a *Agent) Handle() {
 
 	go a.write()
 	go a.heartbeat()
-	select {
-	case <-a.chDie: // agent closed signal
-		return
-	}
+	<-a.chDie // agent closed signal
 }
 
 // IPVersion returns the remote address ip version.
@@ -365,7 +366,15 @@ func (a *Agent) heartbeat() {
 				logger.Log.Debugf("Session heartbeat timeout, LastTime=%d, Deadline=%d", atomic.LoadInt64(&a.lastAt), deadline)
 				return
 			}
-			a.chSend <- pendingWrite{data: hbd}
+
+			// chSend is never closed so we need this to don't block if agent is already closed
+			select {
+			case a.chSend <- pendingWrite{data: hbd}:
+			case <-a.chDie:
+				return
+			case <-a.chStopHeartbeat:
+				return
+			}
 		case <-a.chDie:
 			return
 		case <-a.chStopHeartbeat:
@@ -399,7 +408,6 @@ func (a *Agent) SendHandshakeResponse() error {
 func (a *Agent) write() {
 	// clean func
 	defer func() {
-		close(a.chSend)
 		a.Close()
 	}()