Fixed dnstap panic after graceful restart (send on closed channel) (#1479)

plugin/dnstap/dnstapio/io.go

@@ -24,6 +24,7 @@ type dnstapIO struct {
 	enc      *dnstapEncoder
 	queue    chan tap.Dnstap
 	dropped  uint32
+	quit     chan struct{}
 }
 
 // New returns a new and initialized DnstapIO.
@@ -36,6 +37,7 @@ func New(endpoint string, socket bool) DnstapIO {
 			Bidirectional: true,
 		}),
 		queue: make(chan tap.Dnstap, queueSize),
+		quit:  make(chan struct{}),
 	}
 }
 
@@ -92,7 +94,7 @@ func (dio *dnstapIO) closeConnection() {
 
 // Close waits until the I/O routine is finished to return.
 func (dio *dnstapIO) Close() {
-	close(dio.queue)
+	close(dio.quit)
 }
 
 func (dio *dnstapIO) flushBuffer() {
@@ -124,12 +126,11 @@ func (dio *dnstapIO) serve() {
 	timeout := time.After(flushTimeout)
 	for {
 		select {
-		case payload, ok := <-dio.queue:
-			if !ok {
-				dio.flushBuffer()
-				dio.closeConnection()
-				return
-			}
+		case <-dio.quit:
+			dio.flushBuffer()
+			dio.closeConnection()
+			return
+		case payload := <-dio.queue:
 			dio.write(&payload)
 		case <-timeout:
 			if dropped := atomic.SwapUint32(&dio.dropped, 0); dropped > 0 {