derp: remove the mutex around and closing of send channel

Makes it less complicated.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_server.go

@@ -252,15 +252,6 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) error
 }
 
 func (c *sclient) run() error {
-	defer func() {
-		// Atomically close+remove send queue, so racing writers don't
-		// send to closed channel.
-		c.mu.Lock()
-		close(c.sendQueue)
-		c.sendQueue = nil
-		c.mu.Unlock()
-	}()
-
 	go c.sender()
 
 	for {
@@ -320,16 +311,6 @@ func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 		return nil
 	}
 
-	dst.mu.RLock()
-	defer dst.mu.RUnlock()
-	if dst.sendQueue == nil {
-		s.packetsDropped.Add(1)
-		s.packetsDroppedGone.Add(1)
-		if debug {
-			c.logf("dropping packet for shutdown client %x", dstKey)
-		}
-	}
-
 	msg := sendMsg{
 		bs: contents,
 	}
@@ -340,6 +321,16 @@ func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 	// the queue is full, try to drop from queue head to prioritize
 	// fresher packets.
 	for attempt := 0; attempt < 3; attempt++ {
+		select {
+		case <-dst.done:
+			s.packetsDropped.Add(1)
+			s.packetsDroppedGone.Add(1)
+			if debug {
+				c.logf("dropping packet for shutdown client %x", dstKey)
+			}
+			return nil
+		default:
+		}
 		select {
 		case dst.sendQueue <- msg:
 			return nil
@@ -480,6 +471,7 @@ type sclient struct {
 	logf       logger.Logf
 	done       <-chan struct{} // closed when connection closes
 	remoteAddr string          // usually ip:port from net.Conn.RemoteAddr().String()
+	sendQueue  chan sendMsg    // messages (packets) queued to this client; never closed
 
 	// Owned by run, not thread-safe.
 	br          *bufio.Reader
@@ -488,9 +480,6 @@ type sclient struct {
 
 	// Owned by sender, not thread-safe.
 	bw *bufio.Writer
-
-	mu        sync.RWMutex // guards access to sendQueue for shutdown.
-	sendQueue chan sendMsg // messages (packets) queued to this client
 }
 
 // sendMsg is a request to write a frame to an sclient (usually a data packet).
@@ -540,28 +529,19 @@ func (c *sclient) sender() {
 }
 
 func (c *sclient) sendLoop() error {
-	c.mu.RLock()
-	queue := c.sendQueue
-	c.mu.RUnlock()
-
-	if queue == nil {
-		// Uncommon race, sclient shut down before this loop ever
-		// started. Nothing to do here, move along.
-		return nil
-	}
-
 	defer func() {
 		// Drain the send queue to count dropped packets
 		for {
-			_, ok := <-queue
-			if !ok {
-				break
-			}
+			select {
+			case <-c.sendQueue:
 				c.s.packetsDropped.Add(1)
 				c.s.packetsDroppedGone.Add(1)
 				if debug {
 					c.logf("dropping packet for shutdown %x", c.key)
 				}
+			default:
+				return
+			}
 		}
 	}()
 
@@ -578,7 +558,7 @@ func (c *sclient) sendLoop() error {
 		case <-c.done:
 			return nil
 
-		case msg, ok := <-queue:
+		case msg, ok := <-c.sendQueue:
 			if !ok {
 				return nil
 			}