derp: add sclient.done channel, simplify some context passing

This is mostly prep for a few future CLs, making sure we always have a
close-on-dead done channel available to select on when doing other
channel operations.

derp/derp_server.go

@@ -222,6 +222,9 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) error
 	// At this point we trust the client so we don't time out.
 	nc.SetDeadline(time.Time{})
 
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
 	c := &sclient{
 		s:           s,
 		key:         clientKey,
@@ -229,6 +232,7 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) error
 		br:          br,
 		bw:          bw,
 		logf:        logger.WithPrefix(s.logf, fmt.Sprintf("derp client %v/%x: ", remoteAddr, clientKey)),
+		done:        ctx.Done(),
 		remoteAddr:  remoteAddr,
 		connectedAt: time.Now(),
 		sendQueue:   make(chan pkt, perClientSendQueueDepth),
@@ -248,8 +252,6 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) error
 }
 
 func (c *sclient) run() error {
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
 	defer func() {
 		// Atomically close+remove send queue, so racing writers don't
 		// send to closed channel.
@@ -259,7 +261,7 @@ func (c *sclient) run() error {
 		c.mu.Unlock()
 	}()
 
-	go c.sender(ctx)
+	go c.sender()
 
 	for {
 		ft, fl, err := readFrameHeader(c.br)
@@ -270,9 +272,9 @@ func (c *sclient) run() error {
 		case frameNotePreferred:
 			err = c.handleFrameNotePreferred(ft, fl)
 		case frameSendPacket:
-			err = c.handleFrameSendPacket(ctx, ft, fl)
+			err = c.handleFrameSendPacket(ft, fl)
 		default:
-			err = c.handleUnknownFrame(ctx, ft, fl)
+			err = c.handleUnknownFrame(ft, fl)
 		}
 		if err != nil {
 			return err
@@ -280,7 +282,7 @@ func (c *sclient) run() error {
 	}
 }
 
-func (c *sclient) handleUnknownFrame(ctx context.Context, ft frameType, fl uint32) error {
+func (c *sclient) handleUnknownFrame(ft frameType, fl uint32) error {
 	_, err := io.CopyN(ioutil.Discard, c.br, int64(fl))
 	return err
 }
@@ -297,10 +299,10 @@ func (c *sclient) handleFrameNotePreferred(ft frameType, fl uint32) error {
 	return nil
 }
 
-func (c *sclient) handleFrameSendPacket(ctx context.Context, ft frameType, fl uint32) error {
+func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 	s := c.s
 
-	dstKey, contents, err := s.recvPacket(ctx, c.br, fl)
+	dstKey, contents, err := s.recvPacket(c.br, fl)
 	if err != nil {
 		return fmt.Errorf("client %x: recvPacket: %v", c.key, err)
 	}
@@ -446,7 +448,7 @@ func (s *Server) recvClientKey(br *bufio.Reader) (clientKey key.Public, info *cl
 	return clientKey, info, nil
 }
 
-func (s *Server) recvPacket(ctx context.Context, br *bufio.Reader, frameLen uint32) (dstKey key.Public, contents []byte, err error) {
+func (s *Server) recvPacket(br *bufio.Reader, frameLen uint32) (dstKey key.Public, contents []byte, err error) {
 	if frameLen < keyLen {
 		return key.Public{}, nil, errors.New("short send packet frame")
 	}
@@ -476,7 +478,8 @@ type sclient struct {
 	key        key.Public
 	info       clientInfo
 	logf       logger.Logf
-	remoteAddr string // usually ip:port from net.Conn.RemoteAddr().String()
+	done       <-chan struct{} // closed when connection closes
+	remoteAddr string          // usually ip:port from net.Conn.RemoteAddr().String()
 
 	// Owned by run, not thread-safe.
 	br          *bufio.Reader
@@ -521,16 +524,16 @@ func (c *sclient) setPreferred(v bool) {
 	}
 }
 
-func (c *sclient) sender(ctx context.Context) {
+func (c *sclient) sender() {
 	// If the sender shuts down unilaterally due to an error, close so
 	// that the receive loop unblocks and cleans up the rest.
 	defer c.nc.Close()
-	if err := c.sendLoop(ctx); err != nil {
+	if err := c.sendLoop(); err != nil {
 		c.logf("sender failed: %v", err)
 	}
 }
 
-func (c *sclient) sendLoop(ctx context.Context) error {
+func (c *sclient) sendLoop() error {
 	c.mu.RLock()
 	queue := c.sendQueue
 	c.mu.RUnlock()
@@ -566,7 +569,7 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 
 	for {
 		select {
-		case <-ctx.Done():
+		case <-c.done:
 			return nil
 
 		case pkt, ok := <-queue: