derp: wait for send goroutine to finish before returning from run

I saw a test flake due to the sender goroutine logging (ultimately to
t.Logf) after the server was closed.

This makes sure the all goroutines are cleaned up before Server.Close
returns.

derp/derp_server.go

@@ -24,6 +24,7 @@ import (
 	"time"
 
 	"golang.org/x/crypto/nacl/box"
+	"golang.org/x/sync/errgroup"
 	"tailscale.com/metrics"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
@@ -247,16 +248,35 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) error
 	}
 	defer s.unregisterClient(c)
 
-	return c.run()
+	return c.run(ctx)
 }
 
-func (c *sclient) run() error {
+// run serves the client until there's an error.
-	go c.sender()
+// If the client hangs up or the server is closed, run returns nil, otherwise run returns an error.
+func (c *sclient) run(ctx context.Context) error {
+	// Launch sender, but don't return from run until sender goroutine is done.
+	var grp errgroup.Group
+	sendCtx, cancelSender := context.WithCancel(ctx)
+	grp.Go(func() error { return c.sendLoop(sendCtx) })
+	defer func() {
+		cancelSender()
+		if err := grp.Wait(); err != nil && !c.s.isClosed() {
+			c.logf("sender failed: %v", err)
+		}
+	}()
 
 	for {
 		ft, fl, err := readFrameHeader(c.br)
 		if err != nil {
-			return fmt.Errorf("client %x: readFrameHeader: %v", c.key, err)
+			if errors.Is(err, io.EOF) {
+				c.logf("read EOF")
+				return nil
+			}
+			if c.s.isClosed() {
+				c.logf("closing; server closed")
+				return nil
+			}
+			return fmt.Errorf("client %x: readFrameHeader: %w", c.key, err)
 		}
 		switch ft {
 		case frameNotePreferred:
@@ -518,17 +538,12 @@ func (c *sclient) setPreferred(v bool) {
 	}
 }
 
-func (c *sclient) sender() {
+func (c *sclient) sendLoop(ctx context.Context) error {
-	// 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(); err != nil {
-		c.logf("sender failed: %v", err)
-	}
-}
-
-func (c *sclient) sendLoop() error {
 	defer func() {
+		// If the sender shuts down unilaterally due to an error, close so
+		// that the receive loop unblocks and cleans up the rest.
+		c.nc.Close()
+
 		// Drain the send queue to count dropped packets
 		for {
 			select {
@@ -560,7 +575,7 @@ func (c *sclient) sendLoop() error {
 		// First, a non-blocking select (with a default) that
 		// does as many non-flushing writes as possible.
 		select {
-		case <-c.done:
+		case <-ctx.Done():
 			return nil
 		case msg := <-c.sendQueue:
 			werr = c.sendPacket(msg.src, msg.bs)
@@ -578,7 +593,7 @@ func (c *sclient) sendLoop() error {
 
 		// Then a blocking select with same:
 		select {
-		case <-c.done:
+		case <-ctx.Done():
 			return nil
 		case msg := <-c.sendQueue:
 			werr = c.sendPacket(msg.src, msg.bs)