derp: only flush writes to clients when we're out of things to write

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

derp/derp_server.go

@@ -552,39 +552,57 @@ func (c *sclient) sendLoop() error {
 	keepAliveTick := time.NewTicker(keepAlive + jitter)
 	defer keepAliveTick.Stop()
 
+	var werr error // last write error
 	for {
+		if werr != nil {
+			return werr
+		}
+		// First, a non-blocking select (with a default) that
+		// does as many non-flushing writes as possible.
 		select {
 		case <-c.done:
 			return nil
-
-		case msg, ok := <-c.sendQueue:
-			if !ok {
-				return nil
+		case msg := <-c.sendQueue:
+			werr = c.sendPacket(msg.src, msg.bs)
+			continue
+		case <-keepAliveTick.C:
+			werr = c.sendKeepAlive()
+			continue
+		default:
+			// Flush any writes from the 3 sends above, or from
+			// the blocking loop below.
+			if werr = c.bw.Flush(); werr != nil {
+				return werr
 			}
-			if err := c.sendPacket(msg.src, msg.bs); err != nil {
-				return err
 		}
 
+		// Then a blocking select with same:
+		select {
+		case <-c.done:
+			return nil
+		case msg := <-c.sendQueue:
+			werr = c.sendPacket(msg.src, msg.bs)
 		case <-keepAliveTick.C:
-			if err := c.sendKeepalive(); err != nil {
-				return err
-			}
+			werr = c.sendKeepAlive()
 		}
 	}
 }
 
-func (c *sclient) sendKeepalive() error {
+func (c *sclient) setWriteDeadline() {
 	c.nc.SetWriteDeadline(time.Now().Add(writeTimeout))
-	if err := writeFrame(c.bw, frameKeepAlive, nil); err != nil {
-		return err
-	}
-	return c.bw.Flush()
+}
+
+// sendKeepAlive sends a keep-alive frame, without flushing.
+func (c *sclient) sendKeepAlive() error {
+	c.setWriteDeadline()
+	return writeFrameHeader(c.bw, frameKeepAlive, 0)
 }
 
 // sendPacket writes contents to the client in a RecvPacket frame. If
 // srcKey.IsZero, uses the old DERPv1 framing format, otherwise uses
 // DERPv2. The bytes of contents are only valid until this function
 // returns, do not retain slices.
+// It does not flush its bufio.Writer.
 func (c *sclient) sendPacket(srcKey key.Public, contents []byte) (err error) {
 	defer func() {
 		// Stats update.
@@ -600,7 +618,7 @@ func (c *sclient) sendPacket(srcKey key.Public, contents []byte) (err error) {
 		}
 	}()
 
-	c.nc.SetWriteDeadline(time.Now().Add(writeTimeout))
+	c.setWriteDeadline()
 
 	withKey := !srcKey.IsZero()
 	pktLen := len(contents)
@@ -615,10 +633,8 @@ func (c *sclient) sendPacket(srcKey key.Public, contents []byte) (err error) {
 			return err
 		}
 	}
-	if _, err = c.bw.Write(contents); err != nil {
+	_, err = c.bw.Write(contents)
 	return err
-	}
-	return c.bw.Flush()
 }
 
 func (s *Server) expVarFunc(f func() interface{}) expvar.Func {