derp: use a dedicated queue for disco traffic.

Signed-off-by: David Anderson <danderson@tailscale.com>

derp/derp_server.go

@@ -490,6 +490,7 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string, connN
 		remoteIPPort:   remoteIPPort,
 		connectedAt:    time.Now(),
 		sendQueue:      make(chan pkt, perClientSendQueueDepth),
+		discoSendQueue: make(chan pkt, perClientSendQueueDepth),
 		peerGone:       make(chan key.Public),
 		canMesh:        clientInfo.MeshKey != "" && clientInfo.MeshKey == s.meshKey,
 	}
@@ -739,6 +740,10 @@ func (c *sclient) sendPkt(dst *sclient, p pkt) error {
 	// Attempt to queue for sending up to 3 times. On each attempt, if
 	// the queue is full, try to drop from queue head to prioritize
 	// fresher packets.
+	sendQueue := dst.sendQueue
+	if disco.LooksLikeDiscoWrapper(p.bs) {
+		sendQueue = dst.discoSendQueue
+	}
 	for attempt := 0; attempt < 3; attempt++ {
 		select {
 		case <-dst.done:
@@ -747,13 +752,13 @@ func (c *sclient) sendPkt(dst *sclient, p pkt) error {
 		default:
 		}
 		select {
-		case dst.sendQueue <- p:
+		case sendQueue <- p:
 			return nil
 		default:
 		}
 
 		select {
-		case pkt := <-dst.sendQueue:
+		case pkt := <-sendQueue:
 			s.recordDrop(pkt.bs, c.key, dstKey, dropReasonQueueHead)
 			c.recordQueueTime(pkt.enqueuedAt)
 		default:
@@ -942,6 +947,7 @@ type sclient struct {
 	remoteAddr     string          // usually ip:port from net.Conn.RemoteAddr().String()
 	remoteIPPort   netaddr.IPPort  // zero if remoteAddr is not ip:port.
 	sendQueue      chan pkt        // packets queued to this client; never closed
+	discoSendQueue chan pkt        // important packets queued to this client; never closed
 	peerGone       chan key.Public // write request that a previous sender has disconnected (not used by mesh peers)
 	meshUpdate     chan struct{}   // write request to write peerStateChange
 	canMesh        bool            // clientInfo had correct mesh token for inter-region routing
@@ -1039,6 +1045,8 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 			select {
 			case pkt := <-c.sendQueue:
 				c.s.recordDrop(pkt.bs, pkt.src, c.key, dropReasonGone)
+			case pkt := <-c.discoSendQueue:
+				c.s.recordDrop(pkt.bs, pkt.src, c.key, dropReasonGone)
 			default:
 				return
 			}
@@ -1069,6 +1077,10 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 			werr = c.sendPacket(msg.src, msg.bs)
 			c.recordQueueTime(msg.enqueuedAt)
 			continue
+		case msg := <-c.discoSendQueue:
+			werr = c.sendPacket(msg.src, msg.bs)
+			c.recordQueueTime(msg.enqueuedAt)
+			continue
 		case <-keepAliveTick.C:
 			werr = c.sendKeepAlive()
 			continue
@@ -1092,6 +1104,9 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 		case msg := <-c.sendQueue:
 			werr = c.sendPacket(msg.src, msg.bs)
 			c.recordQueueTime(msg.enqueuedAt)
+		case msg := <-c.discoSendQueue:
+			werr = c.sendPacket(msg.src, msg.bs)
+			c.recordQueueTime(msg.enqueuedAt)
 		case <-keepAliveTick.C:
 			werr = c.sendKeepAlive()
 		}