fix availability signaling of the send queue (quic-go#3597)

Author: marten-seemann

send_queue.go

@@ -36,6 +36,13 @@ func newSendQueue(conn sendConn) sender {
 func (h *sendQueue) Send(p *packetBuffer) {
 	select {
 	case h.queue <- p:
+		// clear available channel if we've reached capacity
+		if len(h.queue) == sendQueueCapacity {
+			select {
+			case <-h.available:
+			default:
+			}
+		}
 	case <-h.runStopped:
 	default:
 		panic("sendQueue.Send would have blocked")

send_queue_test.go

@@ -73,6 +73,48 @@ var _ = Describe("Send Queue", func() {
 		Eventually(done).Should(BeClosed())
 	})
 
+	It("signals when sending is possible again, when the first write succeeded", func() {
+		write := make(chan struct{}, 1)
+		written := make(chan struct{}, 100)
+		// now start sending out packets. This should free up queue space.
+		c.EXPECT().Write(gomock.Any()).DoAndReturn(func(b []byte) error {
+			<-write
+			written <- struct{}{}
+			return nil
+		}).AnyTimes()
+		// allow the first packet to be sent immediately
+		write <- struct{}{}
+
+		done := make(chan struct{})
+		go func() {
+			defer GinkgoRecover()
+			q.Run()
+			close(done)
+		}()
+
+		q.Send(getPacket([]byte("foobar")))
+		<-written
+
+		// now fill up the send queue
+		for i := 0; i < sendQueueCapacity+1; i++ {
+			Expect(q.WouldBlock()).To(BeFalse())
+			q.Send(getPacket([]byte("foobar")))
+		}
+
+		Expect(q.WouldBlock()).To(BeTrue())
+		Consistently(q.Available()).ShouldNot(Receive())
+		write <- struct{}{}
+		Eventually(q.Available()).Should(Receive())
+
+		// test shutdown
+		for i := 0; i < sendQueueCapacity; i++ {
+			write <- struct{}{}
+		}
+
+		q.Close()
+		Eventually(done).Should(BeClosed())
+	})
+
 	It("does not block pending send after the queue has stopped running", func() {
 		done := make(chan struct{})
 		go func() {