quic: implement stream flush

Do not commit data written to a stream to the network until
the user explicitly flushes the stream, the stream output
buffer fills, or the output buffer contains enough data to
fill a packet.

We could write data immediately (as net.TCPConn does),
but this can require the user to put their own buffer in
front of the stream. Since we necessarily need to maintain
a retransmit buffer in the stream, this is redundant.

We could do something like Nagle's algorithm, but nobody
wants that.

So make flushes explicit.

For golang/go#58547

Change-Id: I29dc9d79556c7a358a360ef79beb38b45040b6bc
Reviewed-on: https://go-review.googlesource.com/c/net/+/543083
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/quic/conn.go

@@ -136,12 +136,10 @@ func newConn(now time.Time, side connSide, cids newServerConnIDs, peerAddr netip
 		}
 	}
 
-	// The smallest allowed maximum QUIC datagram size is 1200 bytes.
 	// TODO: PMTU discovery.
-	const maxDatagramSize = 1200
 	c.logConnectionStarted(cids.originalDstConnID, peerAddr)
 	c.keysAppData.init()
-	c.loss.init(c.side, maxDatagramSize, now)
+	c.loss.init(c.side, smallestMaxDatagramSize, now)
 	c.streamsInit()
 	c.lifetimeInit()
 	c.restartIdleTimer(now)

internal/quic/conn_flow_test.go

@@ -262,6 +262,7 @@ func TestConnOutflowBlocked(t *testing.T) {
 	if n != len(data) || err != nil {
 		t.Fatalf("s.Write() = %v, %v; want %v, nil", n, err, len(data))
 	}
+	s.Flush()
 
 	tc.wantFrame("stream writes data up to MAX_DATA limit",
 		packetType1RTT, debugFrameStream{
@@ -310,6 +311,7 @@ func TestConnOutflowMaxDataDecreases(t *testing.T) {
 	if n != len(data) || err != nil {
 		t.Fatalf("s.Write() = %v, %v; want %v, nil", n, err, len(data))
 	}
+	s.Flush()
 
 	tc.wantFrame("stream writes data up to MAX_DATA limit",
 		packetType1RTT, debugFrameStream{
@@ -337,7 +339,9 @@ func TestConnOutflowMaxDataRoundRobin(t *testing.T) {
 	}
 
 	s1.Write(make([]byte, 10))
+	s1.Flush()
 	s2.Write(make([]byte, 10))
+	s2.Flush()
 
 	tc.writeFrames(packetType1RTT, debugFrameMaxData{
 		max: 1,
@@ -378,6 +382,7 @@ func TestConnOutflowMetaAndData(t *testing.T) {
 
 	data := makeTestData(32)
 	s.Write(data)
+	s.Flush()
 
 	s.CloseRead()
 	tc.wantFrame("CloseRead sends a STOP_SENDING, not flow controlled",
@@ -405,6 +410,7 @@ func TestConnOutflowResentData(t *testing.T) {
 
 	data := makeTestData(15)
 	s.Write(data[:8])
+	s.Flush()
 	tc.wantFrame("data is under MAX_DATA limit, all sent",
 		packetType1RTT, debugFrameStream{
 			id:   s.id,
@@ -421,6 +427,7 @@ func TestConnOutflowResentData(t *testing.T) {
 		})
 
 	s.Write(data[8:])
+	s.Flush()
 	tc.wantFrame("new data is sent up to the MAX_DATA limit",
 		packetType1RTT, debugFrameStream{
 			id:   s.id,

internal/quic/conn_loss_test.go

@@ -183,7 +183,7 @@ func TestLostStreamFrameEmpty(t *testing.T) {
 		if err != nil {
 			t.Fatalf("NewStream: %v", err)
 		}
-		c.Write(nil) // open the stream
+		c.Flush() // open the stream
 		tc.wantFrame("created bidirectional stream 0",
 			packetType1RTT, debugFrameStream{
 				id:   newStreamID(clientSide, bidiStream, 0),
@@ -213,13 +213,15 @@ func TestLostStreamWithData(t *testing.T) {
 			p.initialMaxStreamDataUni = 1 << 20
 		})
 		s.Write(data[:4])
+		s.Flush()
 		tc.wantFrame("send [0,4)",
 			packetType1RTT, debugFrameStream{
 				id:   s.id,
 				off:  0,
 				data: data[:4],
 			})
 		s.Write(data[4:8])
+		s.Flush()
 		tc.wantFrame("send [4,8)",
 			packetType1RTT, debugFrameStream{
 				id:   s.id,
@@ -263,6 +265,7 @@ func TestLostStreamPartialLoss(t *testing.T) {
 	})
 	for i := range data {
 		s.Write(data[i : i+1])
+		s.Flush()
 		tc.wantFrame(fmt.Sprintf("send STREAM frame with byte %v", i),
 			packetType1RTT, debugFrameStream{
 				id:   s.id,

internal/quic/conn_streams_test.go

@@ -19,33 +19,33 @@ func TestStreamsCreate(t *testing.T) {
 	tc := newTestConn(t, clientSide, permissiveTransportParameters)
 	tc.handshake()
 
-	c, err := tc.conn.NewStream(ctx)
+	s, err := tc.conn.NewStream(ctx)
 	if err != nil {
 		t.Fatalf("NewStream: %v", err)
 	}
-	c.Write(nil) // open the stream
+	s.Flush() // open the stream
 	tc.wantFrame("created bidirectional stream 0",
 		packetType1RTT, debugFrameStream{
 			id:   0, // client-initiated, bidi, number 0
 			data: []byte{},
 		})
 
-	c, err = tc.conn.NewSendOnlyStream(ctx)
+	s, err = tc.conn.NewSendOnlyStream(ctx)
 	if err != nil {
 		t.Fatalf("NewStream: %v", err)
 	}
-	c.Write(nil) // open the stream
+	s.Flush() // open the stream
 	tc.wantFrame("created unidirectional stream 0",
 		packetType1RTT, debugFrameStream{
 			id:   2, // client-initiated, uni, number 0
 			data: []byte{},
 		})
 
-	c, err = tc.conn.NewStream(ctx)
+	s, err = tc.conn.NewStream(ctx)
 	if err != nil {
 		t.Fatalf("NewStream: %v", err)
 	}
-	c.Write(nil) // open the stream
+	s.Flush() // open the stream
 	tc.wantFrame("created bidirectional stream 1",
 		packetType1RTT, debugFrameStream{
 			id:   4, // client-initiated, uni, number 4
@@ -177,11 +177,11 @@ func TestStreamsStreamSendOnly(t *testing.T) {
 	tc := newTestConn(t, serverSide, permissiveTransportParameters)
 	tc.handshake()
 
-	c, err := tc.conn.NewSendOnlyStream(ctx)
+	s, err := tc.conn.NewSendOnlyStream(ctx)
 	if err != nil {
 		t.Fatalf("NewStream: %v", err)
 	}
-	c.Write(nil) // open the stream
+	s.Flush() // open the stream
 	tc.wantFrame("created unidirectional stream 0",
 		packetType1RTT, debugFrameStream{
 			id:   3, // server-initiated, uni, number 0

internal/quic/quic.go

@@ -64,10 +64,14 @@ const defaultKeepAlivePeriod = 0
 // https://www.rfc-editor.org/rfc/rfc9002.html#section-6.1.2-6
 const timerGranularity = 1 * time.Millisecond
 
+// The smallest allowed maximum datagram size.
+// https://www.rfc-editor.org/rfc/rfc9000#section-14
+const smallestMaxDatagramSize = 1200
+
 // Minimum size of a UDP datagram sent by a client carrying an Initial packet,
 // or a server containing an ack-eliciting Initial packet.
 // https://www.rfc-editor.org/rfc/rfc9000#section-14.1
-const paddedInitialDatagramSize = 1200
+const paddedInitialDatagramSize = smallestMaxDatagramSize
 
 // Maximum number of streams of a given type which may be created.
 // https://www.rfc-editor.org/rfc/rfc9000.html#section-4.6-2

internal/quic/stream.go

@@ -38,10 +38,11 @@ type Stream struct {
 	// the write will fail.
 	outgate      gate
 	out          pipe            // buffered data to send
+	outflushed   int64           // offset of last flush call
 	outwin       int64           // maximum MAX_STREAM_DATA received from the peer
 	outmaxsent   int64           // maximum data offset we've sent to the peer
 	outmaxbuf    int64           // maximum amount of data we will buffer
-	outunsent    rangeset[int64] // ranges buffered but not yet sent
+	outunsent    rangeset[int64] // ranges buffered but not yet sent (only flushed data)
 	outacked     rangeset[int64] // ranges sent and acknowledged
 	outopened    sentVal         // set if we should open the stream
 	outclosed    sentVal         // set by CloseWrite
@@ -240,8 +241,6 @@ func (s *Stream) Write(b []byte) (n int, err error) {
 // WriteContext writes data to the stream write buffer.
 // Buffered data is only sent when the buffer is sufficiently full.
 // Call the Flush method to ensure buffered data is sent.
-//
-// TODO: Implement Flush.
 func (s *Stream) WriteContext(ctx context.Context, b []byte) (n int, err error) {
 	if s.IsReadOnly() {
 		return 0, errors.New("write to read-only stream")
@@ -269,10 +268,6 @@ func (s *Stream) WriteContext(ctx context.Context, b []byte) (n int, err error)
 			s.outUnlock()
 			return n, errors.New("write to closed stream")
 		}
-		// We set outopened here rather than below,
-		// so if this is a zero-length write we still
-		// open the stream despite not writing any data to it.
-		s.outopened.set()
 		if len(b) == 0 {
 			break
 		}
@@ -282,13 +277,26 @@ func (s *Stream) WriteContext(ctx context.Context, b []byte) (n int, err error)
 		// Amount to write is min(the full buffer, data up to the write limit).
 		// This is a number of bytes.
 		nn := min(int64(len(b)), lim-s.out.end)
-		// Copy the data into the output buffer and mark it as unsent.
-		if s.out.end <= s.outwin {
-			s.outunsent.add(s.out.end, min(s.out.end+nn, s.outwin))
-		}
+		// Copy the data into the output buffer.
 		s.out.writeAt(b[:nn], s.out.end)
 		b = b[nn:]
 		n += int(nn)
+		// Possibly flush the output buffer.
+		// We automatically flush if:
+		//   - We have enough data to consume the send window.
+		//     Sending this data may cause the peer to extend the window.
+		//   - We have buffered as much data as we're willing do.
+		//     We need to send data to clear out buffer space.
+		//   - We have enough data to fill a 1-RTT packet using the smallest
+		//     possible maximum datagram size (1200 bytes, less header byte,
+		//     connection ID, packet number, and AEAD overhead).
+		const autoFlushSize = smallestMaxDatagramSize - 1 - connIDLen - 1 - aeadOverhead
+		shouldFlush := s.out.end >= s.outwin || // peer send window is full
+			s.out.end >= lim || // local send buffer is full
+			(s.out.end-s.outflushed) >= autoFlushSize // enough data buffered
+		if shouldFlush {
+			s.flushLocked()
+		}
 		if s.out.end > s.outwin {
 			// We're blocked by flow control.
 			// Send a STREAM_DATA_BLOCKED frame to let the peer know.
@@ -301,6 +309,23 @@ func (s *Stream) WriteContext(ctx context.Context, b []byte) (n int, err error)
 	return n, nil
 }
 
+// Flush flushes data written to the stream.
+// It does not wait for the peer to acknowledge receipt of the data.
+// Use CloseContext to wait for the peer's acknowledgement.
+func (s *Stream) Flush() {
+	s.outgate.lock()
+	defer s.outUnlock()
+	s.flushLocked()
+}
+
+func (s *Stream) flushLocked() {
+	s.outopened.set()
+	if s.outflushed < s.outwin {
+		s.outunsent.add(s.outflushed, min(s.outwin, s.out.end))
+	}
+	s.outflushed = s.out.end
+}
+
 // Close closes the stream.
 // See CloseContext for more details.
 func (s *Stream) Close() error {
@@ -363,6 +388,7 @@ func (s *Stream) CloseWrite() {
 	s.outgate.lock()
 	defer s.outUnlock()
 	s.outclosed.set()
+	s.flushLocked()
 }
 
 // Reset aborts writes on the stream and notifies the peer
@@ -612,8 +638,8 @@ func (s *Stream) handleMaxStreamData(maxStreamData int64) error {
 	if maxStreamData <= s.outwin {
 		return nil
 	}
-	if s.out.end > s.outwin {
-		s.outunsent.add(s.outwin, min(maxStreamData, s.out.end))
+	if s.outflushed > s.outwin {
+		s.outunsent.add(s.outwin, min(maxStreamData, s.outflushed))
 	}
 	s.outwin = maxStreamData
 	if s.out.end > s.outwin {
@@ -741,10 +767,11 @@ func (s *Stream) appendOutFramesLocked(w *packetWriter, pnum packetNumber, pto b
 	}
 	for {
 		// STREAM
-		off, size := dataToSend(min(s.out.start, s.outwin), min(s.out.end, s.outwin), s.outunsent, s.outacked, pto)
+		off, size := dataToSend(min(s.out.start, s.outwin), min(s.outflushed, s.outwin), s.outunsent, s.outacked, pto)
 		if end := off + size; end > s.outmaxsent {
 			// This will require connection-level flow control to send.
 			end = min(end, s.outmaxsent+s.conn.streams.outflow.avail())
+			end = max(end, off)
 			size = end - off
 		}
 		fin := s.outclosed.isSet() && off+size == s.out.end