quic: return error from Stream.Flush

Report errors (stream closed, stream reset) when flushing.

Change-Id: I94b7a35fe874372eb7969aacebc861c5f2e29b3c
Reviewed-on: https://go-review.googlesource.com/c/net/+/640796
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

quic/stream.go

@@ -445,10 +445,20 @@ func (s *Stream) flushFastOutputBuffer() {
 // Flush flushes data written to the stream.
 // It does not wait for the peer to acknowledge receipt of the data.
 // Use Close to wait for the peer's acknowledgement.
-func (s *Stream) Flush() {
+func (s *Stream) Flush() error {
+	if s.IsReadOnly() {
+		return errors.New("flush of read-only stream")
+	}
 	s.outgate.lock()
 	defer s.outUnlock()
+	if s.outreset.isSet() {
+		return errors.New("write to reset stream")
+	}
+	if s.outclosed.isSet() {
+		return errors.New("write to closed stream")
+	}
 	s.flushLocked()
+	return nil
 }
 
 func (s *Stream) flushLocked() {

quic/stream_test.go

@@ -1324,6 +1324,54 @@ func TestStreamFlushExplicit(t *testing.T) {
 	})
 }
 
+func TestStreamFlushClosedStream(t *testing.T) {
+	_, s := newTestConnAndLocalStream(t, clientSide, bidiStream,
+		permissiveTransportParameters)
+	s.Close()
+	if err := s.Flush(); err == nil {
+		t.Errorf("s.Flush of closed stream = nil, want error")
+	}
+}
+
+func TestStreamFlushResetStream(t *testing.T) {
+	_, s := newTestConnAndLocalStream(t, clientSide, bidiStream,
+		permissiveTransportParameters)
+	s.Reset(0)
+	if err := s.Flush(); err == nil {
+		t.Errorf("s.Flush of reset stream = nil, want error")
+	}
+}
+
+func TestStreamFlushStreamAfterPeerStopSending(t *testing.T) {
+	tc, s := newTestConnAndLocalStream(t, clientSide, bidiStream,
+		permissiveTransportParameters)
+	s.Flush() // create the stream
+	tc.wantFrame("stream created after flush",
+		packetType1RTT, debugFrameStream{
+			id:   s.id,
+			data: []byte{},
+		})
+
+	// Peer sends a STOP_SENDING.
+	tc.writeFrames(packetType1RTT, debugFrameStopSending{
+		id: s.id,
+	})
+	if err := s.Flush(); err == nil {
+		t.Errorf("s.Flush of stream reset by peer = nil, want error")
+	}
+}
+
+func TestStreamFlushStreamAfterConnectionClosed(t *testing.T) {
+	tc, s := newTestConnAndLocalStream(t, clientSide, bidiStream,
+		permissiveTransportParameters)
+	tc.writeFrames(packetType1RTT, debugFrameConnectionCloseApplication{
+		code: 0,
+	})
+	if err := s.Flush(); err == nil {
+		t.Errorf("s.Flush of stream on closed connection = nil, want error")
+	}
+}
+
 func TestStreamFlushImplicitExact(t *testing.T) {
 	testStreamTypes(t, "", func(t *testing.T, styp streamType) {
 		const writeBufferSize = 4