protect against concurrent use of Stream.Read (#3380)

receive_stream.go

@@ -47,6 +47,7 @@ type receiveStream struct {
 	resetRemotely     bool // set when HandleResetStreamFrame() is called
 
 	readChan chan struct{}
+	readOnce chan struct{} // cap: 1, to protect against concurrent use of Read
 	deadline time.Time
 
 	flowController flowcontrol.StreamFlowController
@@ -70,6 +71,7 @@ func newReceiveStream(
 		flowController: flowController,
 		frameQueue:     newFrameSorter(),
 		readChan:       make(chan struct{}, 1),
+		readOnce:       make(chan struct{}, 1),
 		finalOffset:    protocol.MaxByteCount,
 		version:        version,
 	}
@@ -81,6 +83,12 @@ func (s *receiveStream) StreamID() protocol.StreamID {
 
 // Read implements io.Reader. It is not thread safe!
 func (s *receiveStream) Read(p []byte) (int, error) {
+	// Concurrent use of Read is not permitted (and doesn't make any sense),
+	// but sometimes people do it anyway.
+	// Make sure that we only execute one call at any given time to avoid hard to debug failures.
+	s.readOnce <- struct{}{}
+	defer func() { <-s.readOnce }()
+
 	s.mutex.Lock()
 	completed, n, err := s.readImpl(p)
 	s.mutex.Unlock()
@@ -105,7 +113,7 @@ func (s *receiveStream) readImpl(p []byte) (bool /*stream completed */, int, err
 		return false, 0, s.closeForShutdownErr
 	}
 
-	bytesRead := 0
+	var bytesRead int
 	var deadlineTimer *utils.Timer
 	for bytesRead < len(p) {
 		if s.currentFrame == nil || s.readPosInFrame >= len(s.currentFrame) {