Update comment

internal/multiplex/session.go

@@ -195,7 +195,9 @@ func (sesh *Session) closeStream(s *Stream, active bool) error {
 		log.Tracef("stream %v passively closed", s.id)
 	}
 
-	// id may or may not exist as this is user input, if we use Delete(s.id) here it will panic
+	// We set it as nil to signify that the stream id had existed before.
+	// If we Delete(s.id) straight away, later on in recvDataFromRemote, it will not be able to tell
+	// if the frame it received was from a new stream or a dying stream whose frame arrived late
 	sesh.streams.Store(s.id, nil)
 	if sesh.streamCountDecr() == 0 {
 		if sesh.Singleplex {

internal/multiplex/session_test.go

@@ -237,8 +237,9 @@ func TestRecvDataFromRemote_Closing_InOrder(t *testing.T) {
 	if s1I != nil {
 		t.Error("stream 1 exists after receiving stream close for the second time")
 	}
-	if sesh.streamCount() != 1 {
-		t.Error("stream count isn't 1 after stream 1 closed twice")
+	streamCount := sesh.streamCount()
+	if streamCount != 1 {
+		t.Errorf("stream count is %v after stream 1 closed twice, expected 1", streamCount)
 	}
 
 	// close session