fix busy-looping in OpenStreamSync

streams_map_outgoing_bidi.go

@@ -106,6 +106,7 @@ func (m *outgoingBidiStreamsMap) OpenStreamSync(ctx context.Context) (streamI, e
 		}
 		str := m.openStream()
 		delete(m.openQueue, queuePos)
+		m.lowestInQueue = queuePos + 1
 		m.unblockOpenSync()
 		return str, nil
 	}
@@ -172,8 +173,10 @@ func (m *outgoingBidiStreamsMap) SetMaxStream(num protocol.StreamNum) {
 	m.maxStream = num
 	m.blockedSent = false
 	m.unblockOpenSync()
+	// TODO(#2826): it might be necessary to send a STREAMS_BLOCKED frame
 }
 
+// unblockOpenSync unblocks the next OpenStreamSync go-routine to open a new stream
 func (m *outgoingBidiStreamsMap) unblockOpenSync() {
 	if len(m.openQueue) == 0 {
 		return
@@ -183,9 +186,12 @@ func (m *outgoingBidiStreamsMap) unblockOpenSync() {
 		if !ok { // entry was deleted because the context was canceled
 			continue
 		}
-		close(c)
-		m.openQueue[qp] = nil
-		m.lowestInQueue = qp + 1
+		// unblockOpenSync is called both from OpenStreamSync and from SetMaxStream.
+		// It's sufficient to only unblock OpenStreamSync once.
+		select {
+		case c <- struct{}{}:
+		default:
+		}
 		return
 	}
 }