enqueue method

Future<void> enqueue(Future<void> send())

Admits send into the window, awaiting room first, and runs it after every previously admitted frame has completed.

A send failure is contained: it neither breaks the chain for later frames nor surfaces here (the transport layer owns delivery errors).

Implementation

Future<void> enqueue(Future<void> Function() send) async {
  while (_outstanding >= capacity) {
    final gate = Completer<void>();
    _gates.add(gate);
    await gate.future;
  }
  _outstanding++;
  _chain = _chain.then((_) async {
    try {
      await send();
    } catch (_) {
      // Delivery errors belong to the transport; the stream continues.
    } finally {
      _outstanding--;
      if (_gates.isNotEmpty) _gates.removeFirst().complete();
    }
  });
}