tryCollect method

@visibleForTesting Future<int?> tryCollect(CryptoContext context)

collectUnused, answering null when it was refused because sync had not caught up, the one refusal a later pass can overcome; a pass that does not look for another reason answers 0, as collectUnused does.

Passes run one at a time, and one asked for while another waits to start joins it. A pass checks sync and scans local storage without holding up cuts, and takes their turn only to delete.

Visible so a test can tell a pass sync refused from one that answered.

Implementation

@visibleForTesting
Future<int?> tryCollect(CryptoContext context) {
  final waiting = _waitingPass;
  if (waiting != null) return waiting;
  final pass = _collecting.then((_) {
    _waitingPass = null;
    return _collectUnused(context);
  });
  _waitingPass = pass;
  _collecting = pass.then((_) {}, onError: (_) {});
  return pass;
}