code: @Override
    public void cancel(final Status reason) {
      checkNotNull(reason, "reason");
      boolean delegateToRealStream = true;
      ClientStreamListener listenerToClose = null;
      synchronized (this) {
        // If realStream != null, then either setStream() or cancel() has been called
        if (realStream == null) {
          realStream = NoopClientStream.INSTANCE;
          delegateToRealStream = false;
  
          // If listener == null, then start() will later call listener with 'error'
          listenerToClose = listener;
          error = reason;
        }
      }
      if (delegateToRealStream) {
        delayOrExecute(new Runnable() {
          @Override
          public void run() {
            realStream.cancel(reason);
          }
        });
      } else {
        if (listenerToClose != null) {
          listenerToClose.closed(reason, new Metadata());
        }
        drainPendingCalls();
      }
    }
docs: When this method returns, passThrough is guaranteed to be true