var Sink = function(expected, timeout, callback) {
  var defaultTimeout = 1000;
  if(typeof timeout == 'function') {
    callback = timeout;
    timeout = defaultTimeout;
  }
  timeout = timeout || defaultTimeout;
  var internalCount = 0;
  var kill = function() {
    assert.ok(false, "Did not reach expected " + expected + " with an idle timeout of " + timeout);
  }
  var killTimeout = setTimeout(kill, timeout);
  return {
    add: function(count) {
      count = count || 1;
      internalCount += count;
      clearTimeout(killTimeout)
      if(internalCount < expected) {
        killTimeout = setTimeout(kill, timeout)
      }
      else {
        assert.equal(internalCount, expected);
        callback();
      }
    }
  }
}