Fix a race condition in DefaultPromise

[trustin]

.. which occurs when a user adds a listener from different threads after the promise is done and the notifications for the listeners, that were added before the promise is done, is in progress. For instance:

1. Thread-1: p.addListener(listenerA);
2. Thread-1: p.setSuccess(null);
3. Thread-2: p.addListener(listenerB);
4. Thread-2: p.executor.execute(taskNotifyListenerB);
5. Thread-1: p.executor.execute(taskNotifyListenerA);

taskNotifyListenerB should not really notify listenerB until taskNotifyListenerA is finished.

To fix this issue:

• Change the semantic of (listeners == null) to determine if the early listeners [1] were notified
• If a late listener is added before the early listeners are notified, the notification of the late listener is deferred until the early listeners are notified (i.e. until listeners == null)
• The late listeners with deferred notifications are stored in a lazily instantiated queue to preserve ordering, and then are notified once the early listeners are notified.

[1] the listeners that were added before the promise is done
[2] the listeners that were added after the promise is done

[trustin]

mvn test passes without any problem.

[ghost]

Build result for #2186 at 3cfa75f: Failure

[trustin]

As you noticed, this didn't work out. Thinking of a proper fix.

[trustin]

With c97f2d2, the test did not fail with 20000000 runs. I'd say it's fixed really.

[ghost]

Build result for #2186 at c97f2d2: Failure

[ghost]

Build result for #2186 at a53805a: Success

[normanmaurer]

@trustin looks good... Please squash and cherry-pick or should I take care for you so you can get some sleep ;) ?

[trustin]

Merged via 309ee68. Thanks for reviewing @normanmaurer