Add specs for Promise#zip/Promise.zip ordering

[ianks]

I noticed there were not any specs on the ordering of Promise.zip. Since it is part of the spec (as it applies to Promise.all), I figured I could add some tests for that.

For more info: https://stackoverflow.com/a/28066851

[hobodave]

Shouldn't rely on sleep for thread synchronization and ordering. Use concurrency primitives instead.

[hobodave]

You shouldn't rely on sleep for thread synchronization/ordering. Instead use concurrency primites:

running = Mutex.new
cond = ConditionVariable.new
cond2 = ConditionVariable.new

p1 = Concurrent::Promise.execute do
  running.synchronize {
    cond.wait(running)
    'one'
  }
end

p2 = Concurrent::Promise.execute do
  running.synchronize {
    cond2.wait(running)
    'two'
  }
end

p3 = Concurrent::Promise.execute do
  running.synchronize {
    'three'
  }
end

cond2.signal
cond.signal

result = Concurrent::Promise.zip(p1, p2, p3).value
expect(result) .to eq(['one', 'two', 'three'])

[pitr-ch]

@hobodave is right. @ianks could you update the spec? I would like to include it since this is indeed the expected behavior.

[ianks]

@hobodave @pitr-ch strangely, i am getting deadlocks using the cv implementation in the spec.

[pitr-ch]

@ianks could you use Countdouwnlatch.new(1) instead? It's what we usually use and it could resolve the deadlock without digging in.

[pitr-ch]

I've updated the PR to use count-down-latch and rebased.