Race condition in Get()

[ssmccoy]

Should Get() be called by a goroutine which yields execution between checking promise.state and calling promise.waitGroup.Wait(), it will hang as the waitgroup will never be woken up.

This issue arises approximately 25% of the time in the following example.

	waiterChan := make(chan Thenable)

	waiterPromises := make([]Thenable, 0, WAITERS)

	// Create 100 waiters...
	for i := 0; i < WAITERS; i++ {
		go (func() {
			waiterChan <- promise.Then(func(value interface{}) interface{} {
				atomic.AddUint64(&counter, 1)

				return nil
			})
		})()
	}

	go (func() {
		promise.Complete(true)
	})()

	// Wait for all of the promises created to be gathered over the channel.
	// It's important to only mutate the slice in a single goroutine, so that
	// happens here.
	for i := 0; i < WAITERS; i++ {
		waiterPromises = append(waiterPromises, <-waiterChan)
	}

	// The promises will now be in varying states of completion, observation
	// has shown ¼th of the time the final promise will be being completed
	// while this Get() method is running, causing a race condition whereby
	// Get() never returned.
	_, err := All(waiterPromises...).Get()