Example Code

This version waits for Do() to finish if it is currently executing:

func (o *Once) Done() bool {
	o.m.Lock()
	defer o.m.Unlock()

	return atomic.LoadUint32(&o.done) == 1
}

_{Note: Not tested}

This version returns true if Do() is currently executing (probably not what we want to do):

func (o *Once) Done() bool {
	if o.m.TryLock() {
		defer o.m.Unlock()
		return atomic.LoadUint32(&o.done) == 1
	}
	return false
}

_{Note: Not tested}

Reason

I sometimes find myself needing to check if a goroutine which executes a sync.Once.Do() has done so yet. This could be done with channels or a separate variable but that can introduce a lot of unnecessary boilerplate. Not to mention that this means we're storing the same state twice which, depending on the code, could possibly get out of sync (ironically, given the package we're dealing with).