type waitWithContextFunc func(ctx context.Context) <-chan struct{}

// waitForWithContext continually checks 'fn' as driven by 'wait'.
//
// waitForWithContext gets a channel from 'wait()”, and then invokes 'fn'
// once for every value placed on the channel and once more when the
// channel is closed. If the channel is closed and 'fn'
// returns false without error, waitForWithContext returns ErrWaitTimeout.
//