class CancellationQueue

final class CancellationQueue {
    
    /** @var bool */
    private $started = false;
    
    /** @var object[] */
    private $queue = [];
    public function __invoke() : void {
        if ($this->started) {
            return;
        }
        $this->started = true;
        $this->drain();
    }
    
    /**
     * @param mixed $cancellable
     */
    public function enqueue($cancellable) : void {
        if (!\is_object($cancellable) || !\method_exists($cancellable, 'then') || !\method_exists($cancellable, 'cancel')) {
            return;
        }
        $length = \array_push($this->queue, $cancellable);
        if ($this->started && 1 === $length) {
            $this->drain();
        }
    }
    private function drain() : void {
        for ($i = \key($this->queue); isset($this->queue[$i]); $i++) {
            $cancellable = $this->queue[$i];
            assert(\method_exists($cancellable, 'cancel'));
            $exception = null;
            try {
                $cancellable->cancel();
            } catch (\Throwable $exception) {
            }
            unset($this->queue[$i]);
            if ($exception) {
                throw $exception;
            }
        }
        $this->queue = [];
    }

}