private function popAndRunBusyCallback() {
		if ( $this->busyCallbacks ) {
			reset( $this->busyCallbacks );
			$key = key( $this->busyCallbacks );
			/** @var callable $workCallback */
			$workCallback =& $this->busyCallbacks[$key];
			try {
				$workCallback();
			} catch ( \Exception $e ) {
				$workCallback = function () use ( $e ) {
					throw $e;
				};
			}
			unset( $this->busyCallbacks[$key] ); // consume

			return true;
		}

		return false;
	}