Make it possible to specify a new queue instance

[joshdifabio]

In order to enable efficient integrations with reactors, it would be very helpful to be able to provide a custom task queue.

Here's a simple example of what would be possible after merging this PR:

use Amp\Reactor;
use GuzzleHttp\Promise\TaskQueue;

class AmpTaskQueue extends TaskQueue
{
    private $reactor;

    public function __construct(Reactor $reactor)
    {
        $this->reactor = $reactor;
    }

    public function add(callable $task)
    {
        if ($this->isEmpty()) {
            $this->reactor->immediately([$this, 'run']);
        }

        parent::add($task);
    }
}

\GuzzleHttp\Promise\queue(new AmpTaskQueue($reactor));

Simply calling $reactor->immedately($task); would really be preferable but, unfortunately, this would prevent the run() and isEmpty() methods from working as expected.

[joshdifabio]

Note that this is the approach react/promise is likely to take when they introduce a task queue in v3.

https://github.com/reactphp/promise/blob/master/src/functions.php#L203

[joshdifabio]

Any thoughts on this @mtdowling? It would be great for optimisation where the curl handler isn't being used.

[mtdowling]

This should be ok considering it would be an application concern to modify this. Do you think it would be better to introduce an interface here?

[joshdifabio]

Do you think it would be better to introduce an interface here?

Probably, yes. Would you like me to add one?

[mtdowling]

That would be great!

[joshdifabio]

I've added an interface. The only question in my mind was whether disableShutdown() should be included in the interface, but for BC I think it probably should.

[joshdifabio]

What are your thoughts on this @mtdowling? We've been using this in production for a while and it's been working well for us. Referencing a fork branch in our Composer configs is getting pretty tricky however.

[mtdowling]

I wonder if this should just be an implementation detail of the default TaskQueue.

[joshdifabio]

I had the same thought. However, would this not be a breaking change? (E.g. queue()->disableShutdown() would potentially no longer work.)

[mtdowling]

I think anyone calling that method has already coupled themselves to a specific implementation detail though... I would be in favor of removing this from the interface but keeping it on the TaskQueue class.

[joshdifabio]

How do you feel about merging this if I update the PR to remove disableShutdown() from TaskQueueInterface? Requiring this forked version in our code base is getting more and more tricky.

[mtdowling]

Yes. Removing this from the interface is the right direction in my opinion. Will be merged after.

[joshdifabio]

@mtdowling I've removed disableShutdown() from TaskQueueInterface as requested.

P.S. Thanks for your responsiveness here and in general, I really appreciate it.

[mtdowling]

Thanks! This looks great.