Follower cancellation propagation

[jsor]

This PR implements cancellation propagation for promises following another promise in the same way currently implemented for child promises created by then().

Consider the following code:

$root = new Promise(/* ...*/);

$child = $root->then();

$follower = new Promise(function($resolve) use ($root) {
    $resolve($root);
});

The behaviour is identical regarding resolution. Both $child and $follower follow $root which means, both will be fulfilled or rejected in the same way as $root.

The behaviour is different regarding cancellation though.

• If $child->cancel() is called, $root is cancelled although $follower is still interested in the result.
• If $follower->cancel() is called, cancellation is not propagated to $root.

This PR fixes that, so $follower propagates cancellation in the same way as $child.

[WyriHaximus]

Are you backporting this to 1.x as well?

[jsor]

@WyriHaximus I haven't planned that. I think you're asking for reactphp/http#213. If a version of react/promise is supported which doesn't have this patch, then cancellation propagation must be manually implemented, like here: https://github.com/reactphp/http/pull/213/files#diff-45da203c5746d523d3d75762c539b707R56

I took care to be backward compatible, so that a canceller registered with a follower takes precedence over the canceller from the parent promise: https://github.com/reactphp/promise/pull/99/files#diff-71f7b9a6df57aaf364b33eacad82e245R116

[WyriHaximus]

LGTM

[clue]

Good catch, change LGTM! 👍

Shouldn't this be filed against current master and then be backported for the v2 branch? Either way, I'm good with this

[jsor]

@clue I'm not sure tbh what's common practice here. I've done that always by starting a patch for the latest released version and then porting down to older and/or up to unreleased versions.

[clue]

For reference only: This PR has been reverted via #122 for this release and is planned for a future release instead.