Support promise cancellation#9

Merged

WyriHaximus merged 3 commits intoreactphp:mainfrom

clue-labs:cancellation

Nov 15, 2021

Commits on Nov 15, 2021

Ensure `parallel()` function stops invoking additional tasks on error
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Ensure `parallel()` function cancels parallel tasks on error
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Calling `cancel()` on resulting promise should cancel all pending tasks
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