Implement Promise.allSettled

[boingoing]

Implement Promise.allSettled

This is now in Stage 3 and supported in V8 and JSC. See spec:
https://tc39.github.io/proposal-promise-allSettled/

Fixes: #6056

[zenparsing]

At https://tc39.github.io/proposal-promise-allSettled/#sec-performpromiseallsettled step 6.d.iii.2, it seems that if this call throws, then it is "caught" by allSettled step 7, which then rejects the resulting promise with the exception value.

I believe the following:

function FakePromise(fn) {
  function resolve() { print('resolve called'); throw new Error('oops'); }
  function reject(e) { print('reject called: ' + e.message) }
  fn(resolve, reject);
  this.then = function(onResolve, onReject) {};
}

FakePromise.resolve = function() {};

Promise.allSettled.call(FakePromise, []);

should print:

resolve called
reject called: oops

[boingoing]

I think you are right, good catch. We have the same issue with Promise.all, it looks like. I'll make the fix to both.

[zenparsing]

Can we retrigger CI for those jobs that failed?

[fatcerberus]

it seems that if this call throws, then it is "caught" by allSettled step 7, which then rejects the resulting promise with the exception value.

Wait, is that actually right? I thought the whole deal with Promise.allSettled was that it never rejects, so that you can see the results of all promises regardless of fulfill/reject unlike Promise.all?

[boingoing]

@zenparsing I don't actually know how to retrigger the tests which failed in the new CI. These failures aren't related to my change, though, they've been failing across the CI lately. I'll rebase the branch which will trigger them to rerun, at least.

[boingoing]

@fatcerberus Yes, this reading of the spec is correct. We call IfAbruptRejectPromise mostly on errors thrown out of the iterator or on errors invoking the resolve method. The case @zenparsing caught is what happens when the iterable argument is empty and an error is thrown while trying to execute resolve to get a promise resolved with the empty array.