Patch v8 to support promise cross-context resolution

[jasnell]

Draft for now until I add tests tests and more tests

export default {
  async fetch(req, env, ctx) {
    if (globalThis.resolve === undefined) {

      {
        const { promise, resolve } = Promise.withResolvers();
        setTimeout(resolve, 1000);
        ctx.waitUntil(promise);
      }

      // This is our first request. We will create a new promise and resolver
      // pair and store it in a global. This request will then wait for the
      // promise to be resolved before continuing.
      const { promise, resolve } = Promise.withResolvers();
      globalThis.resolve = resolve;
      globalThis.promise = promise;
      const ab = AbortSignal.abort();
      console.log(ab.aborted);
      promise.then(() => {
        // This will be run within the correct IoContext now...
        try {
          console.log('test1', ab.aborted);
        } catch (err) {
          // We would get here if the IoContext was incorrect because
          // of the call to ab.aborted
          console.log(err.message);
        }
      });
    } else {
      // This is our second request. We will resolve the promise created in the
      // first request.
      console.log('....');
      globalThis.resolve();
      globalThis.resolve = undefined;
      console.log('test2');
    }

    return new Response("Hello World\n");
  }
};

Some details on how this works:

• Per the original fixup, every IoContext has an associated opaque JS object called the "promise tag"
• Originally, this promise tag was just a simple, ordinary JS object. With this PR it becomes an opaque wrapper around an kj::Own<IoContext::WeakRef>
• When an IoContext becomes current/locked, that IoContexts promise tag is associated with the v8::Isolate as the "current promise tag"
• Per the original fixup, whenever a Promise is created, the Isolate's "current promise tag" is associated with the Promise in an internal field.
• In the original fixup, when a Promise is followed (.then(...), .catch(...), etc), we check to see if the current promise tag matches the promises context tag. If they are not the same, then the promise is a "cross-context promise" and we arrange for proper cross-context signaling.
• This PR adds that when the resolve(...) or reject(...) methods are called, and we determine that the promise is a cross-context promise, scheduling of the promise continuations will be deferred until after we re-enter the promise's original IoContext.
• If the promise's original IoContext has been destroyed, we drop the continuations on the floor and emit a warning.

[jasnell]

TODOs

• Handle reject() ... Currently this only deals with the resolve case. Needs to handle the reject case also but that will follow the identical pattern.
• Tests! Make sure all the various ways promises are resolved are handled. Async iterators, custom thenables, etc
• Documentation ... We should add documentation to the website

[kentonv]

The workerd C++ code seems fine to me. I'd like to defer to @erikcorry for the V8 side as I don't feel like I understand it.

Is there some way we can make it easier to review the patch? Maybe fork the v8 repo, commit it in your fork, and link the commit from here?

[jasnell]

CI is failing due to a transient issue downloading stuff....

[erikcorry]

Here's a diff to the diff file, which uses Handles more, and raw Tagged pointers less. Feel free to use it, or I'll just do a separate change tomorrow (where I'll probably also unify the 0007 and 0019 patches to one file).
handles.txt

[kentonv]

Not quite done reviewing but need to go to meetings, so here's my comments so far.

[kentonv]

OK I actually read through everything this time (except the V8 patch).

[kentonv]

Approving workerd code... trusting @erikcorry to review V8 code.

[jasnell]

Rebased and squashed

[erikcorry]

LGTM