An experiment with defining and using a SafeResolve operation

[mgaudet]

Here's an exploration .

Add a new operation SafeResolvePromise(p, val) defined as follows (this isn’t spec language, but should be followable; in specification this would be an alternative [[Resolve]] internal slot for a promise capability)

SafeResolvePromise(p, resolutionVal): 
  if RequiresDefer(resolutionVal):
    job = new DeferJob(promise, resolutionVal)
    HostEnqueuePromiseJob(job, job.realm());
    return

  FulfillPromise(promise, resolution)
  return

RequiresDefer(resolutionVal): 
  // Traps could run
  if IsProxy(resolutionVal):
    return true

  if HasThenProperty(resolutionVal):
    return true

  if HasGetterConstructor(resolutionVal):
    return true

  return false

Basically, if a resolution value could be expected to run script, rather than running the script synchronously, enqueue a job in the microtask queue to do resolution, potentially running script, in a future microtask.

Experiment: WebIDL

Q: Can we use SafeResolve to replace the promise resolution steps in WebIDL?

Experiment (slightly more broad): Run WPT with the Firefox DOM Promise resolve steps replaced with SafeResolve.

Results:

The vast majority of tests (as expected) pass.

Timeout Failures:

1. /css/css-overflow/scroll-marker-in-display-none-column-crash.html This is the only test where I'm not entirely sure what's going on.
2. /custom-elements/when-defined-reentry-crash.html -- this one is using a then on Object.prototype for nefarious aims. In a sense this is exactly the kind of issue we’re trying to address. https://issues.chromium.org/issues/40061097

Unexpected Pass:

1. /fetch/api/response/response-body-read-task-handling.html - This test is using then to get insight into execution order. The test no longer tests what it thinks it is testing anymore; however the test -also- was created to address this kind of thennable issue.

Test Failures

1. /streams/readable-byte-streams/patched-global.any.js -- Explicitly using then to peek into execution state we’d probably prefer to not be observable.
2. /document-picture-in-picture/returns-window-with-document.https.html | requestWindow timing - assert\_equals: Got the expected order of actions expected "requestWindow,microtask,enter" but got "microtask,requestWindow,enter" -- The job timing changes because it’s resolving a promise with a window (WindowProxy) object, which causes an extra tick.
3. /web-animations/interfaces/Animation/cancel.html; observing event timing with thenable.

[mgaudet]

(cc @mhofman )