Bug 140043 – js/dom/Promise.html is flaky

• Component: Tools / Tests
• Status: NEW

[Alexey Proskuryakov]

js/dom/Promise.html is flaky, frequently failing in debug builds. Not sure if it's a test deficiency, or a legitimate bug.

About to run test - promiseAsync
PASS foo is foo
PASS foo is foo
PASS foo is foo
FAIL foo should be bar

[Alexey Proskuryakov]

Marked as flaky in <http://trac.webkit.org/r177865>.

[Alexey Proskuryakov]

Are promise and setTimeout timers on different event queues?

function promiseAsync() {
  var global = "foo";
  var f = new Promise(function(r1, r2) {
    is(global, "foo", "Global should be foo");
    r1(42);
    is(global, "foo", "Global should still be foo");
    setTimeout(function() {
      is(global, "bar", "Global should still be bar!");
      runTest();
    }, 0);
  }).then(function() {
    global = "bar";
  });
  is(global, "foo", "Global should still be foo (2)");
}

[Sam Weinig]

(In reply to comment #2)
> Are promise and setTimeout timers on different event queues?

They are...kind of.

[Sam Weinig]

(In reply to comment #3)
> (In reply to comment #2)
> > Are promise and setTimeout timers on different event queues?
> 
> They are...kind of.

I don't think anything in our implementation ensures this ordering, but it really should. This is a great example of why it would be great if we have a unified event loop mechanism.

In this case, the bug is that the .then() should run at the next microtask checkpoint, which is defined as being before the next chance for a timer to fire. Our implementation of Promises does not use the microtask checkpoint, but rather uses ScriptExecutionContext::postTask() mechanism, which is not quite the same thing.

[Ryan Haddad]

The test is also flaky on release, so I removed the debug flag in http://trac.webkit.org/projects/webkit/changeset/210898

https://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=js%2Fdom%2FPromise.html