Make prioritization stricter

[shaseley]

(Work in progess)

• Make ordering guaranteed between schedulers associated with the same event loop. All other scheduling decisions are per-agent (% timer ordering, which uses method context), so removing this simplifies things and reduces interop risk.
• Make background priority tasks have lower event loop priority than tasks in the event loop's queues.
• Make user-blocking tasks have a higher event loop priority than timers and postMessage (other scheduling mechanisms), and add a note about intent of user-blocking tasks.

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Preview | Diff

[shaseley]

Hey @domenic, mind taking a look? Thanks.

[domenic]

This will delay behind all posted messages, not just same-Window ones. The below note doesn't seem to align with that.

If you wanted to restrict to same-window ones, you'd need to mark the queued tasks in some way, presumably at the time they're enqueued. I might try something like moneypatching https://html.spec.whatwg.org/multipage/web-messaging.html#posted-message-task-source to contain "If ..., the [task] queued in such a way is a same-window posted message task", and then checking that here.

[domenic]

Quotes around strings like user-blocking, here and elsewhere.

[domenic]

I would add a note here: something like

This means we won't select |scheduler queue|, instead selecting from the non-empty |queues| set.

[domenic]

Task queues don't have associated task sources. Instead, "every task source must be associated with a specific task queue". So e.g. a given task queue could have some timer task source-tasks, some networking tasks, etc.

I think what you want to check here is the task source of the next runnable task in the task queue. That's kind of weird, but I'm not sure there's a better option...

[shaseley]

ACK, ugh I knew that :(. I'm OOO next week, I'll work on cleaning it up after. Thanks for the review - much appreciated!