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Layering: Fix the jobs infrastructure

Presenter: Daniel Ehrenberg

Author: Domenic Denicola

June 2019 TC39 meeting

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What are Jobs?

  • When a Promise is resolved or rejected,�a callback runs
    • Later
      • After the current code finishes
        • In a "Job"
          • Sometimes called "microtask"

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When do Jobs run?

  • In HTML and Node.js, all Promise jobs run before the rest of the "event loop"
    • Event loop responsible for asynchronous I/O, updates to the DOM, etc.

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When do Jobs run?

  • Algorithm in HTML and Node:
    • Run the initial JS code
    • Run all the Promise jobs
    • Return to the event loop
      • Which may call back into JS

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This is a huge simplification

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Goal: Let job ordering be specified precisely, to ensure interoperability between implementations

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Embedder-specific ordering of event loops

  • In HTML
  • In Node.js

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JavaScript specifies an algorithm full of implementation-defined steps.

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Ordering, priority remain vague.

No clean "host hooks" for other specifications to use.

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HTML doesn't use this part of the JavaScript specification

It specifies its own, precise algorithms

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Why does this matter?

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Some proposals may touch the job queue

  • Currently: Promises
  • WeakRefs
  • Atomics.waitAsync
  • (more?)
  • Plus, the current mismatch is just confusing
    • Including for implementers

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Let's let embedders cleanly specify their own event loop and how the job queue interacts with it

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The general approach

  • Ask the embedder to define its event loop, including the job queue, following specified invariants
  • HostEnqueueJob ( queueName, job, arguments )
  • Provides invariants matching current specification
  • Matches current layering in HTML
  • PR includes a large note with self-contained usage

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Cross-environment invariants

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Alternative: One-off callbacks

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Approach taken by WeakRefs, Atomics.waitAsync

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HostCleanupFinalizationGroup expected to queue a call to CleanupFinalizationGroup. No use of Jobs.

We could do this with Promises similarly

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These are all normatively equivalent to RunJobs;

Editorial alterantives allow more precise use by embedders

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Thoughts?

Adopt this notation?