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🌒 ShadowRealm 🌘

for Stage 3

Philip Chimento Igalia, in partnership with Salesforce
TC39 December 2024
—
NOTE - checklists will continue to be updated until the meeting

Recap: What is ShadowRealm?

A mechanism to execute JavaScript code within the context of a new global object and set of built-ins.

Goal is not security, but integrity: complete control over the execution environment.

globalThis.someValue = 1;
const realm = new ShadowRealm();
realm.evaluate(`globalThis.someValue = 2;`);
console.assert(globalThis.someValue === 1);

bg contain

Recap: Proposal history

Recap: Proposal history

  • Sept. 2023: Stage 2, readvancement pending explicit support from two implementations that the testing and list of APIs to be exposed to ShadowRealms are sufficient
  • Feb. 2024: Stage 2.7, with the understanding that stage 3 requires signoff from HTML folks on the HTML integration, as well as resolution of Mozilla's concerns about test coverage
  • Not an opportunity to relitigate previous design decisions

Today: Exposed web APIs

Which web APIs should be present inside ShadowRealm?

  • New W3C TAG design principle:
    • Only purely computational features are exposed everywhere
      • not performing I/O
      • not affecting the state of the user agent or the user's device
    • Exception: Anything relying on an event loop is not exposed
    • Expose conservatively: features primarily useful for unexposed feature are not exposed

Today: Exposed web APIs

  • Developed based on a number of conversations with implementors and web platform experts
  • Clear criterion to decide whether something is in or out
  • Distinction between "which standards body standardized a feature" is irrelevant

Update: Full list of 1300+ global properties, which are in/out, & why: 📋spreadsheet

Today: HTML integration

  • Pull request: whatwg/html#9893
  • Design settled and reviews have occurred
  • Fixing details
  • Some mechanical work needed in specs downstream of HTML to use new terminology

Today: Test coverage

Today: Test coverage

  • ✅ Abort: AbortController, AbortSignal (except AbortSignal.timeout not exposed) (PR)
  • ✅ Base64: atob, btoa (PR)
  • ✅ console (PR 1, 2)
  • ✅ crypto.getRandomValues, crypto.randomUUID (PR - pending acceptance by WebCrypto spec)
  • ✅ DOMException (PR)
  • ✅ Encoding: TextDecoder, TextDecoderStream, TextEncoder, TextEncoderStream (PR 1, 2 - pending review)

Today: Test coverage

  • ✅ Error handler: ErrorEvent, onerror, reportError (PR - pending review)
  • ✅ Events: addEventListener, CustomEvent, dispatchEvent, Event, EventTarget (including globalThis being one), removeEventListener (PR - pending review)
  • ✅ isSecureContext (PR - pending review)
  • ✅ Observable, Subscriber (PR - pending review)
  • ✅ Promise rejection: onrejectionhandled, onunhandledrejection, PromiseRejectionEvent (PR - pending review)

Today: Test coverage

  • ✅ queueMicrotask and evaluation order: (PR - pending review)
  • ✅ self (PR)
  • ✅ Streams: ByteLengthQueuingStrategy, CompressionStream, CountQueueingStrategy, ReadableByteStreamController, ReadableStream, ReadableStreamBYOBReader, ReadableStreamBYOBRequest, ReadableStreamDefaultController, ReadableStreamDefaultReader, TransformStream, WritableStream, WritableStreamDefaultController, WritableStreamDefaultWriter (PR 1, 2 - pending review)

Today: Test coverage

  • ✅ Structured clone: DataCloneError, structuredClone (PR - pending review)
  • ✅ URL, URLSearchParams (PR - pending Interop question)
  • ✅ URLPattern (PR - pending acceptance by URLPattern spec)
  • ✅ WebAssembly (PR 1, 2) (except compileStreaming and instantiateStreaming not exposed)
  • ✅ WebAssembly ESM imports (PR - pending review)

Today: Test coverage

  • ✅ Remove tests for things no longer exposed under the criterion
  • ❌ Not currently covered in WPT at all: TransformStreamDefaultController, WebTransportWriter

Requirements for stage 3

  • ✅ The feature has sufficient testing and appropriate pre-implementation experience

Specific to ShadowRealm

  • Explicit support from two implementations that the testing and list of APIs to be exposed to ShadowRealms are sufficient
  • Signoff from HTML folks on the HTML integration
  • Resolution of Mozilla's concerns about test coverage

Questions?

Consensus to move the proposal to stage 3?

Proposed summary for notes

(to be added)