Add browser-compatible Worker polyfill and WPT coverage

[matthargett]

Dependency and stack

This is the Worker follow-up to BabylonJS/JsRuntimeHost#189. It is built on the refreshed napi-v7 head (231fa40), which incorporates the issue-7 Node-API work, the latest audited CTS updates, and the deterministic JavaScriptCore follow-up collection needed by the CTS finalizer contract. The refreshed base is merged as a parent so the existing Worker review history remains intact; the resulting tree exactly matches the locally tested restack.

The Worker branch composes the independently reviewable browser-polyfill work while those PRs are pending:

• #208 Streams
• #209 Blob streaming
• #210 Headers/Response
• #211 CompressionStream
• #212 data-URL fetch
• #10 QuickJS throw-status fix
• discrete IndexedDB branch

Worker initializes those targets in browser-compatible dependency order. It no longer embeds reduced private copies of Streams, Blob.stream, Response, decompression, or IndexedDB, and no longer links zlib itself.

Browser-compatible Worker surface

• One native thread, AppRuntime, engine realm, and event loop per dedicated Worker.
• Worker, EventTarget, Event, MessageEvent, ErrorEvent, DOMException, handler attributes, postMessage, and terminate.
• DedicatedWorkerGlobalScope with self, location, name, navigator, close, synchronous importScripts, timers, fetch/XHR, URL, Blob/File, text codecs, WebSocket, performance, abort APIs, Streams, CompressionStream, and IndexedDB.
• Structured clone for cyclic/shared graphs, special numbers, BigInt, Date, RegExp, Map, Set, Error, ArrayBuffer/DataView, and typed arrays.
• Transfer-list validation and N-API v7 ArrayBuffer detachment, including duplicate, invalid, and already-detached DataCloneError paths.
• Root-confined app:/// and relative scripts/assets, explicit file://, percent-encoded data:, and a thread-safe packaged-asset resolver.
• Worker-relative fetch inputs resolved against the Worker location.

System JavaScriptCore uses its execution-time-limit ABI to interrupt top-level infinite evaluation. No JSC binary is downloaded or vendored: the Linux lanes install the OS libjavascriptcoregtk-4.1-dev package. QuickJS remains the additional race/conformance test engine.

Visualization startup and playback integration

The reduced integration fixture is pinned to rebeckerspecialties/webapp@3765b13, specifically GithubPortfolio.worker.ts, its parent interop, and IndexedDBCache.

The source trace found two blockers beyond the base Worker interface:

1. pipelineCreate synchronously constructs the IndexedDB cache, so a missing indexedDB stops startup before playback can fall back to live data.
2. Prerecorded playback fetches a Worker-relative ${owner}.${repo}.gz, then runs Blob.stream().pipeThrough(new DecompressionStream("gzip")), constructs a Response, parses the cache, and writes the records in one IndexedDB transaction.

The fixture constructs a named { type: "module" } Worker, queues pipelineCreate and subscribe while the bundle is evaluating, hydrates rebeckerLoaderCacheStore, and plays three 128-item streams. It verifies relative fetch, gzip rejection behavior, transaction completion, Date/Map/Set/cyclic/shared-alias clone fidelity, 384 updated items, abort reasons, and browser-style constructor feature detection.

The deployed GitHub Pages origin itself was not launched in this environment, so this is a source-pinned, app-derived integration test rather than a claim about an end-to-end XR render. The tested contract reaches pipelineCreated, emits the three playback streams, and preserves the data required by the XR consumer.

Browser regression hardening

The regression fixtures carry their source links and guard known browser implementation failures:

• WPT close() behavior: same-task messages/errors survive close(); later tasks are discarded.
• WPT startup ordering: an immediate parent postMessage() is queued and delivered asynchronously.
• Firefox pre-start queue and dead-worker cleanup: terminate-before-start cannot leak or later deliver pending events.
• WPT terminate-during-evaluation and Chromium dead-context fix: JSC interrupts top-level tight loops and callbacks do not use a terminated context.
• WebKit July 2026 termination UAF fix: Worker-owned state is destroyed on the Worker thread before the final parent reference is released; a repeated pending-task termination case stresses the shape.
• WPT corrupt compressed input: truncated and trailing-junk gzip must reject.
• WebKit and Chromium Blob/Response/IndexedDB regressions are covered in the standalone tests, with the exact bug/commit URLs beside each adaptation.

The JSC sanitizer pass also exposed a per-stream implementation cost in Blob.stream(): capturing node-addon-api callbacks attach hidden callback-data properties to each generated function, causing two extra wrappers and JSC Function structure transitions per stream. The standalone Blob branch now uses a shared internal stream-source prototype, so its native pull/cancel callbacks are created once per realm instead of once per stream; the same commit is composed here and keeps the existing BYOB, cancellation, and lifetime tests.

The restack also caught an integration-specific variant of the same “polyfill replaces a host primitive” antipattern: Worker event glue replaced IndexedDB’s existing DOMException, breaking DataCloneError instanceof DOMException. Worker now preserves a host constructor, with a dedicated native regression test.

Tests

The vendored Worker subset pins web-platform-tests/wpt@4809b72, its license, and testharness.js.

Fresh local QuickJS results after the restack:

• Worker host/composition + WPT/app-derived integration: 2/2 pass.
• Streams/Compression/IndexedDB/Fetch initializer and host-preservation tests: 6/6 pass.
• QuickJS Node-API throw-status contract: 1/1 pass.
• Combined native Worker/browser-runtime regression filter: 9/9 pass.
• Focused Headers/Response/Streams/Compression/Blob/IndexedDB JavaScript regressions: 81/81 pass.
• The target builds cleanly with the full Worker dependency graph.
• GitHub Worker matrix: 3/3 pass (system JSC, JSC ASan/UBSan, and QuickJS TSan).

The general JavaScript suite was also started, but this container’s external XHR cases block on network access before reaching the remaining suites; the focused pass above runs the complete changed-feature set. GitHub Actions supplies the system-JSC, ASan/UBSan, and QuickJS-TSan gates.

Deliberate first-cut boundaries

• type: "module" accepts self-contained script-compatible bundles; imports/exports must be flattened because public JSC has no module-loader hook.
• IndexedDB is complete enough for object stores, indexes, cursors, key ranges, upgrades, rollback, and storage clone, but remains per-realm and in-memory. Durable or cross-realm storage is a separate backend concern.
• The existing native fetch transport is still not the complete Fetch standard.
• unhandledrejection synthesis, SharedArrayBuffer, MessagePort transfer, Blob object URLs, nested Workers, and full module-graph loading are not included yet.
• The legacy JSI adapter has no v7 detachable-ArrayBuffer ABI, so Worker remains unavailable there.
• Tight-loop interruption is implemented for system JSC first; other engines stop cooperatively between dispatches until their native interrupt hooks are connected.