[mkotelnikov]

Problem

On an in-browser site (createProjectSite), modules were served with native await. cellb's node:async_hooks polyfill can only carry a store across an await that goes through Promise.prototype.then. As a result, the site fell back to run-at-a-time stores, and two objects answering at once could read each other's AsyncLocalStorage store. Bundles already avoided this, because bundleWorkerForBrowser lowers await with esbuild.

Change

• src/project/lower-await.ts (new, built on acorn): lowers async functions to generators driven by a small runner.

  • await X becomes (yield X).

  • for await becomes a loop over yield it.next(). It closes the iterator on early exit and handles labels.

  • Functions keep their this and arguments.

  • Arrows forward arguments only when they read them.

  • Left as they are:

    • async generators;
    • async functions that use super or new.target;
    • top-level await.

    After an await, these see no store rather than another event's.

• createProjectSite: with nodeCompat, runs webrun-modules' default transform and then lowerAwait, for both the project's modules and loaded Workers.
• browser-host: installAsyncContext() now runs for every nodejs_compat project, sites included, not only bundles.
• Docs: gaps.md, browser-host.md, the polyfill's header comment and the flue-example README are updated. A site's nodeCompat must match the project's nodejs_compat flag; this is now documented.

Tests

• test/p17-lower-await.test.ts: checks that the lowering keeps values, errors, this, arguments and ordering, and covers for await (async and sync iterables, break, labels, closing the iterator). With the polyfill installed, four concurrent runs each keep their own store.
• site-projects.spec: a new test runs test/async-context-worker.ts (16 objects at once, every kind of continuation) on a site.
  • Without the lowering it fails: stores are lost after the first await.
  • With the lowering it passes.
• Local runs:
  • Lint and typecheck are clean.
  • Unit tests: 346 passed. The 4 unhandled errors from p09-celld-conformance also occur on main.
  • Chromium: 207 passed and 15 skipped. Two tests failed in that run with "execution context destroyed" because I edited files mid-run and the dev server reloaded the pages. Rerun 3× each, both pass.

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