[mcollina]

Reverts the two commits that moved parked TransformStream writes off the [[backpressureChangePromise]] reaction, which regressed AsyncLocalStorage propagation into transform() / flush() (#66906):

• bbb0055 "stream: avoid promise allocation for parked transform writes" (stream: keep webstream stream states in fast-mode objects #65625, v24.22.0): transform() for every chunk after a parked write, and flush(), run in the reader's context.
• 86d1196 "stream: decouple transform backpressure changes" (stream: decouple transform backpressure changes #65143, v24.21.0): the first parked transform() runs in the reader's context.

The fix on main (#66907) relies on AsyncContextFrame alone, which doesn't cover --no-async-context-frame / the async_hooks-based AsyncLocalStorage that v24.x still has, so reverting is the safer option for the release line.

Conflicts with later commits are resolved by keeping their state record classes (TransformStreamState now carries backpressureChange again) and the kParkedAlgorithmResult handling still used by ReadableStream.from(). test-whatwg-transformstream-parked-write-error.js is kept: it covers spec behavior and passes on the reverted code. The pipe-through benchmark is kept too.

Verified locally on this branch:

repro (issue):                  a:writer b:writer c:writer d:writer e:writer flush:writer
repro, --no-async-context-frame: a:writer b:writer c:writer d:writer e:writer flush:writer
reverse (reader in als.run):     a:none b:none c:none flush:none   (both modes)

The regression test from #66907 passes in both modes; whatwg/webstream tests, WPT streams, async-local-storage and async-hooks suites pass.

Refs: #66906
Refs: #66907

---

AI-generated, reviewed by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (9e39360) to head (beeec2e).
⚠️ Report is 615 commits behind head on v24.x-staging.

Additional details and impacted files

@@                Coverage Diff                @@
##           v24.x-staging   #66913      +/-   ##
=================================================
- Coverage          90.31%   90.28%   -0.03%     
=================================================
  Files                711      718       +7     
  Lines             228425   238195    +9770     
  Branches           43160    45293    +2133     
=================================================
+ Hits              206293   215055    +8762     
- Misses             14090    14637     +547     
- Partials            8042     8503     +461     

Files with missing lines	Coverage Δ
lib/internal/webstreams/transformstream.js	98.58% <100.00%> (+0.02%)	⬆️

... and 263 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.