stream: trim TransformStream construction costs

[mcollina]

Round 22 of the webstreams performance work (follows #66424). It targets what a TransformStream costs to construct.

Profiling the creation benchmarks showed they are dominated by the scavenger rather than by JavaScript: a start step pending in the microtask queue keeps the whole stream graph live across young-generation collections, so the cost of a creation burst follows the bytes each stream leaves pending and retained (a TransformStream went from 6.1 µs to 0.9 µs per construction with a 128 MB semi-space). A TransformStream without start() left about three kilobytes behind: five closures for the sink and source algorithms of its two sides, and per side a wrapper promise, a thenable job, two reactions and two closures to adopt the start promise.

Shared sink and source algorithms

The five algorithms are now module-level functions. They reach the transform stream through a transformStream field on the state of the readable and writable controller they serve, and the close, abort and cancel algorithms receive the controller as a trailing argument for that (the wrappers around user sinks and sources ignore it).

One start delivery for both sides

Per spec the start promise is resolved with the transformer's start result at the end of construction, and each side then adopts it through a wrapper promise. transformStreamStart() takes the same microtask hops with reactions on one shared promise: three after construction for a non-thenable start result, two after the adopting promise settles for a thenable one, with the rejection path erroring both sides in the same order as before. The readable and writable controllers expose their post-start steps for that (readableStreamDefaultControllerStarted(), writableStreamDefaultControllerStarted() / StartFailed()), and a kDeferredStart start result tells the setup to leave the step to the caller. User-facing streams keep the existing wrapper.

Lazy transfer state

The transfer record of ReadableStream and WritableStream state is materialized on first transfer instead of per stream.

Tests

A 27-scenario start-timing probe (no start(), start() returning undefined, a resolved, pending, late-resolved, late-rejected or rejected promise, a thenable object, a sync throw, close/cancel/abort/terminate/enqueue/error during start, pipe-through, backpressure, writable and readable starts, transfer) logs identical microtask ticks against main. WPT streams/encoding/compression and the webstreams parallel batch are green.

Benchmark

node benchmark/compare.js --runs 20 over benchmark/webstreams:

                                                                       confidence improvement accuracy (*)    (**)   (***)
webstreams/adapters.js kind='readable-from-web' n=100000                              -1.27 %       ±2.95%  ±3.95%  ±5.19%
webstreams/adapters.js kind='readable-to-web' n=100000                                -1.67 %       ±3.26%  ±4.37%  ±5.75%
webstreams/adapters.js kind='writable-from-web' n=100000                               1.80 %       ±5.60%  ±7.54% ±10.02%
webstreams/adapters.js kind='writable-to-web' n=100000                                -1.43 %       ±4.00%  ±5.36%  ±7.05%
webstreams/creation.js kind='ReadableStream.tee' n=50000                      ***      9.04 %       ±3.85%  ±5.19%  ±6.90%
webstreams/creation.js kind='ReadableStream' n=50000                                   7.47 %      ±10.48% ±14.05% ±18.53%
webstreams/creation.js kind='ReadableStreamBYOBReader' n=50000                         0.93 %       ±9.81% ±13.20% ±17.47%
webstreams/creation.js kind='ReadableStreamDefaultReader' n=50000                      1.76 %       ±8.73% ±11.69% ±15.39%
webstreams/creation.js kind='TransformStream' n=50000                         ***     66.74 %       ±8.17% ±10.99% ±14.55%
webstreams/creation.js kind='WritableStream' n=50000                                   0.97 %       ±3.86%  ±5.17%  ±6.79%
webstreams/encoding-streams.js len=1024 kind='decode' n=100000                         2.17 %       ±4.77%  ±6.40%  ±8.45%
webstreams/encoding-streams.js len=1024 kind='encode' n=100000                         1.12 %       ±3.02%  ±4.05%  ±5.34%
webstreams/encoding-streams.js len=16 kind='decode' n=100000                           2.32 %       ±5.21%  ±7.00%  ±9.25%
webstreams/encoding-streams.js len=16 kind='encode' n=100000                          -1.38 %       ±4.10%  ±5.52%  ±7.33%
webstreams/from.js kind='async' n=1000000                                             -2.42 %       ±4.79%  ±6.42%  ±8.46%
webstreams/from.js kind='sync' n=1000000                                              -0.61 %       ±6.29%  ±8.43% ±11.08%
webstreams/js_transfer.js n=10000 payload='ReadableStream'                            -1.46 %       ±2.59%  ±3.48%  ±4.59%
webstreams/js_transfer.js n=10000 payload='TransformStream'                           -1.22 %       ±2.10%  ±2.82%  ±3.71%
webstreams/js_transfer.js n=10000 payload='WritableStream'                             0.22 %       ±2.39%  ±3.20%  ±4.22%
webstreams/lifecycle.js kind='async-iterator' n=50000                                  0.85 %       ±3.17%  ±4.25%  ±5.59%
webstreams/lifecycle.js kind='pipe-through' n=50000                           ***     13.89 %       ±3.69%  ±4.98%  ±6.62%
webstreams/lifecycle.js kind='pipe-to' n=50000                                         3.68 %       ±6.69%  ±8.97% ±11.82%
webstreams/lifecycle.js kind='readable' n=50000                                       -0.09 %       ±2.94%  ±3.94%  ±5.18%
webstreams/pipe-through.js kind='default' n=500000                                    -0.84 %       ±1.69%  ±2.26%  ±2.98%
webstreams/pipe-through.js kind='transform' n=500000                                  -1.35 %       ±2.08%  ±2.79%  ±3.66%
webstreams/pipe-to.js highWaterMarkW=1 highWaterMarkR=1 n=500000                       0.93 %       ±2.70%  ±3.64%  ±4.83%
webstreams/pipe-to.js highWaterMarkW=1 highWaterMarkR=1024 n=500000                    0.12 %       ±2.04%  ±2.74%  ±3.62%
webstreams/pipe-to.js highWaterMarkW=1 highWaterMarkR=4096 n=500000                   -0.41 %       ±2.39%  ±3.20%  ±4.21%
webstreams/pipe-to.js highWaterMarkW=1024 highWaterMarkR=1 n=500000                    1.03 %       ±1.87%  ±2.50%  ±3.30%
webstreams/pipe-to.js highWaterMarkW=1024 highWaterMarkR=1024 n=500000                -0.99 %       ±1.59%  ±2.13%  ±2.80%
webstreams/pipe-to.js highWaterMarkW=1024 highWaterMarkR=4096 n=500000                 1.03 %       ±1.60%  ±2.15%  ±2.82%
webstreams/pipe-to.js highWaterMarkW=4096 highWaterMarkR=1 n=500000                    0.29 %       ±2.16%  ±2.89%  ±3.81%
webstreams/pipe-to.js highWaterMarkW=4096 highWaterMarkR=1024 n=500000                -1.95 %       ±2.31%  ±3.11%  ±4.10%
webstreams/pipe-to.js highWaterMarkW=4096 highWaterMarkR=4096 n=500000                -0.99 %       ±2.20%  ±2.94%  ±3.87%
webstreams/readable-async-iterator.js type='bytes' n=100000                     *     -2.82 %       ±2.12%  ±2.83%  ±3.73%
webstreams/readable-async-iterator.js type='normal' n=100000                           0.37 %       ±5.63%  ±7.55%  ±9.94%
webstreams/readable-read-buffered.js bufferSize=1 n=100000                             4.10 %       ±5.03%  ±6.74%  ±8.87%
webstreams/readable-read-buffered.js bufferSize=10 n=100000                     *      6.22 %       ±6.03%  ±8.08% ±10.62%
webstreams/readable-read-buffered.js bufferSize=100 n=100000                          -1.66 %       ±5.49%  ±7.37%  ±9.71%
webstreams/readable-read-buffered.js bufferSize=1000 n=100000                          0.58 %       ±5.46%  ±7.31%  ±9.63%
webstreams/readable-read.js type='byob' n=100000                                      -1.00 %       ±1.83%  ±2.45%  ±3.23%
webstreams/readable-read.js type='normal' n=100000                                    -1.96 %       ±4.01%  ±5.39%  ±7.12%
webstreams/tee.js type='bytes' n=100000                                                0.10 %       ±1.85%  ±2.47%  ±3.26%
webstreams/tee.js type='normal' n=100000                                               0.91 %       ±2.58%  ±3.46%  ±4.55%
webstreams/writable-write.js type='await' n=100000                                    -1.02 %       ±4.02%  ±5.39%  ±7.10%
webstreams/writable-write.js type='queued' n=100000                                   -0.45 %       ±1.97%  ±2.64%  ±3.49%

The readable-async-iterator.js type='bytes' row (untouched code) re-run directly with 30 samples:

                                                             confidence improvement accuracy (*)   (**)  (***)
webstreams/readable-async-iterator.js type='bytes' n=100000                 -0.57 %       ±2.02% ±2.69% ±3.51%
webstreams/readable-async-iterator.js type='normal' n=100000                -1.41 %       ±3.55% ±4.73% ±6.19%

The per-chunk rows are flat as expected: the savings are per stream. A local harness that constructs a TransformStream measured +67 %.

---

AI generated, humanly reviewed.

[codecov]

Codecov Report

❌ Patch coverage is 98.54651% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.46%. Comparing base (4e09bf3) to head (334b151).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/webstreams/readablestream.js	98.00%	4 Missing ⚠️
lib/internal/webstreams/writablestream.js	97.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66643      +/-   ##
==========================================
+ Coverage   90.43%   90.46%   +0.02%     
==========================================
  Files         791      791              
  Lines      276605   276996     +391     
  Branches    53117    53230     +113     
==========================================
+ Hits       250155   250577     +422     
+ Misses      16866    16836      -30     
+ Partials     9584     9583       -1     

Files with missing lines	Coverage Δ
lib/internal/webstreams/transformstream.js	98.78% <100.00%> (+0.06%)	⬆️
lib/internal/webstreams/util.js	97.20% <100.00%> (+0.12%)	⬆️
lib/internal/webstreams/writablestream.js	99.52% <97.87%> (+<0.01%)	⬆️
lib/internal/webstreams/readablestream.js	98.11% <98.00%> (+0.02%)	⬆️

... and 28 files with indirect coverage changes

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