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stream: fix pipeTo to defer writes per WHATWG spec #61800

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stream: fix pipeTo to defer writes per WHATWG spec #61800

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11 changes: 8 additions & 3 deletions lib/internal/webstreams/readablestream.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1584,9 +1584,14 @@ class PipeToReadableStreamReadRequest {
}

[kChunk](chunk) {
this.state.currentWrite = writableStreamDefaultWriterWrite(this.writer, chunk);
setPromiseHandled(this.state.currentWrite);
this.promise.resolve(false);
// Per spec, pipeTo must queue a microtask for the write to avoid
// synchronous write during enqueue(). See WHATWG Streams spec
// "ReadableStreamPipeTo" step 15's "chunk steps".
queueMicrotask(() => {
this.state.currentWrite = writableStreamDefaultWriterWrite(this.writer, chunk);
setPromiseHandled(this.state.currentWrite);
this.promise.resolve(false);
});
}

[kClose]() {
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8 changes: 0 additions & 8 deletions test/wpt/status/streams.json
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Original file line number Diff line number Diff line change
Expand Up @@ -2,14 +2,6 @@
"idlharness-shadowrealm.window.js": {
"skip": "ShadowRealm support is not enabled"
},
"piping/general-addition.any.js": {
"fail": {
"note": "Blocked on https://github.com/whatwg/streams/issues/1243",
"expected": [
"enqueue() must not synchronously call write algorithm"
]
}
},
Comment on lines -5 to -12
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@MattiasBuelens MattiasBuelens Feb 13, 2026

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Even the reference implementation skips this test. 😅

Is this extra microtask now a "de facto" standard? Looking at wpt.fyi, all major browsers are already passing this test, even though the spec text doesn't require it...

"queuing-strategies-size-function-per-global.window.js": {
"skip": "Browser-specific test"
},
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