stream: decouple transform backpressure changes#65143

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Merged
nodejs-github-bot merged 1 commit into from
Aug 20, 2026
+156 −44
Merged

stream: decouple transform backpressure changes #65143

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38 changes: 38 additions & 0 deletions benchmark/webstreams/pipe-through.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
ReadableStream,
TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
n: [5e5],
kind: ['default', 'transform'],
});

async function main({ n, kind }) {
const b = Buffer.alloc(64);
let i = 0;
const rs = new ReadableStream({
pull(controller) {
if (i++ < n) {
controller.enqueue(b);
} else {
controller.close();
}
},
});
const ts = kind === 'default' ?
new TransformStream() :
new TransformStream({
transform(chunk, controller) { controller.enqueue(chunk); },
});

const reader = rs.pipeThrough(ts).getReader();
bench.start();
for (;;) {
const { done } = await reader.read();
if (done) break;
}
bench.end(n);
}
153 changes: 109 additions & 44 deletions lib/internal/webstreams/transformstream.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
ObjectDefineProperties,
ObjectSetPrototypeOf,
PromisePrototypeThen,
PromiseReject,
PromiseResolve,
PromiseWithResolvers,
Symbol,
SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
customInspect,
extractHighWaterMark,
extractSizeAlgorithm,
getNonWritablePropertyDescriptor,
isBrandCheck,
kParkedAlgorithmResult,
kResolvedPromise,
kState,
kType,
nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
readable: undefined,
writable: undefined,
backpressure: undefined,
backpressureChange: undefined,
pullPending: false,
pendingWrite: undefined,
pendingWriteChunk: undefined,
writeContinuation: undefined,
controller: undefined,
};
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
writable,
controller: undefined,
backpressure: undefined,
backpressureChange: undefined,
// Continuation slots replacing the spec's
// [[backpressureChangePromise]]; see transformStreamSetBackpressure.
pullPending: false,
pendingWrite: undefined,
pendingWriteChunk: undefined,
writeContinuation: undefined,
};

transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
const state = stream[kState];
return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
const state = stream[kState];
assert(state.backpressure !== backpressure);
const backpressureChange = state.backpressureChange;
if (backpressureChange !== undefined) {
state.backpressureChange = undefined;
backpressureChange.resolve();
}
state.backpressure = backpressure;
if (backpressure) {
if (state.pullPending) {
state.pullPending = false;
// The pull-fulfilled step exists: a pull parked it (see
// transformStreamDefaultSourcePullAlgorithm), and the readable
// controller creates it before invoking the pull algorithm.
PromisePrototypeThen(
kResolvedPromise,
state.readable[kState].controller[kState].pullFulfilled);
}
} else if (state.pendingWrite !== undefined) {
PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
}
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
transformAlgorithm,
flushAlgorithm,
cancelAlgorithm,
performTransformRejected: undefined,
};
stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
const flush = transformer?.flush;
const cancel = transformer?.cancel;
const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
defaultTransformAlgorithm;
const flushAlgorithm = flush ?
createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
const controllerState = controller[kState];
const transformAlgorithm = controllerState.transformAlgorithm;
if (transformAlgorithm === undefined) {
// Algorithms were cleared by a concurrent cancel/abort/close.
return kResolvedPromise;
}
let result;
try {
const transformAlgorithm = controller[kState].transformAlgorithm;
if (transformAlgorithm === undefined) {
// Algorithms were cleared by a concurrent cancel/abort/close.
return;
}
return await transformAlgorithm(chunk, controller);
result = transformAlgorithm(chunk, controller);
} catch (error) {
result = PromiseReject(error);
}
if (result === null ||
(typeof result !== 'object' && typeof result !== 'function')) {
result = kResolvedPromise;
} else {
result = PromiseResolve(result);
}
controllerState.performTransformRejected ??= (error) => {
transformStreamError(controller[kState].stream, error);
throw error;
}
};
return PromisePrototypeThen(
result,
undefined,
controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
const state = stream[kState];
const {
writable,
controller,
} = stream[kState];
} = state;
assert(writable[kState].state === 'writable');
if (stream[kState].backpressure) {
const backpressureChange = transformStreamBackpressureChangePromise(stream);
return PromisePrototypeThen(
backpressureChange,
() => {
const {
writable,
} = stream[kState];
if (writable[kState].state === 'erroring')
throw writable[kState].storedError;
assert(writable[kState].state === 'writable');
return transformStreamDefaultControllerPerformTransform(
if (state.backpressure) {
// Park the chunk and one promise record; the backpressure -> false
// flip delivers the cached continuation (see
// transformStreamSetBackpressure) at the same microtask position as
// the old [[backpressureChangePromise]] reaction. The continuation
// resolves the sink promise with the perform-transform promise, so
// adoption reproduces the old derived-chain settle depth exactly.
// The writable dispatches a single write at a time, so one pending
// slot suffices.
assert(state.pendingWrite === undefined);
const pendingWrite = PromiseWithResolvers();
state.pendingWrite = pendingWrite;
state.pendingWriteChunk = chunk;
state.writeContinuation ??= () => {
const pending = state.pendingWrite;
const pendingChunk = state.pendingWriteChunk;
state.pendingWrite = undefined;
state.pendingWriteChunk = undefined;
const writableState = state.writable[kState];
if (writableState.state === 'erroring') {
pending.reject(writableState.storedError);
return;
}
assert(writableState.state === 'writable');
pending.resolve(
transformStreamDefaultControllerPerformTransform(
controller,
chunk);
});
pendingChunk));
};
return pendingWrite.promise;
}
return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
assert(stream[kState].backpressure);
const state = stream[kState];
assert(state.backpressure);
transformStreamSetBackpressure(stream, false);
return transformStreamBackpressureChangePromise(stream);
// Park the pull: the next backpressure -> true flip delivers the
// pull-fulfilled step (see transformStreamSetBackpressure). The old
// [[backpressureChangePromise]] this replaces was only ever resolved,
// so the parked pull needs no rejection delivery.
state.pullPending = true;
return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down
9 changes: 9 additions & 0 deletions lib/internal/webstreams/util.js
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
if (result === kParkedAlgorithmResult)
return;
if (result === null ||
(typeof result !== 'object' && typeof result !== 'function')) {
PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
isBrandCheck,
isPromisePending,
kEmptyQueue,
kParkedAlgorithmResult,
kResolvedPromise,
kState,
kType,
Expand Down
Loading
, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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stream: decouple transform backpressure changes#65143

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to your account

 Merged

nodejs-github-bot
merged 1 commit into

from

Aug 20, 2026

+156

−44

 Merged


stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
stream: decouple transform backpressure changes#65143

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Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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privacy statement. We’ll occasionally send you account related emails.

Already on GitHub?
Sign in
to your account

 Merged

nodejs-github-bot
merged 1 commit into

from

Aug 20, 2026

+156

−44

 Merged


stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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stream: decouple transform backpressure changes#65143

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merged 1 commit into

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Aug 20, 2026

+156

−44

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stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
stream: decouple transform backpressure changes#65143

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Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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privacy statement. We’ll occasionally send you account related emails.

Already on GitHub?
Sign in
to your account

 Merged

nodejs-github-bot
merged 1 commit into

from

Aug 20, 2026

+156

−44

 Merged


stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

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Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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stream: decouple transform backpressure changes#65143

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to your account

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merged 1 commit into

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Aug 20, 2026

+156

−44

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stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
stream: decouple transform backpressure changes#65143

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Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

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privacy statement. We’ll occasionally send you account related emails.

Already on GitHub?
Sign in
to your account

 Merged

nodejs-github-bot
merged 1 commit into

from

Aug 20, 2026

+156

−44

 Merged


stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

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merged 1 commit into

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Aug 20, 2026

+156

−44

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stream: decouple transform backpressure changes
#65143
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38 changes: 38 additions & 0 deletions
benchmark/webstreams/pipe-through.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,38 @@
'use strict';
const common = require('../common.js');
const {
 ReadableStream,
 TransformStream,
} = require('node:stream/web');

const bench = common.createBenchmark(main, {
 n: [5e5],
 kind: ['default', 'transform'],
});

async function main({ n, kind }) {
 const b = Buffer.alloc(64);
 let i = 0;
 const rs = new ReadableStream({
 pull(controller) {
 if (i++ < n) {
 controller.enqueue(b);
 } else {
 controller.close();
 }
 },
 });
 const ts = kind === 'default' ?
 new TransformStream() :
 new TransformStream({
 transform(chunk, controller) { controller.enqueue(chunk); },
 });

 const reader = rs.pipeThrough(ts).getReader();
 bench.start();
 for (;;) {
 const { done } = await reader.read();
 if (done) break;
 }
 bench.end(n);
}

153 changes: 109 additions & 44 deletions
lib/internal/webstreams/transformstream.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -5,6 +5,8 @@ const {
 ObjectDefineProperties,
 ObjectSetPrototypeOf,
 PromisePrototypeThen,
 PromiseReject,
 PromiseResolve,
 PromiseWithResolvers,
 Symbol,
 SymbolToStringTag,
Expand DownExpand Up@@ -44,12 +46,14 @@ const {

const {
 createPromiseCallback1Param,
createPromiseCallback2Params,
createRawCallback2Params,
 customInspect,
 extractHighWaterMark,
 extractSizeAlgorithm,
 getNonWritablePropertyDescriptor,
 isBrandCheck,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
 nonOpCancel,
Expand DownExpand Up@@ -258,7 +262,10 @@ function InternalTransferredTransformStream() {
 readable: undefined,
 writable: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 controller: undefined,
 };
}
Expand DownExpand Up@@ -348,7 +355,9 @@ const isTransformStream =
const isTransformStreamDefaultController =
 isBrandCheck('TransformStreamDefaultController');

async function defaultTransformAlgorithm(chunk, controller) {
// Raw callback (see createRawCallback*): invoked inside the try/catch of
// transformStreamDefaultControllerPerformTransform.
function defaultTransformAlgorithm(chunk, controller) {
 transformStreamDefaultControllerEnqueue(controller, chunk);
}

Expand DownExpand Up@@ -385,7 +394,12 @@ function initializeTransformStream(
 writable,
 controller: undefined,
 backpressure: undefined,
 backpressureChange: undefined,
 // Continuation slots replacing the spec's
 // [[backpressureChangePromise]]; see transformStreamSetBackpressure.
 pullPending: false,
 pendingWrite: undefined,
 pendingWriteChunk: undefined,
 writeContinuation: undefined,
 };

 transformStreamSetBackpressure(stream, true);
Expand DownExpand Up@@ -422,24 +436,30 @@ function transformStreamUnblockWrite(stream) {
// The spec's [[backpressureChangePromise]] is only ever observed by the
// source pull algorithm (settles when backpressure next becomes true) and
// by a sink write arriving while backpressure is set (settles when
// backpressure next becomes false). Instead of allocating a fresh promise
// record on every flip, the record is materialized lazily on first
// observation and dropped once settled; flips nobody is waiting on
// allocate nothing.
function transformStreamBackpressureChangePromise(stream) {
 const state = stream[kState];
 return (state.backpressureChange ??= PromiseWithResolvers()).promise;
}

// backpressure next becomes false). Both observers are internal, so the
// promise record is replaced by continuation slots: a parked pull is
// completed by delivering the readable controller's pull-fulfilled step,
// and a parked write by the cached write continuation (see
// transformStreamDefaultSinkWriteAlgorithm). Each is enqueued on the
// shared resolved promise at the exact microtask position the old
// record's reaction would have had.
function transformStreamSetBackpressure(stream, backpressure) {
 const state = stream[kState];
 assert(state.backpressure !== backpressure);
 const backpressureChange = state.backpressureChange;
 if (backpressureChange !== undefined) {
 state.backpressureChange = undefined;
 backpressureChange.resolve();
 }
 state.backpressure = backpressure;
 if (backpressure) {
 if (state.pullPending) {
 state.pullPending = false;
 // The pull-fulfilled step exists: a pull parked it (see
 // transformStreamDefaultSourcePullAlgorithm), and the readable
 // controller creates it before invoking the pull algorithm.
 PromisePrototypeThen(
 kResolvedPromise,
 state.readable[kState].controller[kState].pullFulfilled);
 }
 } else if (state.pendingWrite !== undefined) {
 PromisePrototypeThen(kResolvedPromise, state.writeContinuation);
 }
}

function setupTransformStreamDefaultController(
Expand All@@ -456,6 +476,7 @@ function setupTransformStreamDefaultController(
 transformAlgorithm,
 flushAlgorithm,
 cancelAlgorithm,
 performTransformRejected: undefined,
 };
 stream[kState].controller = controller;
}
Expand All@@ -468,7 +489,7 @@ function setupTransformStreamDefaultControllerFromTransformer(
 const flush = transformer?.flush;
 const cancel = transformer?.cancel;
 const transformAlgorithm = transform ?
createPromiseCallback2Params('transformer.transform', transform, transformer) :
createRawCallback2Params('transformer.transform', transform, transformer) :
 defaultTransformAlgorithm;
 const flushAlgorithm = flush ?
 createPromiseCallback1Param('transformer.flush', flush, transformer) :
Expand DownExpand Up@@ -521,18 +542,40 @@ function transformStreamDefaultControllerError(controller, error) {
 transformStreamError(controller[kState].stream, error);
}

async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
// Mirrors the reference implementation's
// `promiseCall(transformAlgorithm, ...).then(undefined, rejectionSteps)`:
// the returned promise settles one microtask after the (coerced) result
// does, and a rejection errors the transform stream before propagating.
// The raw transform callback plus the shared resolved promise for
// non-thenable results replace the previous async wrapper's two implicit
// promises per chunk.
function transformStreamDefaultControllerPerformTransform(controller, chunk) {
 const controllerState = controller[kState];
 const transformAlgorithm = controllerState.transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return kResolvedPromise;
 }
 let result;
 try {
 const transformAlgorithm = controller[kState].transformAlgorithm;
 if (transformAlgorithm === undefined) {
 // Algorithms were cleared by a concurrent cancel/abort/close.
 return;
 }
 return await transformAlgorithm(chunk, controller);
 result = transformAlgorithm(chunk, controller);
 } catch (error) {
 result = PromiseReject(error);
 }
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 result = kResolvedPromise;
 } else {
 result = PromiseResolve(result);
 }
 controllerState.performTransformRejected ??= (error) => {
 transformStreamError(controller[kState].stream, error);
 throw error;
 }
 };
 return PromisePrototypeThen(
 result,
 undefined,
 controllerState.performTransformRejected);
}

function transformStreamDefaultControllerTerminate(controller) {
Expand All@@ -553,26 +596,42 @@ function transformStreamDefaultControllerTerminate(controller) {
}

function transformStreamDefaultSinkWriteAlgorithm(stream, chunk) {
 const state = stream[kState];
 const {
 writable,
 controller,
 } = stream[kState];
 } = state;
 assert(writable[kState].state === 'writable');
 if (stream[kState].backpressure) {
 const backpressureChange = transformStreamBackpressureChangePromise(stream);
 return PromisePrototypeThen(
 backpressureChange,
 () => {
 const {
 writable,
 } = stream[kState];
 if (writable[kState].state === 'erroring')
 throw writable[kState].storedError;
 assert(writable[kState].state === 'writable');
 return transformStreamDefaultControllerPerformTransform(
 if (state.backpressure) {
 // Park the chunk and one promise record; the backpressure -> false
 // flip delivers the cached continuation (see
 // transformStreamSetBackpressure) at the same microtask position as
 // the old [[backpressureChangePromise]] reaction. The continuation
 // resolves the sink promise with the perform-transform promise, so
 // adoption reproduces the old derived-chain settle depth exactly.
 // The writable dispatches a single write at a time, so one pending
 // slot suffices.
 assert(state.pendingWrite === undefined);
 const pendingWrite = PromiseWithResolvers();
 state.pendingWrite = pendingWrite;
 state.pendingWriteChunk = chunk;
 state.writeContinuation ??= () => {
 const pending = state.pendingWrite;
 const pendingChunk = state.pendingWriteChunk;
 state.pendingWrite = undefined;
 state.pendingWriteChunk = undefined;
 const writableState = state.writable[kState];
 if (writableState.state === 'erroring') {
 pending.reject(writableState.storedError);
 return;
 }
 assert(writableState.state === 'writable');
 pending.resolve(
 transformStreamDefaultControllerPerformTransform(
 controller,
 chunk);
 });
 pendingChunk));
 };
 return pendingWrite.promise;
 }
 return transformStreamDefaultControllerPerformTransform(controller, chunk);
}
Expand DownExpand Up@@ -642,9 +701,15 @@ function transformStreamDefaultSinkCloseAlgorithm(stream) {
}

function transformStreamDefaultSourcePullAlgorithm(stream) {
 assert(stream[kState].backpressure);
 const state = stream[kState];
 assert(state.backpressure);
 transformStreamSetBackpressure(stream, false);
 return transformStreamBackpressureChangePromise(stream);
 // Park the pull: the next backpressure -> true flip delivers the
 // pull-fulfilled step (see transformStreamSetBackpressure). The old
 // [[backpressureChangePromise]] this replaces was only ever resolved,
 // so the parked pull needs no rejection delivery.
 state.pullPending = true;
 return kParkedAlgorithmResult;
}

function transformStreamDefaultSourceCancelAlgorithm(stream, reason) {
Expand Down

9 changes: 9 additions & 0 deletions
lib/internal/webstreams/util.js

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -355,6 +355,12 @@ function createRawCallback2Params(name, fn, thisArg) {
// the next microtask checkpoint without allocating a fresh promise.
const kResolvedPromise = PromiseResolve();

// Returned by an internal algorithm to signal that it parked the
// operation and takes responsibility for delivering the fulfilled (or
// rejected) continuation itself later, instead of settling a promise
// (see the transform stream source pull algorithm).
const kParkedAlgorithmResult = { __proto__: null };

// Wires the (possibly non-thenable) result of an underlying algorithm
// callback to its fulfilled/rejected continuations. A non-thenable result
// means fulfillment is guaranteed and no then() lookup is observable, so
Expand All@@ -364,6 +370,8 @@ const kResolvedPromise = PromiseResolve();
// matches the spec's "a promise resolved with" conversion (identity for
// native promises).
function thenAlgorithmResult(result, onFulfilled, onRejected) {
 if (result === kParkedAlgorithmResult)
 return;
 if (result === null ||
 (typeof result !== 'object' && typeof result !== 'function')) {
 PromisePrototypeThen(kResolvedPromise, onFulfilled);
Expand DownExpand Up@@ -457,6 +465,7 @@ module.exports = {
 isBrandCheck,
 isPromisePending,
 kEmptyQueue,
 kParkedAlgorithmResult,
 kResolvedPromise,
 kState,
 kType,
Expand Down
Loading