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stream: refine stream/iter backpressure and error handling #63697

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ff1188a
stream: refine the stream/iter backpressure
jasnell Jun 1, 2026
c29567e
stream: fixup error handling in stream/iter adapters
jasnell Jun 1, 2026
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2 changes: 1 addition & 1 deletion doc/api/stream_iter.md
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Original file line number Diff line number Diff line change
Expand Up @@ -1675,7 +1675,7 @@ Creates a classic [`stream.Writable`][] backed by a stream/iter Writer.

Each `_write()` / `_writev()` call attempts the Writer's synchronous method
first (`writeSync` / `writevSync`), falling back to the async method if the
sync path returns `false` or throws. Similarly, `_final()` tries `endSync()`
sync path returns `false`. Similarly, `_final()` tries `endSync()`
before `end()`. When the sync path succeeds, the callback is deferred via
`queueMicrotask` to preserve the async resolution contract.

Expand Down
67 changes: 16 additions & 51 deletions lib/internal/streams/iter/classic.js
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Expand Up @@ -57,7 +57,6 @@ const {
const {
toAsyncStreamable: kToAsyncStreamable,
kValidatedSource,
kSyncWriteAccepted,
drainableProtocol,
} = require('internal/streams/iter/types');

Expand Down Expand Up @@ -765,41 +764,11 @@ function toWritable(writer) {
const hasEndSync = hasEnd &&
typeof writer.endSync === 'function';
const hasFail = typeof writer.fail === 'function';
const hasSyncWriteAccepted =
typeof writer[kSyncWriteAccepted] === 'function';

function syncWriteAccepted() {
return hasSyncWriteAccepted && writer[kSyncWriteAccepted]();
}

function finishAfterSyncBackpressure(cb) {
let ondrain;
try {
if (typeof writer[drainableProtocol] === 'function') {
ondrain = writer[drainableProtocol]();
}
} catch (err) {
cb(err);
return;
}
if (ondrain !== null && ondrain !== undefined) {
PromisePrototypeThen(ondrain, (drained) => {
if (drained === false) {
cb(new ERR_INVALID_STATE.TypeError('Stream closed by consumer'));
return;
}
cb();
}, cb);
return;
}
queueMicrotask(cb);
}

// Try-sync-first pattern: attempt the synchronous method and fall back to the
// async method if it returns false without accepting the data, or if it
// throws. When the sync path succeeds, the callback is deferred via
// queueMicrotask to preserve the async resolution contract that Writable
// internals expect from _write/_writev/_final callbacks.
// async method if it returns false (data not accepted synchronously).
// When the sync path succeeds, the callback is deferred via queueMicrotask
// to preserve the async resolution contract that Writable internals expect
// from _write/_writev/_final callbacks.

function _write(chunk, encoding, cb) {
const bytes = typeof chunk === 'string' ?
Expand All @@ -810,13 +779,10 @@ function toWritable(writer) {
queueMicrotask(cb);
return;
}
if (syncWriteAccepted()) {
// The chunk was accepted; false only signaled backpressure.
finishAfterSyncBackpressure(cb);
return;
}
} catch {
// Sync path threw -- fall through to async.
// WriteSync returned false: not accepted, fall through to async.
} catch (err) {
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cb(err);
return;
}
}
try {
Expand All @@ -839,13 +805,10 @@ function toWritable(writer) {
queueMicrotask(cb);
return;
}
if (syncWriteAccepted()) {
// The chunks were accepted; false only signaled backpressure.
finishAfterSyncBackpressure(cb);
return;
}
} catch {
// Sync path threw -- fall through to async.
// WritevSync returned false: not accepted, fall through to async.
} catch (err) {
cb(err);
return;
}
}
try {
Expand All @@ -867,8 +830,10 @@ function toWritable(writer) {
queueMicrotask(cb);
return;
}
} catch {
// Sync path threw -- fall through to async.
// Result < 0: can't end synchronously, fall through to async.
} catch (err) {
cb(err);
return;
}
}
try {
Expand Down
45 changes: 35 additions & 10 deletions lib/internal/streams/iter/consumers.js
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Expand Up @@ -480,6 +480,7 @@ function merge(...args) {
);
}

let primaryError;
try {
while (activeCount > 0 || ready.length > 0) {
signal?.throwIfAborted();
Expand All @@ -500,22 +501,46 @@ function merge(...args) {
});
}
}
} catch (err) {
primaryError = err;
} finally {
// Clean up: return all iterators
await SafePromiseAllReturnVoid(iterators, async (iterator) => {
if (iterator.return) {
try {
await iterator.return();
} catch {
// Ignore return errors
}
}
});
// Clean up: return all iterators. Cleanup errors are not
// swallowed - a broken iterator.return() (e.g., failing to
// release a resource) should be visible to the caller.
await cleanupIterators(iterators, primaryError);
}
},
};
}

async function cleanupIterators(iterators, primaryError) {
let cleanupError;
await SafePromiseAllReturnVoid(iterators, async (iterator) => {
if (iterator.return) {
try {
await iterator.return();
} catch (err) {
// Keep the first cleanup error encountered.
cleanupError ??= err;
}
}
});
if (cleanupError !== undefined) {
if (primaryError !== undefined) {
// Both a primary error and a cleanup error occurred.
// Wrap in SuppressedError so neither is lost:
// .error = primaryError, .suppressed = cleanupError.
// eslint-disable-next-line no-restricted-syntax
throw new SuppressedError(primaryError, cleanupError);
}
// No primary error - the cleanup error is the only error.
throw cleanupError;
}
if (primaryError !== undefined) {
throw primaryError;
}
}

module.exports = {
array,
arrayBuffer,
Expand Down
56 changes: 5 additions & 51 deletions lib/internal/streams/iter/pull.js
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Expand Up @@ -63,8 +63,6 @@ const {
} = require('internal/streams/iter/utils');

const {
drainableProtocol,
kSyncWriteAcceptedOnFalse,
kValidatedSource,
kValidatedTransform,
toAsyncStreamable,
Expand Down Expand Up @@ -863,18 +861,6 @@ async function* createAsyncPipeline(source, transforms, signal) {
}
}

/**
* Check if a false sync write result means accepted backpressure.
* @param {object} writer - The writer whose sync method returned.
* @param {*} result - The return value from writeSync() or writevSync().
* @returns {boolean}
*/
function isAcceptedSyncWriteBackpressure(writer, result) {
return result === false &&
writer[kSyncWriteAcceptedOnFalse] === true &&
writer.desiredSize === 0;
}

// =============================================================================
// Public API: pull() and pullSync()
// =============================================================================
Expand Down Expand Up @@ -968,9 +954,7 @@ function pipeToSync(source, ...args) {
break;
}
if (hasWritevSync && batch.length > 1) {
const result = writer.writevSync(batch);
if (result === false &&
!isAcceptedSyncWriteBackpressure(writer, result)) {
if (writer.writevSync(batch) === false) {
break;
}
for (let i = 0; i < batch.length; i++) {
Expand All @@ -979,9 +963,7 @@ function pipeToSync(source, ...args) {
} else {
for (let i = 0; i < batch.length; i++) {
const chunk = batch[i];
const result = writer.writeSync(chunk);
if (result === false &&
!isAcceptedSyncWriteBackpressure(writer, result)) {
if (writer.writeSync(chunk) === false) {
canContinue = false;
break;
}
Expand Down Expand Up @@ -1037,28 +1019,13 @@ async function pipeTo(source, ...args) {
const hasWritevSync = typeof writer.writevSync === 'function';
const hasEndSync = typeof writer.endSync === 'function';

function waitForSyncBackpressure() {
const ondrain = writer[drainableProtocol];
return ondrain?.call(writer);
}

async function writeBatchAfterAcceptedBackpressure(batch, startIndex) {
await waitForSyncBackpressure();
await writeBatchAsyncFallback(batch, startIndex);
}

// Async fallback for writeBatch when sync write fails partway through.
// Continues writing from batch[startIndex] using async write().
async function writeBatchAsyncFallback(batch, startIndex) {
for (let i = startIndex; i < batch.length; i++) {
const chunk = batch[i];
const result = hasWriteSync && writer.writeSync(chunk);
if (result) {
if (hasWriteSync && writer.writeSync(chunk)) {
// Sync retry succeeded
} else if (isAcceptedSyncWriteBackpressure(writer, result)) {
totalBytes += TypedArrayPrototypeGetByteLength(chunk);
await waitForSyncBackpressure();
continue;
} else {
const result = writer.write(
chunk, signal ? { __proto__: null, signal } : undefined);
Expand All @@ -1075,14 +1042,7 @@ async function pipeTo(source, ...args) {
// is required. Callers must check: const p = writeBatch(b); if (p) await p;
function writeBatch(batch) {
if (hasWritev && batch.length > 1) {
const result = hasWritevSync && writer.writevSync(batch);
if (!result) {
if (isAcceptedSyncWriteBackpressure(writer, result)) {
for (let i = 0; i < batch.length; i++) {
totalBytes += TypedArrayPrototypeGetByteLength(batch[i]);
}
return waitForSyncBackpressure();
}
if (!hasWritevSync || !writer.writevSync(batch)) {
const opts = signal ? { __proto__: null, signal } : undefined;
const writevResult = writer.writev(batch, opts);
if (writevResult === undefined) {
Expand All @@ -1104,14 +1064,8 @@ async function pipeTo(source, ...args) {
}
for (let i = 0; i < batch.length; i++) {
const chunk = batch[i];
const result = hasWriteSync && writer.writeSync(chunk);
if (!result) {
if (isAcceptedSyncWriteBackpressure(writer, result)) {
totalBytes += TypedArrayPrototypeGetByteLength(chunk);
return writeBatchAfterAcceptedBackpressure(batch, i + 1);
}
if (!hasWriteSync || !writer.writeSync(chunk)) {
// Sync path failed at index i - fall back to async for the rest.
// Count bytes for chunks already written synchronously (0..i-1).
return writeBatchAsyncFallback(batch, i);
}
totalBytes += TypedArrayPrototypeGetByteLength(chunk);
Expand Down
48 changes: 2 additions & 46 deletions lib/internal/streams/iter/push.js
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Expand Up @@ -33,8 +33,6 @@ const {

const {
drainableProtocol,
kSyncWriteAccepted,
kSyncWriteAcceptedOnFalse,
} = require('internal/streams/iter/types');

const {
Expand Down Expand Up @@ -369,19 +367,6 @@ class PushQueue {
this.#pendingEnd = pending;
}

/**
* Force-enqueue chunks into the slots buffer, bypassing capacity checks.
* Used by PushWriter.writeSync() for 'block' policy where the data is
* accepted but false is returned as a backpressure signal.
*/
forceEnqueue(chunks) {
this.#slots.push(chunks);
for (let i = 0; i < chunks.length; i++) {
this.#bytesWritten += TypedArrayPrototypeGetByteLength(chunks[i]);
}
this.#resolvePendingReads();
}

/**
* Wait for backpressure to clear (desiredSize > 0).
* @returns {Promise<void>}
Expand Down Expand Up @@ -563,16 +548,11 @@ class PushQueue {

class PushWriter {
#queue;
#syncWriteAccepted = false;

constructor(queue) {
this.#queue = queue;
}

[kSyncWriteAccepted]() {
return this.#syncWriteAccepted;
}

[drainableProtocol]() {
const desired = this.desiredSize;
if (desired === null) return null;
Expand All @@ -584,10 +564,6 @@ class PushWriter {
return this.#queue.desiredSize;
}

get [kSyncWriteAcceptedOnFalse]() {
return this.#queue.backpressurePolicy === 'block';
}

write(chunk, options) {
if (!options?.signal && this.#queue.canWriteSync()) {
const bytes = toUint8Array(chunk);
Expand All @@ -612,36 +588,16 @@ class PushWriter {
}

writeSync(chunk) {
this.#syncWriteAccepted = false;
const bytes = toUint8Array(chunk);
const result = this.#queue.writeSync([bytes]);
if (!result && this.#queue.backpressurePolicy === 'block' &&
this.#queue.desiredSize === 0) {
// Block policy: force-enqueue and return false as backpressure signal.
// Data IS accepted; false tells caller to slow down.
this.#queue.forceEnqueue([bytes]);
this.#syncWriteAccepted = true;
return false;
}
this.#syncWriteAccepted = result;
return result;
return this.#queue.writeSync([bytes]);
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}

writevSync(chunks) {
this.#syncWriteAccepted = false;
if (!ArrayIsArray(chunks)) {
throw new ERR_INVALID_ARG_TYPE('chunks', 'Array', chunks);
}
const bytes = convertChunks(chunks);
const result = this.#queue.writeSync(bytes);
if (!result && this.#queue.backpressurePolicy === 'block' &&
this.#queue.desiredSize === 0) {
this.#queue.forceEnqueue(bytes);
this.#syncWriteAccepted = true;
return false;
}
this.#syncWriteAccepted = result;
return result;
return this.#queue.writeSync(bytes);
}

end(options) {
Expand Down
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