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fs: fix flush regression from fast path #60553

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fs: fix flush regression from fast path #60553

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5142f4e
fs: fix flush regression from fast path
ChALkeR Nov 2, 2025
37ef8ab
nit: Remove old comment
ChALkeR Jan 27, 2026
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16 changes: 13 additions & 3 deletions lib/fs.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2381,27 +2381,37 @@ function writeFileSync(path, data, options) {
validateBoolean(flush, 'options.flush');

const flag = options.flag || 'w';
const isUserFd = isFd(path);

// C++ fast path for string data and UTF8 encoding
if (typeof data === 'string' && (options.encoding === 'utf8' || options.encoding === 'utf-8')) {
if (
typeof data === 'string' &&
(options.encoding === 'utf8' || options.encoding === 'utf-8') &&
!(flush && !isUserFd)
) {
if (!isInt32(path)) {
path = getValidatedPath(path);
}

return binding.writeFileUtf8(
binding.writeFileUtf8(
path,
data,
stringToFlags(flag),
parseFileMode(options.mode, 'mode', 0o666),
);

if (flush && isUserFd) {
fs.fsyncSync(path);
}

return;
}

if (!isArrayBufferView(data)) {
validateStringAfterArrayBufferView(data, 'data');
data = Buffer.from(data, options.encoding || 'utf8');
}

const isUserFd = isFd(path); // File descriptor ownership
const fd = isUserFd ? path : fs.openSync(path, flag, options.mode);

let offset = 0;
Expand Down
25 changes: 19 additions & 6 deletions test/parallel/test-fs-append-file-flush.js
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
const fsp = require('node:fs/promises');
const test = require('node:test');
const data = 'foo';
const data2 = 'bar';
let cnt = 0;

function nextFile() {
Expand All @@ -25,15 +26,27 @@

await t.test('performs flush', (t) => {
const spy = t.mock.method(fs, 'fsyncSync');
const checkCalls = (expected) => {
const calls = spy.mock.calls;
assert.strictEqual(calls.length, expected);

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
if (expected === 0) return;
assert.strictEqual(calls.at(-1).result, undefined);

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GitHub Actions / lint-js-and-md 

Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
assert.strictEqual(calls.at(-1).error, undefined);

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GitHub Actions / lint-js-and-md 

Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
assert.strictEqual(calls.at(-1).arguments.length, 1);

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GitHub Actions / lint-js-and-md 

Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
assert.strictEqual(typeof calls.at(-1).arguments[0], 'number');

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
};

const file = nextFile();

checkCalls(0);
fs.appendFileSync(file, data, { flush: true });
const calls = spy.mock.calls;
assert.strictEqual(calls.length, 1);
assert.strictEqual(calls[0].result, undefined);
assert.strictEqual(calls[0].error, undefined);
assert.strictEqual(calls[0].arguments.length, 1);
assert.strictEqual(typeof calls[0].arguments[0], 'number');
checkCalls(1);
assert.strictEqual(fs.readFileSync(file, 'utf8'), data);

checkCalls(1);
fs.appendFileSync(file, data2, { flush: true });
checkCalls(2);
assert.strictEqual(fs.readFileSync(file, 'utf8'), data + data2);
});

await t.test('does not perform flush', (t) => {
Expand Down
25 changes: 19 additions & 6 deletions test/parallel/test-fs-write-file-flush.js
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
const fsp = require('node:fs/promises');
const test = require('node:test');
const data = 'foo';
const data2 = 'bar';
let cnt = 0;

function nextFile() {
Expand All @@ -25,15 +26,27 @@

await t.test('performs flush', (t) => {
const spy = t.mock.method(fs, 'fsyncSync');
const checkCalls = (expected) => {
const calls = spy.mock.calls;
assert.strictEqual(calls.length, expected);

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
if (expected === 0) return;
assert.strictEqual(calls.at(-1).result, undefined);

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
assert.strictEqual(calls.at(-1).error, undefined);

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
assert.strictEqual(calls.at(-1).arguments.length, 1);

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
assert.strictEqual(typeof calls.at(-1).arguments[0], 'number');

Check failure on line 36 in test/parallel/test-fs-write-file-flush.js

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Assertions must be wrapped into `common.mustSucceed`, `common.mustCall` or `common.mustCallAtLeast`
};

const file = nextFile();

checkCalls(0);
fs.writeFileSync(file, data, { flush: true });
const calls = spy.mock.calls;
assert.strictEqual(calls.length, 1);
assert.strictEqual(calls[0].result, undefined);
assert.strictEqual(calls[0].error, undefined);
assert.strictEqual(calls[0].arguments.length, 1);
assert.strictEqual(typeof calls[0].arguments[0], 'number');
checkCalls(1);
assert.strictEqual(fs.readFileSync(file, 'utf8'), data);

checkCalls(1);
fs.writeFileSync(file, data2, { flush: true, encoding: 'utf8' });
checkCalls(2);
assert.strictEqual(fs.readFileSync(file, 'utf8'), data2);
});

await t.test('does not perform flush', (t) => {
Expand Down
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