fs: fix flush regression from fast path

lib/fs.js

@@ -2381,27 +2381,37 @@ function writeFileSync(path, data, options) {
   validateBoolean(flush, 'options.flush');
 
   const flag = options.flag || 'w';
+  const isUserFd = isFd(path); // File descriptor ownership
 
   // 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;

test/parallel/test-fs-append-file-flush.js

@@ -6,6 +6,7 @@ const fs = require('node:fs');
 const fsp = require('node:fs/promises');
 const test = require('node:test');
 const data = 'foo';
+const data2 = 'bar';
 let cnt = 0;
 
 function nextFile() {
@@ -25,15 +26,27 @@ test('synchronous version', async (t) => {
 
   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);
+      if (expected === 0) return;
+      assert.strictEqual(calls.at(-1).result, undefined);
+      assert.strictEqual(calls.at(-1).error, undefined);
+      assert.strictEqual(calls.at(-1).arguments.length, 1);
+      assert.strictEqual(typeof calls.at(-1).arguments[0], 'number');
+    };
+
     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) => {

test/parallel/test-fs-write-file-flush.js

@@ -6,6 +6,7 @@ const fs = require('node:fs');
 const fsp = require('node:fs/promises');
 const test = require('node:test');
 const data = 'foo';
+const data2 = 'bar';
 let cnt = 0;
 
 function nextFile() {
@@ -25,15 +26,27 @@ test('synchronous version', async (t) => {
 
   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);
+      if (expected === 0) return;
+      assert.strictEqual(calls.at(-1).result, undefined);
+      assert.strictEqual(calls.at(-1).error, undefined);
+      assert.strictEqual(calls.at(-1).arguments.length, 1);
+      assert.strictEqual(typeof calls.at(-1).arguments[0], 'number');
+    };
+
     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) => {