fs: honor flush option in writeFileSync utf8 fast path

lib/fs.js

@@ -2844,8 +2844,10 @@ function writeFileSync(path, data, options) {
 
   const flag = options.flag || 'w';
 
-  // C++ fast path for string data and UTF8 encoding
-  if (typeof data === 'string' && (options.encoding === 'utf8' || options.encoding === 'utf-8')) {
+  // C++ fast path for string data and UTF8 encoding. The fast path does
+  // not perform an fsync, so it cannot be used when flush is requested.
+  if (typeof data === 'string' && !flush &&
+      (options.encoding === 'utf8' || options.encoding === 'utf-8')) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }

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

@@ -47,6 +47,19 @@ test('synchronous version', async (t) => {
 
     assert.strictEqual(spy.mock.calls.length, 0);
   });
+
+  await t.test('performs flush with explicit utf8 encoding', (t) => {
+    // Refs: the C++ utf8 fast path must not skip the flush.
+    const spy = t.mock.method(fs, 'fsyncSync');
+
+    for (const encoding of ['utf8', 'utf-8']) {
+      const file = nextFile();
+      fs.appendFileSync(file, data, { encoding, flush: true });
+      assert.strictEqual(fs.readFileSync(file, 'utf8'), data);
+    }
+
+    assert.strictEqual(spy.mock.calls.length, 2);
+  });
 });
 
 test('callback version', async (t) => {

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

@@ -47,6 +47,19 @@ test('synchronous version', async (t) => {
 
     assert.strictEqual(spy.mock.calls.length, 0);
   });
+
+  await t.test('performs flush with explicit utf8 encoding', (t) => {
+    // Refs: the C++ utf8 fast path must not skip the flush.
+    const spy = t.mock.method(fs, 'fsyncSync');
+
+    for (const encoding of ['utf8', 'utf-8']) {
+      const file = nextFile();
+      fs.writeFileSync(file, data, { encoding, flush: true });
+      assert.strictEqual(fs.readFileSync(file, 'utf8'), data);
+    }
+
+    assert.strictEqual(spy.mock.calls.length, 2);
+  });
 });
 
 test('callback version', async (t) => {