fs: honor flush option in writeFileSync utf8 fast path

The C++ utf8 fast path added in 4466dee does not perform an
fsync, but the gate routing writeFileSync through it did not consult
the flush option added in e01c1d7. As a result
writeFileSync(path, data, { encoding: 'utf8', flush: true }) wrote
without flushing, while the same call without an explicit encoding
flushed correctly. Skip the fast path when flush is requested.

appendFileSync delegates to writeFileSync and was affected the same
way. Extend the existing flush tests with the explicit-utf8-encoding
case for both; the new cases fail without this change.

Signed-off-by: Sam Attard <sattard@anthropic.com>

Author: MarshallOfSound

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) => {