fs: accept all valid UTF-8 encoding names in fast paths

The fs.readFileSync and fs.writeFileSync fast paths for UTF-8 encoding
only accepted exact matches: 'utf8', 'utf-8', 'UTF8', 'UTF-8'. However,
Node.js considers all case-insensitive variations as valid UTF-8 (e.g.,
'Utf8', 'Utf-8', 'uTf8', etc.).

This commit adds an isUtf8Encoding() helper function that accepts all
valid UTF-8 encoding names, matching the behavior of normalizeEncoding()
in lib/internal/util.js.

Fixes #49888

Author: wdsmini

lib/fs.js

@@ -171,6 +171,40 @@ function lazyLoadUtf8Stream() {
   Utf8Stream ??= require('internal/streams/fast-utf8-stream');
 }
 
+// Check if encoding is a UTF-8 variant that should use the fast path.
+// This covers all valid UTF-8 encoding names: utf8, utf-8, UTF8, UTF-8,
+// and case-insensitive variations like Utf8, Utf-8, etc.
+function isUtf8Encoding(enc) {
+  if (enc === 'utf8' || enc === 'utf-8') return true;
+  if (enc.length === 4) {
+    // Check for UTF8 (case-insensitive)
+    const c1 = StringPrototypeCharCodeAt(enc, 0);
+    const c2 = StringPrototypeCharCodeAt(enc, 1);
+    const c3 = StringPrototypeCharCodeAt(enc, 2);
+    const c4 = StringPrototypeCharCodeAt(enc, 3);
+    // U/u, T/t, F/f, 8
+    return (c1 === 85 || c1 === 117) && // U/u
+           (c2 === 84 || c2 === 116) && // T/t
+           (c3 === 70 || c3 === 102) && // F/f
+           c4 === 56; // 8
+  }
+  if (enc.length === 5) {
+    // Check for UTF-8 (case-insensitive)
+    const c1 = StringPrototypeCharCodeAt(enc, 0);
+    const c2 = StringPrototypeCharCodeAt(enc, 1);
+    const c3 = StringPrototypeCharCodeAt(enc, 2);
+    const c4 = StringPrototypeCharCodeAt(enc, 3);
+    const c5 = StringPrototypeCharCodeAt(enc, 4);
+    // U/u, T/t, F/f, -, 8
+    return (c1 === 85 || c1 === 117) && // U/u
+           (c2 === 84 || c2 === 116) && // T/t
+           (c3 === 70 || c3 === 102) && // F/f
+           c4 === 45 && // -
+           c5 === 56; // 8
+  }
+  return false;
+}
+
 // Ensure that callbacks run in the global context. Only use this function
 // for callbacks that are passed to the binding layer, callbacks that are
 // invoked from JS already run in the proper scope.
@@ -429,8 +463,7 @@ function tryReadSync(fd, isUserFd, buffer, pos, len) {
 function readFileSync(path, options) {
   options = getOptions(options, { flag: 'r' });
 
-  if (options.encoding === 'utf8' || options.encoding === 'utf-8' ||
-      options.encoding === 'UTF8' || options.encoding === 'UTF-8') {
+  if (isUtf8Encoding(options.encoding)) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }
@@ -2391,8 +2424,7 @@ 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' ||
-      options.encoding === 'UTF8' || options.encoding === 'UTF-8')) {
+  if (typeof data === 'string' && isUtf8Encoding(options.encoding)) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }

test/parallel/test-fs-utf8-encoding-case-insensitive.js

@@ -0,0 +1,74 @@
+'use strict';
+
+// Test that fs.readFileSync and fs.writeFileSync accept all valid
+// UTF-8 encoding names (case-insensitive) for the fast path.
+// Refs: https://github.com/nodejs/node/issues/49888
+
+const common = require('../common');
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+const tmpdir = require('../common/tmpdir');
+
+tmpdir.refresh();
+
+const testFile = path.join(tmpdir.path, 'test-utf8-encoding.txt');
+const testContent = 'Hello, World! 你好，世界！';
+
+// All valid UTF-8 encoding variants that should use the fast path
+const utf8Variants = [
+  'utf8',
+  'utf-8',
+  'UTF8',
+  'UTF-8',
+  'Utf8',
+  'Utf-8',
+  'uTf8',
+  'uTf-8',
+  'utF8',
+  'utF-8',
+  'UTf8',
+  'UTf-8',
+  'uTF8',
+  'uTF-8',
+];
+
+// Test writeFileSync with all UTF-8 variants
+for (const encoding of utf8Variants) {
+  const testPath = path.join(tmpdir.path, `test-write-${encoding}.txt`);
+
+  // Should not throw and should write the file correctly
+  fs.writeFileSync(testPath, testContent, { encoding });
+
+  // Verify the file was written correctly
+  const content = fs.readFileSync(testPath, 'utf8');
+  assert.strictEqual(content, testContent,
+                     `writeFileSync should work with encoding "${encoding}"`);
+}
+
+// Test readFileSync with all UTF-8 variants
+for (const encoding of utf8Variants) {
+  const testPath = path.join(tmpdir.path, `test-read-${encoding}.txt`);
+
+  // Create a test file
+  fs.writeFileSync(testPath, testContent, 'utf8');
+
+  // Should read the file correctly with any UTF-8 variant
+  const content = fs.readFileSync(testPath, { encoding });
+  assert.strictEqual(content, testContent,
+                     `readFileSync should work with encoding "${encoding}"`);
+}
+
+// Test that non-UTF-8 encodings still work correctly
+const otherEncodings = ['ascii', 'base64', 'hex', 'latin1'];
+for (const encoding of otherEncodings) {
+  const testPath = path.join(tmpdir.path, `test-${encoding}.txt`);
+
+  // These should not use the UTF-8 fast path but should still work
+  fs.writeFileSync(testPath, testContent, { encoding });
+  const content = fs.readFileSync(testPath, { encoding });
+  assert.strictEqual(content, testContent,
+                     `Should work with encoding "${encoding}"`);
+}
+
+console.log('All tests passed!');