buffer: handle UCS2 `.fill()` properly on BE

There was a byte-order mismatch for `buffer#fill` on big-endian
platforms. Weirdly, the tests seemed to expect that wrong behaviour.

PR-URL: https://github.com/nodejs/node/pull/9837
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

src/node_buffer.cc

@@ -606,6 +606,9 @@ void Fill(const FunctionCallbackInfo<Value>& args) {
 
   } else if (enc == UCS2) {
     node::TwoByteValue str(env->isolate(), args[1]);
+    if (IsBigEndian())
+      SwapBytes16(reinterpret_cast<char*>(&str[0]), str_length);
+
     memcpy(ts_obj_data + start, *str, MIN(str_length, fill_length));
 
   } else {

test/parallel/test-buffer-fill.js

@@ -2,13 +2,11 @@
 
 require('../common');
 const assert = require('assert');
-const os = require('os');
 const SIZE = 28;
 
 const buf1 = Buffer.allocUnsafe(SIZE);
 const buf2 = Buffer.allocUnsafe(SIZE);
 
-
 // Default encoding
 testBufs('abc');
 testBufs('\u0222aa');
@@ -112,8 +110,7 @@ testBufs('\u0222aa', 8, 1, 'ucs2');
 testBufs('a\u0234b\u0235c\u0236', 4, -1, 'ucs2');
 testBufs('a\u0234b\u0235c\u0236', 4, 1, 'ucs2');
 testBufs('a\u0234b\u0235c\u0236', 12, 1, 'ucs2');
-assert.strictEqual(Buffer.allocUnsafe(1).fill('\u0222', 'ucs2')[0],
-                   os.endianness() === 'LE' ? 0x22 : 0x02);
+assert.strictEqual(Buffer.allocUnsafe(1).fill('\u0222', 'ucs2')[0], 0x22);
 
 
 // HEX
@@ -259,15 +256,6 @@ function writeToFill(string, offset, end, encoding) {
     }
   } while (offset < buf2.length);
 
-  // Correction for UCS2 operations.
-  if (os.endianness() === 'BE' && encoding === 'ucs2') {
-    for (var i = 0; i < buf2.length; i += 2) {
-      var tmp = buf2[i];
-      buf2[i] = buf2[i + 1];
-      buf2[i + 1] = tmp;
-    }
-  }
-
   return buf2;
 }
 
@@ -406,3 +394,12 @@ assert.throws(() => {
   });
   buf.fill('');
 }, /^RangeError: out of range index$/);
+
+
+assert.deepStrictEqual(
+    Buffer.allocUnsafeSlow(16).fill('ab', 'utf16le'),
+    Buffer.from('61006200610062006100620061006200', 'hex'));
+
+assert.deepStrictEqual(
+    Buffer.allocUnsafeSlow(15).fill('ab', 'utf16le'),
+    Buffer.from('610062006100620061006200610062', 'hex'));