buffer: fix odd UTF-16LE indexOf start

UTF-16LE string searches rounded odd forward start offsets down,
allowing a match before the requested range.

Round the start up to the next code-unit boundary while leaving
reverse search and the existing 16-bit implementation unchanged.

Assisted-by: Codex
Signed-off-by: inoway46 <inoueyuya416@gmail.com>
PR-URL: #65960
Fixes: #26448
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: inoway46

src/node_buffer.cc

@@ -1086,6 +1086,10 @@ void IndexOfString(const FunctionCallbackInfo<Value>& args) {
   } else if (is_forward && offset >= search_end) {
     return args.GetReturnValue().Set(-1);
   }
+  if (enc == UCS2 && is_forward) {
+    offset += offset % sizeof(uint16_t);
+    if (offset >= search_end) return args.GetReturnValue().Set(-1);
+  }
   CHECK_LT(offset, haystack_length);
   if ((is_forward && needle_length + offset > search_end) ||
       needle_length > search_end) {

test/parallel/test-buffer-indexof.js

@@ -308,6 +308,12 @@ assert.strictEqual(Buffer.from('aaaa').indexOf('你好', 'ucs2'), -1);
 // Haystack has odd length, but the needle is UCS2.
 assert.strictEqual(Buffer.from('aaaaa').indexOf('b', 'ucs2'), -1);
 
+{
+  const buf = Buffer.from('\u6881\u6882\u6881', 'utf16le');
+  assert.strictEqual(buf.indexOf('\u6881', 1, 'utf16le'), 4);
+  assert.strictEqual(buf.indexOf('\u6881', -1, 'utf16le'), -1);
+}
+
 {
   // Find substrings in Utf8.
   const lengths = [1, 3, 15];  // Single char, simple and complex.