buffer: do not affect memory after target for utf16 write

src/string_bytes.cc

@@ -287,18 +287,19 @@ size_t StringBytes::WriteUCS2(Isolate* isolate,
   CHECK_EQ(reinterpret_cast<uintptr_t>(aligned_dst) % sizeof(*dst), 0);
 
   // Write all but the last char
+  max_chars = std::min(max_chars, static_cast<size_t>(str->Length()));
+  if (max_chars == 0) return 0;
   nchars = str->Write(isolate, aligned_dst, 0, max_chars - 1, flags);
+  CHECK_EQ(nchars, max_chars - 1);
 
   // Shift everything to unaligned-left
   memmove(dst, aligned_dst, nchars * sizeof(*dst));
 
   // One more char to be written
   uint16_t last;
-  if (nchars == max_chars - 1 &&
-      str->Write(isolate, &last, nchars, 1, flags) != 0) {
-    memcpy(buf + nchars * sizeof(*dst), &last, sizeof(last));
-    nchars++;
-  }
+  CHECK_EQ(str->Write(isolate, &last, nchars, 1, flags), 1);
+  memcpy(buf + nchars * sizeof(*dst), &last, sizeof(last));
+  nchars++;
 
   *chars_written = nchars;
   return nchars * sizeof(*dst);

test/parallel/test-buffer-write.js

@@ -91,3 +91,11 @@ assert.strictEqual(Buffer.compare(z, Buffer.alloc(4, 0)), 0);
 // Large overrun could corrupt the process
 assert.strictEqual(Buffer.alloc(4)
   .write('ыыыыыы'.repeat(100), 3, 'utf16le'), 0);
+
+{
+  // .write() does not affect the byte after the written-to slice of the Buffer.
+  // Refs: https://github.com/nodejs/node/issues/26422
+  const buf = Buffer.alloc(8);
+  assert.strictEqual(buf.write('ыы', 1, 'utf16le'), 4);
+  assert.deepStrictEqual([...buf], [0, 0x4b, 0x04, 0x4b, 0x04, 0, 0, 0]);
+}