buffer: validate UTF8 on fast path

Fast API handles invalid UTF differently than the slow API.

Fixes: #54521
PR-URL: #54525

src/node_buffer.cc

@@ -1489,6 +1489,30 @@ uint32_t FastWriteString(Local<Value> receiver,
 
 static v8::CFunction fast_write_string(v8::CFunction::Make(FastWriteString));
 
+uint32_t FastWriteStringUTF8(Local<Value> receiver,
+                         const v8::FastApiTypedArray<uint8_t>& dst,
+                         const v8::FastOneByteString& src,
+                         uint32_t offset,
+                         uint32_t max_length) {
+  uint8_t* dst_data;
+  CHECK(dst.getStorageIfAligned(&dst_data));
+  CHECK(offset <= dst.length());
+  CHECK(dst.length() - offset <= std::numeric_limits<uint32_t>::max());
+
+  const auto size = std::min(
+      {static_cast<uint32_t>(dst.length() - offset), max_length, src.length});
+
+  if (simdutf::validate_utf8(src.data, size)) {
+    memcpy(dst_data + offset, src.data, size);
+  } else {
+    // TODO (fix): Fallback to SlowWriteString
+  }
+
+  return size;
+}
+
+static v8::CFunction fast_write_string_utf8(v8::CFunction::Make(FastWriteStringUTF8));
+
 void Initialize(Local<Object> target,
                 Local<Value> unused,
                 Local<Context> context,
@@ -1568,7 +1592,7 @@ void Initialize(Local<Object> target,
                 target,
                 "utf8WriteStatic",
                 SlowWriteString<UTF8>,
-                &fast_write_string);
+                &fast_write_string_utf8);
 
   SetMethod(context, target, "getZeroFillToggle", GetZeroFillToggle);
 }
@@ -1615,6 +1639,8 @@ void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
   registry->Register(SlowWriteString<UTF8>);
   registry->Register(fast_write_string.GetTypeInfo());
   registry->Register(FastWriteString);
+  registry->Register(fast_write_string_utf8.GetTypeInfo());
+  registry->Register(FastWriteStringUTF8);
   registry->Register(StringWrite<ASCII>);
   registry->Register(StringWrite<BASE64>);
   registry->Register(StringWrite<BASE64URL>);