src: fix inefficient usage of v8_inspector::StringView

v8_inspector::StringView can either be one-byte or two-byte strings.
Node has currently two places where it's unconditionally assumed that
it's a two-byte StringView.

This requires the upstream V8 inspector to unnecessarily create
a copy of the string:

https://source.chromium.org/chromium/chromium/src/+/main:v8/src/inspector/v8-inspector-session-impl.cc;l=192-199;drc=05bacd38e7a31e92afe0fd66081dfa2cc03b934a

This is particularly slow, especially for large CDP messages, as the
serialized JSON is iterated 8-bit char by 8-bit char and each one
widened to a 16-bit char.

This PR introduces a small helper that correctly converts a StringView
to a v8::String instance honoring the "is8Bit" flag. This allows
upstream V8 to remove the unnecessary widening copy.

Author: szuend

src/inspector_js_api.cc

@@ -76,9 +76,7 @@ class JSBindingsConnection : public BaseObject {
       HandleScope handle_scope(isolate);
       Context::Scope context_scope(env_->context());
       Local<Value> argument;
-      if (!String::NewFromTwoByte(isolate, message.characters16(),
-                                  NewStringType::kNormal,
-                                  message.length()).ToLocal(&argument)) return;
+      if (!StringViewToV8String(isolate, message).ToLocal(&argument)) return;
       connection_->OnMessage(argument);
     }
 

src/inspector_profiler.cc

@@ -89,11 +89,7 @@ void V8ProfilerConnection::V8ProfilerSessionDelegate::SendMessageToFrontend(
   const char* type = connection_->type();
   // Convert StringView to a Local<String>.
   Local<String> message_str;
-  if (!String::NewFromTwoByte(isolate,
-                              message.characters16(),
-                              NewStringType::kNormal,
-                              message.length())
-           .ToLocal(&message_str)) {
+  if (!StringViewToV8String(isolate, message).ToLocal(&message_str)) {
     fprintf(
         stderr, "Failed to convert %s profile message to V8 string\n", type);
     return;

src/util-inl.h

@@ -213,6 +213,20 @@ inline v8::Local<v8::String> OneByteString(v8::Isolate* isolate,
       .ToLocalChecked();
 }
 
+v8::MaybeLocal<v8::String> StringViewToV8String(
+    v8::Isolate* isolate,
+    v8_inspector::StringView string) {
+  if (string.is8Bit()) {
+    return v8::String::NewFromOneByte(isolate,
+                                      string.characters8(),
+                                      v8::NewStringType::kNormal,
+                                      string.length());
+  }
+  return v8::String::NewFromTwoByte(isolate, string.characters16(),
+                                    v8::NewStringType::kNormal,
+                                    string.length());
+}
+
 void SwapBytes16(char* data, size_t nbytes) {
   CHECK_EQ(nbytes % 2, 0);
 

src/util.h

@@ -26,6 +26,7 @@
 
 #include "uv.h"
 #include "v8.h"
+#include "v8-inspector.h"
 
 #include "node.h"
 #include "node_exit_code.h"
@@ -353,6 +354,10 @@ inline v8::Local<v8::String> FIXED_ONE_BYTE_STRING(
   return OneByteString(isolate, arr.data(), N - 1);
 }
 
+// Convenience wrapper to handle both one- and two-byte inspector strings.
+inline v8::MaybeLocal<v8::String> StringViewToV8String(
+    v8::Isolate* isolate,
+    v8_inspector::StringView string);
 
 
 // Swaps bytes in place. nbytes is the number of bytes to swap and must be a