src: backport FromV8Array

This backports nodejs#51758
which relies on an V8 API that's not available on v20.x.

Original commit message:

src: implement v8 array iteration using the new callback-based API

Using this to iterate over an array can be faster than calling
Array::Get repeatedly. Local experiment shows that this is faster
once the array size is bigger than 2.

PR-URL: nodejs#51758
Reviewed-By: Yagiz Nizipli <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>

Author: joyeecheung

src/util-inl.h

@@ -402,6 +402,35 @@ inline char* UncheckedCalloc(size_t n) { return UncheckedCalloc<char>(n); }
 // headers than we really need to.
 void ThrowErrStringTooLong(v8::Isolate* isolate);
 
+struct ArrayIterationData {
+  std::vector<v8::Global<v8::Value>>* out;
+  v8::Isolate* isolate = nullptr;
+};
+
+inline void PushItemToVector(uint32_t index,
+                             v8::Local<v8::Value> element,
+                             void* data) {
+  auto vec = static_cast<ArrayIterationData*>(data)->out;
+  auto isolate = static_cast<ArrayIterationData*>(data)->isolate;
+  vec->push_back(v8::Global<v8::Value>(isolate, element));
+}
+
+v8::Maybe<void> FromV8Array(v8::Local<v8::Context> context,
+                            v8::Local<v8::Array> js_array,
+                            std::vector<v8::Global<v8::Value>>* out) {
+  uint32_t count = js_array->Length();
+  out->reserve(count);
+  ArrayIterationData data{out, context->GetIsolate()};
+  for (uint32_t i = 0; i < js_array->Length(); i++) {
+    v8::Local<v8::Value> element;
+    if (!js_array->Get(context, i).ToLocal(&element)) {
+      return v8::Nothing<void>();
+    }
+    PushItemToVector(i, element, &data);
+  }
+  return v8::JustVoid();
+}
+
 v8::MaybeLocal<v8::Value> ToV8Value(v8::Local<v8::Context> context,
                                     std::string_view str,
                                     v8::Isolate* isolate) {

src/util.h

@@ -698,6 +698,16 @@ struct FunctionDeleter {
 template <typename T, void (*function)(T*)>
 using DeleteFnPtr = typename FunctionDeleter<T, function>::Pointer;
 
+// Convert a v8::Array into an std::vector using the callback-based API.
+// This can be faster than calling Array::Get() repeatedly when the array
+// has more than 2 entries.
+// Note that iterating over an array in C++ and performing operations on each
+// element in a C++ loop is still slower than iterating over the array in JS
+// and calling into native in the JS loop repeatedly on each element,
+// as of V8 11.9.
+inline v8::Maybe<void> FromV8Array(v8::Local<v8::Context> context,
+                                   v8::Local<v8::Array> js_array,
+                                   std::vector<v8::Global<v8::Value>>* out);
 std::vector<std::string_view> SplitString(const std::string_view in,
                                           const std::string_view delim);