[api] Add v8::BackingStore::IsShared

The new predicate indicates whether the backing store was created for
an ArrayBuffer or a SharedArrayBuffer. It is useful for some embedders.

Bug: v8:9380
Change-Id: I804063bb8c4c17815defd6538ce6a1b32f6a4531
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1873689
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64479}

include/v8.h

@@ -4868,6 +4868,12 @@ class V8_EXPORT BackingStore : public v8::internal::BackingStoreBase {
    */
   size_t ByteLength() const;
 
+  /**
+   * Indicates whether the backing store was created for an ArrayBuffer or
+   * a SharedArrayBuffer.
+   */
+  bool IsShared() const;
+
  private:
   BackingStore();
 };

src/api/api.cc

@@ -3750,6 +3750,10 @@ size_t v8::BackingStore::ByteLength() const {
   return reinterpret_cast<const i::BackingStore*>(this)->byte_length();
 }
 
+bool v8::BackingStore::IsShared() const {
+  return reinterpret_cast<const i::BackingStore*>(this)->is_shared();
+}
+
 std::shared_ptr<v8::BackingStore> v8::ArrayBuffer::GetBackingStore() {
   i::Handle<i::JSArrayBuffer> self = Utils::OpenHandle(this);
   std::shared_ptr<i::BackingStore> backing_store = self->GetBackingStore();

test/cctest/test-api-array-buffer.cc

@@ -608,3 +608,37 @@ TEST(SharedArrayBuffer_NewBackingStore_CustomDeleter) {
   }
   CHECK(backing_store_custom_called);
 }
+
+THREADED_TEST(BackingStore_NotShared) {
+  LocalContext env;
+  v8::Isolate* isolate = env->GetIsolate();
+  v8::HandleScope handle_scope(isolate);
+  Local<v8::ArrayBuffer> ab = v8::ArrayBuffer::New(isolate, 8);
+  CHECK(!ab->GetBackingStore()->IsShared());
+  CHECK(!v8::ArrayBuffer::NewBackingStore(isolate, 8)->IsShared());
+  backing_store_custom_called = false;
+  backing_store_custom_data = malloc(100);
+  backing_store_custom_length = 100;
+  CHECK(!v8::ArrayBuffer::NewBackingStore(
+             backing_store_custom_data, backing_store_custom_length,
+             BackingStoreCustomDeleter,
+             reinterpret_cast<void*>(backing_store_custom_deleter_data))
+             ->IsShared());
+}
+
+THREADED_TEST(BackingStore_Shared) {
+  LocalContext env;
+  v8::Isolate* isolate = env->GetIsolate();
+  v8::HandleScope handle_scope(isolate);
+  Local<v8::SharedArrayBuffer> ab = v8::SharedArrayBuffer::New(isolate, 8);
+  CHECK(ab->GetBackingStore()->IsShared());
+  CHECK(v8::SharedArrayBuffer::NewBackingStore(isolate, 8)->IsShared());
+  backing_store_custom_called = false;
+  backing_store_custom_data = malloc(100);
+  backing_store_custom_length = 100;
+  CHECK(v8::SharedArrayBuffer::NewBackingStore(
+            backing_store_custom_data, backing_store_custom_length,
+            BackingStoreCustomDeleter,
+            reinterpret_cast<void*>(backing_store_custom_deleter_data))
+            ->IsShared());
+}