[SharedArrayBuffer] Add "AllowShared" extended attribute, used for WebGL

third_party/WebKit/Source/core/dom/ArrayBufferViewHelpers.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef NotShared_h
-#define NotShared_h
+#ifndef ArrayBufferViewHelpers_h
+#define ArrayBufferViewHelpers_h
+
+#include <type_traits>
+#include "core/dom/DOMArrayBufferView.h"
+#include "platform/heap/Handle.h"
+#include "platform/wtf/TypeTraits.h"
+
+namespace blink {
 
 // A wrapper template type that is used to ensure that a TypedArray is not
 // backed by a SharedArrayBuffer.
 //
 // Typically this is used as an annotation on C++ functions that are called by
 // the bindings layer, e.g.:
 //
 //   void Foo(NotShared<DOMUint32Array> param) {
 //     DOMUint32Array* array = param.View();
 //     ...
 //   }
-
-#include "platform/heap/Handle.h"
-
-namespace blink {
-
 template <typename T>
 class NotShared {
+  static_assert(WTF::IsSubclass<typename std::remove_const<T>::type,
+                                DOMArrayBufferView>::value,
+                "NotShared<T> must have T as subclass of DOMArrayBufferView");
   STACK_ALLOCATED();
 
  public:
   using TypedArrayType = T;
 
   NotShared() {}
 
   explicit NotShared(T* typedArray) : typed_array_(typedArray) {
     DCHECK(!(typedArray && typedArray->View()->IsShared()));
   }
@@ skipping 21 matching lines @@
  private:
   // Must use an untraced member here since this object may be constructed on a
   // thread without a ThreadState (e.g. an Audio worklet). It is safe in that
   // case because the pointed-to ArrayBuffer is being kept alive another way
   // (e.g. CrossThreadPersistent).
   //
   // TODO(binji): update to using Member, see crbug.com/710295.
   UntracedMember<T> typed_array_;
 };
 
+// A wrapper template type that specifies that a TypedArray may be backed by a
+// SharedArrayBuffer.
+//
+// Typically this is used as an annotation on C++ functions that are called by
+// the bindings layer, e.g.:
+//
+//   void Foo(MaybeShared<DOMUint32Array> param) {
+//     DOMUint32Array* array = param.View();
+//     ...
+//   }
+template <typename T>
+class MaybeShared {
+  static_assert(WTF::IsSubclass<typename std::remove_const<T>::type,
+                                DOMArrayBufferView>::value,
+                "MaybeShared<T> must have T as subclass of DOMArrayBufferView");
+  STACK_ALLOCATED();
+
+ public:
+  using TypedArrayType = T;
+
+  MaybeShared() {}
+
+  explicit MaybeShared(T* typedArray) : typed_array_(typedArray) {}
+  MaybeShared(const MaybeShared& other) = default;
+  template <typename U>
+  MaybeShared(const MaybeShared<U>& other) : typed_array_(other.View()) {}
+  template <typename U>
+  MaybeShared(const Member<U>& other) {
+    typed_array_ = other.Get();
+  }
+
+  MaybeShared& operator=(const MaybeShared& other) = default;
+  template <typename U>
+  MaybeShared& operator=(const MaybeShared<U>& other) {
+    typed_array_ = other.View();
+    return *this;
+  }
+
+  T* View() const { return typed_array_.Get(); }
+
+  bool operator!() const { return !typed_array_; }
+  explicit operator bool() const { return !!typed_array_; }
+
+ private:
+  Member<T> typed_array_;
+};
+
 }  // namespace blink
 
-#endif  // NotShared_h
+#endif  // ArrayBufferViewHelpers_h