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

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

Index: third_party/WebKit/Source/core/dom/ArrayBufferViewHelpers.h
diff --git a/third_party/WebKit/Source/core/dom/NotShared.h b/third_party/WebKit/Source/core/dom/ArrayBufferViewHelpers.h
similarity index 50%
rename from third_party/WebKit/Source/core/dom/NotShared.h
rename to third_party/WebKit/Source/core/dom/ArrayBufferViewHelpers.h
index 69173aaea4610677a9fcc4a8d9ea73971f742986..0f3c71f1475f9f20bd72aff88081b59fce0ea61e 100644
--- a/third_party/WebKit/Source/core/dom/NotShared.h
+++ b/third_party/WebKit/Source/core/dom/ArrayBufferViewHelpers.h
@@ -2,8 +2,15 @@
 // 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.
@@ -15,13 +22,11 @@
 //     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:
@@ -63,6 +68,53 @@ class NotShared {
   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