base,heap: Introduce MaybeValidTrait

Funnel MaybeValid() through a trait instead of imposing an API on the
receiver.

This allows us to avoid implementing a MaybeValid() API on Persistents
in the Oilpan library.

Bug: 1056170
Change-Id: I05f2497a6a10a8324980e5b82b4c40c785af8331
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2642680
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845677}

base/bind_internal.h

@@ -980,6 +980,16 @@ struct IsWeakReceiver<std::reference_wrapper<T>> : IsWeakReceiver<T> {};
 template <typename T>
 struct IsWeakReceiver<WeakPtr<T>> : std::true_type {};
 
+// An injection point to control how objects are checked for maybe validity,
+// which is an optimistic thread-safe check for full validity.
+template <typename>
+struct MaybeValidTraits {
+  template <typename T>
+  static bool MaybeValid(const T& o) {
+    return o.MaybeValid();
+  }
+};
+
 // An injection point to control how bound objects passed to the target
 // function. BindUnwrapTraits<>::Unwrap() is called for each bound objects right
 // before the target function is invoked.
@@ -1055,7 +1065,7 @@ struct CallbackCancellationTraits<
   static bool MaybeValid(const Functor&,
                          const Receiver& receiver,
                          const Args&...) {
-    return receiver.MaybeValid();
+    return MaybeValidTraits<Receiver>::MaybeValid(receiver);
   }
 };
 
@@ -1072,7 +1082,7 @@ struct CallbackCancellationTraits<OnceCallback<Signature>,
 
   template <typename Functor>
   static bool MaybeValid(const Functor& functor, const BoundArgs&...) {
-    return functor.MaybeValid();
+    return MaybeValidTraits<Functor>::MaybeValid(functor);
   }
 };
 
@@ -1088,7 +1098,7 @@ struct CallbackCancellationTraits<RepeatingCallback<Signature>,
 
   template <typename Functor>
   static bool MaybeValid(const Functor& functor, const BoundArgs&...) {
-    return functor.MaybeValid();
+    return MaybeValidTraits<Functor>::MaybeValid(functor);
   }
 };
 

third_party/blink/renderer/platform/heap/impl/persistent.h

@@ -72,10 +72,6 @@ class PersistentBase {
     return raw_;
   }
 
-  // TODO(https://crbug.com/653394): Consider returning a thread-safe best
-  // guess of validity.
-  bool MaybeValid() const { return true; }
-
   explicit operator bool() const { return Get(); }
   T& operator*() const { return *Get(); }
   operator T*() const { return Get(); }

third_party/blink/renderer/platform/heap/persistent.h

@@ -145,6 +145,20 @@ struct BindUnwrapTraits<blink::CrossThreadWeakPersistent<T>> {
   }
 };
 
+// TODO(https://crbug.com/653394): Consider returning a thread-safe best
+// guess of validity. MaybeValid() can be invoked from an arbitrary thread.
+template <typename T>
+struct MaybeValidTraits<blink::WeakPersistent<T>> {
+  static bool MaybeValid(const blink::WeakPersistent<T>& p) { return true; }
+};
+
+template <typename T>
+struct MaybeValidTraits<blink::CrossThreadWeakPersistent<T>> {
+  static bool MaybeValid(const blink::CrossThreadWeakPersistent<T>& p) {
+    return true;
+  }
+};
+
 }  // namespace base
 
 #endif  // THIRD_PARTY_BLINK_RENDERER_PLATFORM_HEAP_PERSISTENT_H_