[wasm] Managed<T> ensures T's lifetime does not leak past Isolate's

src/managed.h

+// Copyright 2016 the V8 project 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 V8_WASM_MANAGED_H_
+#define V8_WASM_MANAGED_H_
+
+#include "src/factory.h"
+#include "src/global-handles.h"
+#include "src/handles.h"
+#include "src/isolate.h"
+
+namespace v8 {
+namespace internal {
+// An object that wraps a pointer to a C++ object and manages its lifetime.
+// The C++ object will be deleted when the managed wrapper object is
+// garbage collected, or, last resort, if the isolate is torn down before GC,
+// as part of Isolate::Dispose().
+// Managed<CppType> may be used polymorphically as Foreign, where the held
+// address is typed as CppType**. The double indirection is due to the
+// use, by Managed, of Isolate::ManagedObjectFinalizer, which has a CppType*
+// first field.
+template <class CppType>
+class Managed : public Foreign {
+ public:
+  V8_INLINE CppType* get() {
+    return *(reinterpret_cast<CppType**>(foreign_address()));
+  }
+
+  static Managed<CppType>* cast(Object* obj) {
+    SLOW_DCHECK(obj->IsForeign());
+    return reinterpret_cast<Managed<CppType>*>(obj);
+  }
+
+  static Handle<Managed<CppType>> New(Isolate* isolate, CppType* ptr) {
+    Isolate::ManagedObjectFinalizer* node =
+        isolate->RegisterForReleaseAtTeardown(ptr,
+                                              Managed<CppType>::NativeDelete);
+    Handle<Managed<CppType>> handle = Handle<Managed<CppType>>::cast(
+        isolate->factory()->NewForeign(reinterpret_cast<Address>(node)));
+    RegisterWeakCallbackForDelete(isolate, handle);
+    return handle;
+  }
+
+ private:
+  static void RegisterWeakCallbackForDelete(Isolate* isolate,
+                                            Handle<Managed<CppType>> handle) {
+    Handle<Object> global_handle = isolate->global_handles()->Create(*handle);
+    GlobalHandles::MakeWeak(global_handle.location(), global_handle.location(),
+                            &Managed<CppType>::GCDelete,
+                            v8::WeakCallbackType::kFinalizer);
+  }
+
+  static void GCDelete(const v8::WeakCallbackInfo<void>& data) {
+    Managed<CppType>** p =
+        reinterpret_cast<Managed<CppType>**>(data.GetParameter());
+
+    Isolate::ManagedObjectFinalizer* finalizer = (*p)->GetFinalizer();
+
+    Isolate* isolate = reinterpret_cast<Isolate*>(data.GetIsolate());
+    finalizer->Dispose();
+    isolate->UnregisterFromReleaseAtTeardown(&finalizer);
+
+    (*p)->set_foreign_address(static_cast<Address>(nullptr));
+    GlobalHandles::Destroy(reinterpret_cast<Object**>(p));
+  }
+
+  static void NativeDelete(void* value) {
+    CppType* typed_value = reinterpret_cast<CppType*>(value);
+    delete typed_value;
+  }
+
+  Isolate::ManagedObjectFinalizer* GetFinalizer() {
+    return reinterpret_cast<Isolate::ManagedObjectFinalizer*>(
+        foreign_address());
+  }
+};
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_WASM_MANAGED_H_