src/objects/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_OBJECTS_MANAGED_H_
#define V8_OBJECTS_MANAGED_H_

#include <memory>

#include "src/execution/isolate.h"
#include "src/handles/handles.h"
#include "src/heap/factory.h"
#include "src/objects/foreign.h"
#include "src/sandbox/external-pointer-table.h"

namespace v8 {
namespace internal {

// Mechanism for associating an ExternalPointerTag with a C++ type that is
// referenced via a Managed. Every such C++ type must have a unique
// ExternalPointerTag to ensure type-safe access to the external object.
//
// This mechanism supports two ways of associating tags with types:
//
// 1. By adding a 'static constexpr ExternalPointerTag kManagedTag` field to
//    the C++ class (preferred for C++ types defined in V8 code):
//
//      class MyCppClass {
//       public:
//        static constexpr ExternalPointerTag kManagedTag = kMyCppClassTag;
//        ...;
//
// 2. Through the ASSIGN_EXTERNAL_POINTER_TAG_FOR_MANAGED macro, which uses
//    template specialization (necessary for C++ types defined outside of V8):
//
//      ASSIGN_EXTERNAL_POINTER_TAG_FOR_MANAGED(MyCppClass, kMyCppClassTag)
//
//    Note that the struct created by this macro must be visible when the
//    Managed<CppType> is used. In particular, there may be issues if the
//    CppType is only forward declared and the respective header isn't included.
//    Note also that this macro must be used inside the v8::internal namespace.
//
template <typename CppType>
struct TagForManaged {
  static constexpr ExternalPointerTag value = CppType::kManagedTag;
};

#define ASSIGN_EXTERNAL_POINTER_TAG_FOR_MANAGED(CppType, Tag) \
  template <>                                                 \
  struct TagForManaged<CppType> {                             \
    static constexpr ExternalPointerTag value = Tag;          \
  };

// Implements a doubly-linked lists of destructors for the isolate.
struct ManagedPtrDestructor
#ifdef V8_ENABLE_SANDBOX
    : public ExternalPointerTable::ManagedResource {
#else
    : public Malloced {
#endif  // V8_ENABLE_SANDBOX

  // Estimated size of external memory associated with the managed object.
  // This is used to adjust the garbage collector's heuristics upon
  // allocation and deallocation of a managed object.
  size_t estimated_size_ = 0;
  ManagedPtrDestructor* prev_ = nullptr;
  ManagedPtrDestructor* next_ = nullptr;
  void* shared_ptr_ptr_ = nullptr;
  void (*destructor_)(void* shared_ptr) = nullptr;
  Address* global_handle_location_ = nullptr;

  ManagedPtrDestructor(size_t estimated_size, void* shared_ptr_ptr,
                       void (*destructor)(void*))
      : estimated_size_(estimated_size),
        shared_ptr_ptr_(shared_ptr_ptr),
        destructor_(destructor) {}
};

// The GC finalizer of a managed object, which does not depend on
// the template parameter.
V8_EXPORT_PRIVATE void ManagedObjectFinalizer(
    const v8::WeakCallbackInfo<void>& data);

// {Managed<T>} is essentially a {std::shared_ptr<T>} allocated on the heap
// that can be used to manage the lifetime of C++ objects that are shared
// across multiple isolates.
// When a {Managed<T>} object is garbage collected (or an isolate which
// contains {Managed<T>} is torn down), the {Managed<T>} deletes its underlying
// {std::shared_ptr<T>}, thereby decrementing its internal reference count,
// which will delete the C++ object when the reference count drops to 0.
template <class CppType>
class Managed : public Foreign {
 public:
  Managed() : Foreign() {}
  explicit Managed(Address ptr) : Foreign(ptr) {}
  explicit V8_INLINE constexpr Managed(Address ptr, SkipTypeCheckTag)
      : Foreign(ptr, SkipTypeCheckTag{}) {}

  // For every object, add a `->` operator which returns a pointer to this
  // object. This will allow smoother transition between T and Tagged<T>.
  Managed* operator->() { return this; }
  const Managed* operator->() const { return this; }

  // Get a raw pointer to the C++ object.
  V8_INLINE CppType* raw() { return GetSharedPtrPtr()->get(); }

  // Get a reference to the shared pointer to the C++ object.
  V8_INLINE const std::shared_ptr<CppType>& get() { return *GetSharedPtrPtr(); }

  static Tagged<Managed> cast(Tagged<Object> obj) {
    return Tagged<Managed>(Managed(obj.ptr()).ptr());
  }
  static constexpr Tagged<Managed> unchecked_cast(Tagged<Object> obj) {
    return Tagged<Managed>(obj.ptr());
  }

  // Allocate a new {CppType} and wrap it in a {Managed<CppType>}.
  template <typename... Args>
  static Handle<Managed<CppType>> Allocate(Isolate* isolate,
                                           size_t estimated_size,
                                           Args&&... args);

  // Create a {Managed<CppType>} from an existing raw {CppType*}. The returned
  // object will now own the memory pointed to by {CppType}.
  static Handle<Managed<CppType>> FromRawPtr(Isolate* isolate,
                                             size_t estimated_size,
                                             CppType* ptr);

  // Create a {Managed<CppType>} from an existing {std::unique_ptr<CppType>}.
  // The returned object will now own the memory pointed to by {CppType}, and
  // the unique pointer will be released.
  static Handle<Managed<CppType>> FromUniquePtr(
      Isolate* isolate, size_t estimated_size,
      std::unique_ptr<CppType> unique_ptr,
      AllocationType allocation_type = AllocationType::kYoung);

  // Create a {Managed<CppType>} from an existing {std::shared_ptr<CppType>}.
  static Handle<Managed<CppType>> FromSharedPtr(
      Isolate* isolate, size_t estimated_size,
      std::shared_ptr<CppType> shared_ptr,
      AllocationType allocation_type = AllocationType::kYoung);

 private:
  friend class Tagged<Managed>;

  // Internally this {Foreign} object stores a pointer to a new
  // std::shared_ptr<CppType>.
  std::shared_ptr<CppType>* GetSharedPtrPtr() {
    static constexpr ExternalPointerTag kTag = TagForManaged<CppType>::value;
    auto destructor =
        reinterpret_cast<ManagedPtrDestructor*>(foreign_address<kTag>());
    return reinterpret_cast<std::shared_ptr<CppType>*>(
        destructor->shared_ptr_ptr_);
  }

  // Called by either isolate shutdown or the {ManagedObjectFinalizer} in order
  // to actually delete the shared pointer and decrement the shared refcount.
  static void Destructor(void* ptr) {
    auto shared_ptr_ptr = reinterpret_cast<std::shared_ptr<CppType>*>(ptr);
    delete shared_ptr_ptr;
  }
};

}  // namespace internal
}  // namespace v8

#endif  // V8_OBJECTS_MANAGED_H_