tor-browser

observer_list_internal.h (5916B)

---

// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_OBSERVER_LIST_INTERNAL_H_
#define BASE_OBSERVER_LIST_INTERNAL_H_

#include <string>

#include "base/base_export.h"
#include "base/check.h"
#include "base/containers/linked_list.h"
#include "base/dcheck_is_on.h"
#include "base/memory/raw_ptr.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/memory/weak_ptr.h"
#include "base/observer_list_types.h"

#if DCHECK_IS_ON()
#include "base/debug/stack_trace.h"
#endif

namespace base {
namespace internal {

// Adapter for putting raw pointers into an ObserverList<Foo>::Unchecked.
class BASE_EXPORT UncheckedObserverAdapter {
public:
 explicit UncheckedObserverAdapter(const void* observer)
     : ptr_(const_cast<void*>(observer)) {}
 UncheckedObserverAdapter(const UncheckedObserverAdapter&) = delete;
 UncheckedObserverAdapter& operator=(const UncheckedObserverAdapter&) = delete;
 UncheckedObserverAdapter(UncheckedObserverAdapter&& other) = default;
 UncheckedObserverAdapter& operator=(UncheckedObserverAdapter&& other) =
     default;

 void MarkForRemoval() { ptr_ = nullptr; }

 bool IsMarkedForRemoval() const { return !ptr_; }
 bool IsEqual(const void* rhs) const { return ptr_ == rhs; }

 template <class ObserverType>
 static ObserverType* Get(const UncheckedObserverAdapter& adapter) {
   static_assert(
       !std::is_base_of_v<CheckedObserver, ObserverType>,
       "CheckedObserver classes must not use ObserverList<T>::Unchecked.");
   return static_cast<ObserverType*>(adapter.ptr_);
 }

#if DCHECK_IS_ON()
 std::string GetCreationStackString() const {
   return "Observer created at:\n" + stack_.ToString();
 }
#endif  // DCHECK_IS_ON()

private:
 raw_ptr<void, AcrossTasksDanglingUntriaged> ptr_;
#if DCHECK_IS_ON()
 base::debug::StackTrace stack_;
#endif  // DCHECK_IS_ON()
};

// Adapter for CheckedObserver types so that they can use the same syntax as a
// raw pointer when stored in the std::vector of observers in an ObserverList.
// It wraps a WeakPtr<CheckedObserver> and allows a "null" pointer via
// destruction to be distinguished from an observer marked for deferred removal
// whilst an iteration is in progress.
class BASE_EXPORT CheckedObserverAdapter {
public:
 explicit CheckedObserverAdapter(const CheckedObserver* observer);

 // Move-only construction and assignment is required to store this in STL
 // types.
 CheckedObserverAdapter(CheckedObserverAdapter&& other);
 CheckedObserverAdapter& operator=(CheckedObserverAdapter&& other);
 CheckedObserverAdapter(const CheckedObserverAdapter&) = delete;
 CheckedObserverAdapter& operator=(const CheckedObserverAdapter&) = delete;
 ~CheckedObserverAdapter();

 void MarkForRemoval() {
   DCHECK(weak_ptr_);
   weak_ptr_ = nullptr;
 }

 bool IsMarkedForRemoval() const {
   // If |weak_ptr_| was invalidated then this attempt to iterate over the
   // pointer is a UAF. Tip: If it's unclear where the `delete` occurred, try
   // adding CHECK(!IsInObserverList()) to the ~CheckedObserver() (destructor)
   // override. However, note that this is not always a bug: a destroyed
   // observer can exist in an ObserverList so long as nothing iterates over
   // the ObserverList before the list itself is destroyed.
   CHECK(!weak_ptr_.WasInvalidated());
   return weak_ptr_ == nullptr;
 }

 bool IsEqual(const CheckedObserver* rhs) const {
   // Note that inside an iteration, ObserverList::HasObserver() may call this
   // and |weak_ptr_| may be null due to a deferred removal, which is fine.
   return weak_ptr_.get() == rhs;
 }

 template <class ObserverType>
 static ObserverType* Get(const CheckedObserverAdapter& adapter) {
   static_assert(
       std::is_base_of_v<CheckedObserver, ObserverType>,
       "Observers should inherit from base::CheckedObserver. "
       "Use ObserverList<T>::Unchecked to observe with raw pointers.");
   DCHECK(adapter.weak_ptr_);
   return static_cast<ObserverType*>(adapter.weak_ptr_.get());
 }

#if DCHECK_IS_ON()
 std::string GetCreationStackString() const { return stack_.ToString(); }
#endif

private:
 WeakPtr<CheckedObserver> weak_ptr_;
#if DCHECK_IS_ON()
 base::debug::StackTrace stack_;
#endif
};

// Wraps a pointer in a stack-allocated, base::LinkNode. The node is
// automatically removed from the linked list upon destruction (of the node, not
// the pointer). Nodes are detached from the list via Invalidate() in the
// destructor of ObserverList. This invalidates all WeakLinkNodes. There is no
// threading support.
template <class ObserverList>
class WeakLinkNode : public base::LinkNode<WeakLinkNode<ObserverList>> {
public:
 WeakLinkNode() = default;
 explicit WeakLinkNode(ObserverList* list) { SetList(list); }
 WeakLinkNode(const WeakLinkNode&) = delete;
 WeakLinkNode& operator=(const WeakLinkNode&) = delete;

 ~WeakLinkNode() { Invalidate(); }

 bool IsOnlyRemainingNode() const {
   return list_ &&
          list_->live_iterators_.head() == list_->live_iterators_.tail();
 }

 void SetList(ObserverList* list) {
   DCHECK(!list_);
   DCHECK(list);
   list_ = list;
   list_->live_iterators_.Append(this);
 }

 void Invalidate() {
   if (list_) {
     list_ = nullptr;
     this->RemoveFromList();
   }
 }

 ObserverList* get() const {
   if (list_)
     DCHECK_CALLED_ON_VALID_SEQUENCE(list_->iteration_sequence_checker_);
   return list_;
 }
 ObserverList* operator->() const { return get(); }
 explicit operator bool() const { return get(); }

private:
 // `list_` is not a raw_ptr<...> for performance reasons: on-stack pointer +
 // based on analysis of sampling profiler data and tab_search:top100:2020.
 RAW_PTR_EXCLUSION ObserverList* list_ = nullptr;
};

}  // namespace internal
}  // namespace base

#endif  // BASE_OBSERVER_LIST_INTERNAL_H_