// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// WARNING: Thread local storage is a bit tricky to get right. Please make sure
// that this is really the proper solution for what you're trying to achieve.
// Don't prematurely optimize, most likely you can just use a Lock.
//
// These classes implement a wrapper around ThreadLocalStorage::Slot. On
// construction, they will allocate a TLS slot, and free the TLS slot on
// destruction. No memory management (creation or destruction) is handled. This
// means for uses of ThreadLocalPointer, you must correctly manage the memory
// yourself, these classes will not destroy the pointer for you. There are no
// at-thread-exit actions taken by these classes.
//
// ThreadLocalPointer<Type> wraps a Type*. It performs no creation or
// destruction, so memory management must be handled elsewhere. The first call
// to Get() on a thread will return NULL. You can update the pointer with a call
// to Set().
//
// ThreadLocalBoolean wraps a bool. It will default to false if it has never
// been set otherwise with Set().
//
// Thread Safety: An instance of ThreadLocalStorage is completely thread safe
// once it has been created. If you want to dynamically create an instance, you
// must of course properly deal with safety and race conditions.
//
// In Android, the system TLS is limited.
//
// Example usage:
//   // My class is logically attached to a single thread. We cache a pointer
//   // on the thread it was created on, so we can implement current().
//   MyClass::MyClass() {
//     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() == NULL);
//     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(this);
//   }
//
//   MyClass::~MyClass() {
//     DCHECK(Singleton<ThreadLocalPointer<MyClass> >::get()->Get() != NULL);
//     Singleton<ThreadLocalPointer<MyClass> >::get()->Set(NULL);
//   }
//
//   // Return the current MyClass associated with the calling thread, can be
//   // NULL if there isn't a MyClass associated.
//   MyClass* MyClass::current() {
//     return Singleton<ThreadLocalPointer<MyClass> >::get()->Get();
//   }

#ifndef BASE_THREADING_THREAD_LOCAL_H_
#define BASE_THREADING_THREAD_LOCAL_H_

#include <memory>

#include "base/check_op.h"
#include "base/macros.h"
#include "base/memory/ptr_util.h"
#include "base/threading/thread_local_internal.h"
#include "base/threading/thread_local_storage.h"

namespace base {

template <typename T>
class ThreadLocalPointer {
 public:
  ThreadLocalPointer() = default;

  ThreadLocalPointer(const ThreadLocalPointer&) = delete;
  ThreadLocalPointer& operator=(const ThreadLocalPointer&) = delete;

  ~ThreadLocalPointer() = default;

  T* Get() const { return static_cast<T*>(slot_.Get()); }

  void Set(T* ptr) {
    slot_.Set(const_cast<void*>(static_cast<const void*>(ptr)));
  }

 private:
  ThreadLocalStorage::Slot slot_;
};

// A ThreadLocalOwnedPointer<T> is like a ThreadLocalPointer<T> except that
// pointers handed to it are owned and automatically deleted during their
// associated thread's exit phase (or when replaced if Set() is invoked multiple
// times on the same thread).
// The ThreadLocalOwnedPointer instance itself can only be destroyed when no
// threads, other than the one it is destroyed on, have remaining state set in
// it. Typically this means that ThreadLocalOwnedPointer instances are held in
// static storage or at the very least only recycled in the single-threaded
// phase between tests in the same process.
#if DCHECK_IS_ON()
template <typename T>
using ThreadLocalOwnedPointer = internal::CheckedThreadLocalOwnedPointer<T>;
#else   // DCHECK_IS_ON()
template <typename T>
class ThreadLocalOwnedPointer {
 public:
  ThreadLocalOwnedPointer() = default;

  ThreadLocalOwnedPointer(const ThreadLocalOwnedPointer&) = delete;
  ThreadLocalOwnedPointer& operator=(const ThreadLocalOwnedPointer&) = delete;

  ~ThreadLocalOwnedPointer() {
    // Assume that this thread is the only one with potential state left. This
    // is verified in ~CheckedThreadLocalOwnedPointer().
    Set(nullptr);
  }

  T* Get() const { return static_cast<T*>(slot_.Get()); }

  // Sets a new value, returns the old.
  std::unique_ptr<T> Set(std::unique_ptr<T> ptr) {
    auto existing = WrapUnique(Get());
    slot_.Set(const_cast<void*>(static_cast<const void*>(ptr.release())));
    return existing;
  }

  T& operator*() { return *Get(); }

 private:
  static void DeleteTlsPtr(void* ptr) { delete static_cast<T*>(ptr); }

  ThreadLocalStorage::Slot slot_{&DeleteTlsPtr};
};
#endif  // DCHECK_IS_ON()

class ThreadLocalBoolean {
 public:
  ThreadLocalBoolean() = default;

  ThreadLocalBoolean(const ThreadLocalBoolean&) = delete;
  ThreadLocalBoolean& operator=(const ThreadLocalBoolean&) = delete;

  ~ThreadLocalBoolean() = default;

  bool Get() const { return tlp_.Get() != nullptr; }

  void Set(bool val) { tlp_.Set(val ? this : nullptr); }

 private:
  ThreadLocalPointer<void> tlp_;
};

}  // namespace base

#endif  // BASE_THREADING_THREAD_LOCAL_H_
