/*
 * Copyright 2016 Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace folly {

namespace detail {

template <typename T>
template <typename Tag, typename VaultTag>
SingletonHolder<T>& SingletonHolder<T>::singleton() {
  static auto entry =
      createGlobal<SingletonHolder<T>, std::pair<Tag, VaultTag>>([]() {
        return new SingletonHolder<T>({typeid(T), typeid(Tag)},
                                      *SingletonVault::singleton<VaultTag>());
      });
  return *entry;
}

[[noreturn]] void singletonWarnDoubleRegistrationAndAbort(
    const TypeDescriptor& type);

template <typename T>
void SingletonHolder<T>::registerSingleton(CreateFunc c, TeardownFunc t) {
  std::lock_guard<std::mutex> entry_lock(mutex_);

  if (state_ != SingletonHolderState::NotRegistered) {
    /* Possible causes:
     *
     * You have two instances of the same
     * folly::Singleton<Class>. Probably because you define the
     * singleton in a header included in multiple places? In general,
     * folly::Singleton shouldn't be in the header, only off in some
     * anonymous namespace in a cpp file. Code needing the singleton
     * will find it when that code references folly::Singleton<Class>.
     *
     * Alternatively, you could have 2 singletons with the same type
     * defined with a different name in a .cpp (source) file. For
     * example:
     *
     * Singleton<int> a([] { return new int(3); });
     * Singleton<int> b([] { return new int(4); });
     *
     */
    singletonWarnDoubleRegistrationAndAbort(type());
  }

  create_ = std::move(c);
  teardown_ = std::move(t);

  state_ = SingletonHolderState::Dead;
}

template <typename T>
void SingletonHolder<T>::registerSingletonMock(CreateFunc c, TeardownFunc t) {
  if (state_ == SingletonHolderState::NotRegistered) {
    LOG(FATAL) << "Registering mock before singleton was registered: "
               << type().name();
  }
  destroyInstance();

  {
    RWSpinLock::WriteHolder wh(&vault_.mutex_);

    auto it = std::find(
        vault_.creation_order_.begin(), vault_.creation_order_.end(), type());
    if (it != vault_.creation_order_.end()) {
      vault_.creation_order_.erase(it);
    }
  }

  std::lock_guard<std::mutex> entry_lock(mutex_);

  create_ = std::move(c);
  teardown_ = std::move(t);
}

template <typename T>
T* SingletonHolder<T>::get() {
  if (LIKELY(state_.load(std::memory_order_acquire) ==
             SingletonHolderState::Living)) {
    return instance_ptr_;
  }
  createInstance();

  if (instance_weak_.expired()) {
    throw std::runtime_error(
        "Raw pointer to a singleton requested after its destruction."
        " Singleton type is: " +
        type().name());
  }

  return instance_ptr_;
}

template <typename T>
std::weak_ptr<T> SingletonHolder<T>::get_weak() {
  if (UNLIKELY(state_.load(std::memory_order_acquire) !=
               SingletonHolderState::Living)) {
    createInstance();
  }

  return instance_weak_;
}

template <typename T>
std::shared_ptr<T> SingletonHolder<T>::try_get() {
  if (UNLIKELY(state_.load(std::memory_order_acquire) !=
               SingletonHolderState::Living)) {
    createInstance();
  }

  return instance_weak_.lock();
}

template <typename T>
folly::ReadMostlySharedPtr<T> SingletonHolder<T>::try_get_fast() {
  if (UNLIKELY(state_.load(std::memory_order_acquire) !=
               SingletonHolderState::Living)) {
    createInstance();
  }

  return instance_weak_fast_.lock();
}

template <typename T>
bool SingletonHolder<T>::hasLiveInstance() {
  return !instance_weak_.expired();
}

template <typename T>
void SingletonHolder<T>::preDestroyInstance(
    ReadMostlyMainPtrDeleter<>& deleter) {
  instance_copy_ = instance_;
  deleter.add(std::move(instance_));
}

template <typename T>
void SingletonHolder<T>::destroyInstance() {
  state_ = SingletonHolderState::Dead;
  instance_.reset();
  instance_copy_.reset();
  if (destroy_baton_) {
    constexpr std::chrono::seconds kDestroyWaitTime{5};
    auto wait_result = destroy_baton_->timed_wait(
      std::chrono::steady_clock::now() + kDestroyWaitTime);
    if (!wait_result) {
      print_destructor_stack_trace_->store(true);
      LOG(ERROR) << "Singleton of type " << type().name() << " has a "
                 << "living reference at destroyInstances time; beware! Raw "
                 << "pointer is " << instance_ptr_ << ". It is very likely "
                 << "that some other singleton is holding a shared_ptr to it. "
                 << "Make sure dependencies between these singletons are "
                 << "properly defined.";
    }
  }
}

template <typename T>
SingletonHolder<T>::SingletonHolder(
    TypeDescriptor typeDesc,
    SingletonVault& vault)
    : SingletonHolderBase(typeDesc), vault_(vault) {}

template <typename T>
bool SingletonHolder<T>::creationStarted() {
  // If alive, then creation was of course started.
  // This is flipped after creating_thread_ was set, and before it was reset.
  if (state_.load(std::memory_order_acquire) == SingletonHolderState::Living) {
    return true;
  }

  // Not yet built.  Is it currently in progress?
  if (creating_thread_.load(std::memory_order_acquire) != std::thread::id()) {
    return true;
  }

  return false;
}

template <typename T>
void SingletonHolder<T>::createInstance() {
  if (creating_thread_.load(std::memory_order_acquire) ==
        std::this_thread::get_id()) {
    LOG(FATAL) << "circular singleton dependency: " << type().name();
  }

  std::lock_guard<std::mutex> entry_lock(mutex_);
  if (state_.load(std::memory_order_acquire) == SingletonHolderState::Living) {
    return;
  }
  if (state_.load(std::memory_order_acquire) ==
        SingletonHolderState::NotRegistered) {
    auto ptr = SingletonVault::stackTraceGetter().load();
    LOG(FATAL) << "Creating instance for unregistered singleton: "
               << type().name() << "\n"
               << "Stacktrace:"
               << "\n"
               << (ptr ? (*ptr)() : "(not available)");
  }

  if (state_.load(std::memory_order_acquire) == SingletonHolderState::Living) {
    return;
  }

  SCOPE_EXIT {
    // Clean up creator thread when complete, and also, in case of errors here,
    // so that subsequent attempts don't think this is still in the process of
    // being built.
    creating_thread_.store(std::thread::id(), std::memory_order_release);
  };

  creating_thread_.store(std::this_thread::get_id(), std::memory_order_release);

  RWSpinLock::ReadHolder rh(&vault_.stateMutex_);
  if (vault_.state_ == SingletonVault::SingletonVaultState::Quiescing) {
    if (vault_.type_ != SingletonVault::Type::Relaxed) {
      LOG(FATAL) << "Requesting singleton after vault was destroyed.";
    }
    return;
  }

  auto destroy_baton = std::make_shared<folly::Baton<>>();
  auto print_destructor_stack_trace =
    std::make_shared<std::atomic<bool>>(false);
  auto teardown = teardown_;

  // Can't use make_shared -- no support for a custom deleter, sadly.
  std::shared_ptr<T> instance(
    create_(),
    [destroy_baton, print_destructor_stack_trace, teardown, type = type()]
    (T* instance_ptr) mutable {
      teardown(instance_ptr);
      destroy_baton->post();
      if (print_destructor_stack_trace->load()) {
        std::string output = "Singleton " + type.name() + " was destroyed.\n";

        auto stack_trace_getter = SingletonVault::stackTraceGetter().load();
        auto stack_trace = stack_trace_getter ? stack_trace_getter() : "";
        if (stack_trace.empty()) {
          output += "Failed to get destructor stack trace.";
        } else {
          output += "Destructor stack trace:\n";
          output += stack_trace;
        }

        LOG(ERROR) << output;
      }
    });

  // We should schedule destroyInstances() only after the singleton was
  // created. This will ensure it will be destroyed before singletons,
  // not managed by folly::Singleton, which were initialized in its
  // constructor
  SingletonVault::scheduleDestroyInstances();

  instance_weak_ = instance;
  instance_ptr_ = instance.get();
  instance_.reset(std::move(instance));
  instance_weak_fast_ = instance_;

  destroy_baton_ = std::move(destroy_baton);
  print_destructor_stack_trace_ = std::move(print_destructor_stack_trace);

  // This has to be the last step, because once state is Living other threads
  // may access instance and instance_weak w/o synchronization.
  state_.store(SingletonHolderState::Living, std::memory_order_release);

  {
    RWSpinLock::WriteHolder wh(&vault_.mutex_);
    vault_.creation_order_.push_back(type());
  }
}

}

}