/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * 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.
 */

#pragma once

#include <chrono>
#include <mutex>
#include <shared_mutex>
#include <system_error>
#include <type_traits>

#include <folly/Portability.h>
#include <folly/Traits.h>
#include <folly/functional/Invoke.h>
#include <folly/lang/Exception.h>

namespace folly {

namespace access {

//  locks and unlocks
FOLLY_CREATE_MEMBER_INVOKER_SUITE(lock);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_for);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_until);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(lock_shared);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_shared);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_shared_for);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_shared_until);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock_shared);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(lock_upgrade);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_upgrade);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_upgrade_for);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_lock_upgrade_until);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock_upgrade);

//  transitions
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock_and_lock_shared);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock_and_lock_upgrade);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_shared_and_lock);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_shared_and_lock_for);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_shared_and_lock_until);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_shared_and_lock_upgrade);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_shared_and_lock_upgrade_for);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_shared_and_lock_upgrade_until);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock_upgrade_and_lock);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_upgrade_and_lock);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_upgrade_and_lock_for);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(try_unlock_upgrade_and_lock_until);
FOLLY_CREATE_MEMBER_INVOKER_SUITE(unlock_upgrade_and_lock_shared);

} // namespace access

namespace detail {

//  Ordinarily, there would not be any need for this lock_storage class and the
//  lock_base class can just have the entire implementation with a little bit
//  of sfinae to handle with-state v.s. sans-state.
//
//  Unfortunately, vc2017 fails to resolve calls to the adopt_lock ctor of such
//  a lock_base class implemented with sfinae. The only observed workaround is
//  to extract the adopt_lock ctor to a pair of dependent base classes, plus the
//  minimum necessary to make it all work. In particular, none of the locking or
//  unlocking functions are needed here, which makes lock_storage actually quite
//  minimal.
//
//  Unfortunately, this workaround leaves lock_base marvelously odd with extra
//  syntax noise everywhere: c'est la vie.
template <typename Mutex, typename LockState>
struct lock_storage {
  Mutex* mutex_{};
  LockState state_{};

  lock_storage() = default;
  lock_storage(lock_storage&& that) noexcept
      : mutex_{std::exchange(that.mutex_, nullptr)},
        state_{std::exchange(that.state_, LockState{})} {}
  lock_storage(Mutex& mutex, std::adopt_lock_t, LockState const& state)
      : mutex_{std::addressof(mutex)}, state_{state} {
    state_ || (check_fail_(), 0);
  }

  void operator=(lock_storage&&) = delete;

 private:
  [[noreturn]] void FOLLY_NOINLINE check_fail_() {
    auto code = std::errc::operation_not_permitted;
    throw_exception<std::system_error>(std::make_error_code(code));
  }
};
template <typename Mutex>
struct lock_storage<Mutex, void> {
  Mutex* mutex_{};
  bool state_{};

  lock_storage() = default;
  lock_storage(lock_storage&& that) noexcept
      : mutex_{std::exchange(that.mutex_, nullptr)},
        state_{std::exchange(that.state_, false)} {}
  lock_storage(Mutex& mutex, std::adopt_lock_t)
      : mutex_{std::addressof(mutex)}, state_{true} {}

  void operator=(lock_storage&&) = delete;
};

//  A lock base class with a mostly-complete implementation suitable for either
//  unique, shared, or upgrade lock base classes. However, each particular base
//  class specific to each lock category must still be its own class to avoid
//  overly permissive overloads of member and free swap.
template <typename Mutex, typename Policy>
class lock_base //
    : private lock_storage<
          Mutex,
          invoke_result_t<typename Policy::lock_fn, Mutex&>> {
 public:
  using mutex_type = Mutex;
  using state_type = invoke_result_t<typename Policy::lock_fn, mutex_type&>;

  static_assert(
      std::is_same<state_type, std::decay_t<state_type>>::value,
      "state_type, if not void, must be a value type");
  static_assert(
      std::is_void<state_type>::value ||
          (std::is_nothrow_default_constructible<state_type>::value &&
           std::is_nothrow_copy_constructible<state_type>::value &&
           std::is_nothrow_copy_assignable<state_type>::value &&
           std::is_nothrow_destructible<state_type>::value),
      "state_type, if not void, must be noexcept-semiregular");
  static_assert(
      std::is_void<state_type>::value ||
          std::is_constructible<bool, state_type>::value,
      "state_type, if not void, must explicitly convert to bool");

 private:
  using storage = lock_storage<mutex_type, state_type>;
  static constexpr bool has_state_ = !std::is_void<state_type>::value;
  template <bool C>
  using if_ = std::enable_if_t<C, int>;

 public:
  using storage::storage;
  lock_base() = default;
  lock_base(lock_base&&) = default;
  explicit lock_base(mutex_type& mutex) {
    storage::mutex_ = std::addressof(mutex);
    lock();
  }
  lock_base(mutex_type& mutex, std::defer_lock_t) noexcept {
    storage::mutex_ = std::addressof(mutex);
  }
  lock_base(mutex_type& mutex, std::try_to_lock_t) {
    storage::mutex_ = std::addressof(mutex);
    try_lock();
  }
  template <typename Rep, typename Period>
  lock_base(
      mutex_type& mutex, std::chrono::duration<Rep, Period> const& timeout) {
    storage::mutex_ = std::addressof(mutex);
    try_lock_for(timeout);
  }
  template <typename Clock, typename Duration>
  lock_base(
      mutex_type& mutex,
      std::chrono::time_point<Clock, Duration> const& deadline) {
    storage::mutex_ = std::addressof(mutex);
    try_lock_until(deadline);
  }

  ~lock_base() {
    if (owns_lock()) {
      unlock();
    }
  }

  lock_base& operator=(lock_base&& that) noexcept {
    if (owns_lock()) {
      unlock();
    }
    storage::mutex_ = std::exchange(that.mutex_, nullptr);
    storage::state_ = std::exchange(that.state_, decltype(storage::state_){});
    return *this;
  }

  template <bool C = has_state_, if_<!C> = 0>
  void lock() {
    check<false>();
    typename Policy::lock_fn{}(*storage::mutex_);
    storage::state_ = true;
  }
  template <bool C = has_state_, if_<C> = 0>
  void lock() {
    check<false>();
    storage::state_ = typename Policy::lock_fn{}(*storage::mutex_);
  }

  bool try_lock() {
    check<false>();
    storage::state_ = typename Policy::try_lock_fn{}(*storage::mutex_);
    return !!storage::state_;
  }

  template <typename Rep, typename Period>
  bool try_lock_for(std::chrono::duration<Rep, Period> const& timeout) {
    check<false>();
    storage::state_ =
        typename Policy::try_lock_for_fn{}(*storage::mutex_, timeout);
    return !!storage::state_;
  }

  template <typename Clock, typename Duration>
  bool try_lock_until(
      std::chrono::time_point<Clock, Duration> const& deadline) {
    check<false>();
    storage::state_ =
        typename Policy::try_lock_until_fn{}(*storage::mutex_, deadline);
    return !!storage::state_;
  }

  template <bool C = has_state_, if_<!C> = 0>
  void unlock() {
    check<true>();
    typename Policy::unlock_fn{}(*storage::mutex_);
    storage::state_ = decltype(storage::state_){};
  }
  template <bool C = has_state_, if_<C> = 0>
  void unlock() {
    check<true>();
    auto const& state = storage::state_; // prevent unlock from mutating state_
    typename Policy::unlock_fn{}(*storage::mutex_, state);
    storage::state_ = decltype(storage::state_){};
  }

  mutex_type* release() noexcept {
    storage::state_ = {};
    return std::exchange(storage::mutex_, nullptr);
  }

  mutex_type* mutex() const noexcept { return storage::mutex_; }

  template <bool C = has_state_, if_<C> = 0>
  state_type state() const noexcept {
    return storage::state_;
  }

  bool owns_lock() const noexcept { return !!storage::state_; }

  explicit operator bool() const noexcept { return !!storage::state_; }

 protected:
  void swap(lock_base& that) noexcept {
    std::swap(storage::mutex_, that.mutex_);
    std::swap(storage::state_, that.state_);
  }

 private:
  template <bool Owns>
  void check() {
    if (!storage::mutex_ || !storage::state_ == Owns) {
      check_fail_<Owns>();
    }
  }

  template <bool Owns>
  [[noreturn]] FOLLY_NOINLINE void check_fail_() {
    auto perm = std::errc::operation_not_permitted;
    auto dead = std::errc::resource_deadlock_would_occur;
    auto code = !storage::mutex_ || !storage::state_ ? perm : dead;
    throw_exception<std::system_error>(std::make_error_code(code));
  }
};

template <typename Mutex, typename Policy>
class lock_guard_base {
 private:
  using lock_type_ = lock_base<Mutex, Policy>;
  using lock_state_type_ = typename lock_type_::state_type;

  static constexpr bool has_state_ = !std::is_void<lock_state_type_>::value;
  using state_type_ = conditional_t<has_state_, lock_state_type_, bool>;
  template <bool C>
  using if_ = std::enable_if_t<C, int>;

 public:
  using mutex_type = Mutex;

  lock_guard_base(lock_guard_base const&) = delete;
  lock_guard_base(lock_guard_base&&) = delete;
  explicit lock_guard_base(mutex_type& mutex) : lock_{mutex} {}
  template <bool C = has_state_, if_<!C> = 0>
  lock_guard_base(mutex_type& mutex, std::adopt_lock_t)
      : lock_{mutex, std::adopt_lock} {}
  template <bool C = has_state_, if_<C> = 0>
  lock_guard_base(
      mutex_type& mutex, std::adopt_lock_t, state_type_ const& state)
      : lock_{mutex, std::adopt_lock, state} {}

  void operator=(lock_guard_base const&) = delete;
  void operator=(lock_guard_base&&) = delete;

 private:
  lock_type_ lock_;
};

struct lock_policy_unique {
  using lock_fn = access::lock_fn;
  using try_lock_fn = access::try_lock_fn;
  using try_lock_for_fn = access::try_lock_for_fn;
  using try_lock_until_fn = access::try_lock_until_fn;
  using unlock_fn = access::unlock_fn;
};

struct lock_policy_shared {
  using lock_fn = access::lock_shared_fn;
  using try_lock_fn = access::try_lock_shared_fn;
  using try_lock_for_fn = access::try_lock_shared_for_fn;
  using try_lock_until_fn = access::try_lock_shared_until_fn;
  using unlock_fn = access::unlock_shared_fn;
};

struct lock_policy_upgrade {
  using lock_fn = access::lock_upgrade_fn;
  using try_lock_fn = access::try_lock_upgrade_fn;
  using try_lock_for_fn = access::try_lock_upgrade_for_fn;
  using try_lock_until_fn = access::try_lock_upgrade_until_fn;
  using unlock_fn = access::unlock_upgrade_fn;
};

template <typename Mutex>
using lock_policy_hybrid = conditional_t<
    is_invocable_v<access::lock_shared_fn, Mutex&>,
    lock_policy_shared,
    lock_policy_unique>;

template <typename Mutex>
using lock_base_unique = lock_base<Mutex, lock_policy_unique>;

template <typename Mutex>
using lock_base_shared = lock_base<Mutex, lock_policy_shared>;

template <typename Mutex>
using lock_base_upgrade = lock_base<Mutex, lock_policy_upgrade>;

template <typename Mutex>
using lock_base_hybrid = lock_base<Mutex, lock_policy_hybrid<Mutex>>;

} // namespace detail

//  unique_lock_base
//
//  A lock-holder base which holds exclusive locks, usable with any mutex type.
//
//  Works with both lockable mutex types and lockable-with-state mutex types.
//
//  When defining lockable-with-state mutex types, specialize std::unique_lock
//  to derive this. See the example with upgrade_lock.
//
//  A lockable-with-state mutex type is signalled by the return type of mutex
//  member function lock. Members try_lock, try_lock_for, and try_lock_until
//  all return this type and member unlock accepts this type.
template <typename Mutex>
class unique_lock_base : public detail::lock_base_unique<Mutex> {
 private:
  using base = detail::lock_base_unique<Mutex>;
  using self = unique_lock_base;

 public:
  using base::base;

  void swap(self& that) noexcept { base::swap(that); }

  friend void swap(self& a, self& b) noexcept { a.swap(b); }
};

//  shared_lock_base
//
//  A lock-holder base which holds shared locks, usable with any shared mutex
//  type.
//
//  Works with both shared-lockable mutex types and shared-lockable-with-state
//  mutex types.
//
//  When defining shared-lockable-with-state mutex types, specialize
//  std::shared_lock to derive this. See the example with upgrade_lock.
//
//  A shared-lockable-with-state mutex type is signalled by the return type of
//  mutex member function lock_shared. Members try_lock_shared,
//  try_lock_shared_for, and try_lock_shared_until all return this type and
//  member unlock_shared accepts this type. Likewise for mutex member
//  transition functions.
template <typename Mutex>
class shared_lock_base : public detail::lock_base_shared<Mutex> {
 private:
  using base = detail::lock_base_shared<Mutex>;
  using self = shared_lock_base;

 public:
  using base::base;

  void swap(self& that) noexcept { base::swap(that); }

  friend void swap(self& a, self& b) noexcept { a.swap(b); }
};

//  upgrade_lock_base
//
//  A lock-holder base which holds upgrade locks, usable with any upgrade mutex
//  type.
//
//  Works with both upgrade-lockable mutex types and upgrade-lockable-with-state
//  mutex types.
//
//  There are no use-cases except the one below.
//
//  An upgrade-lockable-with-state mutex type is signalled by the return type of
//  mutex member function lock_upgrade. Members try_lock_upgrade,
//  try_lock_upgrade_for, and try_lock_upgrade_until all return this type and
//  member unlock_upgrade accepts this type. Likewise for mutex member
//  transition functions.
template <typename Mutex>
class upgrade_lock_base : public detail::lock_base_upgrade<Mutex> {
 private:
  using base = detail::lock_base_upgrade<Mutex>;
  using self = upgrade_lock_base;

 public:
  using base::base;

  void swap(self& that) noexcept { base::swap(that); }

  friend void swap(self& a, self& b) noexcept { a.swap(b); }
};

//  hybrid_lock_base
//
//  A lock-holder base which holds shared locks for shared mutex types or
//  exclusive locks otherwise.
//
//  See unique_lock_base and shared_lock_base.
template <typename Mutex>
class hybrid_lock_base : public detail::lock_base_hybrid<Mutex> {
 private:
  using base = detail::lock_base_hybrid<Mutex>;
  using self = hybrid_lock_base;

 public:
  using base::base;

  void swap(self& that) noexcept { base::swap(that); }

  friend void swap(self& a, self& b) noexcept { a.swap(b); }
};

//  unique_lock
//
//  Alias to std::unique_lock.
using std::unique_lock;

//  shared_lock
//
//  Alias to std::shared_lock.
using std::shared_lock;

//  upgrade_lock
//
//  A lock-holder type which holds upgrade locks, usable with any upgrade mutex
//  type. An upgrade mutex is a shared mutex which supports the upgrade state.
//
//  Works with both upgrade-lockable mutex types and upgrade-lockable-with-state
//  mutex types.
//
//  Upgrade locks are not useful by themselves; they are primarily useful since
//  upgrade locks may be transitioned atomically to exclusive locks. This lock-
//  holder type works with the transition_to_... functions below to facilitate
//  atomic transition from ugprade lock to exclusive lock.
template <typename Mutex>
class upgrade_lock : public upgrade_lock_base<Mutex> {
 public:
  using upgrade_lock_base<Mutex>::upgrade_lock_base;
};

//  hybrid_lock
//
//  A lock-holder type which holds shared locks for shared mutex types or
//  exclusive locks otherwise.
//
//  See unique_lock and shared_lock.
template <typename Mutex>
class hybrid_lock : public hybrid_lock_base<Mutex> {
 public:
  using hybrid_lock_base<Mutex>::hybrid_lock_base;
};

#if __cpp_deduction_guides >= 201611
template <typename Mutex, typename... A>
explicit hybrid_lock(Mutex&, A const&...) -> hybrid_lock<Mutex>;
#endif

//  lock_guard_base
//
//  A lock-guard which holds exclusive locks, usable with any mutex type.
//
//  Works with both lockable mutex types and lockable-with-state mutex types.
//
//  When defining lockable-with-state mutex types, specialize std::lock_guard
//  to derive this.
template <typename Mutex>
class unique_lock_guard_base
    : public detail::lock_guard_base<Mutex, detail::lock_policy_unique> {
 private:
  using base = detail::lock_guard_base<Mutex, detail::lock_policy_unique>;

 public:
  using base::base;
};

//  unique_lock_guard
//
//  Alias to std::lock_guard.
template <typename Mutex>
using unique_lock_guard = std::lock_guard<Mutex>;

//  shared_lock_guard
//
//  A lock-guard which holds shared locks, usable with any shared mutex type.
//
//  Works with both lockable mutex types and lockable-with-state mutex types.
template <typename Mutex>
class shared_lock_guard
    : public detail::lock_guard_base<Mutex, detail::lock_policy_shared> {
 private:
  using base = detail::lock_guard_base<Mutex, detail::lock_policy_shared>;

 public:
  using base::base;
};

//  hybrid_lock_guard
//
//  For shared mutex types, effectively shared_lock_guard; otherwise,
//  effectively unique_lock_guard.
template <typename Mutex>
class hybrid_lock_guard
    : public detail::lock_guard_base<Mutex, detail::lock_policy_hybrid<Mutex>> {
 private:
  using base =
      detail::lock_guard_base<Mutex, detail::lock_policy_hybrid<Mutex>>;

 public:
  using base::base;
};

#if __cpp_deduction_guides >= 201611
template <typename Mutex, typename... A>
explicit hybrid_lock_guard(Mutex&, A const&...) -> hybrid_lock_guard<Mutex>;
#endif

//  make_unique_lock
//
//  Returns a unique_lock constructed with the given arguments. Deduces the
//  mutex type.
template <typename Mutex, typename... A>
FOLLY_NODISCARD unique_lock<Mutex> make_unique_lock(Mutex& mutex, A&&... a) {
  return unique_lock<Mutex>{mutex, static_cast<A&&>(a)...};
}

//  make_shared_lock
//
//  Returns a shared_lock constructed with the given arguments. Deduces the
//  mutex type.
template <typename Mutex, typename... A>
FOLLY_NODISCARD shared_lock<Mutex> make_shared_lock(Mutex& mutex, A&&... a) {
  return shared_lock<Mutex>{mutex, static_cast<A&&>(a)...};
}

//  make_upgrade_lock
//
//  Returns an upgrade_lock constructed with the given arguments. Deduces the
//  mutex type.
template <typename Mutex, typename... A>
FOLLY_NODISCARD upgrade_lock<Mutex> make_upgrade_lock(Mutex& mutex, A&&... a) {
  return upgrade_lock<Mutex>{mutex, static_cast<A&&>(a)...};
}

//  make_hybrid_lock
//
//  Returns a hybrid_lock constructed with the given arguments. Deduces the
//  mutex type.
template <typename Mutex, typename... A>
FOLLY_NODISCARD hybrid_lock<Mutex> make_hybrid_lock(Mutex& mutex, A&&... a) {
  return hybrid_lock<Mutex>{mutex, static_cast<A&&>(a)...};
}

namespace detail {

template <typename L>
using lock_state_type_of_t_ = typename L::state_type;
template <typename L>
using lock_state_type_of_t = detected_or_t<void, lock_state_type_of_t_, L>;

template <typename State>
struct transition_lock_result_ {
  template <typename Transition, typename Mutex, typename... A>
  using apply = invoke_result_t<Transition, Mutex&, State const&, A const&...>;
};
template <>
struct transition_lock_result_<void> {
  template <typename Transition, typename Mutex, typename... A>
  using apply = invoke_result_t<Transition, Mutex&, A const&...>;
};
template <typename From, typename Transition, typename... A>
using transition_lock_result_t_ =
    typename transition_lock_result_<lock_state_type_of_t<From>>::
        template apply<Transition, typename From::mutex_type&, A...>;

template <
    typename From,
    typename Transition,
    typename... A,
    typename FromState = lock_state_type_of_t<From>,
    std::enable_if_t<std::is_void<FromState>::value, int> = 0>
auto transition_lock_2_(From& lock, Transition transition, A const&... a) {
  return transition(*lock.mutex(), a...);
}
template <
    typename From,
    typename Transition,
    typename... A,
    typename FromState = lock_state_type_of_t<From>,
    std::enable_if_t<!std::is_void<FromState>::value, int> = 0>
auto transition_lock_2_(From& lock, Transition transition, A const&... a) {
  return transition(*lock.mutex(), lock.state(), a...);
}
template <
    typename From,
    typename Transition,
    typename... A,
    typename Result = transition_lock_result_t_<From, Transition, A...>,
    std::enable_if_t<std::is_void<Result>::value, int> = 0>
auto transition_lock_1_(From& lock, Transition transition, A const&... a) {
  return transition_lock_2_(lock, transition, a...), true;
}
template <
    typename From,
    typename Transition,
    typename... A,
    typename Result = transition_lock_result_t_<From, Transition, A...>,
    std::enable_if_t<!std::is_void<Result>::value, int> = 0>
auto transition_lock_1_(From& lock, Transition transition, A const&... a) {
  return transition_lock_2_(lock, transition, a...);
}
template <
    typename To,
    typename From,
    typename Transition,
    typename... A,
    typename ToState = lock_state_type_of_t<To>,
    std::enable_if_t<std::is_void<ToState>::value, int> = 0>
auto transition_lock_0_(From& lock, Transition transition, A const&... a) {
  auto s = transition_lock_1_(lock, transition, a...);
  return !s ? To{} : To{*lock.release(), std::adopt_lock};
}
template <
    typename To,
    typename From,
    typename Transition,
    typename... A,
    typename ToState = lock_state_type_of_t<To>,
    std::enable_if_t<!std::is_void<ToState>::value, int> = 0>
auto transition_lock_0_(From& lock, Transition transition, A const&... a) {
  auto s = transition_lock_1_(lock, transition, a...);
  return !s ? To{} : To{*lock.release(), std::adopt_lock, s};
}
template <
    template <typename>
    class To,
    template <typename>
    class From,
    typename Mutex,
    typename Transition,
    typename... A>
auto transition_lock_(From<Mutex>& lock, Transition transition, A const&... a) {
  // clang-format off
  return
      !lock.mutex() ? To<Mutex>{} :
      !lock.owns_lock() ? To<Mutex>{*lock.release(), std::defer_lock} :
      transition_lock_0_<To<Mutex>>(lock, transition, a...);
  // clang-format on
}

template <typename, typename>
struct transition_lock_policy;

template <typename Mutex>
struct transition_lock_policy<unique_lock<Mutex>, shared_lock<Mutex>> {
  using transition_fn = access::unlock_and_lock_shared_fn;
};
template <typename Mutex>
struct transition_lock_policy<unique_lock<Mutex>, upgrade_lock<Mutex>> {
  using transition_fn = access::unlock_and_lock_upgrade_fn;
};
template <typename Mutex>
struct transition_lock_policy<shared_lock<Mutex>, unique_lock<Mutex>> {
  using try_transition_fn = access::try_unlock_shared_and_lock_fn;
  using try_transition_for_fn = access::try_unlock_shared_and_lock_for_fn;
  using try_transition_until_fn = access::try_unlock_shared_and_lock_until_fn;
};
template <typename Mutex>
struct transition_lock_policy<shared_lock<Mutex>, upgrade_lock<Mutex>> {
  using try_transition_fn = access::try_unlock_shared_and_lock_upgrade_fn;
  using try_transition_for_fn =
      access::try_unlock_shared_and_lock_upgrade_for_fn;
  using try_transition_until_fn =
      access::try_unlock_shared_and_lock_upgrade_until_fn;
};
template <typename Mutex>
struct transition_lock_policy<upgrade_lock<Mutex>, unique_lock<Mutex>> {
  using transition_fn = access::unlock_upgrade_and_lock_fn;
  using try_transition_fn = access::try_unlock_upgrade_and_lock_fn;
  using try_transition_for_fn = access::try_unlock_upgrade_and_lock_for_fn;
  using try_transition_until_fn = access::try_unlock_upgrade_and_lock_until_fn;
};
template <typename Mutex>
struct transition_lock_policy<upgrade_lock<Mutex>, shared_lock<Mutex>> {
  using transition_fn = access::unlock_upgrade_and_lock_shared_fn;
};

} // namespace detail

//  transition_lock
//
//  Represents an atomic transition from the from-lock to the to-lock. Waits
//  unboundedly for the transition to become available.
template <
    template <typename>
    class ToLock,
    typename Mutex,
    template <typename>
    class FromLock>
ToLock<Mutex> transition_lock(FromLock<Mutex>& lock) {
  using policy = detail::transition_lock_policy<FromLock<Mutex>, ToLock<Mutex>>;
  auto _ = typename policy::transition_fn{};
  return detail::transition_lock_<ToLock>(lock, _);
}

//  try_transition_lock
//
//  Represents an atomic transition attempt from the from-lock to the to-lock.
//  Does not wait if the transition is not immediately available.
template <
    template <typename>
    class ToLock,
    typename Mutex,
    template <typename>
    class FromLock>
ToLock<Mutex> try_transition_lock(FromLock<Mutex>& lock) {
  using policy = detail::transition_lock_policy<FromLock<Mutex>, ToLock<Mutex>>;
  auto _ = typename policy::try_transition_fn{};
  return detail::transition_lock_<ToLock>(lock, _);
}

//  try_transition_lock_for
//
//  Represents an atomic transition attempt from the from-lock to the to-lock
//  bounded by a timeout. Waits up to the timeout for the transition to become
//  available.
template <
    template <typename>
    class ToLock,
    typename Mutex,
    template <typename>
    class FromLock,
    typename Rep,
    typename Period>
ToLock<Mutex> try_transition_lock_for(
    FromLock<Mutex>& lock, std::chrono::duration<Rep, Period> const& timeout) {
  using policy = detail::transition_lock_policy<FromLock<Mutex>, ToLock<Mutex>>;
  auto _ = typename policy::try_transition_for_fn{};
  return detail::transition_lock_<ToLock>(lock, _, timeout);
}

//  try_transition_lock_until
//
//  Represents an atomic transition attempt from the from-lock to the to-lock
//  bounded by a deadline. Waits up to the deadline for the transition to become
//  available.
template <
    template <typename>
    class ToLock,
    typename Mutex,
    template <typename>
    class FromLock,
    typename Clock,
    typename Duration>
ToLock<Mutex> try_transition_lock_until(
    FromLock<Mutex>& lock,
    std::chrono::time_point<Clock, Duration> const& deadline) {
  using policy = detail::transition_lock_policy<FromLock<Mutex>, ToLock<Mutex>>;
  auto _ = typename policy::try_transition_until_fn{};
  return detail::transition_lock_<ToLock>(lock, _, deadline);
}

//  transition_to_shared_lock(unique_lock)
//
//  Wraps mutex member function unlock_and_lock_shared.
//
//  Represents an immediate atomic downgrade transition from exclusive lock to
//  to shared lock.
template <typename Mutex>
shared_lock<Mutex> transition_to_shared_lock(unique_lock<Mutex>& lock) {
  return transition_lock<shared_lock>(lock);
}

//  transition_to_shared_lock(upgrade_lock)
//
//  Wraps mutex member function unlock_upgrade_and_lock_shared.
//
//  Represents an immediate atomic downgrade transition from upgrade lock to
//  shared lock.
template <typename Mutex>
shared_lock<Mutex> transition_to_shared_lock(upgrade_lock<Mutex>& lock) {
  return transition_lock<shared_lock>(lock);
}

//  transition_to_upgrade_lock(unique_lock)
//
//  Wraps mutex member function unlock_and_lock_upgrade.
//
//  Represents an immediate atomic downgrade transition from unique lock to
//  upgrade lock.
template <typename Mutex>
upgrade_lock<Mutex> transition_to_upgrade_lock(unique_lock<Mutex>& lock) {
  return transition_lock<upgrade_lock>(lock);
}

//  transition_to_unique_lock(upgrade_lock)
//
//  Wraps mutex member function unlock_upgrade_and_lock.
//
//  Represents an eventual atomic upgrade transition from upgrade lock to unique
//  lock.
template <typename Mutex>
unique_lock<Mutex> transition_to_unique_lock(upgrade_lock<Mutex>& lock) {
  return transition_lock<unique_lock>(lock);
}

//  try_transition_to_unique_lock(upgrade_lock)
//
//  Wraps mutex member function try_unlock_upgrade_and_lock.
//
//  Represents an immediate attempted atomic upgrade transition from upgrade
//  lock to unique lock.
template <typename Mutex>
unique_lock<Mutex> try_transition_to_unique_lock(upgrade_lock<Mutex>& lock) {
  return transition_lock<unique_lock>(lock);
}

//  try_transition_to_unique_lock_for(upgrade_lock)
//
//  Wraps mutex member function try_unlock_upgrade_and_lock_for.
//
//  Represents an eventual attempted atomic upgrade transition from upgrade
//  lock to unique lock.
template <typename Mutex, typename Rep, typename Period>
unique_lock<Mutex> try_transition_to_unique_lock_for(
    upgrade_lock<Mutex>& lock,
    std::chrono::duration<Rep, Period> const& timeout) {
  return try_transition_lock_for<unique_lock>(lock, timeout);
}

//  try_transition_to_unique_lock_until(upgrade_lock)
//
//  Wraps mutex member function try_unlock_upgrade_and_lock_until.
//
//  Represents an eventual attempted atomic upgrade transition from upgrade
//  lock to unique lock.
template <typename Mutex, typename Clock, typename Duration>
unique_lock<Mutex> try_transition_to_unique_lock_until(
    upgrade_lock<Mutex>& lock,
    std::chrono::time_point<Clock, Duration> const& deadline) {
  return try_transition_lock_until<unique_lock>(lock, deadline);
}

//  try_transition_to_unique_lock(shared_lock)
//
//  Wraps mutex member function try_unlock_shared_and_lock.
//
//  Represents an immediate attempted atomic upgrade transition from shared
//  lock to unique lock.
template <typename Mutex>
unique_lock<Mutex> try_transition_to_unique_lock(shared_lock<Mutex>& lock) {
  return try_transition_lock<unique_lock>(lock);
}

//  try_transition_to_unique_lock_for(shared_lock)
//
//  Wraps mutex member function try_unlock_shared_and_lock_for.
//
//  Represents an eventual attempted atomic upgrade transition from shared
//  lock to unique lock.
template <typename Mutex, typename Rep, typename Period>
unique_lock<Mutex> try_transition_to_unique_lock_for(
    shared_lock<Mutex>& lock,
    std::chrono::duration<Rep, Period> const& timeout) {
  return try_transition_lock_for<unique_lock>(lock, timeout);
}

//  try_transition_to_unique_lock_until(shared_lock)
//
//  Wraps mutex member function try_unlock_shared_and_lock_until.
//
//  Represents an eventual attempted atomic upgrade transition from shared
//  lock to unique lock.
template <typename Mutex, typename Clock, typename Duration>
unique_lock<Mutex> try_transition_to_unique_lock_until(
    shared_lock<Mutex>& lock,
    std::chrono::time_point<Clock, Duration> const& deadline) {
  return try_transition_lock_until<unique_lock>(lock, deadline);
}

//  try_transition_to_upgrade_lock(shared_lock)
//
//  Wraps mutex member function try_unlock_shared_and_lock_upgrade.
//
//  Represents an immediate attempted atomic upgrade transition from shared
//  lock to upgrade lock.
template <typename Mutex>
upgrade_lock<Mutex> try_transition_to_upgrade_lock(shared_lock<Mutex>& lock) {
  return try_transition_lock<upgrade_lock>(lock);
}

//  try_transition_to_upgrade_lock_for(shared_lock)
//
//  Wraps mutex member function try_unlock_shared_and_lock_upgrade_for.
//
//  Represents an eventual attempted atomic upgrade transition from shared
//  lock to upgrade lock.
template <typename Mutex, typename Rep, typename Period>
upgrade_lock<Mutex> try_transition_to_upgrade_lock_for(
    shared_lock<Mutex>& lock,
    std::chrono::duration<Rep, Period> const& timeout) {
  return try_transition_lock_for<upgrade_lock>(lock, timeout);
}

//  try_transition_to_upgrade_lock_until(shared_lock)
//
//  Wraps mutex member function try_unlock_shared_and_lock_upgrade_until.
//
//  Represents an eventual attempted atomic upgrade transition from shared
//  lock to upgrade lock.
template <typename Mutex, typename Clock, typename Duration>
upgrade_lock<Mutex> try_transition_to_upgrade_lock_until(
    shared_lock<Mutex>& lock,
    std::chrono::time_point<Clock, Duration> const& deadline) {
  return try_transition_lock_until<upgrade_lock>(lock, deadline);
}

} // namespace folly