ScopeGuard.h

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/*
 * Copyright 2011-present 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.
 */

#pragma once

#include <cstddef>
#include <cstdlib>
#include <functional>
#include <new>
#include <type_traits>
#include <utility>

#include <folly/Portability.h>
#include <folly/Preprocessor.h>
#include <folly/Utility.h>
#include <folly/lang/UncaughtExceptions.h>

namespace folly {

namespace detail {

class ScopeGuardImplBase {
 public:
  void dismiss() noexcept {
    dismissed_ = true;
  }

 protected:
  ScopeGuardImplBase() noexcept : dismissed_(false) {}

  static void warnAboutToCrash() noexcept;
  static ScopeGuardImplBase makeEmptyScopeGuard() noexcept {
    return ScopeGuardImplBase{};
  }

  template <typename T>
  static const T& asConst(const T& t) noexcept {
    return t;
  }

  bool dismissed_;
};

template <typename FunctionType, bool InvokeNoexcept>
class ScopeGuardImpl : public ScopeGuardImplBase {
 public:
  explicit ScopeGuardImpl(FunctionType& fn) noexcept(
      std::is_nothrow_copy_constructible<FunctionType>::value)
      : ScopeGuardImpl(
            asConst(fn),
            makeFailsafe(
                std::is_nothrow_copy_constructible<FunctionType>{},
                &fn)) {}

  explicit ScopeGuardImpl(const FunctionType& fn) noexcept(
      std::is_nothrow_copy_constructible<FunctionType>::value)
      : ScopeGuardImpl(
            fn,
            makeFailsafe(
                std::is_nothrow_copy_constructible<FunctionType>{},
                &fn)) {}

  explicit ScopeGuardImpl(FunctionType&& fn) noexcept(
      std::is_nothrow_move_constructible<FunctionType>::value)
      : ScopeGuardImpl(
            std::move_if_noexcept(fn),
            makeFailsafe(
                std::is_nothrow_move_constructible<FunctionType>{},
                &fn)) {}

  ScopeGuardImpl(ScopeGuardImpl&& other) noexcept(
      std::is_nothrow_move_constructible<FunctionType>::value)
      : function_(std::move_if_noexcept(other.function_)) {
    // If the above line attempts a copy and the copy throws, other is
    // left owning the cleanup action and will execute it (or not) depending
    // on the value of other.dismissed_. The following lines only execute
    // if the move/copy succeeded, in which case *this assumes ownership of
    // the cleanup action and dismisses other.
    dismissed_ = exchange(other.dismissed_, true);
  }

  ~ScopeGuardImpl() noexcept(InvokeNoexcept) {
    if (!dismissed_) {
      execute();
    }
  }

 private:
  static ScopeGuardImplBase makeFailsafe(std::true_type, const void*) noexcept {
    return makeEmptyScopeGuard();
  }

  template <typename Fn>
  static auto makeFailsafe(std::false_type, Fn* fn) noexcept
      -> ScopeGuardImpl<decltype(std::ref(*fn)), InvokeNoexcept> {
    return ScopeGuardImpl<decltype(std::ref(*fn)), InvokeNoexcept>{
        std::ref(*fn)};
  }

  template <typename Fn>
  explicit ScopeGuardImpl(Fn&& fn, ScopeGuardImplBase&& failsafe)
      : ScopeGuardImplBase{}, function_(std::forward<Fn>(fn)) {
    failsafe.dismiss();
  }

  void* operator new(std::size_t) = delete;

  void execute() noexcept(InvokeNoexcept) {
    if (InvokeNoexcept) {
      try {
        function_();
      } catch (...) {
        warnAboutToCrash();
        std::terminate();
      }
    } else {
      function_();
    }
  }

  FunctionType function_;
};

template <typename F, bool INE>
using ScopeGuardImplDecay = ScopeGuardImpl<typename std::decay<F>::type, INE>;

} // namespace detail

template <typename F>
FOLLY_NODISCARD detail::ScopeGuardImplDecay<F, true> makeGuard(F&& f) noexcept(
    noexcept(detail::ScopeGuardImplDecay<F, true>(static_cast<F&&>(f)))) {
  return detail::ScopeGuardImplDecay<F, true>(static_cast<F&&>(f));
}

namespace detail {

#if defined(FOLLY_EXCEPTION_COUNT_USE_CXA_GET_GLOBALS) || \
    defined(FOLLY_EXCEPTION_COUNT_USE_GETPTD) ||          \
    defined(FOLLY_EXCEPTION_COUNT_USE_STD)

template <typename FunctionType, bool ExecuteOnException>
class ScopeGuardForNewException {
 public:
  explicit ScopeGuardForNewException(const FunctionType& fn) : guard_(fn) {}

  explicit ScopeGuardForNewException(FunctionType&& fn)
      : guard_(std::move(fn)) {}

  ScopeGuardForNewException(ScopeGuardForNewException&& other) = default;

  ~ScopeGuardForNewException() noexcept(ExecuteOnException) {
    if (ExecuteOnException != (exceptionCounter_ < uncaught_exceptions())) {
      guard_.dismiss();
    }
  }

 private:
  void* operator new(std::size_t) = delete;
  void operator delete(void*) = delete;

  ScopeGuardImpl<FunctionType, ExecuteOnException> guard_;
  int exceptionCounter_{uncaught_exceptions()};
};

enum class ScopeGuardOnFail {};

template <typename FunctionType>
ScopeGuardForNewException<typename std::decay<FunctionType>::type, true>
operator+(detail::ScopeGuardOnFail, FunctionType&& fn) {
  return ScopeGuardForNewException<
      typename std::decay<FunctionType>::type,
      true>(std::forward<FunctionType>(fn));
}

enum class ScopeGuardOnSuccess {};

template <typename FunctionType>
ScopeGuardForNewException<typename std::decay<FunctionType>::type, false>
operator+(ScopeGuardOnSuccess, FunctionType&& fn) {
  return ScopeGuardForNewException<
      typename std::decay<FunctionType>::type,
      false>(std::forward<FunctionType>(fn));
}

#endif // native uncaught_exception() supported

enum class ScopeGuardOnExit {};

template <typename FunctionType>
ScopeGuardImpl<typename std::decay<FunctionType>::type, true> operator+(
    detail::ScopeGuardOnExit,
    FunctionType&& fn) {
  return ScopeGuardImpl<typename std::decay<FunctionType>::type, true>(
      std::forward<FunctionType>(fn));
}
} // namespace detail

} // namespace folly

#define SCOPE_EXIT                               \
  auto FB_ANONYMOUS_VARIABLE(SCOPE_EXIT_STATE) = \
      ::folly::detail::ScopeGuardOnExit() + [&]() noexcept

#if defined(FOLLY_EXCEPTION_COUNT_USE_CXA_GET_GLOBALS) || \
    defined(FOLLY_EXCEPTION_COUNT_USE_GETPTD) ||          \
    defined(FOLLY_EXCEPTION_COUNT_USE_STD)
#define SCOPE_FAIL                               \
  auto FB_ANONYMOUS_VARIABLE(SCOPE_FAIL_STATE) = \
      ::folly::detail::ScopeGuardOnFail() + [&]() noexcept

#define SCOPE_SUCCESS                               \
  auto FB_ANONYMOUS_VARIABLE(SCOPE_SUCCESS_STATE) = \
      ::folly::detail::ScopeGuardOnSuccess() + [&]()
#endif // native uncaught_exception() supported