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/*
* Copyright 2017 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 <functional>
#include <new>
#include <type_traits>
#include <utility>
#include <folly/Portability.h>
#include <folly/Preprocessor.h>
#include <folly/detail/UncaughtExceptionCounter.h>
namespace folly {
/**
* ScopeGuard is a general implementation of the "Initialization is
* Resource Acquisition" idiom. Basically, it guarantees that a function
* is executed upon leaving the currrent scope unless otherwise told.
*
* The makeGuard() function is used to create a new ScopeGuard object.
* It can be instantiated with a lambda function, a std::function<void()>,
* a functor, or a void(*)() function pointer.
*
*
* Usage example: Add a friend to memory if and only if it is also added
* to the db.
*
* void User::addFriend(User& newFriend) {
* // add the friend to memory
* friends_.push_back(&newFriend);
*
* // If the db insertion that follows fails, we should
* // remove it from memory.
* // (You could also declare this as "auto guard = makeGuard(...)")
* ScopeGuard guard = makeGuard([&] { friends_.pop_back(); });
*
* // this will throw an exception upon error, which
* // makes the ScopeGuard execute UserCont::pop_back()
* // once the Guard's destructor is called.
* db_->addFriend(GetName(), newFriend.GetName());
*
* // an exception was not thrown, so don't execute
* // the Guard.
* guard.dismiss();
* }
*
* Examine ScopeGuardTest.cpp for some more sample usage.
*
* Stolen from:
* Andrei's and Petru Marginean's CUJ article:
* http://drdobbs.com/184403758
* and the loki library:
* http://loki-lib.sourceforge.net/index.php?n=Idioms.ScopeGuardPointer
* and triendl.kj article:
* http://www.codeproject.com/KB/cpp/scope_guard.aspx
*/
class ScopeGuardImplBase {
public:
void dismiss() noexcept {
dismissed_ = true;
}
template <typename T>
FOLLY_ALWAYS_INLINE static void runAndWarnAboutToCrashOnException(
T& function) noexcept {
try {
function();
} catch (...) {
warnAboutToCrash();
std::terminate();
}
}
protected:
ScopeGuardImplBase() noexcept : dismissed_(false) {}
static ScopeGuardImplBase makeEmptyScopeGuard() noexcept {
return ScopeGuardImplBase{};
}
template <typename T>
static const T& asConst(const T& t) noexcept {
return t;
}
bool dismissed_;
private:
static void warnAboutToCrash() noexcept;
};
template <typename FunctionType>
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_ = other.dismissed_;
other.dismissed_ = true;
}
~ScopeGuardImpl() noexcept {
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))> {
return ScopeGuardImpl<decltype(std::ref(*fn))>{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 {
runAndWarnAboutToCrashOnException(function_);
}
FunctionType function_;
};
template <typename FunctionType>
ScopeGuardImpl<typename std::decay<FunctionType>::type>
makeGuard(FunctionType&& fn) noexcept(
std::is_nothrow_constructible<typename std::decay<FunctionType>::type,
FunctionType>::value) {
return ScopeGuardImpl<typename std::decay<FunctionType>::type>(
std::forward<FunctionType>(fn));
}
/**
* This is largely unneeded if you just use auto for your guards.
*/
typedef ScopeGuardImplBase&& ScopeGuard;
namespace detail {
#if defined(FOLLY_EXCEPTION_COUNT_USE_CXA_GET_GLOBALS) || \
defined(FOLLY_EXCEPTION_COUNT_USE_GETPTD) || \
defined(FOLLY_EXCEPTION_COUNT_USE_STD)
/**
* ScopeGuard used for executing a function when leaving the current scope
* depending on the presence of a new uncaught exception.
*
* If the executeOnException template parameter is true, the function is
* executed if a new uncaught exception is present at the end of the scope.
* If the parameter is false, then the function is executed if no new uncaught
* exceptions are present at the end of the scope.
*
* Used to implement SCOPE_FAIL and SCOPE_SUCCESS below.
*/
template <typename FunctionType, bool executeOnException>
class ScopeGuardForNewException {
public:
explicit ScopeGuardForNewException(const FunctionType& fn)
: function_(fn) {
}
explicit ScopeGuardForNewException(FunctionType&& fn)
: function_(std::move(fn)) {
}
ScopeGuardForNewException(ScopeGuardForNewException&& other)
: function_(std::move(other.function_))
, exceptionCounter_(std::move(other.exceptionCounter_)) {
}
~ScopeGuardForNewException() noexcept(executeOnException) {
if (executeOnException == exceptionCounter_.isNewUncaughtException()) {
if (executeOnException) {
ScopeGuardImplBase::runAndWarnAboutToCrashOnException(function_);
} else {
function_();
}
}
}
private:
ScopeGuardForNewException(const ScopeGuardForNewException& other) = delete;
void* operator new(std::size_t) = delete;
FunctionType function_;
UncaughtExceptionCounter exceptionCounter_;
};
/**
* Internal use for the macro SCOPE_FAIL below
*/
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));
}
/**
* Internal use for the macro SCOPE_SUCCESS below
*/
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
/**
* Internal use for the macro SCOPE_EXIT below
*/
enum class ScopeGuardOnExit {};
template <typename FunctionType>
ScopeGuardImpl<typename std::decay<FunctionType>::type>
operator+(detail::ScopeGuardOnExit, FunctionType&& fn) {
return ScopeGuardImpl<typename std::decay<FunctionType>::type>(
std::forward<FunctionType>(fn));
}
} // namespace detail
} // 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