folly/folly/ScopeGuard.h

/*
 * 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.
 */

//
// Docs: https://fburl.com/fbcref_scopeguard
//

/**
 * ScopeGuard is a general implementation of the "Initialization is
 * Resource Acquisition" idiom.  It guarantees that a function
 * is executed upon leaving the current scope.
 *
 * @file ScopeGuard.h
 * @refcode folly/docs/examples/folly/ScopeGuard.cpp
 */
/*
 * 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.
 *   auto 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();
 * }
 *
 * It is also possible to create a guard in dismissed state with
 * makeDismissedGuard(), and later rehire it with the rehire()
 * method.
 *
 * makeDismissedGuard() is not just syntactic sugar for creating a guard and
 * immediately dismissing it, but it has a subtle behavior difference if
 * move-construction of the passed function can throw: if it does, the function
 * will be called by makeGuard(), but not by makeDismissedGuard().
 *
 * 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
 */
#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/Exception.h>
#include <folly/lang/UncaughtExceptions.h>

namespace folly {

namespace detail {

struct ScopeGuardDismissed { … };

class ScopeGuardImplBase { … };

template <typename FunctionType, bool InvokeNoexcept>
class ScopeGuardImpl : public ScopeGuardImplBase { … };

ScopeGuardImplDecay;

} // namespace detail

/**
 * Create a scope guard.
 *
 * The returned object has methods .dismiss() and .rehire(), which will
 * deactivate/reactivate the calling of the function upon destruction.
 *
 * The return value of this function must be captured. Otherwise, since it is a
 * temporary, it will be destroyed immediately, thus calling the function.
 *
 *     auto guard = makeGuard(...); // good
 *
 *     makeGuard(...); // bad
 *
 * @param f  The function to execute upon the guard's destruction.
 * @refcode folly/docs/examples/folly/ScopeGuard2.cpp
 */
template <typename F>
FOLLY_NODISCARD detail::ScopeGuardImplDecay<F, true> makeGuard(F&& f) noexcept(
    noexcept(detail::ScopeGuardImplDecay<F, true>(static_cast<F&&>(f)))) { … }

/**
 * Create a scope guard in the dismissed state.
 *
 * The guard can be enabled using .rehire().
 *
 * @see makeGuard
 * @refcode folly/docs/examples/folly/ScopeGuard2.cpp
 */
template <typename F>
FOLLY_NODISCARD detail::ScopeGuardImplDecay<F, true>
makeDismissedGuard(F&& f) noexcept(
    noexcept(detail::ScopeGuardImplDecay<F, true>(
        static_cast<F&&>(f), detail::ScopeGuardDismissed{ … }

namespace detail {

/**
 * 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 { … };

/**
 * 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) { … }

/**
 * 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) { … }

/**
 * Internal use for the macro SCOPE_EXIT below
 */
enum class ScopeGuardOnExit { … };

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

} // namespace folly

//  SCOPE_EXIT
//
//  Example:
//
//      /* open scope */ {
//
//        some_resource_t resource;
//        some_resource_init(resource);
//        SCOPE_EXIT { some_resource_fini(resource); };
//
//        if (!cond)
//          throw 0; // the cleanup happens at end of the scope
//        else
//          return; // the cleanup happens at end of the scope
//
//        use_some_resource(resource); // may throw; cleanup will happen
//
//      } /* close scope */
//
//  The code in the braces passed to SCOPE_EXIT executes at the end of the
//  containing scope as if the code is the content of the destructor of an
//  object instantiated at the point of the SCOPE_EXIT, where the destructor
//  reference-captures all local variables it uses.
//
//  The cleanup code - the code in the braces passed to SCOPE_EXIT - always
//  executes at the end of the scope, regardless of whether the scope exits
//  normally or erroneously as if via the throw statement.
//
//  Caution: Suitable for coroutine functions only when the cleanup code does
//  not use captured references to thread-local objects. Recall that there is
//  no assumption that coroutines resume from co-await, co-yield, or co-return
//  in the same thread as the one in which they suspend.
//
//  Caution: May not execute if the scope exits erroneously but stack unwinding
//  is skipped, or if the scope does not exit at all such as with std::abort or
//  setcontext, which fibers use.

/**
 * Capture code that shall be run when the current scope exits.
 *
 * The code within SCOPE_EXIT's braces shall execute as if the code was in the
 * destructor of an object instantiated at the point of SCOPE_EXIT.
 *
 * Variables used within SCOPE_EXIT are captured by reference.
 *
 * @def SCOPE_EXIT
 */
#define SCOPE_EXIT …

//  SCOPE_FAIL
//
//  May be useful in situations where the caller requests a resource where
//  initializations of the resource is multi-step and may fail.
//
//  Example:
//
//      some_resource_t resource;
//      some_resource_init(resource);
//      SCOPE_FAIL { some_resource_fini(resource); };
//
//      if (do_throw)
//        throw 0; // the cleanup happens at the end of the scope
//      else
//        return resource; // the cleanup does not happen
//
//  Warning: Not suitable for coroutine functions.

/**
 * Capture code to run if the scope exits with an exception.
 *
 * Like SCOPE_EXIT, but only executes the code if the scope exited due to an
 * exception.
 *
 * @def SCOPE_FAIL
 */
#define SCOPE_FAIL …

//  SCOPE_SUCCESS
//
//  In a sense, the opposite of SCOPE_FAIL.
//
//  Example:
//
//      folly::stop_watch<> watch;
//      SCOPE_FAIL { log_failure(watch.elapsed(); };
//      SCOPE_SUCCESS { log_success(watch.elapsed(); };
//
//      if (do_throw)
//        throw 0; // the cleanup does not happen; log failure
//      else
//        return; // the cleanup happens at the end of the scope; log success
//
//  Warning: Not suitable for coroutine functions.

/**
 * Capture code to run if the scope exits without an exception.
 *
 * Like SCOPE_EXIT, but does not execute the code if the scope exited due to an
 * exception.
 *
 * @def SCOPE_SUCCESS
 */
#define SCOPE_SUCCESS …