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 //===--- CrashRecoveryContext.h - Crash Recovery ----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_CRASHRECOVERYCONTEXT_H
 #define LLVM_SUPPORT_CRASHRECOVERYCONTEXT_H
 
 #include "llvm/ADT/STLFunctionalExtras.h"
 
 namespace llvm {
 class CrashRecoveryContextCleanup;
 
 /// Crash recovery helper object.
 ///
 /// This class implements support for running operations in a safe context so
 /// that crashes (memory errors, stack overflow, assertion violations) can be
 /// detected and control restored to the crashing thread. Crash detection is
 /// purely "best effort", the exact set of failures which can be recovered from
 /// is platform dependent.
 ///
 /// Clients make use of this code by first calling
 /// CrashRecoveryContext::Enable(), and then executing unsafe operations via a
 /// CrashRecoveryContext object. For example:
 ///
 /// \code
 ///    void actual_work(void *);
 ///
 ///    void foo() {
 ///      CrashRecoveryContext CRC;
 ///
 ///      if (!CRC.RunSafely(actual_work, 0)) {
 ///         ... a crash was detected, report error to user ...
 ///      }
 ///
 ///      ... no crash was detected ...
 ///    }
 /// \endcode
 ///
 /// To assist recovery the class allows specifying set of actions that will be
 /// executed in any case, whether crash occurs or not. These actions may be used
 /// to reclaim resources in the case of crash.
 class CrashRecoveryContext {
   void *Impl = nullptr;
   CrashRecoveryContextCleanup *head = nullptr;
 
 public:
   CrashRecoveryContext();
   ~CrashRecoveryContext();
 
   /// Register cleanup handler, which is used when the recovery context is
   /// finished.
   /// The recovery context owns the handler.
   void registerCleanup(CrashRecoveryContextCleanup *cleanup);
 
   void unregisterCleanup(CrashRecoveryContextCleanup *cleanup);
 
   /// Enable crash recovery.
   static void Enable();
 
   /// Disable crash recovery.
   static void Disable();
 
   /// Return the active context, if the code is currently executing in a
   /// thread which is in a protected context.
   static CrashRecoveryContext *GetCurrent();
 
   /// Return true if the current thread is recovering from a crash.
   static bool isRecoveringFromCrash();
 
   /// Execute the provided callback function (with the given arguments) in
   /// a protected context.
   ///
   /// \return True if the function completed successfully, and false if the
   /// function crashed (or HandleCrash was called explicitly). Clients should
   /// make as little assumptions as possible about the program state when
   /// RunSafely has returned false.
   bool RunSafely(function_ref<void()> Fn);
   bool RunSafely(void (*Fn)(void*), void *UserData) {
     return RunSafely([&]() { Fn(UserData); });
   }
 
   /// Execute the provide callback function (with the given arguments) in
   /// a protected context which is run in another thread (optionally with a
   /// requested stack size).
   ///
   /// See RunSafely().
   ///
   /// On Darwin, if PRIO_DARWIN_BG is set on the calling thread, it will be
   /// propagated to the new thread as well.
   bool RunSafelyOnThread(function_ref<void()>, unsigned RequestedStackSize = 0);
   bool RunSafelyOnThread(void (*Fn)(void*), void *UserData,
                          unsigned RequestedStackSize = 0) {
     return RunSafelyOnThread([&]() { Fn(UserData); }, RequestedStackSize);
   }
 
   /// Explicitly trigger a crash recovery in the current process, and
   /// return failure from RunSafely(). This function does not return.
   [[noreturn]] void HandleExit(int RetCode);
 
   /// Return true if RetCode indicates that a signal or an exception occurred.
   static bool isCrash(int RetCode);
 
   /// Throw again a signal or an exception, after it was catched once by a
   /// CrashRecoveryContext.
   static bool throwIfCrash(int RetCode);
 
   /// In case of a crash, this is the crash identifier.
   int RetCode = 0;
 
   /// Selects whether handling of failures should be done in the same way as
   /// for regular crashes. When this is active, a crash would print the
   /// callstack, clean-up any temporary files and create a coredump/minidump.
   bool DumpStackAndCleanupOnFailure = false;
 };
 
 /// Abstract base class of cleanup handlers.
 ///
 /// Derived classes override method recoverResources, which makes actual work on
 /// resource recovery.
 ///
 /// Cleanup handlers are stored in a double list, which is owned and managed by
 /// a crash recovery context.
 class CrashRecoveryContextCleanup {
 protected:
   CrashRecoveryContext *context = nullptr;
   CrashRecoveryContextCleanup(CrashRecoveryContext *context)
       : context(context) {}
 
 public:
   bool cleanupFired = false;
 
   virtual ~CrashRecoveryContextCleanup();
   virtual void recoverResources() = 0;
 
   CrashRecoveryContext *getContext() const {
     return context;
   }
 
 private:
   friend class CrashRecoveryContext;
   CrashRecoveryContextCleanup *prev = nullptr, *next = nullptr;
 };
 
 /// Base class of cleanup handler that controls recovery of resources of the
 /// given type.
 ///
 /// \tparam Derived Class that uses this class as a base.
 /// \tparam T Type of controlled resource.
 ///
 /// This class serves as a base for its template parameter as implied by
 /// Curiously Recurring Template Pattern.
 ///
 /// This class factors out creation of a cleanup handler. The latter requires
 /// knowledge of the current recovery context, which is provided by this class.
 template<typename Derived, typename T>
 class CrashRecoveryContextCleanupBase : public CrashRecoveryContextCleanup {
 protected:
   T *resource;
   CrashRecoveryContextCleanupBase(CrashRecoveryContext *context, T *resource)
       : CrashRecoveryContextCleanup(context), resource(resource) {}
 
 public:
   /// Creates cleanup handler.
   /// \param x Pointer to the resource recovered by this handler.
   /// \return New handler or null if the method was called outside a recovery
   ///         context.
   static Derived *create(T *x) {
     if (x) {
       if (CrashRecoveryContext *context = CrashRecoveryContext::GetCurrent())
         return new Derived(context, x);
     }
     return nullptr;
   }
 };
 
 /// Cleanup handler that reclaims resource by calling destructor on it.
 template <typename T>
 class CrashRecoveryContextDestructorCleanup : public
   CrashRecoveryContextCleanupBase<CrashRecoveryContextDestructorCleanup<T>, T> {
 public:
   CrashRecoveryContextDestructorCleanup(CrashRecoveryContext *context,
                                         T *resource)
       : CrashRecoveryContextCleanupBase<
             CrashRecoveryContextDestructorCleanup<T>, T>(context, resource) {}
 
   void recoverResources() override {
     this->resource->~T();
   }
 };
 
 /// Cleanup handler that reclaims resource by calling 'delete' on it.
 template <typename T>
 class CrashRecoveryContextDeleteCleanup : public
   CrashRecoveryContextCleanupBase<CrashRecoveryContextDeleteCleanup<T>, T> {
 public:
   CrashRecoveryContextDeleteCleanup(CrashRecoveryContext *context, T *resource)
     : CrashRecoveryContextCleanupBase<
         CrashRecoveryContextDeleteCleanup<T>, T>(context, resource) {}
 
   void recoverResources() override { delete this->resource; }
 };
 
 /// Cleanup handler that reclaims resource by calling its method 'Release'.
 template <typename T>
 class CrashRecoveryContextReleaseRefCleanup : public
   CrashRecoveryContextCleanupBase<CrashRecoveryContextReleaseRefCleanup<T>, T> {
 public:
   CrashRecoveryContextReleaseRefCleanup(CrashRecoveryContext *context,
                                         T *resource)
     : CrashRecoveryContextCleanupBase<CrashRecoveryContextReleaseRefCleanup<T>,
           T>(context, resource) {}
 
   void recoverResources() override { this->resource->Release(); }
 };
 
 /// Helper class for managing resource cleanups.
 ///
 /// \tparam T Type of resource been reclaimed.
 /// \tparam Cleanup Class that defines how the resource is reclaimed.
 ///
 /// Clients create objects of this type in the code executed in a crash recovery
 /// context to ensure that the resource will be reclaimed even in the case of
 /// crash. For example:
 ///
 /// \code
 ///    void actual_work(void *) {
 ///      ...
 ///      std::unique_ptr<Resource> R(new Resource());
 ///      CrashRecoveryContextCleanupRegistrar D(R.get());
 ///      ...
 ///    }
 ///
 ///    void foo() {
 ///      CrashRecoveryContext CRC;
 ///
 ///      if (!CRC.RunSafely(actual_work, 0)) {
 ///         ... a crash was detected, report error to user ...
 ///      }
 /// \endcode
 ///
 /// If the code of `actual_work` in the example above does not crash, the
 /// destructor of CrashRecoveryContextCleanupRegistrar removes cleanup code from
 /// the current CrashRecoveryContext and the resource is reclaimed by the
 /// destructor of std::unique_ptr. If crash happens, destructors are not called
 /// and the resource is reclaimed by cleanup object registered in the recovery
 /// context by the constructor of CrashRecoveryContextCleanupRegistrar.
 template <typename T, typename Cleanup = CrashRecoveryContextDeleteCleanup<T> >
 class CrashRecoveryContextCleanupRegistrar {
   CrashRecoveryContextCleanup *cleanup;
 
 public:
   CrashRecoveryContextCleanupRegistrar(T *x)
     : cleanup(Cleanup::create(x)) {
     if (cleanup)
       cleanup->getContext()->registerCleanup(cleanup);
   }
 
   ~CrashRecoveryContextCleanupRegistrar() { unregister(); }
 
   void unregister() {
     if (cleanup && !cleanup->cleanupFired)
       cleanup->getContext()->unregisterCleanup(cleanup);
     cleanup = nullptr;
   }
 };
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_CRASHRECOVERYCONTEXT_H

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