/* * 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. */ /* * Author: Eric Niebler */ #pragma once #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #define FOLLY_EXCEPTION_WRAPPER_H_INCLUDED namespace folly { #define FOLLY_REQUIRES_DEF(...) \ std::enable_if_t(__VA_ARGS__), long> #define FOLLY_REQUIRES(...) FOLLY_REQUIRES_DEF(__VA_ARGS__) = __LINE__ //! Throwing exceptions can be a convenient way to handle errors. Storing //! exceptions in an `exception_ptr` makes it easy to handle exceptions in a //! different thread or at a later time. `exception_ptr` can also be used in a //! very generic result/exception wrapper. //! //! However, there are some issues with throwing exceptions and //! `std::exception_ptr`. These issues revolve around `throw` being expensive, //! particularly in a multithreaded environment (see //! ExceptionWrapperBenchmark.cpp). //! //! Imagine we have a library that has an API which returns a result/exception //! wrapper. Let's consider some approaches for implementing this wrapper. //! First, we could store a `std::exception`. This approach loses the derived //! exception type, which can make exception handling more difficult for users //! that prefer rethrowing the exception. We could use a `folly::dynamic` for //! every possible type of exception. This is not very flexible - adding new //! types of exceptions requires a change to the result/exception wrapper. We //! could use an `exception_ptr`. However, constructing an `exception_ptr` as //! well as accessing the error requires a call to throw. That means that there //! will be two calls to throw in order to process the exception. For //! performance sensitive applications, this may be unacceptable. //! //! `exception_wrapper` is designed to handle exception management for both //! convenience and high performance use cases. `make_exception_wrapper` is //! templated on derived type, allowing us to rethrow the exception properly for //! users that prefer convenience. These explicitly named exception types can //! therefore be handled without any performance penalty. `exception_wrapper` is //! also flexible enough to accept any type. If a caught exception is not of an //! explicitly named type, then `std::exception_ptr` is used to preserve the //! exception state. For performance sensitive applications, the accessor //! methods can test or extract a pointer to a specific exception type with very //! little overhead. //! //! \par Example usage: //! \code //! exception_wrapper globalExceptionWrapper; //! //! // Thread1 //! void doSomethingCrazy() { //! int rc = doSomethingCrazyWithLameReturnCodes(); //! if (rc == NAILED_IT) { //! globalExceptionWrapper = exception_wrapper(); //! } else if (rc == FACE_PLANT) { //! globalExceptionWrapper = make_exception_wrapper(); //! } else if (rc == FAIL_WHALE) { //! globalExceptionWrapper = make_exception_wrapper(); //! } //! } //! //! // Thread2: Exceptions are ok! //! void processResult() { //! try { //! globalExceptionWrapper.throw_exception(); //! } catch (const FacePlantException& e) { //! LOG(ERROR) << "FACEPLANT!"; //! } catch (const FailWhaleException& e) { //! LOG(ERROR) << "FAILWHALE!"; //! } //! } //! //! // Thread2: Exceptions are bad! //! void processResult() { //! globalExceptionWrapper.handle( //! [&](FacePlantException& faceplant) { //! LOG(ERROR) << "FACEPLANT"; //! }, //! [&](FailWhaleException& failwhale) { //! LOG(ERROR) << "FAILWHALE!"; //! }, //! [](...) { //! LOG(FATAL) << "Unrecognized exception"; //! }); //! } //! \endcode class exception_wrapper final { private: struct FOLLY_EXPORT AnyException : std::exception { std::type_info const* typeinfo_; template /* implicit */ AnyException(T&& t) noexcept : typeinfo_(&typeid(t)) {} }; template struct arg_type_; template using arg_type = _t>; struct with_exception_from_fn_; struct with_exception_from_ex_; // exception_wrapper is implemented as a simple variant over four // different representations: // 0. Empty, no exception. // 1. An small object stored in-situ. // 2. A larger object stored on the heap and referenced with a // std::shared_ptr. // 3. A std::exception_ptr. // This is accomplished with the help of a union and a pointer to a hand- // rolled virtual table. This virtual table contains pointers to functions // that know which field of the union is active and do the proper action. // The class invariant ensures that the vtable ptr and the union stay in sync. struct VTable { void (*copy_)(exception_wrapper const*, exception_wrapper*); void (*move_)(exception_wrapper*, exception_wrapper*); void (*delete_)(exception_wrapper*); void (*throw_)(exception_wrapper const*); std::type_info const* (*type_)(exception_wrapper const*); std::exception const* (*get_exception_)(exception_wrapper const*); exception_wrapper (*get_exception_ptr_)(exception_wrapper const*); }; [[noreturn]] static void onNoExceptionError(char const* name); template static Ret noop_(Args...); static std::type_info const* uninit_type_(exception_wrapper const*); static VTable const uninit_; template using IsStdException = std::is_base_of>; template using IsCatchAll = std::is_same>, AnyException>; // Sadly, with the gcc-4.9 platform, std::logic_error and std::runtime_error // do not fit here. They also don't have noexcept copy-ctors, so the internal // storage wouldn't be used anyway. For the gcc-5 platform, both logic_error // and runtime_error can be safely stored internally. struct Buffer { using Storage = std::aligned_storage_t<2 * sizeof(void*), alignof(std::exception)>; Storage buff_; Buffer() : buff_{} {} template Buffer(in_place_type_t, As&&... as_); template Ex& as() noexcept; template Ex const& as() const noexcept; }; struct ThrownTag {}; struct InSituTag {}; struct OnHeapTag {}; template using PlacementOf = std::conditional_t< !IsStdException::value, ThrownTag, std::conditional_t< sizeof(T) <= sizeof(Buffer::Storage) && alignof(T) <= alignof(Buffer::Storage)&& noexcept( T(std::declval< T&&>()))&& noexcept(T(std::declval())), InSituTag, OnHeapTag>>; static std::exception const* as_exception_or_null_(std::exception const& ex); static std::exception const* as_exception_or_null_(AnyException); struct ExceptionPtr { std::exception_ptr ptr_; static void copy_(exception_wrapper const* from, exception_wrapper* to); static void move_(exception_wrapper* from, exception_wrapper* to); static void delete_(exception_wrapper* that); [[noreturn]] static void throw_(exception_wrapper const* that); static std::type_info const* type_(exception_wrapper const* that); static std::exception const* get_exception_(exception_wrapper const* that); static exception_wrapper get_exception_ptr_(exception_wrapper const* that); static VTable const ops_; }; template struct InPlace { static_assert(IsStdException::value, "only deriving std::exception"); static void copy_(exception_wrapper const* from, exception_wrapper* to); static void move_(exception_wrapper* from, exception_wrapper* to); static void delete_(exception_wrapper* that); [[noreturn]] static void throw_(exception_wrapper const* that); static std::type_info const* type_(exception_wrapper const*); static std::exception const* get_exception_(exception_wrapper const* that); static exception_wrapper get_exception_ptr_(exception_wrapper const* that); static constexpr VTable const ops_{ copy_, move_, delete_, throw_, type_, get_exception_, get_exception_ptr_}; }; struct SharedPtr { struct Base { std::type_info const* info_; Base() = delete; explicit Base(std::type_info const& info) : info_(&info) {} virtual ~Base() {} virtual void throw_() const = 0; virtual std::exception const* get_exception_() const noexcept = 0; virtual exception_wrapper get_exception_ptr_() const noexcept = 0; }; template struct Impl final : public Base { static_assert(IsStdException::value, "only deriving std::exception"); Ex ex_; Impl() : Base{typeid(Ex)}, ex_() {} // clang-format off template explicit Impl(As&&... as) : Base{typeid(Ex)}, ex_(std::forward(as)...) {} [[noreturn]] void throw_() const override; // clang-format on std::exception const* get_exception_() const noexcept override; exception_wrapper get_exception_ptr_() const noexcept override; }; std::shared_ptr ptr_; static void copy_(exception_wrapper const* from, exception_wrapper* to); static void move_(exception_wrapper* from, exception_wrapper* to); static void delete_(exception_wrapper* that); [[noreturn]] static void throw_(exception_wrapper const* that); static std::type_info const* type_(exception_wrapper const* that); static std::exception const* get_exception_(exception_wrapper const* that); static exception_wrapper get_exception_ptr_(exception_wrapper const* that); static VTable const ops_; }; union { Buffer buff_{}; ExceptionPtr eptr_; SharedPtr sptr_; }; VTable const* vptr_{&uninit_}; template exception_wrapper(ThrownTag, in_place_type_t, As&&... as); template exception_wrapper(OnHeapTag, in_place_type_t, As&&... as); template exception_wrapper(InSituTag, in_place_type_t, As&&... as); template struct IsRegularExceptionType : StrictConjunction< std::is_copy_constructible, Negation>, Negation>> {}; template static bool with_exception_(This& this_, Fn fn_, tag_t); template static bool with_exception_(This& this_, Fn fn_, tag_t); template static bool with_exception_(This& this_, Fn fn_); template static void handle_(This& this_, char const* name, CatchFns&... fns); public: static exception_wrapper from_exception_ptr( std::exception_ptr const& eptr) noexcept; static exception_wrapper from_exception_ptr( std::exception_ptr&& eptr) noexcept; //! Default-constructs an empty `exception_wrapper` //! \post `type() == none()` exception_wrapper() noexcept {} //! Move-constructs an `exception_wrapper` //! \post `*this` contains the value of `that` prior to the move //! \post `that.type() == none()` exception_wrapper(exception_wrapper&& that) noexcept; //! Copy-constructs an `exception_wrapper` //! \post `*this` contains a copy of `that`, and `that` is unmodified //! \post `type() == that.type()` exception_wrapper(exception_wrapper const& that) noexcept; //! Move-assigns an `exception_wrapper` //! \pre `this != &that` //! \post `*this` contains the value of `that` prior to the move //! \post `that.type() == none()` exception_wrapper& operator=(exception_wrapper&& that) noexcept; //! Copy-assigns an `exception_wrapper` //! \post `*this` contains a copy of `that`, and `that` is unmodified //! \post `type() == that.type()` exception_wrapper& operator=(exception_wrapper const& that) noexcept; ~exception_wrapper(); //! \post `!ptr || bool(*this)` explicit exception_wrapper(std::exception_ptr const& ptr) noexcept; explicit exception_wrapper(std::exception_ptr&& ptr) noexcept; //! \pre `ptr` holds a reference to `ex`. //! \post `bool(*this)` //! \post `type() == typeid(ex)` template exception_wrapper(std::exception_ptr const& ptr, Ex& ex) noexcept; template exception_wrapper(std::exception_ptr&& ptr, Ex& ex) noexcept; //! \pre `typeid(ex) == typeid(typename decay::type)` //! \post `bool(*this)` //! \post `type() == typeid(ex)` //! \note Exceptions of types derived from `std::exception` can be implicitly //! converted to an `exception_wrapper`. template < class Ex, class Ex_ = std::decay_t, FOLLY_REQUIRES( Conjunction, IsRegularExceptionType>::value)> /* implicit */ exception_wrapper(Ex&& ex); //! \pre `typeid(ex) == typeid(typename decay::type)` //! \post `bool(*this)` //! \post `type() == typeid(ex)` //! \note Exceptions of types not derived from `std::exception` can still be //! used to construct an `exception_wrapper`, but you must specify //! `folly::in_place` as the first parameter. template < class Ex, class Ex_ = std::decay_t, FOLLY_REQUIRES(IsRegularExceptionType::value)> exception_wrapper(in_place_t, Ex&& ex); template < class Ex, typename... As, FOLLY_REQUIRES(IsRegularExceptionType::value)> exception_wrapper(in_place_type_t, As&&... as); //! Swaps the value of `*this` with the value of `that` void swap(exception_wrapper& that) noexcept; //! \return `true` if `*this` is holding an exception. explicit operator bool() const noexcept; //! \return `!bool(*this)` bool operator!() const noexcept; //! Make this `exception_wrapper` empty //! \post `!*this` void reset(); //! \return `true` if this `exception_wrapper` holds a reference to an //! exception that was thrown (i.e., if it was constructed with //! a `std::exception_ptr`, or if `to_exception_ptr()` was called on a //! (non-const) reference to `*this`). bool has_exception_ptr() const noexcept; //! \return a pointer to the `std::exception` held by `*this`, if it holds //! one; otherwise, returns `nullptr`. //! \note This function does not mutate the `exception_wrapper` object. //! \note This function never causes an exception to be thrown. std::exception* get_exception() noexcept; //! \overload std::exception const* get_exception() const noexcept; //! \returns a pointer to the `Ex` held by `*this`, if it holds an object //! whose type `From` permits `std::is_convertible`; //! otherwise, returns `nullptr`. //! \note This function does not mutate the `exception_wrapper` object. //! \note This function may cause an exception to be thrown and immediately //! caught internally, affecting runtime performance. template Ex* get_exception() noexcept; //! \overload template Ex const* get_exception() const noexcept; //! \return A `std::exception_ptr` that references either the exception held //! by `*this`, or a copy of same. //! \note This function may need to throw an exception to complete the action. //! \note The non-const overload of this function mutates `*this` to cache the //! computed `std::exception_ptr`; that is, this function may cause //! `has_exception_ptr()` to change from `false` to `true`. std::exception_ptr to_exception_ptr() noexcept; //! \overload std::exception_ptr to_exception_ptr() const noexcept; //! \return the `typeid` of an unspecified type used by //! `exception_wrapper::type()` to denote an empty `exception_wrapper`. static std::type_info const& none() noexcept; //! Returns the `typeid` of the wrapped exception object. If there is no //! wrapped exception object, returns `exception_wrapper::none()`. std::type_info const& type() const noexcept; //! \return If `get_exception() != nullptr`, `class_name() + ": " + //! get_exception()->what()`; otherwise, `class_name()`. folly::fbstring what() const; //! \return If `!*this`, the empty string; otherwise, //! the result of `type().name()` after demangling. folly::fbstring class_name() const; //! \tparam Ex The expression type to check for compatibility with. //! \return `true` if and only if `*this` wraps an exception that would be //! caught with a `catch(Ex const&)` clause. //! \note If `*this` is empty, this function returns `false`. template bool is_compatible_with() const noexcept; //! Throws the wrapped expression. //! \pre `bool(*this)` [[noreturn]] void throw_exception() const; //! Terminates the process with the wrapped expression. [[noreturn]] void terminate_with() const noexcept { throw_exception(); } //! Throws the wrapped expression nested into another exception. //! \pre `bool(*this)` //! \param ex Exception in *this will be thrown nested into ex; // see std::throw_with_nested() for details on this semantic. template [[noreturn]] void throw_with_nested(Ex&& ex) const; //! Call `fn` with the wrapped exception (if any), if `fn` can accept it. //! \par Example //! \code //! exception_wrapper ew{std::runtime_error("goodbye cruel world")}; //! //! assert( ew.with_exception([](std::runtime_error& e){/*...*/}) ); //! //! assert( !ew.with_exception([](int& e){/*...*/}) ); //! //! assert( !exception_wrapper{}.with_exception([](int& e){/*...*/}) ); //! \endcode //! \tparam Ex Optionally, the type of the exception that `fn` accepts. //! \tparam Fn The type of a monomophic function object. //! \param fn A function object to call with the wrapped exception //! \return `true` if and only if `fn` was called. //! \note Optionally, you may explicitly specify the type of the exception //! that `fn` expects, as in //! \code //! ew.with_exception([](auto&& e) { /*...*/; }); //! \endcode //! \note The handler is not invoked with an active exception. //! **Do not try to rethrow the exception with `throw;` from within your //! handler -- that is, a throw expression with no operand.** This may //! cause your process to terminate. (It is perfectly ok to throw from //! a handler so long as you specify the exception to throw, as in //! `throw e;`.) template bool with_exception(Fn fn); //! \overload template bool with_exception(Fn fn) const; //! Handle the wrapped expression as if with a series of `catch` clauses, //! propagating the exception if no handler matches. //! \par Example //! \code //! exception_wrapper ew{std::runtime_error("goodbye cruel world")}; //! //! ew.handle( //! [&](std::logic_error const& e) { //! LOG(DFATAL) << "ruh roh"; //! ew.throw_exception(); // rethrow the active exception without //! // slicing it. Will not be caught by other //! // handlers in this call. //! }, //! [&](std::exception const& e) { //! LOG(ERROR) << ew.what(); //! }); //! \endcode //! In the above example, any exception _not_ derived from `std::exception` //! will be propagated. To specify a catch-all clause, pass a lambda that //! takes a C-style ellipses, as in: //! \code //! ew.handle(/*...* /, [](...) { /* handle unknown exception */ } ) //! \endcode //! \pre `!*this` //! \tparam CatchFns A pack of unary monomorphic function object types. //! \param fns A pack of unary monomorphic function objects to be treated as //! an ordered list of potential exception handlers. //! \note The handlers are not invoked with an active exception. //! **Do not try to rethrow the exception with `throw;` from within your //! handler -- that is, a throw expression with no operand.** This may //! cause your process to terminate. (It is perfectly ok to throw from //! a handler so long as you specify the exception to throw, as in //! `throw e;`.) template void handle(CatchFns... fns); //! \overload template void handle(CatchFns... fns) const; }; template constexpr exception_wrapper::VTable exception_wrapper::InPlace::ops_; /** * \return An `exception_wrapper` that wraps an instance of type `Ex` * that has been constructed with arguments `std::forward(as)...`. */ template exception_wrapper make_exception_wrapper(As&&... as) { return exception_wrapper{in_place_type, std::forward(as)...}; } /** * Inserts `ew.what()` into the ostream `sout`. * \return `sout` */ template std::basic_ostream& operator<<( std::basic_ostream& sout, exception_wrapper const& ew) { return sout << ew.what(); } /** * Swaps the value of `a` with the value of `b`. */ inline void swap(exception_wrapper& a, exception_wrapper& b) noexcept { a.swap(b); } // For consistency with exceptionStr() functions in ExceptionString.h fbstring exceptionStr(exception_wrapper const& ew); //! `try_and_catch` is a convenience for `try {} catch(...) {}`` that returns an //! `exception_wrapper` with the thrown exception, if any. template exception_wrapper try_and_catch(F&& fn) noexcept { auto x = [&] { return void(static_cast(fn)()), std::exception_ptr{}; }; return exception_wrapper{catch_exception(x, std::current_exception)}; } } // namespace folly #include #undef FOLLY_REQUIRES #undef FOLLY_REQUIRES_DEF