ExceptionWrapper.h

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/*
 * Copyright 2014-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.
 */
/*
 * Author: Eric Niebler <eniebler@fb.com>
 */

#pragma once

#include <cassert>
#include <cstdint>
#include <exception>
#include <iosfwd>
#include <memory>
#include <new>
#include <type_traits>
#include <typeinfo>
#include <utility>

#include <folly/CPortability.h>
#include <folly/Demangle.h>
#include <folly/ExceptionString.h>
#include <folly/FBString.h>
#include <folly/Portability.h>
#include <folly/Traits.h>
#include <folly/Utility.h>
#include <folly/lang/Assume.h>

#ifdef __GNUC__
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wpragmas"
#pragma GCC diagnostic ignored "-Wpotentially-evaluated-expression"
// GCC gets confused about lambda scopes and issues shadow-local warnings for
// parameters in totally different functions.
FOLLY_GCC_DISABLE_NEW_SHADOW_WARNINGS
#endif

#define FOLLY_EXCEPTION_WRAPPER_H_INCLUDED

namespace folly {

#define FOLLY_REQUIRES_DEF(...) \
  _t<std::enable_if<static_cast<bool>(__VA_ARGS__), long>>

#define FOLLY_REQUIRES(...) FOLLY_REQUIRES_DEF(__VA_ARGS__) = __LINE__

namespace exception_wrapper_detail {

template <template <class> class T, class... As>
using AllOf = StrictConjunction<T<As>...>;

template <bool If, class T>
using AddConstIf = _t<std::conditional<If, const T, T>>;

template <class Fn, class A>
FOLLY_ALWAYS_INLINE FOLLY_ATTR_VISIBILITY_HIDDEN auto fold(Fn&&, A&& a) {
  return static_cast<A&&>(a);
}

template <class Fn, class A, class B, class... Bs>
FOLLY_ALWAYS_INLINE FOLLY_ATTR_VISIBILITY_HIDDEN auto
fold(Fn&& fn, A&& a, B&& b, Bs&&... bs) {
  return fold(
      // This looks like a use of fn after a move of fn, but in reality, this is
      // just a cast and not a move. That's because regardless of which fold
      // overload is selected, fn gets bound to a &&. Had fold taken fn by value
      // there would indeed be a problem here.
      static_cast<Fn&&>(fn),
      static_cast<Fn&&>(fn)(static_cast<A&&>(a), static_cast<B&&>(b)),
      static_cast<Bs&&>(bs)...);
}

} // namespace exception_wrapper_detail

class exception_wrapper final {
 private:
  struct FOLLY_EXPORT AnyException : std::exception {
    std::type_info const* typeinfo_;
    template <class T>
    /* implicit */ AnyException(T&& t) noexcept : typeinfo_(&typeid(t)) {}
  };

  template <class Fn>
  struct arg_type_;
  template <class Fn>
  using arg_type = _t<arg_type_<Fn>>;

  // 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, together with either:
  //       a. A pointer to the referenced std::exception object, or
  //       b. A pointer to a std::type_info object for the referenced exception,
  //          or for an unspecified type if the type is unknown.
  // 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 <class Ret, class... Args>
  static Ret noop_(Args...);

  static std::type_info const* uninit_type_(exception_wrapper const*);

  static VTable const uninit_;

  template <class Ex>
  using IsStdException = std::is_base_of<std::exception, _t<std::decay<Ex>>>;
  template <bool B, class T>
  using AddConstIf = exception_wrapper_detail::AddConstIf<B, T>;
  template <class CatchFn>
  using IsCatchAll =
      std::is_same<arg_type<_t<std::decay<CatchFn>>>, AnyException>;

  struct Unknown {};

  // 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 =
        _t<std::aligned_storage<2 * sizeof(void*), alignof(std::exception)>>;
    Storage buff_;

    Buffer() : buff_{} {}

    template <class Ex, typename... As>
    Buffer(in_place_type_t<Ex>, As&&... as_);
    template <class Ex>
    Ex& as() noexcept;
    template <class Ex>
    Ex const& as() const noexcept;
  };

  struct ThrownTag {};
  struct InSituTag {};
  struct OnHeapTag {};

  template <class T>
  using PlacementOf = _t<std::conditional<
      !IsStdException<T>::value,
      ThrownTag,
      _t<std::conditional<
          sizeof(T) <= sizeof(Buffer::Storage) &&
              alignof(T) <= alignof(Buffer::Storage) &&
              noexcept(T(std::declval<T&&>())) &&
              noexcept(T(std::declval<T const&>())),
          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_;
    std::uintptr_t exception_or_type_; // odd for type_info
    static_assert(
        1 < alignof(std::exception) && 1 < alignof(std::type_info),
        "Surprise! std::exception and std::type_info don't have alignment "
        "greater than one. as_int_ below will not work!");

    static std::uintptr_t as_int_(
        std::exception_ptr const& ptr,
        std::exception const& e) noexcept;
    static std::uintptr_t as_int_(
        std::exception_ptr const& ptr,
        AnyException e) noexcept;
    bool has_exception_() const;
    std::exception const* as_exception_() const;
    std::type_info const* as_type_() const;
    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 <class Ex>
  struct InPlace {
    static_assert(IsStdException<Ex>::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() = default;
      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 <class Ex>
    struct Impl final : public Base {
      static_assert(IsStdException<Ex>::value, "only deriving std::exception");
      Ex ex_;
      Impl() = default;
      // clang-format off
      template <typename... As>
      explicit Impl(As&&... as)
          : Base{typeid(Ex)}, ex_(std::forward<As>(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<Base> 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 <class Ex, typename... As>
  exception_wrapper(ThrownTag, in_place_type_t<Ex>, As&&... as);

  template <class Ex, typename... As>
  exception_wrapper(OnHeapTag, in_place_type_t<Ex>, As&&... as);

  template <class Ex, typename... As>
  exception_wrapper(InSituTag, in_place_type_t<Ex>, As&&... as);

  template <class T>
  struct IsRegularExceptionType
      : StrictConjunction<
            std::is_copy_constructible<T>,
            Negation<std::is_base_of<exception_wrapper, T>>,
            Negation<std::is_abstract<T>>> {};

  template <class CatchFn, bool IsConst = false>
  struct ExceptionTypeOf;

  template <bool IsConst>
  struct HandleReduce;

  template <bool IsConst>
  struct HandleStdExceptReduce;

  template <class This, class... CatchFns>
  static void handle_(std::false_type, This& this_, CatchFns&... fns);

  template <class This, class... CatchFns>
  static void handle_(std::true_type, This& this_, CatchFns&... fns);

  template <class Ex, class This, class Fn>
  static bool with_exception_(This& this_, Fn fn_);

 public:
  static exception_wrapper from_exception_ptr(
      std::exception_ptr const& eptr) noexcept;

  exception_wrapper() noexcept {}

  exception_wrapper(exception_wrapper&& that) noexcept;

  exception_wrapper(exception_wrapper const& that) noexcept;

  exception_wrapper& operator=(exception_wrapper&& that) noexcept;

  exception_wrapper& operator=(exception_wrapper const& that) noexcept;

  ~exception_wrapper();

  explicit exception_wrapper(std::exception_ptr ptr) noexcept;

  template <class Ex>
  exception_wrapper(std::exception_ptr ptr, Ex& ex) noexcept;

  template <
      class Ex,
      class Ex_ = _t<std::decay<Ex>>,
      FOLLY_REQUIRES(
          Conjunction<IsStdException<Ex_>, IsRegularExceptionType<Ex_>>::value)>
  /* implicit */ exception_wrapper(Ex&& ex);

  template <
      class Ex,
      class Ex_ = _t<std::decay<Ex>>,
      FOLLY_REQUIRES(IsRegularExceptionType<Ex_>::value)>
  exception_wrapper(in_place_t, Ex&& ex);

  template <
      class Ex,
      typename... As,
      FOLLY_REQUIRES(IsRegularExceptionType<Ex>::value)>
  exception_wrapper(in_place_type_t<Ex>, As&&... as);

  void swap(exception_wrapper& that) noexcept;

  explicit operator bool() const noexcept;

  bool operator!() const noexcept;

  void reset();

  bool has_exception_ptr() const noexcept;

  std::exception* get_exception() noexcept;
  std::exception const* get_exception() const noexcept;

  template <typename Ex>
  Ex* get_exception() noexcept;
  template <typename Ex>
  Ex const* get_exception() const noexcept;

  std::exception_ptr const& to_exception_ptr() noexcept;
  std::exception_ptr to_exception_ptr() const noexcept;

  static std::type_info const& none() noexcept;
  static std::type_info const& unknown() noexcept;

  std::type_info const& type() const noexcept;

  folly::fbstring what() const;

  folly::fbstring class_name() const;

  template <class Ex>
  bool is_compatible_with() const noexcept;

  [[noreturn]] void throw_exception() const;

  //      see std::throw_with_nested() for details on this semantic.
  template <class Ex>
  [[noreturn]] void throw_with_nested(Ex&& ex) const;

  template <class Ex = void const, class Fn>
  bool with_exception(Fn fn);
  template <class Ex = void const, class Fn>
  bool with_exception(Fn fn) const;

  template <class... CatchFns>
  void handle(CatchFns... fns);
  template <class... CatchFns>
  void handle(CatchFns... fns) const;
};

template <class Ex>
constexpr exception_wrapper::VTable exception_wrapper::InPlace<Ex>::ops_;

template <class Ex, typename... As>
exception_wrapper make_exception_wrapper(As&&... as) {
  return exception_wrapper{in_place_type<Ex>, std::forward<As>(as)...};
}

template <class Ch>
std::basic_ostream<Ch>& operator<<(
    std::basic_ostream<Ch>& sout,
    exception_wrapper const& ew) {
  return sout << ew.what();
}

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);

namespace detail {
template <typename F>
inline exception_wrapper try_and_catch_(F&& f) {
  return (f(), exception_wrapper());
}

template <typename F, typename Ex, typename... Exs>
inline exception_wrapper try_and_catch_(F&& f) {
  try {
    return try_and_catch_<F, Exs...>(std::forward<F>(f));
  } catch (Ex& ex) {
    return exception_wrapper(std::current_exception(), ex);
  }
}
} // namespace detail

template <typename... Exceptions, typename F>
exception_wrapper try_and_catch(F&& fn) {
  return detail::try_and_catch_<F, Exceptions...>(std::forward<F>(fn));
}
} // namespace folly

#include <folly/ExceptionWrapper-inl.h>

#undef FOLLY_REQUIRES
#undef FOLLY_REQUIRES_DEF
#ifdef __GNUC__
#pragma GCC diagnostic pop
#endif