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/**
* Copyright (C) 2018 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the GNU Affero General Public License in all respects
* for all of the code used other than as permitted herein. If you modify
* file(s) with this exception, you may extend this exception to your
* version of the file(s), but you are not obligated to do so. If you do not
* wish to do so, delete this exception statement from your version. If you
* delete this exception statement from all source files in the program,
* then also delete it in the license file.
*/
#pragma once
#include <functional>
#include "mongo/base/init.h"
#include "mongo/config.h"
/**
* The `SHIM` mechanism allows for the creation of "weak-symbol-like" functions which can have their
* actual implementation injected in the final binary without creating a link dependency upon any
* actual implementation. One uses it like this:
*
* In a header:
* ```
* class MyClass {
* public:
* static MONGO_DECLARE_SHIM((int)->std::string) helloWorldFunction;
* };
* ```
*
* In the corresponding C++ file (which is a link dependency):
* ```
* MONGO_DEFINE_SHIM(MyClass::helloWorldFunction);
* ```
*
* And in any number of implementation files:
* ```
* MONGO_REGISTER_SHIM(MyClass::helloWorldFunction)(int value)->std::string {
* if (value == 42) {
* return "Hello World";
* } else {
* return "No way!";
* }
* }
* ```
*
* This can be useful for making auto-registering and auto-constructing mock and release class
* factories, among other useful things
*/
namespace mongo {
template <typename T>
struct PrivateCall;
/**
* When declaring shim functions that should be private, they really need to be public; however,
* this type can be used as a parameter to permit the function to only be called by the type
* specified in the template parameter.
*/
template <typename T>
struct PrivateTo {
private:
friend PrivateCall<T>;
PrivateTo() = default;
};
/**
* When calling shim functions that should be private, you pass an immediately created instance of
* the type `PrivateCall< T >`, where `T` is the type that `PrivateTo` requires as a template
* parameter.
*/
template <typename T>
struct PrivateCall {
private:
friend T;
PrivateCall() {}
public:
operator PrivateTo<T>() {
return {};
}
};
} // namespace mongo
namespace shim_detail {
/**
* This type, `storage`, is used as a workaround for needing C++17 `inline` variables. The template
* static member is effectively `inline` already.
*/
template <typename T, typename tag = void>
struct storage {
static T data;
};
template <typename T, typename tag>
T storage<T, tag>::data = {};
} // namespace shim_detail
#define MONGO_SHIM_DEPENDENTS ("ShimHooks")
namespace mongo {
#ifdef MONGO_CONFIG_CHECK_SHIM_DEPENDENCIES
const bool checkShimsViaTUHook = true;
#define MONGO_SHIM_TU_HOOK(name) \
name {}
#else
const bool checkShimsViaTUHook = false;
#define MONGO_SHIM_TU_HOOK(name)
#endif
} // namespace mongo
/**
* Declare a shimmable function with signature `SHIM_SIGNATURE`. Declare such constructs in a C++
* header as static members of a class.
*/
#define MONGO_DECLARE_SHIM(/*SHIM_SIGNATURE*/...) MONGO_DECLARE_SHIM_1(__LINE__, __VA_ARGS__)
#define MONGO_DECLARE_SHIM_1(LN, ...) MONGO_DECLARE_SHIM_2(LN, __VA_ARGS__)
#define MONGO_DECLARE_SHIM_2(LN, ...) \
const struct ShimBasis_##LN { \
ShimBasis_##LN() = default; \
struct MongoShimImplGuts { \
template <bool required = mongo::checkShimsViaTUHook> \
struct AbiCheckType { \
AbiCheckType() = default; \
}; \
using AbiCheck = AbiCheckType<>; \
struct LibTUHookTypeBase { \
LibTUHookTypeBase(); \
}; \
template <bool required = true> \
struct LibTUHookType : LibTUHookTypeBase {}; \
using LibTUHook = LibTUHookType<>; \
struct ImplTUHookTypeBase { \
ImplTUHookTypeBase(); \
}; \
template <bool required = mongo::checkShimsViaTUHook> \
struct ImplTUHookType : ImplTUHookTypeBase {}; \
using ImplTUHook = ImplTUHookType<>; \
\
static auto functionTypeHelper __VA_ARGS__; \
/* Workaround for Microsoft -- by taking the address of this function pointer, we \
* avoid the problems that their compiler has with default * arguments in deduced \
* typedefs. */ \
using function_type_pointer = decltype(&MongoShimImplGuts::functionTypeHelper); \
using function_type = std::remove_pointer_t<function_type_pointer>; \
MongoShimImplGuts* abi(const AbiCheck* const) { \
return this; \
} \
MongoShimImplGuts* lib(const LibTUHook* const) { \
LibTUHook{}; \
return this; \
} \
MongoShimImplGuts* impl(const ImplTUHook* const) { \
MONGO_SHIM_TU_HOOK(ImplTUHook); \
return this; \
} \
virtual auto implementation __VA_ARGS__ = 0; \
\
using tag = \
std::tuple<MongoShimImplGuts::function_type, AbiCheck, LibTUHook, ImplTUHook>; \
}; \
\
using storage = shim_detail::storage<MongoShimImplGuts*, MongoShimImplGuts::tag>; \
\
/* TODO: When the dependency graph is fixed, add the `impl()->` call to the call chain */ \
template <typename... Args> \
auto operator()(Args&&... args) const \
noexcept(noexcept(storage::data->abi(nullptr)->lib(nullptr)->implementation( \
std::forward<Args>(args)...))) \
-> decltype(storage::data->abi(nullptr)->lib(nullptr)->implementation( \
std::forward<Args>(args)...)) { \
return storage::data->abi(nullptr)->lib(nullptr)->implementation( \
std::forward<Args>(args)...); \
} \
}
/**
* Define a shimmable function with name `SHIM_NAME`, returning a value of type `RETURN_TYPE`, with
* any arguments. This shim definition macro should go in the associated C++ file to the header
* where a SHIM was defined. This macro does not emit a function definition, only the customization
* point's machinery.
*/
#define MONGO_DEFINE_SHIM(/*SHIM_NAME*/...) MONGO_DEFINE_SHIM_1(__LINE__, __VA_ARGS__)
#define MONGO_DEFINE_SHIM_1(LN, ...) MONGO_DEFINE_SHIM_2(LN, __VA_ARGS__)
#define MONGO_DEFINE_SHIM_2(LN, ...) \
namespace { \
namespace shim_namespace##LN { \
using ShimType = decltype(__VA_ARGS__); \
} /*namespace shim_namespace*/ \
} /*namespace*/ \
shim_namespace##LN::ShimType::MongoShimImplGuts::LibTUHookTypeBase::LibTUHookTypeBase() = \
default; \
shim_namespace##LN::ShimType __VA_ARGS__{};
#define MONGO_SHIM_EVIL_STRINGIFY_(args) #args
/**
* Define an implementation of a shimmable function with name `SHIM_NAME`. The compiler will check
* supplied parameters for correctness. This shim registration macro should go in the associated
* C++ implementation file to the header where a SHIM was defined. Such a file would be a mock
* implementation or a real implementation, for example
*/
#define MONGO_REGISTER_SHIM(/*SHIM_NAME*/...) MONGO_REGISTER_SHIM_1(__LINE__, __VA_ARGS__)
#define MONGO_REGISTER_SHIM_1(LN, ...) MONGO_REGISTER_SHIM_2(LN, __VA_ARGS__)
#define MONGO_REGISTER_SHIM_2(LN, ...) \
namespace { \
namespace shim_namespace##LN { \
using ShimType = decltype(__VA_ARGS__); \
\
class Implementation final : public ShimType::MongoShimImplGuts { \
/* Some compilers don't work well with the trailing `override` in this kind of \
* function declaration. */ \
ShimType::MongoShimImplGuts::function_type implementation; /* override */ \
}; \
\
::mongo::Status createInitializerRegistration(::mongo::InitializerContext* const) { \
static Implementation impl; \
ShimType::storage::data = &impl; \
return Status::OK(); \
} \
\
const ::mongo::GlobalInitializerRegisterer registrationHook{ \
std::string(MONGO_SHIM_EVIL_STRINGIFY_(__VA_ARGS__)), \
{}, \
{MONGO_SHIM_DEPENDENTS}, \
mongo::InitializerFunction(createInitializerRegistration)}; \
} /*namespace shim_namespace*/ \
} /*namespace*/ \
\
shim_namespace##LN::ShimType::MongoShimImplGuts::ImplTUHookTypeBase::ImplTUHookTypeBase() = \
default; \
\
auto shim_namespace##LN::Implementation::implementation /* After this point someone just \
writes the signature's arguments \
and return value (using arrow \
notation). Then they write the \
body. */
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