threading.hpp

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// Copyright (C) 2015-2023 Jonathan Müller and foonathan/memory contributors
// SPDX-License-Identifier: Zlib
 
#ifndef WPI_MEMORY_THREADING_HPP_INCLUDED
#define WPI_MEMORY_THREADING_HPP_INCLUDED
 
/// \file
/// The mutex types.
 
#include <type_traits>
 
#include "allocator_traits.hpp"
#include "config.hpp"
 
#if WPI_HOSTED_IMPLEMENTATION
#include <mutex>
#endif
 
namespace wpi
{
    namespace memory
    {
        /// A dummy \c Mutex class that does not lock anything.
        /// It is a valid \c Mutex and can be used to disable locking anywhere a \c Mutex is requested.
        /// \ingroup memory_core
        struct no_mutex
        {
            void lock() noexcept {}
 
            bool try_lock() noexcept
            {
                return true;
            }
 
            void unlock() noexcept {}
        };
 
        /// Specifies whether or not a RawAllocator is thread safe as-is.
        /// This allows to use \ref no_mutex as an optimization.
        /// Note that stateless allocators are implictly thread-safe.
        /// Specialize it only for your own stateful allocators.
        /// \ingroup memory_core
        template <class RawAllocator>
        struct is_thread_safe_allocator
        : std::integral_constant<bool, !allocator_traits<RawAllocator>::is_stateful::value>
        {
        };
 
        namespace detail
        {
            // selects a mutex for an Allocator
            // stateless allocators don't need locking
            template <class RawAllocator, class Mutex>
            using mutex_for =
                typename std::conditional<is_thread_safe_allocator<RawAllocator>::value, no_mutex,
                                          Mutex>::type;
 
            // storage for mutexes to use EBO
            // it provides const lock/unlock function, inherit from it
            template <class Mutex>
            class mutex_storage
            {
            public:
                mutex_storage() noexcept = default;
                mutex_storage(const mutex_storage&) noexcept {}
 
                mutex_storage& operator=(const mutex_storage&) noexcept
                {
                    return *this;
                }
 
                void lock() const
                {
                    mutex_.lock();
                }
 
                void unlock() const noexcept
                {
                    mutex_.unlock();
                }
 
            protected:
                ~mutex_storage() noexcept = default;
 
            private:
                mutable Mutex mutex_;
            };
 
            template <>
            class mutex_storage<no_mutex>
            {
            public:
                mutex_storage() noexcept = default;
 
                void lock() const noexcept {}
                void unlock() const noexcept {}
 
            protected:
                ~mutex_storage() noexcept = default;
            };
 
            // non changeable pointer to an Allocator that keeps a lock
            // I don't think EBO is necessary here...
            template <class Alloc, class Mutex>
            class locked_allocator
            {
            public:
                locked_allocator(Alloc& alloc, Mutex& m) noexcept : mutex_(&m), alloc_(&alloc)
                {
                    mutex_->lock();
                }
 
                locked_allocator(locked_allocator&& other) noexcept
                : mutex_(other.mutex_), alloc_(other.alloc_)
                {
                    other.mutex_ = nullptr;
                    other.alloc_ = nullptr;
                }
 
                ~locked_allocator() noexcept
                {
                    if (mutex_)
                        mutex_->unlock();
                }
 
                locked_allocator& operator=(locked_allocator&& other) noexcept = delete;
 
                Alloc& operator*() const noexcept
                {
                    WPI_MEMORY_ASSERT(alloc_);
                    return *alloc_;
                }
 
                Alloc* operator->() const noexcept
                {
                    WPI_MEMORY_ASSERT(alloc_);
                    return alloc_;
                }
 
            private:
                Mutex* mutex_; // don't use unqiue_lock to avoid dependency
                Alloc* alloc_;
            };
 
            template <class Alloc, class Mutex>
            locked_allocator<Alloc, Mutex> lock_allocator(Alloc& a, Mutex& m)
            {
                return {a, m};
            }
        } // namespace detail
    }     // namespace memory
} // namespace wpi
 
#endif // WPI_MEMORY_THREADING_HPP_INCLUDED