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159 changes: 159 additions & 0 deletions include/xrpl/basics/Mutex.hpp
Original file line number Diff line number Diff line change
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/*
This file is part of clio: https://github.com/XRPLF/clio
Copyright (c) 2024, the clio developers.

Permission to use, copy, modify, and distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
//==============================================================================

#pragma once

#include <mutex>
#include <type_traits>

namespace xrpl {

template <typename ProtectedDataType, typename MutexType>
class Mutex;

/**
* @brief A lock on a mutex that provides access to the protected data.
*
* @tparam ProtectedDataType data type to hold
* @tparam LockType type of lock
* @tparam MutexType type of mutex
*/
template <typename ProtectedDataType, template <typename...> typename LockType, typename MutexType>
class Lock
{
LockType<MutexType> lock_;
ProtectedDataType& data_;

public:
/** @cond */
ProtectedDataType const&
operator*() const
{
return data_;
}

ProtectedDataType&
operator*()
{
return data_;
}

ProtectedDataType const&
get() const
{
return data_;
}

ProtectedDataType&
get()
{
return data_;
}

ProtectedDataType const*
operator->() const
{
return &data_;
}

ProtectedDataType*
operator->()
{
return &data_;
}

operator LockType<MutexType>&()
{
return lock_;
}
/** @endcond */

private:
friend class Mutex<std::remove_const_t<ProtectedDataType>, MutexType>;

Lock(MutexType& mutex, ProtectedDataType& data) : lock_(mutex), data_(data)
{
}
};

/**
* @brief A container for data that is protected by a mutex. Inspired by Mutex in Rust.
*
* @tparam ProtectedDataType data type to hold
* @tparam MutexType type of mutex
*/
template <typename ProtectedDataType, typename MutexType = std::mutex>
class Mutex
{
mutable MutexType mutex_;
ProtectedDataType data_;

public:
Mutex() = default;

/**
* @brief Construct a new Mutex object with the given data
*
* @param data The data to protect
*/
explicit Mutex(ProtectedDataType data) : data_(std::move(data))
{
}

/**
* @brief Make a new Mutex object with the given data
*
* @tparam Args The types of the arguments to forward to the constructor of the protected data
* @param args The arguments to forward to the constructor of the protected data
* @return The Mutex object that protects the given data
*/
template <typename... Args>
static Mutex
make(Args&&... args)
{
return Mutex{ProtectedDataType{std::forward<Args>(args)...}};
}

/**
* @brief Lock the mutex and get a lock object allowing access to the protected data
*
* @tparam LockType The type of lock to use
* @return A lock on the mutex and a reference to the protected data
*/
template <template <typename...> typename LockType = std::lock_guard>
Lock<ProtectedDataType const, LockType, MutexType>
lock() const
{
return {mutex_, data_};
}

/**
* @brief Lock the mutex and get a lock object allowing access to the protected data
*
* @tparam LockType The type of lock to use
* @return A lock on the mutex and a reference to the protected data
*/
template <template <typename...> typename LockType = std::lock_guard>
Lock<ProtectedDataType, LockType, MutexType>
lock()
{
return {mutex_, data_};
}
Comment on lines +118 to +156
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Copilot AI Feb 27, 2026

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This introduces a new concurrency utility but there are no accompanying unit tests. Since the basics module already has gtest coverage for similar utilities, consider adding a small test that exercises make(...), lock() (const/non-const), and access operators to catch integration/const-correctness regressions.

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I agree with Copilot. Let's add basic checks (no need to test the standard mutex itself):

  • Mutex::make(...)
  • non-const lock() and mutable access through *, ->, and get()
  • const lock() and confirming it exposes const access
  • the templated lock() path / conversion to the underlying lock type
// src/tests/libxrpl/basics/Mutex_test.cpp

#include <xrpl/basics/Mutex.hpp> 
#include <gtest/gtest.h>

#include <string>
#include <type_traits>

namespace {

struct Widget
{
    std::string name;
    int value;

    Widget(std::string n, int v) : name(n), value(v) {}

    void
    bump()
    {
        ++value;
    }
};

TEST(MutexTest, MakeAndMutableAccess)
{
    auto m = xrpl::Mutex<Widget>::make("alice", 7);

    {
        auto lock = m.lock();

        static_assert(std::is_same_v<decltype(*lock), Widget&>);
        static_assert(std::is_same_v<decltype(lock.get()), Widget&>);
        static_assert(std::is_same_v<decltype(lock.operator->()), Widget*>);

        EXPECT_EQ(lock->name, "alice");
        EXPECT_EQ((*lock).value, 7);

        lock.get().value = 8;
        lock->bump();

        EXPECT_EQ(lock->value, 9);
    }

    auto verify = m.lock();
    EXPECT_EQ(verify->value, 9);
}

TEST(MutexTest, ConstLockReturnsConstView)
{
    xrpl::Mutex<Widget> const m{Widget{"bob", 3}};

    auto lock = m.lock();

    static_assert(std::is_same_v<decltype(*lock), Widget const&>);
    static_assert(std::is_same_v<decltype(lock.get()), Widget const&>);
    static_assert(std::is_same_v<decltype(lock.operator->()), Widget const*>);

    EXPECT_EQ(lock->name, "bob");
    EXPECT_EQ(lock->value, 3);
}

TEST(MutexTest, SupportsAlternateLockType)
{
    xrpl::Mutex<Widget> m{Widget{"carol", 0}};

    {
        auto lock = m.lock<std::unique_lock>();
        std::unique_lock<std::mutex>& underlying = lock;

        EXPECT_TRUE(underlying.owns_lock());

        lock->bump();
        EXPECT_EQ(lock->value, 1);
    }
}

}  // namespace

};

} // namespace xrpl
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