16697765559

Committed 02 Aug 2025 09:09PM UTC coverage: 87.996%. First build

Build # 16697765559

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Pull #365

github

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web-flow

Commit Message

Merge 761522df6 into 10ca7de0a

Pull Request Pull Request #365: coro::shared_mutex use coro::mutex instead of std::mutex

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84.95

/include/coro/shared_mutex.hpp

#pragma once

#include "coro/concepts/executor.hpp"
#include "coro/mutex.hpp"
#include "coro/task.hpp"

#include <atomic>
#include <coroutine>

namespace coro
{
template<concepts::executor executor_type>
class shared_mutex;

/**
 * A scoped RAII lock holder for a coro::shared_mutex.  It will call the appropriate unlock() or
 * unlock_shared() based on how the coro::shared_mutex was originally acquired, either shared or
 * exclusive modes.
 */
template<concepts::executor executor_type>
class shared_scoped_lock
{
public:
    shared_scoped_lock(shared_mutex<executor_type>& sm, bool exclusive) : m_shared_mutex(&sm), m_exclusive(exclusive) {}

    /**
     * Unlocks the mutex upon this shared scoped lock destructing.
     */
    ~shared_scoped_lock() { unlock(); }

    shared_scoped_lock(const shared_scoped_lock&) = delete;
    shared_scoped_lock(shared_scoped_lock&& other)
        : m_shared_mutex(std::exchange(other.m_shared_mutex, nullptr)),
          m_exclusive(other.m_exclusive)
    {
    }

    auto operator=(const shared_scoped_lock&) -> shared_scoped_lock& = delete;
    auto operator=(shared_scoped_lock&& other) noexcept -> shared_scoped_lock&
    {
        if (std::addressof(other) != this)
        {
            m_shared_mutex = std::exchange(other.m_shared_mutex, nullptr);
            m_exclusive    = other.m_exclusive;
        }
        return *this;
    }

    /**
     * Unlocks the shared mutex prior to this lock going out of scope.
     */
    auto unlock() -> void
    {
        if (m_shared_mutex != nullptr)
        {
            if (!m_shared_mutex->m_executor->spawn(make_unlock_task(m_shared_mutex, m_exclusive)))
            {
                // If for some reason it fails to spawn block so this mutex isn't unusable.
                coro::sync_wait(make_unlock_task(m_shared_mutex, m_exclusive));
            }
        }
    }

private:
    shared_mutex<executor_type>* m_shared_mutex{nullptr};
    bool                         m_exclusive{false};

    static auto make_unlock_task(shared_mutex<executor_type>* shared_mutex, bool exclusive) -> coro::task<void>
    {
        std::cerr << "make_unlock_task() start\n";
        // This function is spawned and detached from the lifetime of the unlocking scoped lock.

        if (shared_mutex != nullptr)
        {
            if (exclusive)
            {
                std::cerr << "unlock() exclusive\n";
                co_await shared_mutex->unlock();
            }
            else
            {
                std::cerr << "unlock() shared\n";
                co_await shared_mutex->unlock_shared();
            }

            shared_mutex = nullptr;
        }
        else
        {
            std::cerr << "unlock() nullptr\n";
        }
        std::cerr << "make_unlock_task() end\n";
    }
};

namespace detail
{
template<concepts::executor executor_type>
struct shared_lock_operation_base
{
    explicit shared_lock_operation_base(coro::shared_mutex<executor_type>& shared_mutex, bool exclusive)
        : m_shared_mutex(shared_mutex),
          m_exclusive(exclusive)
    {}
    virtual ~shared_lock_operation_base() = default;

    shared_lock_operation_base(const shared_lock_operation_base&) = delete;
    shared_lock_operation_base(shared_lock_operation_base&&) = delete;
    auto operator=(const shared_lock_operation_base&) -> shared_lock_operation_base& = delete;
    auto operator=(shared_lock_operation_base&&) -> shared_lock_operation_base& = delete;

    auto await_ready() const noexcept -> bool
    {
        // If either mode can be acquired, unlock the internal mutex and resume.

        if (m_exclusive)
        {
            if (m_shared_mutex.try_lock_locked())
            {
                m_shared_mutex.m_mutex.unlock();
                return true;
            }
        }
        else if (m_shared_mutex.try_lock_shared_locked())
        {
            m_shared_mutex.m_mutex.unlock();
            return true;
        }

        return false;
    }

    auto await_suspend(std::coroutine_handle<> awaiting_coroutine) noexcept -> bool
    {
        // For sure the lock is currently held in a manner that it cannot be acquired, suspend ourself
        // at the end of the waiter list.

        auto* tail_waiter = m_shared_mutex.m_tail_waiter.load(std::memory_order::acquire);

        if (tail_waiter == nullptr)
        {
            m_shared_mutex.m_head_waiter = this;
            m_shared_mutex.m_tail_waiter = this;
        }
        else
        {
            tail_waiter->m_next = this;
            m_shared_mutex.m_tail_waiter         = this;
        }

        // If this is an exclusive lock acquire then mark it as so so that shared locks after this
        // exclusive one will also suspend so this exclusive lock doesn't get starved.
        if (m_exclusive)
        {
            ++m_shared_mutex.m_exclusive_waiters;
        }

        m_awaiting_coroutine = awaiting_coroutine;
        m_shared_mutex.m_mutex.unlock();
        return true;
    }

protected:
    friend class coro::shared_mutex<executor_type>;

    std::coroutine_handle<> m_awaiting_coroutine;
    shared_lock_operation_base* m_next{nullptr};
    coro::shared_mutex<executor_type>& m_shared_mutex;
    bool m_exclusive{false};
};

template<typename return_type, concepts::executor executor_type>
struct shared_lock_operation : public shared_lock_operation_base<executor_type>
{
    explicit shared_lock_operation(coro::shared_mutex<executor_type>& shared_mutex, bool exclusive)
        : shared_lock_operation_base<executor_type>(shared_mutex, exclusive)
    {}
    ~shared_lock_operation() override = default;

    shared_lock_operation(const shared_lock_operation&) = delete;
    shared_lock_operation(shared_lock_operation&&) = delete;
    auto operator=(const shared_lock_operation&) -> shared_lock_operation& = delete;
    auto operator=(shared_lock_operation&&) -> shared_lock_operation& = delete;

    auto await_resume() noexcept -> return_type
    {
        if constexpr (std::is_same_v<shared_scoped_lock<executor_type>, return_type>)
        {
            return shared_scoped_lock<executor_type>{this->m_shared_mutex, this->m_exclusive};
        }
        else
        {
            return;
        }
    }
};
} // namespace detail

template<concepts::executor executor_type>
class shared_mutex
{
public:
    /**
     * @param e The executor for when multiple shared waiters can be woken up at the same time,
     *          each shared waiter will be scheduled to immediately run on this executor in
     *          parallel.
     */
    explicit shared_mutex(std::shared_ptr<executor_type> e) : m_executor(std::move(e))
    {
        if (m_executor == nullptr)
        {
            throw std::runtime_error{"coro::shared_mutex cannot have a nullptr executor"};
        }
    }
    ~shared_mutex() = default;

    shared_mutex(const shared_mutex&)                    = delete;
    shared_mutex(shared_mutex&&)                         = delete;
    auto operator=(const shared_mutex&) -> shared_mutex& = delete;
    auto operator=(shared_mutex&&) -> shared_mutex&      = delete;

    /**
     * Locks the mutex in a shared state.  If there are any exclusive waiters then the shared waiters
     * will also wait so the exclusive waiters are not starved.
     */
    // [[nodiscard]] auto scoped_lock_shared() -> coro::task<detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>>
    // {
    //     co_await m_mutex.lock();
    //     co_return detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>{*this, false};
    // }
    //
    // /**
    //  * Locks the mutex in an exclusive state.
    //  */
    // [[nodiscard]] auto scoped_lock() -> coro::task<detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>>
    // {
    //     co_await m_mutex.lock();
    //     co_return detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>{*this, true};
    // }

    [[nodiscard]] auto lock_shared() -> coro::task<void>
    {
        co_await m_mutex.lock();
        co_await detail::shared_lock_operation<void, executor_type>{*this, false};
        co_return;

    }

    [[nodiscard]] auto lock() -> coro::task<void>
    {
        co_await m_mutex.lock();
        co_await detail::shared_lock_operation<void, executor_type>{*this, true};
        co_return;
    }

    /**
     * @return True if the lock could immediately be acquired in a shared state.
     */
    [[nodiscard]] auto try_lock_shared() -> bool
    {
        // To acquire the shared lock the state must be one of two states:
        //   1) unlocked
        //   2) shared locked with zero exclusive waiters
        //          Zero exclusive waiters prevents exclusive starvation if shared locks are
        //          always continuously happening.

        if (m_mutex.try_lock())
        {
            coro::scoped_lock lk{m_mutex};
            return try_lock_shared_locked();
        }
        return false;
    }

    /**
     * @return True if the lock could immediately be acquired in an exclusive state.
     */
    [[nodiscard]] auto try_lock() -> bool
    {
        // To acquire the exclusive lock the state must be unlocked.
        if (m_mutex.try_lock())
        {
            coro::scoped_lock lk{m_mutex};
            return try_lock_locked();
        }
        return false;
    }

    /**
     * Unlocks a single shared state user.  *REQUIRES* that the lock was first acquired exactly once
     * via `lock_shared()` or `try_lock_shared() -> True` before being called, otherwise undefined
     * behavior.
     *
     * If the shared user count drops to zero and this lock has an exclusive waiter then the exclusive
     * waiter acquires the lock.
     */
    [[nodiscard]] auto unlock_shared() -> coro::task<void>
    {
        auto lk = co_await m_mutex.scoped_lock();
        --m_shared_users;

        // Only wake waiters from shared state if all shared users have completed.
        if (m_shared_users == 0)
        {
            if (m_head_waiter != nullptr)
            {
                wake_waiters(lk);
            }
            else
            {
                m_state = state::unlocked;
            }
        }

        co_return;
    }

    /**
     * Unlocks the mutex from its exclusive state.  If there is a following exclusive watier then
     * that exclusive waiter acquires the lock.  If there are 1 or more shared waiters then all the
     * shared waiters acquire the lock in a shared state in parallel and are resumed on the original
     * executor this shared mutex was created with.
     */
    [[nodiscard]] auto unlock() -> coro::task<void>
    {
        auto lk = co_await m_mutex.scoped_lock();
        if (m_head_waiter != nullptr)
        {
            wake_waiters(lk);
        }
        else
        {
            m_state = state::unlocked;
        }

        co_return;
    }

    /**
     * @brief Gets the executor that drives the shared mutex.
     *
     * @return std::shared_ptr<executor_type>
     */
    [[nodiscard]] auto executor() -> std::shared_ptr<executor_type>
    {
        return m_executor;
    }

private:
    friend struct detail::shared_lock_operation_base<executor_type>;
    friend class shared_scoped_lock<executor_type>;

    enum class state
    {
        /// @brief The shared mutex is unlocked.
        unlocked,
        /// @brief The shared mutex is locked in shared mode.
        locked_shared,
        /// @brief The shared mutex is locked in exclusive mode.
        locked_exclusive
    };

    /// @brief This executor is for resuming multiple shared waiters.
    std::shared_ptr<executor_type> m_executor{nullptr};
    /// @brief Exclusive access for mutating the shared mutex's state.
    coro::mutex m_mutex;
    /// @brief The current state of the shared mutex.
    std::atomic<state> m_state{state::unlocked};

    /// @brief The current number of shared users that have acquired the lock.
    std::atomic<uint64_t> m_shared_users{0};
    /// @brief The current number of exclusive waiters waiting to acquire the lock.  This is used to block
    ///        new incoming shared lock attempts so the exclusive waiter is not starved.
    std::atomic<uint64_t> m_exclusive_waiters{0};

    std::atomic<detail::shared_lock_operation_base<executor_type>*> m_head_waiter{nullptr};
    std::atomic<detail::shared_lock_operation_base<executor_type>*> m_tail_waiter{nullptr};

    auto try_lock_shared_locked() -> bool
    {
        if (m_state == state::unlocked)
        {
            // If the shared mutex is unlocked put it into shared mode and add ourself as using the lock.
            m_state = state::locked_shared;
            ++m_shared_users;
            return true;
        }
        else if (m_state == state::locked_shared && m_exclusive_waiters == 0)
        {
            // If the shared mutex is in a shared locked state and there are no exclusive waiters
            // the add ourself as using the lock.
            ++m_shared_users;
            return true;
        }

        // If the lock is in shared mode but there are exclusive waiters then we will also wait so
        // the writers are not starved.

        // If the lock is in exclusive mode already then we need to wait.

        return false;
    }

    auto try_lock_locked() -> bool
    {
        if (m_state == state::unlocked)
        {
            m_state = state::locked_exclusive;
            return true;
        }
        return false;
    }

    auto wake_waiters(coro::scoped_lock& lk) -> void
    {
        // First determine what the next lock state will be based on the first waiter.
        if (m_head_waiter.load()->m_exclusive)
        {
            // If its exclusive then only this waiter can be woken up.
            m_state                   = state::locked_exclusive;
            detail::shared_lock_operation_base<executor_type>* to_resume = m_head_waiter.load();
            m_head_waiter             = m_head_waiter.load()->m_next;
            --m_exclusive_waiters;
            if (m_head_waiter == nullptr)
            {
                m_tail_waiter = nullptr;
            }

            // Since this is an exclusive lock waiting we can resume it directly.
            lk.unlock();
            to_resume->m_awaiting_coroutine.resume();
        }
        else
        {
            // If its shared then we will scan forward and awake all shared waiters onto the given
            // thread pool so they can run in parallel.
            m_state = state::locked_shared;
            do
            {
                detail::shared_lock_operation_base<executor_type>* to_resume = m_head_waiter.load();
                m_head_waiter             = m_head_waiter.load()->m_next;
                if (m_head_waiter == nullptr)
                {
                    m_tail_waiter = nullptr;
                }
                ++m_shared_users;

                m_executor->resume(to_resume->m_awaiting_coroutine);
            } while (m_head_waiter != nullptr && !m_head_waiter.load()->m_exclusive);

            // Cannot unlock until the entire set of shared waiters has been traversed.  I think this
            // makes more sense than allocating space for all the shared waiters, unlocking, and then
            // resuming in a batch?
            lk.unlock();
        }
    }
};

} // namespace coro

1	#pragma once
2
3	#include "coro/concepts/executor.hpp"
4	#include "coro/mutex.hpp"
5	#include "coro/task.hpp"
6
7	#include <atomic>
8	#include <coroutine>
9
10	namespace coro
11	{
12	template<concepts::executor executor_type>
13	class shared_mutex;
14
15	/**
16	* A scoped RAII lock holder for a coro::shared_mutex. It will call the appropriate unlock() or
17	* unlock_shared() based on how the coro::shared_mutex was originally acquired, either shared or
18	* exclusive modes.
19	*/
20	template<concepts::executor executor_type>
21	class shared_scoped_lock
22	{
23	public:
24	shared_scoped_lock(shared_mutex<executor_type>& sm, bool exclusive) : m_shared_mutex(&sm), m_exclusive(exclusive) {}
25
26	/**
27	* Unlocks the mutex upon this shared scoped lock destructing.
28	*/
29	~shared_scoped_lock() { unlock(); }
30
31	shared_scoped_lock(const shared_scoped_lock&) = delete;
32	shared_scoped_lock(shared_scoped_lock&& other)
33	: m_shared_mutex(std::exchange(other.m_shared_mutex, nullptr)),
34	m_exclusive(other.m_exclusive)
35	{
36	}
37
38	auto operator=(const shared_scoped_lock&) -> shared_scoped_lock& = delete;
39	auto operator=(shared_scoped_lock&& other) noexcept -> shared_scoped_lock&
40	{
41	if (std::addressof(other) != this)
42	{
43	m_shared_mutex = std::exchange(other.m_shared_mutex, nullptr);
44	m_exclusive = other.m_exclusive;
45	}
46	return *this;
47	}
48
49	/**
50	* Unlocks the shared mutex prior to this lock going out of scope.
51	*/
52	auto unlock() -> void
53	{
54	if (m_shared_mutex != nullptr)
55	{
56	if (!m_shared_mutex->m_executor->spawn(make_unlock_task(m_shared_mutex, m_exclusive)))
57	{
58	// If for some reason it fails to spawn block so this mutex isn't unusable.
59	coro::sync_wait(make_unlock_task(m_shared_mutex, m_exclusive));
60	}
61	}
62	}
63
64	private:
65	shared_mutex<executor_type>* m_shared_mutex{nullptr};
66	bool m_exclusive{false};
67
68	static auto make_unlock_task(shared_mutex<executor_type>* shared_mutex, bool exclusive) -> coro::task<void>
69	{
70	std::cerr << "make_unlock_task() start\n";
71	// This function is spawned and detached from the lifetime of the unlocking scoped lock.
72
73	if (shared_mutex != nullptr)
74	{
75	if (exclusive)
76	{
77	std::cerr << "unlock() exclusive\n";
78	co_await shared_mutex->unlock();
79	}
80	else
81	{
82	std::cerr << "unlock() shared\n";
83	co_await shared_mutex->unlock_shared();
84	}
85
86	shared_mutex = nullptr;
87	}
88	else
89	{
90	std::cerr << "unlock() nullptr\n";
91	}
92	std::cerr << "make_unlock_task() end\n";
93	}
94	};
95
96	namespace detail
97	{
98	template<concepts::executor executor_type>
99	struct shared_lock_operation_base
100	{
101	explicit shared_lock_operation_base(coro::shared_mutex<executor_type>& shared_mutex, bool exclusive)	81✔
102	: m_shared_mutex(shared_mutex),	81✔
103	m_exclusive(exclusive)	81✔
104	{}	81✔
105	virtual ~shared_lock_operation_base() = default;	81✔
106
107	shared_lock_operation_base(const shared_lock_operation_base&) = delete;
108	shared_lock_operation_base(shared_lock_operation_base&&) = delete;
109	auto operator=(const shared_lock_operation_base&) -> shared_lock_operation_base& = delete;
110	auto operator=(shared_lock_operation_base&&) -> shared_lock_operation_base& = delete;
111
112	auto await_ready() const noexcept -> bool	81✔
113	{
114	// If either mode can be acquired, unlock the internal mutex and resume.
115
116	if (m_exclusive)	81✔
117	{
118	if (m_shared_mutex.try_lock_locked())	2✔
119	{
120	m_shared_mutex.m_mutex.unlock();	2✔
121	return true;	2✔
122	}
123	}
124	else if (m_shared_mutex.try_lock_shared_locked())	79✔
125	{
126	m_shared_mutex.m_mutex.unlock();	38✔
127	return true;	38✔
128	}
129
130	return false;	41✔
131	}
132
133	auto await_suspend(std::coroutine_handle<> awaiting_coroutine) noexcept -> bool	41✔
134	{
135	// For sure the lock is currently held in a manner that it cannot be acquired, suspend ourself
136	// at the end of the waiter list.
137
138	auto* tail_waiter = m_shared_mutex.m_tail_waiter.load(std::memory_order::acquire);	41✔
139
140	if (tail_waiter == nullptr)	41✔
141	{
142	m_shared_mutex.m_head_waiter = this;	1✔
143	m_shared_mutex.m_tail_waiter = this;	1✔
144	}
145	else
146	{
147	tail_waiter->m_next = this;	40✔
148	m_shared_mutex.m_tail_waiter = this;	40✔
149	}
150
151	// If this is an exclusive lock acquire then mark it as so so that shared locks after this
152	// exclusive one will also suspend so this exclusive lock doesn't get starved.
153	if (m_exclusive)	41✔
154	{
NEW 155	++m_shared_mutex.m_exclusive_waiters;	×
156	}
157
158	m_awaiting_coroutine = awaiting_coroutine;	41✔
159	m_shared_mutex.m_mutex.unlock();	41✔
160	return true;	41✔
161	}
162
163	protected:
164	friend class coro::shared_mutex<executor_type>;
165
166	std::coroutine_handle<> m_awaiting_coroutine;
167	shared_lock_operation_base* m_next{nullptr};
168	coro::shared_mutex<executor_type>& m_shared_mutex;
169	bool m_exclusive{false};
170	};
171
172	template<typename return_type, concepts::executor executor_type>
173	struct shared_lock_operation : public shared_lock_operation_base<executor_type>
174	{
175	explicit shared_lock_operation(coro::shared_mutex<executor_type>& shared_mutex, bool exclusive)	81✔
176	: shared_lock_operation_base<executor_type>(shared_mutex, exclusive)	81✔
177	{}	81✔
178	~shared_lock_operation() override = default;	81✔
179
180	shared_lock_operation(const shared_lock_operation&) = delete;
181	shared_lock_operation(shared_lock_operation&&) = delete;
182	auto operator=(const shared_lock_operation&) -> shared_lock_operation& = delete;
183	auto operator=(shared_lock_operation&&) -> shared_lock_operation& = delete;
184
185	auto await_resume() noexcept -> return_type	81✔
186	{
187	if constexpr (std::is_same_v<shared_scoped_lock<executor_type>, return_type>)
188	{
189	return shared_scoped_lock<executor_type>{this->m_shared_mutex, this->m_exclusive};
190	}
191	else
192	{
193	return;	81✔
194	}
195	}
196	};
197	} // namespace detail
198
199	template<concepts::executor executor_type>
200	class shared_mutex
201	{
202	public:
203	/**
204	* @param e The executor for when multiple shared waiters can be woken up at the same time,
205	* each shared waiter will be scheduled to immediately run on this executor in
206	* parallel.
207	*/
208	explicit shared_mutex(std::shared_ptr<executor_type> e) : m_executor(std::move(e))	3✔
209	{
210	if (m_executor == nullptr)	3✔
211	{
212	throw std::runtime_error{"coro::shared_mutex cannot have a nullptr executor"};	×
213	}
214	}	3✔
215	~shared_mutex() = default;	3✔
216
217	shared_mutex(const shared_mutex&) = delete;
218	shared_mutex(shared_mutex&&) = delete;
219	auto operator=(const shared_mutex&) -> shared_mutex& = delete;
220	auto operator=(shared_mutex&&) -> shared_mutex& = delete;
221
222	/**
223	* Locks the mutex in a shared state. If there are any exclusive waiters then the shared waiters
224	* will also wait so the exclusive waiters are not starved.
225	*/
226	// [[nodiscard]] auto scoped_lock_shared() -> coro::task<detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>>
227	// {
228	// co_await m_mutex.lock();
229	// co_return detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>{*this, false};
230	// }
231	//
232	// /**
233	// * Locks the mutex in an exclusive state.
234	// */
235	// [[nodiscard]] auto scoped_lock() -> coro::task<detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>>
236	// {
237	// co_await m_mutex.lock();
238	// co_return detail::shared_lock_operation<shared_scoped_lock<executor_type>, executor_type>{*this, true};
239	// }
240
241	[[nodiscard]] auto lock_shared() -> coro::task<void>	79✔
242	{
243	co_await m_mutex.lock();
244	co_await detail::shared_lock_operation<void, executor_type>{*this, false};
245	co_return;
246
247	}	158✔
248
249	[[nodiscard]] auto lock() -> coro::task<void>	2✔
250	{
251	co_await m_mutex.lock();
252	co_await detail::shared_lock_operation<void, executor_type>{*this, true};
253	co_return;
254	}	4✔
255
256	/**
257	* @return True if the lock could immediately be acquired in a shared state.
258	*/
259	[[nodiscard]] auto try_lock_shared() -> bool	3✔
260	{
261	// To acquire the shared lock the state must be one of two states:
262	// 1) unlocked
263	// 2) shared locked with zero exclusive waiters
264	// Zero exclusive waiters prevents exclusive starvation if shared locks are
265	// always continuously happening.
266
267	if (m_mutex.try_lock())	3✔
268	{
269	coro::scoped_lock lk{m_mutex};	3✔
270	return try_lock_shared_locked();	3✔
271	}	3✔
NEW 272	return false;	×
273	}
274
275	/**
276	* @return True if the lock could immediately be acquired in an exclusive state.
277	*/
278	[[nodiscard]] auto try_lock() -> bool	6✔
279	{
280	// To acquire the exclusive lock the state must be unlocked.
281	if (m_mutex.try_lock())	6✔
282	{
283	coro::scoped_lock lk{m_mutex};	6✔
284	return try_lock_locked();	6✔
285	}	6✔
NEW 286	return false;	×
287	}
288
289	/**
290	* Unlocks a single shared state user. REQUIRES that the lock was first acquired exactly once
291	* via `lock_shared()` or `try_lock_shared() -> True` before being called, otherwise undefined
292	* behavior.
293	*
294	* If the shared user count drops to zero and this lock has an exclusive waiter then the exclusive
295	* waiter acquires the lock.
296	*/
297	[[nodiscard]] auto unlock_shared() -> coro::task<void>	81✔
298	{
299	auto lk = co_await m_mutex.scoped_lock();
300	--m_shared_users;
301
302	// Only wake waiters from shared state if all shared users have completed.
303	if (m_shared_users == 0)
304	{
305	if (m_head_waiter != nullptr)
306	{
307	wake_waiters(lk);
308	}
309	else
310	{
311	m_state = state::unlocked;
312	}
313	}
314
315	co_return;
316	}	162✔
317
318	/**
319	* Unlocks the mutex from its exclusive state. If there is a following exclusive watier then
320	* that exclusive waiter acquires the lock. If there are 1 or more shared waiters then all the
321	* shared waiters acquire the lock in a shared state in parallel and are resumed on the original
322	* executor this shared mutex was created with.
323	*/
324	[[nodiscard]] auto unlock() -> coro::task<void>	6✔
325	{
326	auto lk = co_await m_mutex.scoped_lock();
327	if (m_head_waiter != nullptr)
328	{
329	wake_waiters(lk);
330	}
331	else
332	{
333	m_state = state::unlocked;
334	}
335
336	co_return;
337	}	12✔
338
339	/**
340	* @brief Gets the executor that drives the shared mutex.
341	*
342	* @return std::shared_ptr<executor_type>
343	*/
NEW 344	[[nodiscard]] auto executor() -> std::shared_ptr<executor_type>	×
345	{
NEW 346	return m_executor;	×
347	}
348
349	private:
350	friend struct detail::shared_lock_operation_base<executor_type>;
351	friend class shared_scoped_lock<executor_type>;
352
353	enum class state
354	{
355	/// @brief The shared mutex is unlocked.
356	unlocked,
357	/// @brief The shared mutex is locked in shared mode.
358	locked_shared,
359	/// @brief The shared mutex is locked in exclusive mode.
360	locked_exclusive
361	};
362
363	/// @brief This executor is for resuming multiple shared waiters.
364	std::shared_ptr<executor_type> m_executor{nullptr};
365	/// @brief Exclusive access for mutating the shared mutex's state.
366	coro::mutex m_mutex;
367	/// @brief The current state of the shared mutex.
368	std::atomic<state> m_state{state::unlocked};
369
370	/// @brief The current number of shared users that have acquired the lock.
371	std::atomic<uint64_t> m_shared_users{0};
372	/// @brief The current number of exclusive waiters waiting to acquire the lock. This is used to block
373	/// new incoming shared lock attempts so the exclusive waiter is not starved.
374	std::atomic<uint64_t> m_exclusive_waiters{0};
375
376	std::atomic<detail::shared_lock_operation_base<executor_type>*> m_head_waiter{nullptr};
377	std::atomic<detail::shared_lock_operation_base<executor_type>*> m_tail_waiter{nullptr};
378
379	auto try_lock_shared_locked() -> bool	82✔
380	{
381	if (m_state == state::unlocked)	82✔
382	{
383	// If the shared mutex is unlocked put it into shared mode and add ourself as using the lock.
384	m_state = state::locked_shared;	36✔
385	++m_shared_users;	36✔
386	return true;	36✔
387	}
388	else if (m_state == state::locked_shared && m_exclusive_waiters == 0)	46✔
389	{
390	// If the shared mutex is in a shared locked state and there are no exclusive waiters
391	// the add ourself as using the lock.
392	++m_shared_users;	4✔
393	return true;	4✔
394	}
395
396	// If the lock is in shared mode but there are exclusive waiters then we will also wait so
397	// the writers are not starved.
398
399	// If the lock is in exclusive mode already then we need to wait.
400
401	return false;	42✔
402	}
403
404	auto try_lock_locked() -> bool	8✔
405	{
406	if (m_state == state::unlocked)	8✔
407	{
408	m_state = state::locked_exclusive;	6✔
409	return true;	6✔
410	}
411	return false;	2✔
412	}
413
414	auto wake_waiters(coro::scoped_lock& lk) -> void	1✔
415	{
416	// First determine what the next lock state will be based on the first waiter.
417	if (m_head_waiter.load()->m_exclusive)	1✔
418	{
419	// If its exclusive then only this waiter can be woken up.
420	m_state = state::locked_exclusive;	×
421	detail::shared_lock_operation_base<executor_type>* to_resume = m_head_waiter.load();	×
422	m_head_waiter = m_head_waiter.load()->m_next;	×
423	--m_exclusive_waiters;	×
424	if (m_head_waiter == nullptr)	×
425	{
426	m_tail_waiter = nullptr;	×
427	}
428
429	// Since this is an exclusive lock waiting we can resume it directly.
430	lk.unlock();	×
431	to_resume->m_awaiting_coroutine.resume();	×
432	}
433	else
434	{
435	// If its shared then we will scan forward and awake all shared waiters onto the given
436	// thread pool so they can run in parallel.
437	m_state = state::locked_shared;	1✔
438	do
439	{
440	detail::shared_lock_operation_base<executor_type>* to_resume = m_head_waiter.load();	41✔
441	m_head_waiter = m_head_waiter.load()->m_next;	41✔
442	if (m_head_waiter == nullptr)	41✔
443	{
444	m_tail_waiter = nullptr;	1✔
445	}
446	++m_shared_users;	41✔
447
448	m_executor->resume(to_resume->m_awaiting_coroutine);	41✔
449	} while (m_head_waiter != nullptr && !m_head_waiter.load()->m_exclusive);	41✔
450
451	// Cannot unlock until the entire set of shared waiters has been traversed. I think this
452	// makes more sense than allocating space for all the shared waiters, unlocking, and then
453	// resuming in a batch?
454	lk.unlock();	1✔
455	}
456	}	1✔
457	};
458
459	} // namespace coro

jbaldwin / libcoro / 16697765559

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