libcrosscoro/examples/coro_shared_mutex.cpp

#include <coro/coro.hpp>
#include <iostream>

int main()
{
    // Shared mutexes require an excutor type to be able to wake up multiple shared waiters when
    // there is an exclusive lock holder releasing the lock.  This example uses a single thread
    // to also show the interleaving of coroutines acquiring the shared lock in shared and
    // exclusive mode as they resume and suspend in a linear manner.  Ideally the thread pool
    // executor would have more than 1 thread to resume all shared waiters in parallel.
    auto               tp = std::make_shared<coro::thread_pool>(coro::thread_pool::options{.thread_count = 1});
    coro::shared_mutex mutex{tp};

    auto make_shared_task = [&](uint64_t i) -> coro::task<void> {
        co_await tp->schedule();
        {
            std::cerr << "shared task " << i << " lock_shared()\n";
            auto scoped_lock = co_await mutex.lock_shared();
            std::cerr << "shared task " << i << " lock_shared() acquired\n";
            /// Immediately yield so the other shared tasks also acquire in shared state
            /// while this task currently holds the mutex in shared state.
            co_await tp->yield();
            std::cerr << "shared task " << i << " unlock_shared()\n";
        }
        co_return;
    };

    auto make_exclusive_task = [&]() -> coro::task<void> {
        co_await tp->schedule();

        std::cerr << "exclusive task lock()\n";
        auto scoped_lock = co_await mutex.lock();
        std::cerr << "exclusive task lock() acquired\n";
        // Do the exclusive work..
        std::cerr << "exclusive task unlock()\n";
        co_return;
    };

    // Create 3 shared tasks that will acquire the mutex in a shared state.
    const size_t                  num_tasks{3};
    std::vector<coro::task<void>> tasks{};
    for (size_t i = 1; i <= num_tasks; ++i)
    {
        tasks.emplace_back(make_shared_task(i));
    }
    // Create an exclusive task.
    tasks.emplace_back(make_exclusive_task());
    // Create 3 more shared tasks that will be blocked until the exclusive task completes.
    for (size_t i = num_tasks + 1; i <= num_tasks * 2; ++i)
    {
        tasks.emplace_back(make_shared_task(i));
    }

    coro::sync_wait(coro::when_all(std::move(tasks)));
}