semaphore.hh

/*
 * This file is open source software, licensed to you under the terms
 * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
 * distributed with this work for additional information regarding copyright
 * ownership.  You may not use this file except in compliance with the License.
 *
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
/*
 * Copyright (C) 2014 Cloudius Systems, Ltd.
 */
 
#pragma once
 
#include <seastar/core/future.hh>
#include <seastar/core/chunked_fifo.hh>
#include <stdexcept>
#include <exception>
#include <optional>
#include <seastar/core/timer.hh>
#include <seastar/core/expiring_fifo.hh>
#include <seastar/core/timed_out_error.hh>
 
namespace seastar {
 
 
class broken_semaphore : public std::exception {
public:
    virtual const char* what() const noexcept;
};
 
class semaphore_timed_out : public timed_out_error {
public:
    virtual const char* what() const noexcept;
};
 
struct semaphore_default_exception_factory {
    static semaphore_timed_out timeout() noexcept;
    static broken_semaphore broken() noexcept;
};
 
class named_semaphore_timed_out : public semaphore_timed_out {
    sstring _msg;
public:
    named_semaphore_timed_out(std::string_view msg) noexcept;
    virtual const char* what() const noexcept;
};
 
class broken_named_semaphore : public broken_semaphore {
    sstring _msg;
public:
    broken_named_semaphore(std::string_view msg) noexcept;
    virtual const char* what() const noexcept;
};
 
// A factory of semaphore exceptions that contain additional context: the semaphore name
// auto sem = named_semaphore(0, named_semaphore_exception_factory{"file_opening_limit_semaphore"});
struct named_semaphore_exception_factory {
    sstring name;
    named_semaphore_timed_out timeout() const noexcept;
    broken_named_semaphore broken() const noexcept;
};
 
template<typename ExceptionFactory, typename Clock = typename timer<>::clock>
class basic_semaphore : private ExceptionFactory {
public:
    using duration = typename timer<Clock>::duration;
    using clock = typename timer<Clock>::clock;
    using time_point = typename timer<Clock>::time_point;
    using exception_factory = ExceptionFactory;
private:
    ssize_t _count;
    std::exception_ptr _ex;
    struct entry {
        promise<> pr;
        size_t nr;
        entry(promise<>&& pr_, size_t nr_) noexcept : pr(std::move(pr_)), nr(nr_) {}
    };
    using expiry_handler = std::function<void (entry&)>;
    expiring_fifo<entry, expiry_handler, clock> _wait_list;
    expiry_handler make_expiry_handler() noexcept {
        return [this] (entry& e) noexcept {
            try {
                e.pr.set_exception(this->timeout());
            } catch (...) {
                e.pr.set_exception(semaphore_timed_out());
            }
        };
    }
    bool has_available_units(size_t nr) const noexcept {
        return _count >= 0 && (static_cast<size_t>(_count) >= nr);
    }
    bool may_proceed(size_t nr) const noexcept {
        return has_available_units(nr) && _wait_list.empty();
    }
public:
    static constexpr size_t max_counter() noexcept {
        return std::numeric_limits<decltype(_count)>::max();
    }
 
    basic_semaphore(size_t count) noexcept(std::is_nothrow_default_constructible_v<exception_factory>)
        : exception_factory()
        , _count(count),
        _wait_list(make_expiry_handler())
    {}
    basic_semaphore(size_t count, exception_factory&& factory) noexcept(std::is_nothrow_move_constructible_v<exception_factory>)
        : exception_factory(std::move(factory))
        , _count(count)
        , _wait_list(make_expiry_handler())
    {
        static_assert(std::is_nothrow_move_constructible_v<expiry_handler>);
    }
    future<> wait(size_t nr = 1) noexcept {
        return wait(time_point::max(), nr);
    }
    future<> wait(time_point timeout, size_t nr = 1) noexcept {
        if (may_proceed(nr)) {
            _count -= nr;
            return make_ready_future<>();
        }
        if (_ex) {
            return make_exception_future(_ex);
        }
        entry e(promise<>(), nr);
        auto fut = e.pr.get_future();
        try {
            _wait_list.push_back(std::move(e), timeout);
        } catch (...) {
            e.pr.set_exception(std::current_exception());
        }
        return fut;
    }
 
    future<> wait(duration timeout, size_t nr = 1) noexcept {
        return wait(clock::now() + timeout, nr);
    }
    void signal(size_t nr = 1) noexcept {
        if (_ex) {
            return;
        }
        _count += nr;
        while (!_wait_list.empty() && has_available_units(_wait_list.front().nr)) {
            auto& x = _wait_list.front();
            _count -= x.nr;
            x.pr.set_value();
            _wait_list.pop_front();
        }
    }
 
    //
    void consume(size_t nr = 1) noexcept {
        if (_ex) {
            return;
        }
        _count -= nr;
    }
 
    bool try_wait(size_t nr = 1) noexcept {
        if (may_proceed(nr)) {
            _count -= nr;
            return true;
        } else {
            return false;
        }
    }
    size_t current() const noexcept { return std::max(_count, ssize_t(0)); }
 
    ssize_t available_units() const noexcept { return _count; }
 
    size_t waiters() const noexcept { return _wait_list.size(); }
 
    void broken() noexcept {
        std::exception_ptr ep;
        try {
            ep = std::make_exception_ptr(exception_factory::broken());
        } catch (...) {
            ep = std::make_exception_ptr(broken_semaphore());
        }
        broken(std::move(ep));
    }
 
    template <typename Exception>
    void broken(const Exception& ex) noexcept {
        broken(std::make_exception_ptr(ex));
    }
 
    void broken(std::exception_ptr ex) noexcept;
 
    void ensure_space_for_waiters(size_t n) {
        _wait_list.reserve(n);
    }
};
 
template<typename ExceptionFactory, typename Clock>
inline
void
basic_semaphore<ExceptionFactory, Clock>::broken(std::exception_ptr xp) noexcept {
    static_assert(std::is_nothrow_copy_constructible_v<std::exception_ptr>);
    _ex = xp;
    _count = 0;
    while (!_wait_list.empty()) {
        auto& x = _wait_list.front();
        x.pr.set_exception(xp);
        _wait_list.pop_front();
    }
}
 
template<typename ExceptionFactory = semaphore_default_exception_factory, typename Clock = typename timer<>::clock>
class semaphore_units {
    basic_semaphore<ExceptionFactory, Clock>* _sem;
    size_t _n;
 
    semaphore_units(basic_semaphore<ExceptionFactory, Clock>* sem, size_t n) noexcept : _sem(sem), _n(n) {}
public:
    semaphore_units() noexcept : semaphore_units(nullptr, 0) {}
    semaphore_units(basic_semaphore<ExceptionFactory, Clock>& sem, size_t n) noexcept : semaphore_units(&sem, n) {}
    semaphore_units(semaphore_units&& o) noexcept : _sem(o._sem), _n(std::exchange(o._n, 0)) {
    }
    semaphore_units& operator=(semaphore_units&& o) noexcept {
        _sem = o._sem;
        _n = std::exchange(o._n, 0);
        return *this;
    }
    semaphore_units(const semaphore_units&) = delete;
    ~semaphore_units() noexcept {
        return_all();
    }
    size_t return_units(size_t units) {
        if (units > _n) {
            throw std::invalid_argument("Cannot take more units than those protected by the semaphore");
        }
        _n -= units;
        _sem->signal(units);
        return _n;
    }
    void return_all() noexcept {
        if (_n) {
            _sem->signal(_n);
            _n = 0;
        }
    }
    size_t release() noexcept {
        return std::exchange(_n, 0);
    }
    semaphore_units split(size_t units) {
        if (units > _n) {
            throw std::invalid_argument("Cannot take more units than those protected by the semaphore");
        }
        _n -= units;
        return semaphore_units(_sem, units);
    }
    void adopt(semaphore_units&& other) noexcept {
        assert(other._sem == _sem);
        _n += other.release();
    }
 
    size_t count() const noexcept {
        return _n;
    }
};
 
template<typename ExceptionFactory, typename Clock = typename timer<>::clock>
future<semaphore_units<ExceptionFactory, Clock>>
get_units(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units) noexcept {
    return sem.wait(units).then([&sem, units] {
        return semaphore_units<ExceptionFactory, Clock>{ sem, units };
    });
}
 
template<typename ExceptionFactory, typename Clock = typename timer<>::clock>
future<semaphore_units<ExceptionFactory, Clock>>
get_units(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units, typename basic_semaphore<ExceptionFactory, Clock>::time_point timeout) noexcept {
    return sem.wait(timeout, units).then([&sem, units] {
        return semaphore_units<ExceptionFactory, Clock>{ sem, units };
    });
}
 
template<typename ExceptionFactory, typename Clock>
future<semaphore_units<ExceptionFactory, Clock>>
get_units(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units, typename basic_semaphore<ExceptionFactory, Clock>::duration timeout) noexcept {
    return sem.wait(timeout, units).then([&sem, units] {
        return semaphore_units<ExceptionFactory, Clock>{ sem, units };
    });
}
 
template<typename ExceptionFactory, typename Clock = typename timer<>::clock>
std::optional<semaphore_units<ExceptionFactory, Clock>>
try_get_units(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units) noexcept {
    if (!sem.try_wait(units)) {
        return std::nullopt;
    }
    return std::make_optional<semaphore_units<ExceptionFactory, Clock>>(sem, units);
}
 
template<typename ExceptionFactory, typename Clock = typename timer<>::clock>
semaphore_units<ExceptionFactory, Clock>
consume_units(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units) noexcept {
    sem.consume(units);
    return semaphore_units<ExceptionFactory, Clock>{ sem, units };
}
 
template <typename ExceptionFactory, typename Func, typename Clock = typename timer<>::clock>
inline
futurize_t<std::result_of_t<Func()>>
with_semaphore(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units, Func&& func) noexcept {
    return get_units(sem, units).then([func = std::forward<Func>(func)] (auto units) mutable {
        return futurize_invoke(std::forward<Func>(func)).finally([units = std::move(units)] {});
    });
}
 
template <typename ExceptionFactory, typename Clock, typename Func>
inline
futurize_t<std::result_of_t<Func()>>
with_semaphore(basic_semaphore<ExceptionFactory, Clock>& sem, size_t units, typename basic_semaphore<ExceptionFactory, Clock>::duration timeout, Func&& func) noexcept {
    return get_units(sem, units, timeout).then([func = std::forward<Func>(func)] (auto units) mutable {
        return futurize_invoke(std::forward<Func>(func)).finally([units = std::move(units)] {});
    });
}
 
using semaphore = basic_semaphore<semaphore_default_exception_factory>;
using named_semaphore = basic_semaphore<named_semaphore_exception_factory>;
 
 
}