scylladb/core/reactor.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 2014 Cloudius Systems
 */

#ifndef REACTOR_HH_
#define REACTOR_HH_

#include "seastar.hh"
#include "iostream.hh"
#include <memory>
#include <type_traits>
#include <libaio.h>
#include <sys/epoll.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unordered_map>
#include <netinet/ip.h>
#include <cstring>
#include <cassert>
#include <stdexcept>
#include <iostream>
#include <unistd.h>
#include <vector>
#include <queue>
#include <algorithm>
#include <thread>
#include <system_error>
#include <chrono>
#include <ratio>
#include <atomic>
#include <experimental/optional>
#include <boost/lockfree/spsc_queue.hpp>
#include <boost/optional.hpp>
#include <boost/program_options.hpp>
#include <set>
#include "util/eclipse.hh"
#include "future.hh"
#include "posix.hh"
#include "apply.hh"
#include "sstring.hh"
#include "deleter.hh"
#include "net/api.hh"
#include "temporary_buffer.hh"
#include "circular_buffer.hh"
#include "file.hh"
#include "semaphore.hh"
#include "core/scattered_message.hh"
#include "core/enum.hh"
#include <boost/range/irange.hpp>
#include "timer.hh"

#ifdef HAVE_OSV
#include <osv/sched.hh>
#include <osv/mutex.h>
#include <osv/condvar.h>
#include <osv/newpoll.hh>
#endif

namespace scollectd { class registration; }

class reactor;
class pollable_fd;
class pollable_fd_state;

struct free_deleter {
    void operator()(void* p) { ::free(p); }
};

template <typename CharType>
inline
std::unique_ptr<CharType[], free_deleter> allocate_aligned_buffer(size_t size, size_t align) {
    static_assert(sizeof(CharType) == 1, "must allocate byte type");
    void* ret;
    auto r = posix_memalign(&ret, align, size);
    assert(r == 0);
    return std::unique_ptr<CharType[], free_deleter>(reinterpret_cast<CharType*>(ret));
}

class lowres_clock {
public:
    typedef int64_t rep;
    // The lowres_clock's resolution is 10ms. However, to make it is easier to
    // do calcuations with std::chrono::milliseconds, we make the clock's
    // period to 1ms instead of 10ms.
    typedef std::ratio<1, 1000> period;
    typedef std::chrono::duration<rep, period> duration;
    typedef std::chrono::time_point<lowres_clock, duration> time_point;
    lowres_clock();
    static time_point now() {
        auto nr = _now.load(std::memory_order_relaxed);
        return time_point(duration(nr));
    }
private:
    static void update();
    // _now is updated by cpu0 and read by other cpus. Make _now on its own
    // cache line to avoid false sharing.
    static std::atomic<rep> _now [[gnu::aligned(64)]];
    // High resolution timer to drive this low resolution clock
    timer<> _timer [[gnu::aligned(64)]];
    // High resolution timer expires every 10 milliseconds
    static constexpr std::chrono::milliseconds _granularity{10};
};

class pollable_fd_state {
public:
    struct speculation {
        int events = 0;
        explicit speculation(int epoll_events_guessed = 0) : events(epoll_events_guessed) {}
    };
    ~pollable_fd_state();
    explicit pollable_fd_state(file_desc fd, speculation speculate = speculation())
        : fd(std::move(fd)), events_known(speculate.events) {}
    pollable_fd_state(const pollable_fd_state&) = delete;
    void operator=(const pollable_fd_state&) = delete;
    void speculate_epoll(int events) { events_known |= events; }
    file_desc fd;
    int events_requested = 0; // wanted by pollin/pollout promises
    int events_epoll = 0;     // installed in epoll
    int events_known = 0;     // returned from epoll
    promise<> pollin;
    promise<> pollout;
    friend class reactor;
    friend class pollable_fd;
};

inline
size_t iovec_len(const std::vector<iovec>& iov)
{
    size_t ret = 0;
    for (auto&& e : iov) {
        ret += e.iov_len;
    }
    return ret;
}

class pollable_fd {
public:
    using speculation = pollable_fd_state::speculation;
    pollable_fd(file_desc fd, speculation speculate = speculation())
        : _s(std::make_unique<pollable_fd_state>(std::move(fd), speculate)) {}
public:
    pollable_fd(pollable_fd&&) = default;
    pollable_fd& operator=(pollable_fd&&) = default;
    future<size_t> read_some(char* buffer, size_t size);
    future<size_t> read_some(uint8_t* buffer, size_t size);
    future<size_t> read_some(const std::vector<iovec>& iov);
    future<> write_all(const char* buffer, size_t size);
    future<> write_all(const uint8_t* buffer, size_t size);
    future<size_t> write_some(net::packet& p);
    future<> write_all(net::packet& p);
    future<> readable();
    future<> writeable();
    void abort_reader(std::exception_ptr ex);
    void abort_writer(std::exception_ptr ex);
    future<pollable_fd, socket_address> accept();
    future<size_t> sendmsg(struct msghdr *msg);
    future<size_t> recvmsg(struct msghdr *msg);
    future<size_t> sendto(socket_address addr, const void* buf, size_t len);
    file_desc& get_file_desc() const { return _s->fd; }
    void shutdown(int how) { _s->fd.shutdown(how); }
    void close() { _s.reset(); }
protected:
    int get_fd() const { return _s->fd.get(); }
    friend class reactor;
    friend class readable_eventfd;
    friend class writeable_eventfd;
private:
    std::unique_ptr<pollable_fd_state> _s;
};

class connected_socket_impl {
public:
    virtual ~connected_socket_impl() {}
    virtual input_stream<char> input() = 0;
    virtual output_stream<char> output() = 0;
    virtual void shutdown_input() = 0;
    virtual void shutdown_output() = 0;
};

/// \addtogroup networking-module
/// @{

/// A TCP (or other stream-based protocol) connection.
///
/// A \c connected_socket represents a full-duplex stream between
/// two endpoints, a local endpoint and a remote endpoint.
class connected_socket {
    std::unique_ptr<connected_socket_impl> _csi;
public:
    /// Constructs a \c connected_socket not corresponding to a connection
    connected_socket() {};
    /// \cond internal
    explicit connected_socket(std::unique_ptr<connected_socket_impl> csi)
        : _csi(std::move(csi)) {}
    /// \endcond
    /// Moves a \c connected_socket object.
    connected_socket(connected_socket&& cs) = default;
    /// Move-assigns a \c connected_socket object.
    connected_socket& operator=(connected_socket&& cs) = default;
    /// Gets the input stream.
    ///
    /// Gets an object returning data sent from the remote endpoint.
    input_stream<char> input();
    /// Gets the output stream.
    ///
    /// Gets an object that sends data to the remote endpoint.
    output_stream<char> output();
    /// Disables output to the socket.
    ///
    /// Current or future writes that have not been successfully flushed
    /// will immediately fail with an error.  This is useful to abort
    /// operations on a socket that is not making progress due to a
    /// peer failure.
    void shutdown_output();
    /// Disables input from the socket.
    ///
    /// Current or future reads will immediately fail with an error.
    /// This is useful to abort operations on a socket that is not making
    /// progress due to a peer failure.
    void shutdown_input();
    /// Disables socket input and output.
    ///
    /// Equivalent to \ref shutdown_input() and \ref shutdown_output().
};
/// @}

/// \cond internal
class server_socket_impl {
public:
    virtual ~server_socket_impl() {}
    virtual future<connected_socket, socket_address> accept() = 0;
    virtual void abort_accept() = 0;
};
/// \endcond

namespace std {

template <>
struct hash<::sockaddr_in> {
    size_t operator()(::sockaddr_in a) const {
        return a.sin_port ^ a.sin_addr.s_addr;
    }
};

}

bool operator==(const ::sockaddr_in a, const ::sockaddr_in b);

/// \addtogroup networking-module
/// @{

/// A listening socket, waiting to accept incoming network connections.
class server_socket {
    std::unique_ptr<server_socket_impl> _ssi;
public:
    /// Constructs a \c server_socket not corresponding to a connection
    server_socket() {}
    /// \cond internal
    explicit server_socket(std::unique_ptr<server_socket_impl> ssi)
        : _ssi(std::move(ssi)) {}
    /// \endcond
    /// Moves a \c server_socket object.
    server_socket(server_socket&& ss) = default;
    /// Move-assigns a \c server_socket object.
    server_socket& operator=(server_socket&& cs) = default;

    /// Accepts the next connection to successfully connect to this socket.
    ///
    /// \return a \ref connected_socket representing the connection, and
    ///         a \ref socket_address describing the remote endpoint.
    ///
    /// \see listen(socket_address sa)
    /// \see listen(socket_address sa, listen_options opts)
    future<connected_socket, socket_address> accept() {
        return _ssi->accept();
    }

    /// Stops any \ref accept() in progress.
    ///
    /// Current and future \ref accept() calls will terminate immediately
    /// with an error.
    void abort_accept() {
        return _ssi->abort_accept();
    }
};
/// @}

class network_stack {
public:
    virtual ~network_stack() {}
    virtual server_socket listen(socket_address sa, listen_options opts) = 0;
    // FIXME: local parameter assumes ipv4 for now, fix when adding other AF
    virtual future<connected_socket> connect(socket_address sa, socket_address local = socket_address(::sockaddr_in{AF_INET, INADDR_ANY, 0})) = 0;
    virtual net::udp_channel make_udp_channel(ipv4_addr addr = {}) = 0;
    virtual future<> initialize() {
        return make_ready_future();
    }
    virtual bool has_per_core_namespace() = 0;
};

class network_stack_registry {
public:
    using options = boost::program_options::variables_map;
private:
    static std::unordered_map<sstring,
            std::function<future<std::unique_ptr<network_stack>> (options opts)>>& _map() {
        static std::unordered_map<sstring,
                std::function<future<std::unique_ptr<network_stack>> (options opts)>> map;
        return map;
    }
    static sstring& _default() {
        static sstring def;
        return def;
    }
public:
    static boost::program_options::options_description& options_description() {
        static boost::program_options::options_description opts;
        return opts;
    }
    static void register_stack(sstring name,
            boost::program_options::options_description opts,
            std::function<future<std::unique_ptr<network_stack>> (options opts)> create,
            bool make_default = false);
    static sstring default_stack();
    static std::vector<sstring> list();
    static future<std::unique_ptr<network_stack>> create(options opts);
    static future<std::unique_ptr<network_stack>> create(sstring name, options opts);
};

class network_stack_registrator {
public:
    using options = boost::program_options::variables_map;
    explicit network_stack_registrator(sstring name,
            boost::program_options::options_description opts,
            std::function<future<std::unique_ptr<network_stack>> (options opts)> factory,
            bool make_default = false) {
        network_stack_registry::register_stack(name, opts, factory, make_default);
    }
};

class writeable_eventfd;

class readable_eventfd {
    pollable_fd _fd;
public:
    explicit readable_eventfd(size_t initial = 0) : _fd(try_create_eventfd(initial)) {}
    readable_eventfd(readable_eventfd&&) = default;
    writeable_eventfd write_side();
    future<size_t> wait();
    int get_write_fd() { return _fd.get_fd(); }
private:
    explicit readable_eventfd(file_desc&& fd) : _fd(std::move(fd)) {}
    static file_desc try_create_eventfd(size_t initial);

    friend class writeable_eventfd;
};

class writeable_eventfd {
    file_desc _fd;
public:
    explicit writeable_eventfd(size_t initial = 0) : _fd(try_create_eventfd(initial)) {}
    writeable_eventfd(writeable_eventfd&&) = default;
    readable_eventfd read_side();
    void signal(size_t nr);
    int get_read_fd() { return _fd.get(); }
private:
    explicit writeable_eventfd(file_desc&& fd) : _fd(std::move(fd)) {}
    static file_desc try_create_eventfd(size_t initial);

    friend class readable_eventfd;
};

// The reactor_notifier interface is a simplified version of Linux's eventfd
// interface (with semaphore behavior off, and signal() always signaling 1).
//
// A call to signal() causes an ongoing wait() to invoke its continuation.
// If no wait() is ongoing, the next wait() will continue immediately.
class reactor_notifier {
public:
    virtual future<> wait() = 0;
    virtual void signal() = 0;
    virtual ~reactor_notifier() {}
};

class thread_pool;
class smp;

class syscall_work_queue {
    static constexpr size_t queue_length = 128;
    struct work_item;
    using lf_queue = boost::lockfree::spsc_queue<work_item*,
                            boost::lockfree::capacity<queue_length>>;
    lf_queue _pending;
    lf_queue _completed;
    writeable_eventfd _start_eventfd;
    semaphore _queue_has_room = { queue_length };
    struct work_item {
        virtual ~work_item() {}
        virtual void process() = 0;
        virtual void complete() = 0;
    };
    template <typename T, typename Func>
    struct work_item_returning :  work_item {
        Func _func;
        promise<T> _promise;
        boost::optional<T> _result;
        work_item_returning(Func&& func) : _func(std::move(func)) {}
        virtual void process() override { _result = this->_func(); }
        virtual void complete() override { _promise.set_value(std::move(*_result)); }
        future<T> get_future() { return _promise.get_future(); }
    };
public:
    syscall_work_queue();
    template <typename T, typename Func>
    future<T> submit(Func func) {
        auto wi = new work_item_returning<T, Func>(std::move(func));
        auto fut = wi->get_future();
        submit_item(wi);
        return fut;
    }
private:
    void work();
    void complete();
    void submit_item(work_item* wi);

    friend class thread_pool;
};

class smp_message_queue {
    static constexpr size_t queue_length = 128;
    static constexpr size_t batch_size = 16;
    static constexpr size_t prefetch_cnt = 2;
    struct work_item;
    using lf_queue = boost::lockfree::spsc_queue<work_item*,
                            boost::lockfree::capacity<queue_length>>;
    lf_queue _pending;
    lf_queue _completed;
    struct alignas(64) {
        size_t _sent = 0;
        size_t _compl = 0;
        size_t _last_snt_batch = 0;
        size_t _last_cmpl_batch = 0;
        size_t _current_queue_length = 0;
    };
    // keep this between two structures with statistics
    // this makes sure that they have at least one cache line
    // between them, so hw prefecther will not accidentally prefetch
    // cache line used by aother cpu.
    std::vector<scollectd::registration> _collectd_regs;
    struct alignas(64) {
        size_t _received = 0;
        size_t _last_rcv_batch = 0;
    };
    struct work_item {
        virtual ~work_item() {}
        virtual future<> process() = 0;
        virtual void complete() = 0;
    };
    template <typename Func>
    struct async_work_item : work_item {
        Func _func;
        using futurator = futurize<std::result_of_t<Func()>>;
        using future_type = typename futurator::type;
        using value_type = typename future_type::value_type;
        std::experimental::optional<value_type> _result;
        std::exception_ptr _ex; // if !_result
        typename futurator::promise_type _promise; // used on local side
        async_work_item(Func&& func) : _func(std::move(func)) {}
        virtual future<> process() override {
            try {
                return futurator::apply(this->_func).then_wrapped([this] (auto&& f) {
                    try {
                        _result = f.get();
                    } catch (...) {
                        _ex = std::current_exception();
                    }
                });
            } catch (...) {
                _ex = std::current_exception();
                return make_ready_future();
            }
        }
        virtual void complete() override {
            if (_result) {
                _promise.set_value(std::move(*_result));
            } else {
                // FIXME: _ex was allocated on another cpu
                _promise.set_exception(std::move(_ex));
            }
        }
        future_type get_future() { return _promise.get_future(); }
    };
    union tx_side {
        tx_side() {}
        ~tx_side() {}
        void init() { new (&a) aa; }
        struct aa {
            std::deque<work_item*> pending_fifo;
        } a;
    } _tx;
    std::vector<work_item*> _completed_fifo;
public:
    smp_message_queue();
    template <typename Func>
    futurize_t<std::result_of_t<Func()>> submit(Func&& func) {
        auto wi = new async_work_item<Func>(std::forward<Func>(func));
        auto fut = wi->get_future();
        submit_item(wi);
        return fut;
    }
    void start(unsigned cpuid);
    template<size_t PrefetchCnt, typename Func>
    size_t process_queue(lf_queue& q, Func process);
    size_t process_incoming();
    size_t process_completions();
private:
    void work();
    void submit_item(work_item* wi);
    void respond(work_item* wi);
    void move_pending();
    void flush_request_batch();
    void flush_response_batch();

    friend class smp;
};

class thread_pool {
    uint64_t _aio_threaded_fallbacks = 0;
#ifndef HAVE_OSV
    // FIXME: implement using reactor_notifier abstraction we used for SMP
    syscall_work_queue inter_thread_wq;
    posix_thread _worker_thread;
    std::atomic<bool> _stopped = { false };
    pthread_t _notify;
public:
    thread_pool();
    ~thread_pool();
    template <typename T, typename Func>
    future<T> submit(Func func) {return inter_thread_wq.submit<T>(std::move(func));}
    uint64_t operation_count() const { return _aio_threaded_fallbacks; }
#else
public:
    template <typename T, typename Func>
    future<T> submit(Func func) { std::cout << "thread_pool not yet implemented on osv\n"; abort(); }
#endif
private:
    void work();
};

// The "reactor_backend" interface provides a method of waiting for various
// basic events on one thread. We have one implementation based on epoll and
// file-descriptors (reactor_backend_epoll) and one implementation based on
// OSv-specific file-descriptor-less mechanisms (reactor_backend_osv).
class reactor_backend {
public:
    virtual ~reactor_backend() {};
    // wait_and_process() waits for some events to become available, and
    // processes one or more of them. If block==false, it doesn't wait,
    // and just processes events that have already happened, if any.
    // After the optional wait, just before processing the events, the
    // pre_process() function is called.
    virtual bool wait_and_process() = 0;
    // Methods that allow polling on file descriptors. This will only work on
    // reactor_backend_epoll. Other reactor_backend will probably abort if
    // they are called (which is fine if no file descriptors are waited on):
    virtual future<> readable(pollable_fd_state& fd) = 0;
    virtual future<> writeable(pollable_fd_state& fd) = 0;
    virtual void forget(pollable_fd_state& fd) = 0;
    // Methods that allow polling on a reactor_notifier. This is currently
    // used only for reactor_backend_osv, but in the future it should really
    // replace the above functions.
    virtual future<> notified(reactor_notifier *n) = 0;
    // Methods for allowing sending notifications events between threads.
    virtual std::unique_ptr<reactor_notifier> make_reactor_notifier() = 0;
};

// reactor backend using file-descriptor & epoll, suitable for running on
// Linux. Can wait on multiple file descriptors, and converts other events
// (such as timers, signals, inter-thread notifications) into file descriptors
// using mechanisms like timerfd, signalfd and eventfd respectively.
class reactor_backend_epoll : public reactor_backend {
private:
    file_desc _epollfd;
    future<> get_epoll_future(pollable_fd_state& fd,
            promise<> pollable_fd_state::* pr, int event);
    void complete_epoll_event(pollable_fd_state& fd,
            promise<> pollable_fd_state::* pr, int events, int event);
    void abort_fd(pollable_fd_state& fd, std::exception_ptr ex,
            promise<> pollable_fd_state::* pr, int event);
public:
    reactor_backend_epoll();
    virtual ~reactor_backend_epoll() override { }
    virtual bool wait_and_process() override;
    virtual future<> readable(pollable_fd_state& fd) override;
    virtual future<> writeable(pollable_fd_state& fd) override;
    virtual void forget(pollable_fd_state& fd) override;
    virtual future<> notified(reactor_notifier *n) override;
    virtual std::unique_ptr<reactor_notifier> make_reactor_notifier() override;
    void abort_reader(pollable_fd_state& fd, std::exception_ptr ex);
    void abort_writer(pollable_fd_state& fd, std::exception_ptr ex);
};

#ifdef HAVE_OSV
// reactor_backend using OSv-specific features, without any file descriptors.
// This implementation cannot currently wait on file descriptors, but unlike
// reactor_backend_epoll it doesn't need file descriptors for waiting on a
// timer, for example, so file descriptors are not necessary.
class reactor_notifier_osv;
class reactor_backend_osv : public reactor_backend {
private:
    osv::newpoll::poller _poller;
    future<> get_poller_future(reactor_notifier_osv *n);
    promise<> _timer_promise;
public:
    reactor_backend_osv();
    virtual ~reactor_backend_osv() override { }
    virtual bool wait_and_process() override;
    virtual future<> readable(pollable_fd_state& fd) override;
    virtual future<> writeable(pollable_fd_state& fd) override;
    virtual void forget(pollable_fd_state& fd) override;
    virtual future<> notified(reactor_notifier *n) override;
    virtual std::unique_ptr<reactor_notifier> make_reactor_notifier() override;
    void enable_timer(clock_type::time_point when);
    friend class reactor_notifier_osv;
};
#endif /* HAVE_OSV */

enum class open_flags {
    rw = O_RDWR,
    ro = O_RDONLY,
    wo = O_WRONLY,
    create = O_CREAT,
    truncate = O_TRUNC,
    exclusive = O_EXCL,
};

inline open_flags operator|(open_flags a, open_flags b) {
    return open_flags(static_cast<unsigned int>(a) | static_cast<unsigned int>(b));
}

class reactor {
private:
    struct pollfn {
        virtual ~pollfn() {}
        virtual bool poll_and_check_more_work() = 0;
    };

public:
    class poller {
        std::unique_ptr<pollfn> _pollfn;
        class registration_task;
        class deregistration_task;
        registration_task* _registration_task;
    public:
        template <typename Func> // signature: bool ()
        explicit poller(Func&& poll_and_check_more_work)
                : _pollfn(make_pollfn(std::forward<Func>(poll_and_check_more_work))) {
            do_register();
        }
        ~poller();
        poller(poller&& x);
        poller& operator=(poller&& x);
        void do_register();
        friend class reactor;
    };

private:
    // FIXME: make _backend a unique_ptr<reactor_backend>, not a compile-time #ifdef.
#ifdef HAVE_OSV
    reactor_backend_osv _backend;
    sched::thread _timer_thread;
    sched::thread *_engine_thread;
    mutable mutex _timer_mutex;
    condvar _timer_cond;
    s64 _timer_due = 0;
#else
    reactor_backend_epoll _backend;
#endif
    std::vector<pollfn*> _pollers;
    static constexpr size_t max_aio = 128;
    promise<> _exit_promise;
    future<> _exit_future;
    unsigned _id = 0;
    bool _stopped = false;
    bool _handle_sigint = true;
    promise<std::unique_ptr<network_stack>> _network_stack_ready_promise;
    int _return = 0;
    timer_t _timer = {};
    promise<> _start_promise;
    semaphore _cpu_started;
    uint64_t _tasks_processed = 0;
    seastar::timer_set<timer<>, &timer<>::_link> _timers;
    seastar::timer_set<timer<>, &timer<>::_link>::timer_list_t _expired_timers;
    seastar::timer_set<timer<lowres_clock>, &timer<lowres_clock>::_link> _lowres_timers;
    seastar::timer_set<timer<lowres_clock>, &timer<lowres_clock>::_link>::timer_list_t _expired_lowres_timers;
    io_context_t _io_context;
    std::vector<struct ::iocb> _pending_aio;
    semaphore _io_context_available;
    uint64_t _aio_reads = 0;
    uint64_t _aio_writes = 0;
    uint64_t _fsyncs = 0;
    circular_buffer<std::unique_ptr<task>> _pending_tasks;
    circular_buffer<std::unique_ptr<task>> _at_destroy_tasks;
    size_t _task_quota;
    std::unique_ptr<network_stack> _network_stack;
    // _lowres_clock will only be created on cpu 0
    std::unique_ptr<lowres_clock> _lowres_clock;
    lowres_clock::time_point _lowres_next_timeout;
    std::experimental::optional<poller> _epoll_poller;
    const bool _reuseport;
    circular_buffer<double> _loads;
    double _load = 0;
private:
    bool flush_pending_aio();
    void abort_on_error(int ret);
    template <typename T, typename E, typename EnableFunc>
    void complete_timers(T&, E&, EnableFunc&& enable_fn);

    /**
     * Returns TRUE if all pollers allow blocking.
     *
     * @return FALSE if at least one of the blockers requires a non-blocking
     *         execution.
     */
    bool poll_once();
    template <typename Func> // signature: bool ()
    static std::unique_ptr<pollfn> make_pollfn(Func&& func);

    class signals {
    public:
        signals();
        ~signals();

        bool poll_signal();
        void handle_signal(int signo, std::function<void ()>&& handler);
        void handle_signal_once(int signo, std::function<void ()>&& handler);
        static void action(int signo, siginfo_t* siginfo, void* ignore);

    private:
        struct signal_handler {
            signal_handler(int signo, std::function<void ()>&& handler);
            std::function<void ()> _handler;
        };
        std::atomic<uint64_t> _pending_signals;
        std::unordered_map<int, signal_handler> _signal_handlers;
    };

    signals _signals;
    thread_pool _thread_pool;
    friend thread_pool;

    void run_tasks(circular_buffer<std::unique_ptr<task>>& tasks, size_t task_quota);
    bool posix_reuseport_detect();
public:
    static boost::program_options::options_description get_options_description();
    reactor();
    reactor(const reactor&) = delete;
    ~reactor();
    void operator=(const reactor&) = delete;

    void configure(boost::program_options::variables_map config);

    server_socket listen(socket_address sa, listen_options opts = {});

    future<connected_socket> connect(socket_address sa);

    pollable_fd posix_listen(socket_address sa, listen_options opts = {});

    bool posix_reuseport_available() const { return _reuseport; }

    future<pollable_fd> posix_connect(socket_address sa, socket_address local);

    future<pollable_fd, socket_address> accept(pollable_fd_state& listen_fd);

    future<size_t> read_some(pollable_fd_state& fd, void* buffer, size_t size);
    future<size_t> read_some(pollable_fd_state& fd, const std::vector<iovec>& iov);

    future<size_t> write_some(pollable_fd_state& fd, const void* buffer, size_t size);

    future<> write_all(pollable_fd_state& fd, const void* buffer, size_t size);

    future<file> open_file_dma(sstring name, open_flags flags);
    future<file> open_directory(sstring name);
    future<> make_directory(sstring name);
    future<std::experimental::optional<directory_entry_type>>  file_type(sstring name);
    future<> remove_file(sstring pathname);
    future<> rename_file(sstring old_pathname, sstring new_pathname);

    template <typename Func>
    future<io_event> submit_io(Func prepare_io);
    template <typename Func>
    future<io_event> submit_io_read(Func prepare_io);
    template <typename Func>
    future<io_event> submit_io_write(Func prepare_io);

    int run();
    void exit(int ret);
    future<> when_started() { return _start_promise.get_future(); }

    template <typename Func>
    void at_exit(Func&& func) {
        _exit_future = _exit_future.then(std::forward<Func>(func));
    }

    template <typename Func>
    void at_destroy(Func&& func) {
        _at_destroy_tasks.push_back(make_task(std::forward<Func>(func)));
    }

    void add_task(std::unique_ptr<task>&& t) { _pending_tasks.push_back(std::move(t)); }

    network_stack& net() { return *_network_stack; }
    unsigned cpu_id() const { return _id; }

    void start_epoll() {
        if (!_epoll_poller) {
            _epoll_poller = poller([this] {
                return wait_and_process();
            });
        }
    }

#ifdef HAVE_OSV
    void timer_thread_func();
    void set_timer(sched::timer &tmr, s64 t);
#endif
private:
    /**
     * Add a new "poller" - a non-blocking function returning a boolean, that
     * will be called every iteration of a main loop.
     * If it returns FALSE then reactor's main loop is forbidden to block in the
     * current iteration.
     *
     * @param fn a new "poller" function to register
     */
    void register_poller(pollfn* p);
    void unregister_poller(pollfn* p);
    void replace_poller(pollfn* old, pollfn* neww);
    struct collectd_registrations;
    collectd_registrations register_collectd_metrics();
    future<> write_all_part(pollable_fd_state& fd, const void* buffer, size_t size, size_t completed);

    bool process_io();

    void add_timer(timer<>*);
    bool queue_timer(timer<>*);
    void del_timer(timer<>*);
    void add_timer(timer<lowres_clock>*);
    bool queue_timer(timer<lowres_clock>*);
    void del_timer(timer<lowres_clock>*);

    future<> run_exit_tasks();
    void stop();
    friend class pollable_fd;
    friend class pollable_fd_state;
    friend class posix_file_impl;
    friend class blockdev_file_impl;
    friend class readable_eventfd;
    friend class timer<>;
    friend class timer<lowres_clock>;
    friend class smp;
    friend class smp_message_queue;
    friend class poller;
public:
    bool wait_and_process() {
        return _backend.wait_and_process();
    }

    future<> readable(pollable_fd_state& fd) {
        return _backend.readable(fd);
    }
    future<> writeable(pollable_fd_state& fd) {
        return _backend.writeable(fd);
    }
    void forget(pollable_fd_state& fd) {
        _backend.forget(fd);
    }
    future<> notified(reactor_notifier *n) {
        return _backend.notified(n);
    }
    void abort_reader(pollable_fd_state& fd, std::exception_ptr ex) {
        return _backend.abort_reader(fd, std::move(ex));
    }
    void abort_writer(pollable_fd_state& fd, std::exception_ptr ex) {
        return _backend.abort_writer(fd, std::move(ex));
    }
    void enable_timer(clock_type::time_point when);
    std::unique_ptr<reactor_notifier> make_reactor_notifier() {
        return _backend.make_reactor_notifier();
    }
};

template <typename Func> // signature: bool ()
inline
std::unique_ptr<reactor::pollfn>
reactor::make_pollfn(Func&& func) {
    struct the_pollfn : pollfn {
        the_pollfn(Func&& func) : func(std::forward<Func>(func)) {}
        Func func;
        virtual bool poll_and_check_more_work() override {
            return func();
        }
    };
    return std::make_unique<the_pollfn>(std::forward<Func>(func));
}

extern __thread reactor* local_engine;
extern __thread size_t task_quota;

inline reactor& engine() {
    return *local_engine;
}

class smp {
#if HAVE_DPDK
    using thread_adaptor = std::function<void ()>;
#else
    using thread_adaptor = posix_thread;
#endif
    static std::vector<thread_adaptor> _threads;
    static smp_message_queue** _qs;
    static std::thread::id _tmain;

    template <typename Func>
    using returns_future = is_future<std::result_of_t<Func()>>;
    template <typename Func>
    using returns_void = std::is_same<std::result_of_t<Func()>, void>;
public:
    static boost::program_options::options_description get_options_description();
    static void configure(boost::program_options::variables_map vm);
    static void cleanup();
    static void join_all();
    static bool main_thread() { return std::this_thread::get_id() == _tmain; }

    /// Runs a function on a remote core.
    ///
    /// \param t designates the core to run the function on (may be a remote
    ///          core or the local core).
    /// \param func a callable to run on core \c t.  If \c func is a temporary object,
    ///          its lifetime will be extended by moving it.  If @func is a reference,
    ///          the caller must guarantee that it will survive the call.
    /// \return whatever \c func returns, as a future<> (if \c func does not return a future,
    ///         submit_to() will wrap it in a future<>).
    template <typename Func>
    static futurize_t<std::result_of_t<Func()>> submit_to(unsigned t, Func&& func) {
        using ret_type = std::result_of_t<Func()>;
        if (t == engine().cpu_id()) {
            if (!is_future<ret_type>::value) {
                // Non-deferring function, so don't worry about func lifetime
                return futurize<ret_type>::apply(std::forward<Func>(func));
            } else if (std::is_lvalue_reference<Func>::value) {
                // func is an lvalue, so caller worries about its lifetime
                return futurize<ret_type>::apply(func);
            } else {
                // Deferring call on rvalue function, make sure to preserve it across call
                auto w = std::make_unique<Func>(std::move(func));
                auto ret = futurize<ret_type>::apply(*w);
                return ret.finally([w = std::move(w)] {});
            }
        } else {
            return _qs[t][engine().cpu_id()].submit(std::forward<Func>(func));
        }
    }
    static bool poll_queues() {
        size_t got = 0;
        for (unsigned i = 0; i < count; i++) {
            if (engine().cpu_id() != i) {
                auto& rxq = _qs[engine().cpu_id()][i];
                rxq.flush_response_batch();
                got += rxq.process_incoming();
                auto& txq = _qs[i][engine()._id];
                txq.flush_request_batch();
                got += txq.process_completions();
            }
        }
        return got != 0;
    }
    static boost::integer_range<unsigned> all_cpus() {
        return boost::irange(0u, count);
    }
private:
    static void start_all_queues();
    static void pin(unsigned cpu_id);
    static void allocate_reactor();
public:
    static unsigned count;
};

inline
pollable_fd_state::~pollable_fd_state() {
    engine().forget(*this);
}

inline
size_t iovec_len(const iovec* begin, size_t len)
{
    size_t ret = 0;
    auto end = begin + len;
    while (begin != end) {
        ret += begin++->iov_len;
    }
    return ret;
}

inline
future<pollable_fd, socket_address>
reactor::accept(pollable_fd_state& listenfd) {
    return readable(listenfd).then([this, &listenfd] () mutable {
        socket_address sa;
        socklen_t sl = sizeof(&sa.u.sas);
        file_desc fd = listenfd.fd.accept(sa.u.sa, sl, SOCK_NONBLOCK | SOCK_CLOEXEC);
        pollable_fd pfd(std::move(fd), pollable_fd::speculation(EPOLLOUT));
        return make_ready_future<pollable_fd, socket_address>(std::move(pfd), std::move(sa));
    });
}

inline
future<size_t>
reactor::read_some(pollable_fd_state& fd, void* buffer, size_t len) {
    return readable(fd).then([this, &fd, buffer, len] () mutable {
        auto r = fd.fd.read(buffer, len);
        if (!r) {
            return read_some(fd, buffer, len);
        }
        if (size_t(*r) == len) {
            fd.speculate_epoll(EPOLLIN);
        }
        return make_ready_future<size_t>(*r);
    });
}

inline
future<size_t>
reactor::read_some(pollable_fd_state& fd, const std::vector<iovec>& iov) {
    return readable(fd).then([this, &fd, iov = iov] () mutable {
        ::msghdr mh = {};
        mh.msg_iov = &iov[0];
        mh.msg_iovlen = iov.size();
        auto r = fd.fd.recvmsg(&mh, 0);
        if (!r) {
            return read_some(fd, iov);
        }
        if (size_t(*r) == iovec_len(iov)) {
            fd.speculate_epoll(EPOLLIN);
        }
        return make_ready_future<size_t>(*r);
    });
}

inline
future<size_t>
reactor::write_some(pollable_fd_state& fd, const void* buffer, size_t len) {
    return writeable(fd).then([this, &fd, buffer, len] () mutable {
        auto r = fd.fd.send(buffer, len, MSG_NOSIGNAL);
        if (!r) {
            return write_some(fd, buffer, len);
        }
        if (size_t(*r) == len) {
            fd.speculate_epoll(EPOLLOUT);
        }
        return make_ready_future<size_t>(*r);
    });
}

inline
future<>
reactor::write_all_part(pollable_fd_state& fd, const void* buffer, size_t len, size_t completed) {
    if (completed == len) {
        return make_ready_future<>();
    } else {
        return write_some(fd, static_cast<const char*>(buffer) + completed, len - completed).then(
                [&fd, buffer, len, completed, this] (size_t part) mutable {
            return write_all_part(fd, buffer, len, completed + part);
        });
    }
}

inline
future<>
reactor::write_all(pollable_fd_state& fd, const void* buffer, size_t len) {
    assert(len);
    return write_all_part(fd, buffer, len, 0);
}

template <typename T, typename E, typename EnableFunc>
void reactor::complete_timers(T& timers, E& expired_timers, EnableFunc&& enable_fn) {
    expired_timers = timers.expire(timers.now());
    for (auto& t : expired_timers) {
        t._expired = true;
    }
    while (!expired_timers.empty()) {
        auto t = &*expired_timers.begin();
        expired_timers.pop_front();
        t->_queued = false;
        if (t->_armed) {
            t->_armed = false;
            if (t->_period) {
                t->readd_periodic();
            }
            t->_callback();
        }
    }
    enable_fn();
}

inline
future<size_t> pollable_fd::read_some(char* buffer, size_t size) {
    return engine().read_some(*_s, buffer, size);
}

inline
future<size_t> pollable_fd::read_some(uint8_t* buffer, size_t size) {
    return engine().read_some(*_s, buffer, size);
}

inline
future<size_t> pollable_fd::read_some(const std::vector<iovec>& iov) {
    return engine().read_some(*_s, iov);
}

inline
future<> pollable_fd::write_all(const char* buffer, size_t size) {
    return engine().write_all(*_s, buffer, size);
}

inline
future<> pollable_fd::write_all(const uint8_t* buffer, size_t size) {
    return engine().write_all(*_s, buffer, size);
}

inline
future<size_t> pollable_fd::write_some(net::packet& p) {
    return engine().writeable(*_s).then([this, &p] () mutable {
        static_assert(offsetof(iovec, iov_base) == offsetof(net::fragment, base) &&
            sizeof(iovec::iov_base) == sizeof(net::fragment::base) &&
            offsetof(iovec, iov_len) == offsetof(net::fragment, size) &&
            sizeof(iovec::iov_len) == sizeof(net::fragment::size) &&
            alignof(iovec) == alignof(net::fragment) &&
            sizeof(iovec) == sizeof(net::fragment)
            , "net::fragment and iovec should be equivalent");

        iovec* iov = reinterpret_cast<iovec*>(p.fragment_array());
        auto r = get_file_desc().writev(iov, p.nr_frags());
        if (!r) {
            return write_some(p);
        }
        if (size_t(*r) == p.len()) {
            _s->speculate_epoll(EPOLLOUT);
        }
        return make_ready_future<size_t>(*r);
    });
}

inline
future<> pollable_fd::write_all(net::packet& p) {
    return write_some(p).then([this, &p] (size_t size) {
        if (p.len() == size) {
            return make_ready_future<>();
        }
        p.trim_front(size);
        return write_all(p);
    });
}

inline
future<> pollable_fd::readable() {
    return engine().readable(*_s);
}

inline
future<> pollable_fd::writeable() {
    return engine().writeable(*_s);
}

inline
void
pollable_fd::abort_reader(std::exception_ptr ex) {
    engine().abort_reader(*_s, std::move(ex));
}

inline
void
pollable_fd::abort_writer(std::exception_ptr ex) {
    engine().abort_writer(*_s, std::move(ex));
}

inline
future<pollable_fd, socket_address> pollable_fd::accept() {
    return engine().accept(*_s);
}

inline
future<size_t> pollable_fd::recvmsg(struct msghdr *msg) {
    return engine().readable(*_s).then([this, msg] {
        auto r = get_file_desc().recvmsg(msg, 0);
        if (!r) {
            return recvmsg(msg);
        }
        // We always speculate here to optimize for throughput in a workload
        // with multiple outstanding requests. This way the caller can consume
        // all messages without resorting to epoll. However this adds extra
        // recvmsg() call when we hit the empty queue condition, so it may
        // hurt request-response workload in which the queue is empty when we
        // initially enter recvmsg(). If that turns out to be a problem, we can
        // improve speculation by using recvmmsg().
        _s->speculate_epoll(EPOLLIN);
        return make_ready_future<size_t>(*r);
    });
};

inline
future<size_t> pollable_fd::sendmsg(struct msghdr* msg) {
    return engine().writeable(*_s).then([this, msg] () mutable {
        auto r = get_file_desc().sendmsg(msg, 0);
        if (!r) {
            return sendmsg(msg);
        }
        // For UDP this will always speculate. We can't know if there's room
        // or not, but most of the time there should be so the cost of mis-
        // speculation is amortized.
        if (size_t(*r) == iovec_len(msg->msg_iov, msg->msg_iovlen)) {
            _s->speculate_epoll(EPOLLOUT);
        }
        return make_ready_future<size_t>(*r);
    });
}

inline
future<size_t> pollable_fd::sendto(socket_address addr, const void* buf, size_t len) {
    return engine().writeable(*_s).then([this, buf, len, addr] () mutable {
        auto r = get_file_desc().sendto(addr, buf, len, 0);
        if (!r) {
            return sendto(std::move(addr), buf, len);
        }
        // See the comment about speculation in sendmsg().
        if (size_t(*r) == len) {
            _s->speculate_epoll(EPOLLOUT);
        }
        return make_ready_future<size_t>(*r);
    });
}

template <typename Clock>
inline
timer<Clock>::timer(callback_t&& callback) : _callback(std::move(callback)) {
}

template <typename Clock>
inline
timer<Clock>::~timer() {
    if (_queued) {
        engine().del_timer(this);
    }
}

template <typename Clock>
inline
void timer<Clock>::set_callback(callback_t&& callback) {
    _callback = std::move(callback);
}

template <typename Clock>
inline
void timer<Clock>::arm_state(time_point until, std::experimental::optional<duration> period) {
    assert(!_armed);
    _period = period;
    _armed = true;
    _expired = false;
    _expiry = until;
    _queued = true;
}

template <typename Clock>
inline
void timer<Clock>::arm(time_point until, std::experimental::optional<duration> period) {
    arm_state(until, period);
    engine().add_timer(this);
}

template <typename Clock>
inline
void timer<Clock>::rearm(time_point until, std::experimental::optional<duration> period) {
    if (_armed) {
        cancel();
    }
    arm(until, period);
}

template <typename Clock>
inline
void timer<Clock>::arm(duration delta) {
    return arm(Clock::now() + delta);
}

template <typename Clock>
inline
void timer<Clock>::arm_periodic(duration delta) {
    arm(Clock::now() + delta, {delta});
}

template <typename Clock>
inline
void timer<Clock>::readd_periodic() {
    arm_state(Clock::now() + _period.value(), {_period.value()});
    engine().queue_timer(this);
}

template <typename Clock>
inline
bool timer<Clock>::cancel() {
    if (!_armed) {
        return false;
    }
    _armed = false;
    if (_queued) {
        engine().del_timer(this);
        _queued = false;
    }
    return true;
}

template <typename Clock>
inline
typename timer<Clock>::time_point timer<Clock>::get_timeout() {
    return _expiry;
}

inline
input_stream<char>
connected_socket::input() {
    return _csi->input();
}

inline
output_stream<char>
connected_socket::output() {
    return _csi->output();
}

inline
void
connected_socket::shutdown_input() {
    return _csi->shutdown_input();
}

inline
void
connected_socket::shutdown_output() {
    return _csi->shutdown_output();
}

#endif /* REACTOR_HH_ */