src/openvpn/event.c

/*
 *  OpenVPN -- An application to securely tunnel IP networks
 *             over a single TCP/UDP port, with support for SSL/TLS-based
 *             session authentication and key exchange,
 *             packet encryption, packet authentication, and
 *             packet compression.
 *
 *  Copyright (C) 2002-2022 OpenVPN Inc <sales@openvpn.net>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  as published by the Free Software Foundation.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#ifdef HAVE_CONFIG_H
#include "config.h"
#elif defined(_MSC_VER)
#include "config-msvc.h"
#endif

#include "syshead.h"

#include "buffer.h"
#include "error.h"
#include "integer.h"
#include "event.h"
#include "fdmisc.h"

#include "memdbg.h"

/*
 * Some OSes will prefer select() over poll()
 * when both are available.
 */
#if defined(TARGET_DARWIN)
#define SELECT_PREFERRED_OVER_POLL
#endif

/*
 * All non-windows OSes are assumed to have select()
 */
#ifdef _WIN32
#define SELECT 0
#else
#define SELECT 1
#endif

/*
 * This should be set to the highest file descriptor
 * which can be used in one of the FD_ macros.
 */
#ifdef FD_SETSIZE
#define SELECT_MAX_FDS FD_SETSIZE
#else
#define SELECT_MAX_FDS 256
#endif

static inline int
tv_to_ms_timeout(const struct timeval *tv)
{
    if (tv->tv_sec == 0 && tv->tv_usec == 0)
    {
        return 0;
    }
    else
    {
        return max_int(tv->tv_sec * 1000 + (tv->tv_usec + 500) / 1000, 1);
    }
}

#ifdef _WIN32

struct we_set
{
    struct event_set_functions func;
    bool fast;
    HANDLE *events;
    struct event_set_return *esr;
    int n_events;
    int capacity;
};

static inline void
we_set_event(struct we_set *wes, int i, event_t event, unsigned int rwflags, void *arg)
{
    ASSERT(i >= 0 && i < wes->capacity);

    if (rwflags == EVENT_READ)
    {
        ASSERT(event->read != NULL);
        wes->events[i] = event->read;
    }
    else if (rwflags == EVENT_WRITE)
    {
        ASSERT(event->write != NULL);
        wes->events[i] = event->write;
    }
    else
    {
        msg(M_FATAL, "fatal error in we_set_events: rwflags=%d", rwflags);
    }

    wes->esr[i].rwflags = rwflags;
    wes->esr[i].arg = arg;
}

static inline bool
we_append_event(struct we_set *wes, event_t event, unsigned int rwflags, void *arg)
{
    if (rwflags & EVENT_WRITE)
    {
        if (wes->n_events < wes->capacity)
        {
            we_set_event(wes, wes->n_events, event, EVENT_WRITE, arg);
            ++wes->n_events;
        }
        else
        {
            return false;
        }
    }
    if (rwflags & EVENT_READ)
    {
        if (wes->n_events < wes->capacity)
        {
            we_set_event(wes, wes->n_events, event, EVENT_READ, arg);
            ++wes->n_events;
        }
        else
        {
            return false;
        }
    }
    return true;
}

static void
we_del_event(struct we_set *wes, event_t event)
{
    int i, j = 0;
    const int len = wes->n_events;

    for (i = 0; i < len; ++i)
    {
        const HANDLE h = wes->events[i];
        if (h == event->read || h == event->write)
        {
            --wes->n_events;
        }
        else
        {
            if (i != j)
            {
                wes->events[j] = wes->events[i];
                wes->esr[j] = wes->esr[i];
            }
            ++j;
        }
    }
}

static void
we_del_index(struct we_set *wes, int index)
{
    int i;
    ASSERT(index >= 0 && index < wes->n_events);
    for (i = index; i < wes->n_events - 1; ++i)
    {
        wes->events[i] = wes->events[i+1];
        wes->esr[i] = wes->esr[i+1];
    }
    --wes->n_events;
}

static void
we_get_rw_indices(struct we_set *wes, event_t event, int *ri, int *wi)
{
    int i;
    *ri = *wi = -1;
    for (i = 0; i < wes->n_events; ++i)
    {
        const HANDLE h = wes->events[i];
        if (h == event->read)
        {
            ASSERT(*ri == -1);
            *ri = i;
        }
        else if (h == event->write)
        {
            ASSERT(*wi == -1);
            *wi = i;
        }
    }
}

static void
we_free(struct event_set *es)
{
    struct we_set *wes = (struct we_set *) es;
    free(wes->events);
    free(wes->esr);
    free(wes);
}

static void
we_reset(struct event_set *es)
{
    struct we_set *wes = (struct we_set *) es;
    ASSERT(wes->fast);
    wes->n_events = 0;
}

static void
we_del(struct event_set *es, event_t event)
{
    struct we_set *wes = (struct we_set *) es;
    ASSERT(!wes->fast);
    we_del_event(wes, event);
}

static void
we_ctl(struct event_set *es, event_t event, unsigned int rwflags, void *arg)
{
    struct we_set *wes = (struct we_set *) es;

    dmsg(D_EVENT_WAIT, "WE_CTL n=%d ev=%p rwflags=0x%04x arg=" ptr_format,
         wes->n_events,
         event,
         rwflags,
         (ptr_type)arg);

    if (wes->fast)
    {
        if (!we_append_event(wes, event, rwflags, arg))
        {
            goto err;
        }
    }
    else
    {
        int ri, wi;
        int one = -1;
        int n = 0;

        we_get_rw_indices(wes, event, &ri, &wi);
        if (wi >= 0)
        {
            one = wi;
            ++n;
        }
        if (ri >= 0)
        {
            one = ri;
            ++n;
        }
        switch (rwflags)
        {
            case 0:
                switch (n)
                {
                    case 0:
                        break;

                    case 1:
                        we_del_index(wes, one);
                        break;

                    case 2:
                        we_del_event(wes, event);
                        break;

                    default:
                        ASSERT(0);
                }
                break;

            case EVENT_READ:
                switch (n)
                {
                    case 0:
                        if (!we_append_event(wes, event, EVENT_READ, arg))
                        {
                            goto err;
                        }
                        break;

                    case 1:
                        we_set_event(wes, one, event, EVENT_READ, arg);
                        break;

                    case 2:
                        we_del_index(wes, wi);
                        break;

                    default:
                        ASSERT(0);
                }
                break;

            case EVENT_WRITE:
                switch (n)
                {
                    case 0:
                        if (!we_append_event(wes, event, EVENT_WRITE, arg))
                        {
                            goto err;
                        }
                        break;

                    case 1:
                        we_set_event(wes, one, event, EVENT_WRITE, arg);
                        break;

                    case 2:
                        we_del_index(wes, ri);
                        break;

                    default:
                        ASSERT(0);
                }
                break;

            case EVENT_READ|EVENT_WRITE:
                switch (n)
                {
                    case 0:
                        if (!we_append_event(wes, event, EVENT_READ|EVENT_WRITE, arg))
                        {
                            goto err;
                        }
                        break;

                    case 1:
                        if (ri == -1)
                        {
                            ASSERT(wi != -1);
                            if (!we_append_event(wes, event, EVENT_READ, arg))
                            {
                                goto err;
                            }
                        }
                        else if (wi == -1)
                        {
                            if (!we_append_event(wes, event, EVENT_WRITE, arg))
                            {
                                goto err;
                            }
                        }
                        else
                        {
                            ASSERT(0);
                        }
                        break;

                    case 2:
                        break;

                    default:
                        ASSERT(0);
                }
                break;

            default:
                msg(M_FATAL, "fatal error in we_ctl: rwflags=%d", rwflags);
        }
    }
    return;

err:
    msg(D_EVENT_ERRORS, "Error: Windows resource limit WSA_MAXIMUM_WAIT_EVENTS (%d) has been exceeded", WSA_MAXIMUM_WAIT_EVENTS);
}

static int
we_wait(struct event_set *es, const struct timeval *tv, struct event_set_return *out, int outlen)
{
    struct we_set *wes = (struct we_set *) es;
    const int timeout = tv_to_ms_timeout(tv);
    DWORD status;

    dmsg(D_EVENT_WAIT, "WE_WAIT enter n=%d to=%d", wes->n_events, timeout);

#ifdef ENABLE_DEBUG
    if (check_debug_level(D_EVENT_WAIT))
    {
        int i;
        for (i = 0; i < wes->n_events; ++i)
        {
            dmsg(D_EVENT_WAIT, "[%d] ev=%p rwflags=0x%04x arg=" ptr_format,
                 i,
                 wes->events[i],
                 wes->esr[i].rwflags,
                 (ptr_type)wes->esr[i].arg);
        }
    }
#endif

    /*
     * First poll our event list with 0 timeout
     */
    status = WSAWaitForMultipleEvents(
        (DWORD) wes->n_events,
        wes->events,
        FALSE,
        (DWORD) 0,
        FALSE);

    /*
     * If at least one event is already set, we must
     * individually poll the whole list.
     */
    if (status >= WSA_WAIT_EVENT_0 && status < WSA_WAIT_EVENT_0 + (DWORD) wes->n_events)
    {
        int i;
        int j = 0;
        for (i = 0; i < wes->n_events; ++i)
        {
            if (j >= outlen)
            {
                break;
            }
            if (WaitForSingleObject(wes->events[i], 0) == WAIT_OBJECT_0)
            {
                *out = wes->esr[i];
                dmsg(D_EVENT_WAIT, "WE_WAIT leave [%d,%d] rwflags=0x%04x arg=" ptr_format,
                     i, j, out->rwflags, (ptr_type)out->arg);
                ++j;
                ++out;
            }
        }
        return j;
    }
    else
    {
        /*
         * If caller specified timeout > 0, we know at this point
         * that no events are set, so wait only for the first event
         * (or timeout) and return at most one event_set_return object.
         *
         * If caller specified timeout == 0, the second call to
         * WSAWaitForMultipleEvents would be redundant -- just
         * return 0 indicating timeout.
         */
        if (timeout > 0)
        {
            status = WSAWaitForMultipleEvents(
                (DWORD) wes->n_events,
                wes->events,
                FALSE,
                (DWORD) timeout,
                FALSE);
        }

        if (outlen >= 1 && status >= WSA_WAIT_EVENT_0 && status < WSA_WAIT_EVENT_0 + (DWORD) wes->n_events)
        {
            *out = wes->esr[status - WSA_WAIT_EVENT_0];
            dmsg(D_EVENT_WAIT, "WE_WAIT leave rwflags=0x%04x arg=" ptr_format,
                 out->rwflags, (ptr_type)out->arg);
            return 1;
        }
        else if (status == WSA_WAIT_TIMEOUT)
        {
            return 0;
        }
        else
        {
            return -1;
        }
    }
}

static struct event_set *
we_init(int *maxevents, unsigned int flags)
{
    struct we_set *wes;

    dmsg(D_EVENT_WAIT, "WE_INIT maxevents=%d flags=0x%08x", *maxevents, flags);

    ALLOC_OBJ_CLEAR(wes, struct we_set);

    /* set dispatch functions */
    wes->func.free = we_free;
    wes->func.reset = we_reset;
    wes->func.del = we_del;
    wes->func.ctl = we_ctl;
    wes->func.wait = we_wait;

    if (flags & EVENT_METHOD_FAST)
    {
        wes->fast = true;
    }
    wes->n_events = 0;

    /* Figure our event capacity */
    ASSERT(*maxevents > 0);
    wes->capacity = min_int(*maxevents * 2, WSA_MAXIMUM_WAIT_EVENTS);
    *maxevents = min_int(*maxevents, WSA_MAXIMUM_WAIT_EVENTS);

    /* Allocate space for Win32 event handles */
    ALLOC_ARRAY_CLEAR(wes->events, HANDLE, wes->capacity);

    /* Allocate space for event_set_return objects */
    ALLOC_ARRAY_CLEAR(wes->esr, struct event_set_return, wes->capacity);

    dmsg(D_EVENT_WAIT, "WE_INIT maxevents=%d capacity=%d",
         *maxevents, wes->capacity);

    return (struct event_set *) wes;
}

#endif /* _WIN32 */

#if EPOLL

struct ep_set
{
    struct event_set_functions func;
    bool fast;
    int epfd;
    int maxevents;
    struct epoll_event *events;
};

static void
ep_free(struct event_set *es)
{
    struct ep_set *eps = (struct ep_set *) es;
    close(eps->epfd);
    free(eps->events);
    free(eps);
}

static void
ep_reset(struct event_set *es)
{
    const struct ep_set *eps = (struct ep_set *) es;
    ASSERT(eps->fast);
}

static void
ep_del(struct event_set *es, event_t event)
{
    struct epoll_event ev;
    struct ep_set *eps = (struct ep_set *) es;

    dmsg(D_EVENT_WAIT, "EP_DEL ev=%d", (int)event);

    ASSERT(!eps->fast);
    CLEAR(ev);
    epoll_ctl(eps->epfd, EPOLL_CTL_DEL, event, &ev);
}

static void
ep_ctl(struct event_set *es, event_t event, unsigned int rwflags, void *arg)
{
    struct ep_set *eps = (struct ep_set *) es;
    struct epoll_event ev;

    CLEAR(ev);

    ev.data.ptr = arg;
    if (rwflags & EVENT_READ)
    {
        ev.events |= EPOLLIN;
    }
    if (rwflags & EVENT_WRITE)
    {
        ev.events |= EPOLLOUT;
    }

    dmsg(D_EVENT_WAIT, "EP_CTL fd=%d rwflags=0x%04x ev=0x%08x arg=" ptr_format,
         (int)event,
         rwflags,
         (unsigned int)ev.events,
         (ptr_type)ev.data.ptr);

    if (epoll_ctl(eps->epfd, EPOLL_CTL_MOD, event, &ev) < 0)
    {
        if (errno == ENOENT)
        {
            if (epoll_ctl(eps->epfd, EPOLL_CTL_ADD, event, &ev) < 0)
            {
                msg(M_ERR, "EVENT: epoll_ctl EPOLL_CTL_ADD failed, sd=%d", (int)event);
            }
        }
        else
        {
            msg(M_ERR, "EVENT: epoll_ctl EPOLL_CTL_MOD failed, sd=%d", (int)event);
        }
    }
}

static int
ep_wait(struct event_set *es, const struct timeval *tv, struct event_set_return *out, int outlen)
{
    struct ep_set *eps = (struct ep_set *) es;
    int stat;

    if (outlen > eps->maxevents)
    {
        outlen = eps->maxevents;
    }

    stat = epoll_wait(eps->epfd, eps->events, outlen, tv_to_ms_timeout(tv));
    ASSERT(stat <= outlen);

    if (stat > 0)
    {
        int i;
        const struct epoll_event *ev = eps->events;
        struct event_set_return *esr = out;
        for (i = 0; i < stat; ++i)
        {
            esr->rwflags = 0;
            if (ev->events & (EPOLLIN|EPOLLPRI|EPOLLERR|EPOLLHUP))
            {
                esr->rwflags |= EVENT_READ;
            }
            if (ev->events & EPOLLOUT)
            {
                esr->rwflags |= EVENT_WRITE;
            }
            esr->arg = ev->data.ptr;
            dmsg(D_EVENT_WAIT, "EP_WAIT[%d] rwflags=0x%04x ev=0x%08x arg=" ptr_format,
                 i, esr->rwflags, ev->events, (ptr_type)ev->data.ptr);
            ++ev;
            ++esr;
        }
    }
    return stat;
}

static struct event_set *
ep_init(int *maxevents, unsigned int flags)
{
    struct ep_set *eps;
    int fd;

    dmsg(D_EVENT_WAIT, "EP_INIT maxevents=%d flags=0x%08x", *maxevents, flags);

    /* open epoll file descriptor */
    fd = epoll_create(*maxevents);
    if (fd < 0)
    {
        return NULL;
    }

    set_cloexec(fd);

    ALLOC_OBJ_CLEAR(eps, struct ep_set);

    /* set dispatch functions */
    eps->func.free = ep_free;
    eps->func.reset = ep_reset;
    eps->func.del = ep_del;
    eps->func.ctl = ep_ctl;
    eps->func.wait = ep_wait;

    /* fast method ("sort of") corresponds to epoll one-shot */
    if (flags & EVENT_METHOD_FAST)
    {
        eps->fast = true;
    }

    /* allocate space for epoll_wait return */
    ASSERT(*maxevents > 0);
    eps->maxevents = *maxevents;
    ALLOC_ARRAY_CLEAR(eps->events, struct epoll_event, eps->maxevents);

    /* set epoll control fd */
    eps->epfd = fd;

    return (struct event_set *) eps;
}
#endif /* EPOLL */

#if POLL

struct po_set
{
    struct event_set_functions func;
    bool fast;
    struct pollfd *events;
    void **args;
    int n_events;
    int capacity;
};

static void
po_free(struct event_set *es)
{
    struct po_set *pos = (struct po_set *) es;
    free(pos->events);
    free(pos->args);
    free(pos);
}

static void
po_reset(struct event_set *es)
{
    struct po_set *pos = (struct po_set *) es;
    ASSERT(pos->fast);
    pos->n_events = 0;
}

static void
po_del(struct event_set *es, event_t event)
{
    struct po_set *pos = (struct po_set *) es;
    int i;

    dmsg(D_EVENT_WAIT, "PO_DEL ev=%d", (int)event);

    ASSERT(!pos->fast);
    for (i = 0; i < pos->n_events; ++i)
    {
        if (pos->events[i].fd == event)
        {
            int j;
            for (j = i; j < pos->n_events - 1; ++j)
            {
                pos->events[j] = pos->events[j+1];
                pos->args[j] = pos->args[j+1];
            }
            --pos->n_events;
            break;
        }
    }
}

static inline void
po_set_pollfd_events(struct pollfd *pfdp, unsigned int rwflags)
{
    pfdp->events = 0;
    if (rwflags & EVENT_WRITE)
    {
        pfdp->events |= POLLOUT;
    }
    if (rwflags & EVENT_READ)
    {
        pfdp->events |= (POLLIN|POLLPRI);
    }
}

static inline bool
po_append_event(struct po_set *pos, event_t event, unsigned int rwflags, void *arg)
{
    if (pos->n_events < pos->capacity)
    {
        struct pollfd *pfdp = &pos->events[pos->n_events];
        pfdp->fd = event;
        pos->args[pos->n_events] = arg;
        po_set_pollfd_events(pfdp, rwflags);
        ++pos->n_events;
        return true;
    }
    else
    {
        return false;
    }
}

static void
po_ctl(struct event_set *es, event_t event, unsigned int rwflags, void *arg)
{
    struct po_set *pos = (struct po_set *) es;

    dmsg(D_EVENT_WAIT, "PO_CTL rwflags=0x%04x ev=%d arg=" ptr_format,
         rwflags, (int)event, (ptr_type)arg);

    if (pos->fast)
    {
        if (!po_append_event(pos, event, rwflags, arg))
        {
            goto err;
        }
    }
    else
    {
        int i;
        for (i = 0; i < pos->n_events; ++i)
        {
            struct pollfd *pfdp = &pos->events[i];
            if (pfdp->fd == event)
            {
                pos->args[i] = arg;
                po_set_pollfd_events(pfdp, rwflags);
                goto done;
            }
        }
        if (!po_append_event(pos, event, rwflags, arg))
        {
            goto err;
        }
    }

done:
    return;

err:
    msg(D_EVENT_ERRORS, "Error: poll: too many I/O wait events");
}

static int
po_wait(struct event_set *es, const struct timeval *tv, struct event_set_return *out, int outlen)
{
    struct po_set *pos = (struct po_set *) es;
    int stat;

    stat = poll(pos->events, pos->n_events, tv_to_ms_timeout(tv));

    ASSERT(stat <= pos->n_events);

    if (stat > 0)
    {
        int i, j = 0;
        const struct pollfd *pfdp = pos->events;
        for (i = 0; i < pos->n_events && j < outlen; ++i)
        {
            if (pfdp->revents & (POLLIN|POLLPRI|POLLERR|POLLHUP|POLLOUT))
            {
                out->rwflags = 0;
                if (pfdp->revents & (POLLIN|POLLPRI|POLLERR|POLLHUP))
                {
                    out->rwflags |= EVENT_READ;
                }
                if (pfdp->revents & POLLOUT)
                {
                    out->rwflags |= EVENT_WRITE;
                }
                out->arg = pos->args[i];
                dmsg(D_EVENT_WAIT, "PO_WAIT[%d,%d] fd=%d rev=0x%08x rwflags=0x%04x arg=" ptr_format " %s",
                     i, j, pfdp->fd, pfdp->revents, out->rwflags, (ptr_type)out->arg, pos->fast ? "" : "[scalable]");
                ++out;
                ++j;
            }
            else if (pfdp->revents)
            {
                msg(D_EVENT_ERRORS, "Error: poll: unknown revents=0x%04x", (unsigned int)pfdp->revents);
            }
            ++pfdp;
        }
        return j;
    }
    return stat;
}

static struct event_set *
po_init(int *maxevents, unsigned int flags)
{
    struct po_set *pos;

    dmsg(D_EVENT_WAIT, "PO_INIT maxevents=%d flags=0x%08x", *maxevents, flags);

    ALLOC_OBJ_CLEAR(pos, struct po_set);

    /* set dispatch functions */
    pos->func.free = po_free;
    pos->func.reset = po_reset;
    pos->func.del = po_del;
    pos->func.ctl = po_ctl;
    pos->func.wait = po_wait;

    if (flags & EVENT_METHOD_FAST)
    {
        pos->fast = true;
    }

    pos->n_events = 0;

    /* Figure our event capacity */
    ASSERT(*maxevents > 0);
    pos->capacity = *maxevents;

    /* Allocate space for pollfd structures to be passed to poll() */
    ALLOC_ARRAY_CLEAR(pos->events, struct pollfd, pos->capacity);

    /* Allocate space for event_set_return objects */
    ALLOC_ARRAY_CLEAR(pos->args, void *, pos->capacity);

    return (struct event_set *) pos;
}
#endif /* POLL */

#if SELECT

struct se_set
{
    struct event_set_functions func;
    bool fast;
    fd_set readfds;
    fd_set writefds;
    void **args; /* allocated to capacity size */
    int maxfd;  /* largest fd seen so far, always < capacity */
    int capacity; /* fixed largest fd + 1 */
};

static void
se_free(struct event_set *es)
{
    struct se_set *ses = (struct se_set *) es;
    free(ses->args);
    free(ses);
}

static void
se_reset(struct event_set *es)
{
    struct se_set *ses = (struct se_set *) es;
    int i;
    ASSERT(ses->fast);

    dmsg(D_EVENT_WAIT, "SE_RESET");

    FD_ZERO(&ses->readfds);
    FD_ZERO(&ses->writefds);
    for (i = 0; i <= ses->maxfd; ++i)
    {
        ses->args[i] = NULL;
    }
    ses->maxfd = -1;
}

static void
se_del(struct event_set *es, event_t event)
{
    struct se_set *ses = (struct se_set *) es;
    ASSERT(!ses->fast);

    dmsg(D_EVENT_WAIT, "SE_DEL ev=%d", (int)event);

    if (event >= 0 && event < ses->capacity)
    {
        FD_CLR(event, &ses->readfds);
        FD_CLR(event, &ses->writefds);
        ses->args[event] = NULL;
    }
    else
    {
        msg(D_EVENT_ERRORS, "Error: select/se_del: too many I/O wait events");
    }
    return;
}

static void
se_ctl(struct event_set *es, event_t event, unsigned int rwflags, void *arg)
{
    struct se_set *ses = (struct se_set *) es;

    dmsg(D_EVENT_WAIT, "SE_CTL rwflags=0x%04x ev=%d fast=%d cap=%d maxfd=%d arg=" ptr_format,
         rwflags, (int)event, (int)ses->fast, ses->capacity, ses->maxfd, (ptr_type)arg);

    if (event >= 0 && event < ses->capacity)
    {
        ses->maxfd = max_int(event, ses->maxfd);
        ses->args[event] = arg;
        if (ses->fast)
        {
            if (rwflags & EVENT_READ)
            {
                openvpn_fd_set(event, &ses->readfds);
            }
            if (rwflags & EVENT_WRITE)
            {
                openvpn_fd_set(event, &ses->writefds);
            }
        }
        else
        {
            if (rwflags & EVENT_READ)
            {
                openvpn_fd_set(event, &ses->readfds);
            }
            else
            {
                FD_CLR(event, &ses->readfds);
            }
            if (rwflags & EVENT_WRITE)
            {
                openvpn_fd_set(event, &ses->writefds);
            }
            else
            {
                FD_CLR(event, &ses->writefds);
            }
        }
    }
    else
    {
        msg(D_EVENT_ERRORS, "Error: select: too many I/O wait events, fd=%d cap=%d",
            (int) event,
            ses->capacity);
    }
}

static int
se_wait_return(struct se_set *ses,
               fd_set *read,
               fd_set *write,
               struct event_set_return *out,
               int outlen)
{
    int i, j = 0;
    for (i = 0; i <= ses->maxfd && j < outlen; ++i)
    {
        const bool r = FD_ISSET(i, read);
        const bool w = FD_ISSET(i, write);
        if (r || w)
        {
            out->rwflags = 0;
            if (r)
            {
                out->rwflags |= EVENT_READ;
            }
            if (w)
            {
                out->rwflags |= EVENT_WRITE;
            }
            out->arg = ses->args[i];
            dmsg(D_EVENT_WAIT, "SE_WAIT[%d,%d] rwflags=0x%04x arg=" ptr_format,
                 i, j, out->rwflags, (ptr_type)out->arg);
            ++out;
            ++j;
        }
    }
    return j;
}

static int
se_wait_fast(struct event_set *es, const struct timeval *tv, struct event_set_return *out, int outlen)
{
    struct se_set *ses = (struct se_set *) es;
    struct timeval tv_tmp = *tv;
    int stat;

    dmsg(D_EVENT_WAIT, "SE_WAIT_FAST maxfd=%d tv=%" PRIi64 "/%ld",
         ses->maxfd,
         (int64_t)tv_tmp.tv_sec,
         (long)tv_tmp.tv_usec);

    stat = select(ses->maxfd + 1, &ses->readfds, &ses->writefds, NULL, &tv_tmp);

    if (stat > 0)
    {
        stat = se_wait_return(ses, &ses->readfds, &ses->writefds, out, outlen);
    }

    return stat;
}

static int
se_wait_scalable(struct event_set *es, const struct timeval *tv, struct event_set_return *out, int outlen)
{
    struct se_set *ses = (struct se_set *) es;
    struct timeval tv_tmp = *tv;
    fd_set read = ses->readfds;
    fd_set write = ses->writefds;
    int stat;

    dmsg(D_EVENT_WAIT, "SE_WAIT_SCALEABLE maxfd=%d tv=%" PRIi64 "/%ld",
         ses->maxfd, (int64_t)tv_tmp.tv_sec, (long)tv_tmp.tv_usec);

    stat = select(ses->maxfd + 1, &read, &write, NULL, &tv_tmp);

    if (stat > 0)
    {
        stat = se_wait_return(ses, &read, &write, out, outlen);
    }

    return stat;
}

static struct event_set *
se_init(int *maxevents, unsigned int flags)
{
    struct se_set *ses;

    dmsg(D_EVENT_WAIT, "SE_INIT maxevents=%d flags=0x%08x", *maxevents, flags);

    ALLOC_OBJ_CLEAR(ses, struct se_set);

    /* set dispatch functions */
    ses->func.free = se_free;
    ses->func.reset = se_reset;
    ses->func.del = se_del;
    ses->func.ctl = se_ctl;
    ses->func.wait = se_wait_scalable;

    if (flags & EVENT_METHOD_FAST)
    {
        ses->fast = true;
        ses->func.wait = se_wait_fast;
    }

    /* Select needs to be passed this value + 1 */
    ses->maxfd = -1;

    /* Set our event capacity */
    ASSERT(*maxevents > 0);
    *maxevents = min_int(*maxevents, SELECT_MAX_FDS);
    ses->capacity = SELECT_MAX_FDS;

    /* Allocate space for event_set_return void * args */
    ALLOC_ARRAY_CLEAR(ses->args, void *, ses->capacity);

    return (struct event_set *) ses;
}
#endif /* SELECT */

static struct event_set *
event_set_init_simple(int *maxevents, unsigned int flags)
{
    struct event_set *ret = NULL;
#ifdef _WIN32
    ret = we_init(maxevents, flags);
#elif POLL && SELECT
#if 0 /* Define to 1 if EVENT_METHOD_US_TIMEOUT should cause select to be favored over poll */
    if (flags & EVENT_METHOD_US_TIMEOUT)
    {
        ret = se_init(maxevents, flags);
    }
#endif
#ifdef SELECT_PREFERRED_OVER_POLL
    if (!ret)
    {
        ret = se_init(maxevents, flags);
    }
    if (!ret)
    {
        ret = po_init(maxevents, flags);
    }
#else  /* ifdef SELECT_PREFERRED_OVER_POLL */
    if (!ret)
    {
        ret = po_init(maxevents, flags);
    }
    if (!ret)
    {
        ret = se_init(maxevents, flags);
    }
#endif
#elif POLL
    ret = po_init(maxevents, flags);
#elif SELECT
    ret = se_init(maxevents, flags);
#else  /* ifdef _WIN32 */
#error At least one of poll, select, or WSAWaitForMultipleEvents must be supported by the kernel
#endif /* ifdef _WIN32 */
    ASSERT(ret);
    return ret;
}

static struct event_set *
event_set_init_scalable(int *maxevents, unsigned int flags)
{
    struct event_set *ret = NULL;
#if EPOLL
    ret = ep_init(maxevents, flags);
    if (!ret)
    {
        msg(M_WARN, "Note: sys_epoll API is unavailable, falling back to poll/select API");
        ret = event_set_init_simple(maxevents, flags);
    }
#else  /* if EPOLL */
    ret = event_set_init_simple(maxevents, flags);
#endif
    ASSERT(ret);
    return ret;
}

struct event_set *
event_set_init(int *maxevents, unsigned int flags)
{
    if (flags & EVENT_METHOD_FAST)
    {
        return event_set_init_simple(maxevents, flags);
    }
    else
    {
        return event_set_init_scalable(maxevents, flags);
    }
}