latch.c

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/*-------------------------------------------------------------------------
 *
 * latch.c
 *    Routines for inter-process latches
 *
 * The poll() implementation uses the so-called self-pipe trick to overcome the
 * race condition involved with poll() and setting a global flag in the signal
 * handler. When a latch is set and the current process is waiting for it, the
 * signal handler wakes up the poll() in WaitLatch by writing a byte to a pipe.
 * A signal by itself doesn't interrupt poll() on all platforms, and even on
 * platforms where it does, a signal that arrives just before the poll() call
 * does not prevent poll() from entering sleep. An incoming byte on a pipe
 * however reliably interrupts the sleep, and causes poll() to return
 * immediately even if the signal arrives before poll() begins.
 *
 * The epoll() implementation overcomes the race with a different technique: it
 * keeps SIGURG blocked and consumes from a signalfd() descriptor instead.  We
 * don't need to register a signal handler or create our own self-pipe.  We
 * assume that any system that has Linux epoll() also has Linux signalfd().
 *
 * The kqueue() implementation waits for SIGURG with EVFILT_SIGNAL.
 *
 * The Windows implementation uses Windows events that are inherited by all
 * postmaster child processes. There's no need for the self-pipe trick there.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/latch.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <fcntl.h>
#include <limits.h>
#include <signal.h>
#include <unistd.h>
#ifdef HAVE_SYS_EPOLL_H
#include <sys/epoll.h>
#endif
#ifdef HAVE_SYS_EVENT_H
#include <sys/event.h>
#endif
#ifdef HAVE_POLL_H
#include <poll.h>
#endif

#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "portability/instr_time.h"
#include "postmaster/postmaster.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/shmem.h"
#include "utils/memutils.h"

/*
 * Select the fd readiness primitive to use. Normally the "most modern"
 * primitive supported by the OS will be used, but for testing it can be
 * useful to manually specify the used primitive.  If desired, just add a
 * define somewhere before this block.
 */
#if defined(WAIT_USE_EPOLL) || defined(WAIT_USE_POLL) || \
    defined(WAIT_USE_KQUEUE) || defined(WAIT_USE_WIN32)
/* don't overwrite manual choice */
#elif defined(HAVE_SYS_EPOLL_H)
#define WAIT_USE_EPOLL
#elif defined(HAVE_KQUEUE)
#define WAIT_USE_KQUEUE
#elif defined(HAVE_POLL)
#define WAIT_USE_POLL
#elif WIN32
#define WAIT_USE_WIN32
#else
#error "no wait set implementation available"
#endif

#ifdef WAIT_USE_EPOLL
#include <sys/signalfd.h>
#endif

/* typedef in latch.h */
struct WaitEventSet
{
    int         nevents;        /* number of registered events */
    int         nevents_space;  /* maximum number of events in this set */

    /*
     * Array, of nevents_space length, storing the definition of events this
     * set is waiting for.
     */
    WaitEvent  *events;

    /*
     * If WL_LATCH_SET is specified in any wait event, latch is a pointer to
     * said latch, and latch_pos the offset in the ->events array. This is
     * useful because we check the state of the latch before performing doing
     * syscalls related to waiting.
     */
    Latch      *latch;
    int         latch_pos;

    /*
     * WL_EXIT_ON_PM_DEATH is converted to WL_POSTMASTER_DEATH, but this flag
     * is set so that we'll exit immediately if postmaster death is detected,
     * instead of returning.
     */
    bool        exit_on_postmaster_death;

#if defined(WAIT_USE_EPOLL)
    int         epoll_fd;
    /* epoll_wait returns events in a user provided arrays, allocate once */
    struct epoll_event *epoll_ret_events;
#elif defined(WAIT_USE_KQUEUE)
    int         kqueue_fd;
    /* kevent returns events in a user provided arrays, allocate once */
    struct kevent *kqueue_ret_events;
    bool        report_postmaster_not_running;
#elif defined(WAIT_USE_POLL)
    /* poll expects events to be waited on every poll() call, prepare once */
    struct pollfd *pollfds;
#elif defined(WAIT_USE_WIN32)

    /*
     * Array of windows events. The first element always contains
     * pgwin32_signal_event, so the remaining elements are offset by one (i.e.
     * event->pos + 1).
     */
    HANDLE     *handles;
#endif
};

/* A common WaitEventSet used to implement WatchLatch() */
static WaitEventSet *LatchWaitSet;

/* The position of the latch in LatchWaitSet. */
#define LatchWaitSetLatchPos 0

#ifndef WIN32
/* Are we currently in WaitLatch? The signal handler would like to know. */
static volatile sig_atomic_t waiting = false;
#endif

#ifdef WAIT_USE_EPOLL
/* On Linux, we'll receive SIGURG via a signalfd file descriptor. */
static int  signal_fd = -1;
#endif

#if defined(WAIT_USE_POLL)
/* Read and write ends of the self-pipe */
static int  selfpipe_readfd = -1;
static int  selfpipe_writefd = -1;

/* Process owning the self-pipe --- needed for checking purposes */
static int  selfpipe_owner_pid = 0;

/* Private function prototypes */
static void latch_sigurg_handler(SIGNAL_ARGS);
static void sendSelfPipeByte(void);
#endif

#if defined(WAIT_USE_POLL) || defined(WAIT_USE_EPOLL)
static void drain(void);
#endif

#if defined(WAIT_USE_EPOLL)
static void WaitEventAdjustEpoll(WaitEventSet *set, WaitEvent *event, int action);
#elif defined(WAIT_USE_KQUEUE)
static void WaitEventAdjustKqueue(WaitEventSet *set, WaitEvent *event, int old_events);
#elif defined(WAIT_USE_POLL)
static void WaitEventAdjustPoll(WaitEventSet *set, WaitEvent *event);
#elif defined(WAIT_USE_WIN32)
static void WaitEventAdjustWin32(WaitEventSet *set, WaitEvent *event);
#endif

static inline int WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
                                        WaitEvent *occurred_events, int nevents);

/*
 * Initialize the process-local latch infrastructure.
 *
 * This must be called once during startup of any process that can wait on
 * latches, before it issues any InitLatch() or OwnLatch() calls.
 */
void
InitializeLatchSupport(void)
{
#if defined(WAIT_USE_POLL)
    int         pipefd[2];

    if (IsUnderPostmaster)
    {
        /*
         * We might have inherited connections to a self-pipe created by the
         * postmaster.  It's critical that child processes create their own
         * self-pipes, of course, and we really want them to close the
         * inherited FDs for safety's sake.
         */
        if (selfpipe_owner_pid != 0)
        {
            /* Assert we go through here but once in a child process */
            Assert(selfpipe_owner_pid != MyProcPid);
            /* Release postmaster's pipe FDs; ignore any error */
            (void) close(selfpipe_readfd);
            (void) close(selfpipe_writefd);
            /* Clean up, just for safety's sake; we'll set these below */
            selfpipe_readfd = selfpipe_writefd = -1;
            selfpipe_owner_pid = 0;
            /* Keep fd.c's accounting straight */
            ReleaseExternalFD();
            ReleaseExternalFD();
        }
        else
        {
            /*
             * Postmaster didn't create a self-pipe ... or else we're in an
             * EXEC_BACKEND build, in which case it doesn't matter since the
             * postmaster's pipe FDs were closed by the action of FD_CLOEXEC.
             * fd.c won't have state to clean up, either.
             */
            Assert(selfpipe_readfd == -1);
        }
    }
    else
    {
        /* In postmaster or standalone backend, assert we do this but once */
        Assert(selfpipe_readfd == -1);
        Assert(selfpipe_owner_pid == 0);
    }

    /*
     * Set up the self-pipe that allows a signal handler to wake up the
     * poll()/epoll_wait() in WaitLatch. Make the write-end non-blocking, so
     * that SetLatch won't block if the event has already been set many times
     * filling the kernel buffer. Make the read-end non-blocking too, so that
     * we can easily clear the pipe by reading until EAGAIN or EWOULDBLOCK.
     * Also, make both FDs close-on-exec, since we surely do not want any
     * child processes messing with them.
     */
    if (pipe(pipefd) < 0)
        elog(FATAL, "pipe() failed: %m");
    if (fcntl(pipefd[0], F_SETFL, O_NONBLOCK) == -1)
        elog(FATAL, "fcntl(F_SETFL) failed on read-end of self-pipe: %m");
    if (fcntl(pipefd[1], F_SETFL, O_NONBLOCK) == -1)
        elog(FATAL, "fcntl(F_SETFL) failed on write-end of self-pipe: %m");
    if (fcntl(pipefd[0], F_SETFD, FD_CLOEXEC) == -1)
        elog(FATAL, "fcntl(F_SETFD) failed on read-end of self-pipe: %m");
    if (fcntl(pipefd[1], F_SETFD, FD_CLOEXEC) == -1)
        elog(FATAL, "fcntl(F_SETFD) failed on write-end of self-pipe: %m");

    selfpipe_readfd = pipefd[0];
    selfpipe_writefd = pipefd[1];
    selfpipe_owner_pid = MyProcPid;

    /* Tell fd.c about these two long-lived FDs */
    ReserveExternalFD();
    ReserveExternalFD();

    pqsignal(SIGURG, latch_sigurg_handler);
#endif

#ifdef WAIT_USE_EPOLL
    sigset_t    signalfd_mask;

    /* Block SIGURG, because we'll receive it through a signalfd. */
    sigaddset(&UnBlockSig, SIGURG);

    /* Set up the signalfd to receive SIGURG notifications. */
    sigemptyset(&signalfd_mask);
    sigaddset(&signalfd_mask, SIGURG);
    signal_fd = signalfd(-1, &signalfd_mask, SFD_NONBLOCK | SFD_CLOEXEC);
    if (signal_fd < 0)
        elog(FATAL, "signalfd() failed");
    ReserveExternalFD();
#endif

#ifdef WAIT_USE_KQUEUE
    /* Ignore SIGURG, because we'll receive it via kqueue. */
    pqsignal(SIGURG, SIG_IGN);
#endif
}

void
InitializeLatchWaitSet(void)
{
    int         latch_pos PG_USED_FOR_ASSERTS_ONLY;

    Assert(LatchWaitSet == NULL);

    /* Set up the WaitEventSet used by WaitLatch(). */
    LatchWaitSet = CreateWaitEventSet(TopMemoryContext, 2);
    latch_pos = AddWaitEventToSet(LatchWaitSet, WL_LATCH_SET, PGINVALID_SOCKET,
                                  MyLatch, NULL);
    if (IsUnderPostmaster)
        AddWaitEventToSet(LatchWaitSet, WL_EXIT_ON_PM_DEATH,
                          PGINVALID_SOCKET, NULL, NULL);

    Assert(latch_pos == LatchWaitSetLatchPos);
}

void
ShutdownLatchSupport(void)
{
#if defined(WAIT_USE_POLL)
    pqsignal(SIGURG, SIG_IGN);
#endif

    if (LatchWaitSet)
    {
        FreeWaitEventSet(LatchWaitSet);
        LatchWaitSet = NULL;
    }

#if defined(WAIT_USE_POLL)
    close(selfpipe_readfd);
    close(selfpipe_writefd);
    selfpipe_readfd = -1;
    selfpipe_writefd = -1;
    selfpipe_owner_pid = InvalidPid;
#endif

#if defined(WAIT_USE_EPOLL)
    close(signal_fd);
    signal_fd = -1;
#endif
}

/*
 * Initialize a process-local latch.
 */
void
InitLatch(Latch *latch)
{
    latch->is_set = false;
    latch->maybe_sleeping = false;
    latch->owner_pid = MyProcPid;
    latch->is_shared = false;

#if defined(WAIT_USE_POLL)
    /* Assert InitializeLatchSupport has been called in this process */
    Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
#elif defined(WAIT_USE_WIN32)
    latch->event = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (latch->event == NULL)
        elog(ERROR, "CreateEvent failed: error code %lu", GetLastError());
#endif                          /* WIN32 */
}

/*
 * Initialize a shared latch that can be set from other processes. The latch
 * is initially owned by no-one; use OwnLatch to associate it with the
 * current process.
 *
 * InitSharedLatch needs to be called in postmaster before forking child
 * processes, usually right after allocating the shared memory block
 * containing the latch with ShmemInitStruct. (The Unix implementation
 * doesn't actually require that, but the Windows one does.) Because of
 * this restriction, we have no concurrency issues to worry about here.
 *
 * Note that other handles created in this module are never marked as
 * inheritable.  Thus we do not need to worry about cleaning up child
 * process references to postmaster-private latches or WaitEventSets.
 */
void
InitSharedLatch(Latch *latch)
{
#ifdef WIN32
    SECURITY_ATTRIBUTES sa;

    /*
     * Set up security attributes to specify that the events are inherited.
     */
    ZeroMemory(&sa, sizeof(sa));
    sa.nLength = sizeof(sa);
    sa.bInheritHandle = TRUE;

    latch->event = CreateEvent(&sa, TRUE, FALSE, NULL);
    if (latch->event == NULL)
        elog(ERROR, "CreateEvent failed: error code %lu", GetLastError());
#endif

    latch->is_set = false;
    latch->maybe_sleeping = false;
    latch->owner_pid = 0;
    latch->is_shared = true;
}

/*
 * Associate a shared latch with the current process, allowing it to
 * wait on the latch.
 *
 * Although there is a sanity check for latch-already-owned, we don't do
 * any sort of locking here, meaning that we could fail to detect the error
 * if two processes try to own the same latch at about the same time.  If
 * there is any risk of that, caller must provide an interlock to prevent it.
 */
void
OwnLatch(Latch *latch)
{
    /* Sanity checks */
    Assert(latch->is_shared);

#if defined(WAIT_USE_POLL)
    /* Assert InitializeLatchSupport has been called in this process */
    Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
#endif

    if (latch->owner_pid != 0)
        elog(ERROR, "latch already owned");

    latch->owner_pid = MyProcPid;
}

/*
 * Disown a shared latch currently owned by the current process.
 */
void
DisownLatch(Latch *latch)
{
    Assert(latch->is_shared);
    Assert(latch->owner_pid == MyProcPid);

    latch->owner_pid = 0;
}

/*
 * Wait for a given latch to be set, or for postmaster death, or until timeout
 * is exceeded. 'wakeEvents' is a bitmask that specifies which of those events
 * to wait for. If the latch is already set (and WL_LATCH_SET is given), the
 * function returns immediately.
 *
 * The "timeout" is given in milliseconds. It must be >= 0 if WL_TIMEOUT flag
 * is given.  Although it is declared as "long", we don't actually support
 * timeouts longer than INT_MAX milliseconds.  Note that some extra overhead
 * is incurred when WL_TIMEOUT is given, so avoid using a timeout if possible.
 *
 * The latch must be owned by the current process, ie. it must be a
 * process-local latch initialized with InitLatch, or a shared latch
 * associated with the current process by calling OwnLatch.
 *
 * Returns bit mask indicating which condition(s) caused the wake-up. Note
 * that if multiple wake-up conditions are true, there is no guarantee that
 * we return all of them in one call, but we will return at least one.
 */
int
WaitLatch(Latch *latch, int wakeEvents, long timeout,
          uint32 wait_event_info)
{
    WaitEvent   event;

    /* Postmaster-managed callers must handle postmaster death somehow. */
    Assert(!IsUnderPostmaster ||
           (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
           (wakeEvents & WL_POSTMASTER_DEATH));

    /*
     * Some callers may have a latch other than MyLatch, or no latch at all,
     * or want to handle postmaster death differently.  It's cheap to assign
     * those, so just do it every time.
     */
    if (!(wakeEvents & WL_LATCH_SET))
        latch = NULL;
    ModifyWaitEvent(LatchWaitSet, LatchWaitSetLatchPos, WL_LATCH_SET, latch);
    LatchWaitSet->exit_on_postmaster_death =
        ((wakeEvents & WL_EXIT_ON_PM_DEATH) != 0);

    if (WaitEventSetWait(LatchWaitSet,
                         (wakeEvents & WL_TIMEOUT) ? timeout : -1,
                         &event, 1,
                         wait_event_info) == 0)
        return WL_TIMEOUT;
    else
        return event.events;
}

/*
 * Like WaitLatch, but with an extra socket argument for WL_SOCKET_*
 * conditions.
 *
 * When waiting on a socket, EOF and error conditions always cause the socket
 * to be reported as readable/writable/connected, so that the caller can deal
 * with the condition.
 *
 * wakeEvents must include either WL_EXIT_ON_PM_DEATH for automatic exit
 * if the postmaster dies or WL_POSTMASTER_DEATH for a flag set in the
 * return value if the postmaster dies.  The latter is useful for rare cases
 * where some behavior other than immediate exit is needed.
 *
 * NB: These days this is just a wrapper around the WaitEventSet API. When
 * using a latch very frequently, consider creating a longer living
 * WaitEventSet instead; that's more efficient.
 */
int
WaitLatchOrSocket(Latch *latch, int wakeEvents, pgsocket sock,
                  long timeout, uint32 wait_event_info)
{
    int         ret = 0;
    int         rc;
    WaitEvent   event;
    WaitEventSet *set = CreateWaitEventSet(CurrentMemoryContext, 3);

    if (wakeEvents & WL_TIMEOUT)
        Assert(timeout >= 0);
    else
        timeout = -1;

    if (wakeEvents & WL_LATCH_SET)
        AddWaitEventToSet(set, WL_LATCH_SET, PGINVALID_SOCKET,
                          latch, NULL);

    /* Postmaster-managed callers must handle postmaster death somehow. */
    Assert(!IsUnderPostmaster ||
           (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
           (wakeEvents & WL_POSTMASTER_DEATH));

    if ((wakeEvents & WL_POSTMASTER_DEATH) && IsUnderPostmaster)
        AddWaitEventToSet(set, WL_POSTMASTER_DEATH, PGINVALID_SOCKET,
                          NULL, NULL);

    if ((wakeEvents & WL_EXIT_ON_PM_DEATH) && IsUnderPostmaster)
        AddWaitEventToSet(set, WL_EXIT_ON_PM_DEATH, PGINVALID_SOCKET,
                          NULL, NULL);

    if (wakeEvents & WL_SOCKET_MASK)
    {
        int         ev;

        ev = wakeEvents & WL_SOCKET_MASK;
        AddWaitEventToSet(set, ev, sock, NULL, NULL);
    }

    rc = WaitEventSetWait(set, timeout, &event, 1, wait_event_info);

    if (rc == 0)
        ret |= WL_TIMEOUT;
    else
    {
        ret |= event.events & (WL_LATCH_SET |
                               WL_POSTMASTER_DEATH |
                               WL_SOCKET_MASK);
    }

    FreeWaitEventSet(set);

    return ret;
}

/*
 * Sets a latch and wakes up anyone waiting on it.
 *
 * This is cheap if the latch is already set, otherwise not so much.
 *
 * NB: when calling this in a signal handler, be sure to save and restore
 * errno around it.  (That's standard practice in most signal handlers, of
 * course, but we used to omit it in handlers that only set a flag.)
 *
 * NB: this function is called from critical sections and signal handlers so
 * throwing an error is not a good idea.
 */
void
SetLatch(Latch *latch)
{
#ifndef WIN32
    pid_t       owner_pid;
#else
    HANDLE      handle;
#endif

    /*
     * The memory barrier has to be placed here to ensure that any flag
     * variables possibly changed by this process have been flushed to main
     * memory, before we check/set is_set.
     */
    pg_memory_barrier();

    /* Quick exit if already set */
    if (latch->is_set)
        return;

    latch->is_set = true;

    pg_memory_barrier();
    if (!latch->maybe_sleeping)
        return;

#ifndef WIN32

    /*
     * See if anyone's waiting for the latch. It can be the current process if
     * we're in a signal handler. We use the self-pipe or SIGURG to ourselves
     * to wake up WaitEventSetWaitBlock() without races in that case. If it's
     * another process, send a signal.
     *
     * Fetch owner_pid only once, in case the latch is concurrently getting
     * owned or disowned. XXX: This assumes that pid_t is atomic, which isn't
     * guaranteed to be true! In practice, the effective range of pid_t fits
     * in a 32 bit integer, and so should be atomic. In the worst case, we
     * might end up signaling the wrong process. Even then, you're very
     * unlucky if a process with that bogus pid exists and belongs to
     * Postgres; and PG database processes should handle excess SIGUSR1
     * interrupts without a problem anyhow.
     *
     * Another sort of race condition that's possible here is for a new
     * process to own the latch immediately after we look, so we don't signal
     * it. This is okay so long as all callers of ResetLatch/WaitLatch follow
     * the standard coding convention of waiting at the bottom of their loops,
     * not the top, so that they'll correctly process latch-setting events
     * that happen before they enter the loop.
     */
    owner_pid = latch->owner_pid;
    if (owner_pid == 0)
        return;
    else if (owner_pid == MyProcPid)
    {
#if defined(WAIT_USE_POLL)
        if (waiting)
            sendSelfPipeByte();
#else
        if (waiting)
            kill(MyProcPid, SIGURG);
#endif
    }
    else
        kill(owner_pid, SIGURG);

#else

    /*
     * See if anyone's waiting for the latch. It can be the current process if
     * we're in a signal handler.
     *
     * Use a local variable here just in case somebody changes the event field
     * concurrently (which really should not happen).
     */
    handle = latch->event;
    if (handle)
    {
        SetEvent(handle);

        /*
         * Note that we silently ignore any errors. We might be in a signal
         * handler or other critical path where it's not safe to call elog().
         */
    }
#endif

}

/*
 * Clear the latch. Calling WaitLatch after this will sleep, unless
 * the latch is set again before the WaitLatch call.
 */
void
ResetLatch(Latch *latch)
{
    /* Only the owner should reset the latch */
    Assert(latch->owner_pid == MyProcPid);
    Assert(latch->maybe_sleeping == false);

    latch->is_set = false;

    /*
     * Ensure that the write to is_set gets flushed to main memory before we
     * examine any flag variables.  Otherwise a concurrent SetLatch might
     * falsely conclude that it needn't signal us, even though we have missed
     * seeing some flag updates that SetLatch was supposed to inform us of.
     */
    pg_memory_barrier();
}

/*
 * Create a WaitEventSet with space for nevents different events to wait for.
 *
 * These events can then be efficiently waited upon together, using
 * WaitEventSetWait().
 */
WaitEventSet *
CreateWaitEventSet(MemoryContext context, int nevents)
{
    WaitEventSet *set;
    char       *data;
    Size        sz = 0;

    /*
     * Use MAXALIGN size/alignment to guarantee that later uses of memory are
     * aligned correctly. E.g. epoll_event might need 8 byte alignment on some
     * platforms, but earlier allocations like WaitEventSet and WaitEvent
     * might not sized to guarantee that when purely using sizeof().
     */
    sz += MAXALIGN(sizeof(WaitEventSet));
    sz += MAXALIGN(sizeof(WaitEvent) * nevents);

#if defined(WAIT_USE_EPOLL)
    sz += MAXALIGN(sizeof(struct epoll_event) * nevents);
#elif defined(WAIT_USE_KQUEUE)
    sz += MAXALIGN(sizeof(struct kevent) * nevents);
#elif defined(WAIT_USE_POLL)
    sz += MAXALIGN(sizeof(struct pollfd) * nevents);
#elif defined(WAIT_USE_WIN32)
    /* need space for the pgwin32_signal_event */
    sz += MAXALIGN(sizeof(HANDLE) * (nevents + 1));
#endif

    data = (char *) MemoryContextAllocZero(context, sz);

    set = (WaitEventSet *) data;
    data += MAXALIGN(sizeof(WaitEventSet));

    set->events = (WaitEvent *) data;
    data += MAXALIGN(sizeof(WaitEvent) * nevents);

#if defined(WAIT_USE_EPOLL)
    set->epoll_ret_events = (struct epoll_event *) data;
    data += MAXALIGN(sizeof(struct epoll_event) * nevents);
#elif defined(WAIT_USE_KQUEUE)
    set->kqueue_ret_events = (struct kevent *) data;
    data += MAXALIGN(sizeof(struct kevent) * nevents);
#elif defined(WAIT_USE_POLL)
    set->pollfds = (struct pollfd *) data;
    data += MAXALIGN(sizeof(struct pollfd) * nevents);
#elif defined(WAIT_USE_WIN32)
    set->handles = (HANDLE) data;
    data += MAXALIGN(sizeof(HANDLE) * nevents);
#endif

    set->latch = NULL;
    set->nevents_space = nevents;
    set->exit_on_postmaster_death = false;

#if defined(WAIT_USE_EPOLL)
    if (!AcquireExternalFD())
    {
        /* treat this as though epoll_create1 itself returned EMFILE */
        elog(ERROR, "epoll_create1 failed: %m");
    }
    set->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
    if (set->epoll_fd < 0)
    {
        ReleaseExternalFD();
        elog(ERROR, "epoll_create1 failed: %m");
    }
#elif defined(WAIT_USE_KQUEUE)
    if (!AcquireExternalFD())
    {
        /* treat this as though kqueue itself returned EMFILE */
        elog(ERROR, "kqueue failed: %m");
    }
    set->kqueue_fd = kqueue();
    if (set->kqueue_fd < 0)
    {
        ReleaseExternalFD();
        elog(ERROR, "kqueue failed: %m");
    }
    if (fcntl(set->kqueue_fd, F_SETFD, FD_CLOEXEC) == -1)
    {
        int         save_errno = errno;

        close(set->kqueue_fd);
        ReleaseExternalFD();
        errno = save_errno;
        elog(ERROR, "fcntl(F_SETFD) failed on kqueue descriptor: %m");
    }
    set->report_postmaster_not_running = false;
#elif defined(WAIT_USE_WIN32)

    /*
     * To handle signals while waiting, we need to add a win32 specific event.
     * We accounted for the additional event at the top of this routine. See
     * port/win32/signal.c for more details.
     *
     * Note: pgwin32_signal_event should be first to ensure that it will be
     * reported when multiple events are set.  We want to guarantee that
     * pending signals are serviced.
     */
    set->handles[0] = pgwin32_signal_event;
    StaticAssertStmt(WSA_INVALID_EVENT == NULL, "");
#endif

    return set;
}

/*
 * Free a previously created WaitEventSet.
 *
 * Note: preferably, this shouldn't have to free any resources that could be
 * inherited across an exec().  If it did, we'd likely leak those resources in
 * many scenarios.  For the epoll case, we ensure that by setting EPOLL_CLOEXEC
 * when the FD is created.  For the Windows case, we assume that the handles
 * involved are non-inheritable.
 */
void
FreeWaitEventSet(WaitEventSet *set)
{
#if defined(WAIT_USE_EPOLL)
    close(set->epoll_fd);
    ReleaseExternalFD();
#elif defined(WAIT_USE_KQUEUE)
    close(set->kqueue_fd);
    ReleaseExternalFD();
#elif defined(WAIT_USE_WIN32)
    WaitEvent  *cur_event;

    for (cur_event = set->events;
         cur_event < (set->events + set->nevents);
         cur_event++)
    {
        if (cur_event->events & WL_LATCH_SET)
        {
            /* uses the latch's HANDLE */
        }
        else if (cur_event->events & WL_POSTMASTER_DEATH)
        {
            /* uses PostmasterHandle */
        }
        else
        {
            /* Clean up the event object we created for the socket */
            WSAEventSelect(cur_event->fd, NULL, 0);
            WSACloseEvent(set->handles[cur_event->pos + 1]);
        }
    }
#endif

    pfree(set);
}

/* ---
 * Add an event to the set. Possible events are:
 * - WL_LATCH_SET: Wait for the latch to be set
 * - WL_POSTMASTER_DEATH: Wait for postmaster to die
 * - WL_SOCKET_READABLE: Wait for socket to become readable,
 *   can be combined in one event with other WL_SOCKET_* events
 * - WL_SOCKET_WRITEABLE: Wait for socket to become writeable,
 *   can be combined with other WL_SOCKET_* events
 * - WL_SOCKET_CONNECTED: Wait for socket connection to be established,
 *   can be combined with other WL_SOCKET_* events (on non-Windows
 *   platforms, this is the same as WL_SOCKET_WRITEABLE)
 * - WL_EXIT_ON_PM_DEATH: Exit immediately if the postmaster dies
 *
 * Returns the offset in WaitEventSet->events (starting from 0), which can be
 * used to modify previously added wait events using ModifyWaitEvent().
 *
 * In the WL_LATCH_SET case the latch must be owned by the current process,
 * i.e. it must be a process-local latch initialized with InitLatch, or a
 * shared latch associated with the current process by calling OwnLatch.
 *
 * In the WL_SOCKET_READABLE/WRITEABLE/CONNECTED cases, EOF and error
 * conditions cause the socket to be reported as readable/writable/connected,
 * so that the caller can deal with the condition.
 *
 * The user_data pointer specified here will be set for the events returned
 * by WaitEventSetWait(), allowing to easily associate additional data with
 * events.
 */
int
AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd, Latch *latch,
                  void *user_data)
{
    WaitEvent  *event;

    /* not enough space */
    Assert(set->nevents < set->nevents_space);

    if (events == WL_EXIT_ON_PM_DEATH)
    {
        events = WL_POSTMASTER_DEATH;
        set->exit_on_postmaster_death = true;
    }

    if (latch)
    {
        if (latch->owner_pid != MyProcPid)
            elog(ERROR, "cannot wait on a latch owned by another process");
        if (set->latch)
            elog(ERROR, "cannot wait on more than one latch");
        if ((events & WL_LATCH_SET) != WL_LATCH_SET)
            elog(ERROR, "latch events only support being set");
    }
    else
    {
        if (events & WL_LATCH_SET)
            elog(ERROR, "cannot wait on latch without a specified latch");
    }

    /* waiting for socket readiness without a socket indicates a bug */
    if (fd == PGINVALID_SOCKET && (events & WL_SOCKET_MASK))
        elog(ERROR, "cannot wait on socket event without a socket");

    event = &set->events[set->nevents];
    event->pos = set->nevents++;
    event->fd = fd;
    event->events = events;
    event->user_data = user_data;
#ifdef WIN32
    event->reset = false;
#endif

    if (events == WL_LATCH_SET)
    {
        set->latch = latch;
        set->latch_pos = event->pos;
#if defined(WAIT_USE_POLL)
        event->fd = selfpipe_readfd;
#elif defined(WAIT_USE_EPOLL)
        event->fd = signal_fd;
#else
        event->fd = PGINVALID_SOCKET;
#ifdef WAIT_USE_EPOLL
        return event->pos;
#endif
#endif
    }
    else if (events == WL_POSTMASTER_DEATH)
    {
#ifndef WIN32
        event->fd = postmaster_alive_fds[POSTMASTER_FD_WATCH];
#endif
    }

    /* perform wait primitive specific initialization, if needed */
#if defined(WAIT_USE_EPOLL)
    WaitEventAdjustEpoll(set, event, EPOLL_CTL_ADD);
#elif defined(WAIT_USE_KQUEUE)
    WaitEventAdjustKqueue(set, event, 0);
#elif defined(WAIT_USE_POLL)
    WaitEventAdjustPoll(set, event);
#elif defined(WAIT_USE_WIN32)
    WaitEventAdjustWin32(set, event);
#endif

    return event->pos;
}

/*
 * Change the event mask and, in the WL_LATCH_SET case, the latch associated
 * with the WaitEvent.  The latch may be changed to NULL to disable the latch
 * temporarily, and then set back to a latch later.
 *
 * 'pos' is the id returned by AddWaitEventToSet.
 */
void
ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch)
{
    WaitEvent  *event;
#if defined(WAIT_USE_KQUEUE)
    int         old_events;
#endif

    Assert(pos < set->nevents);

    event = &set->events[pos];
#if defined(WAIT_USE_KQUEUE)
    old_events = event->events;
#endif

    /*
     * If neither the event mask nor the associated latch changes, return
     * early. That's an important optimization for some sockets, where
     * ModifyWaitEvent is frequently used to switch from waiting for reads to
     * waiting on writes.
     */
    if (events == event->events &&
        (!(event->events & WL_LATCH_SET) || set->latch == latch))
        return;

    if (event->events & WL_LATCH_SET &&
        events != event->events)
    {
        elog(ERROR, "cannot modify latch event");
    }

    if (event->events & WL_POSTMASTER_DEATH)
    {
        elog(ERROR, "cannot modify postmaster death event");
    }

    /* FIXME: validate event mask */
    event->events = events;

    if (events == WL_LATCH_SET)
    {
        if (latch && latch->owner_pid != MyProcPid)
            elog(ERROR, "cannot wait on a latch owned by another process");
        set->latch = latch;

        /*
         * On Unix, we don't need to modify the kernel object because the
         * underlying pipe (if there is one) is the same for all latches so we
         * can return immediately.  On Windows, we need to update our array of
         * handles, but we leave the old one in place and tolerate spurious
         * wakeups if the latch is disabled.
         */
#if defined(WAIT_USE_WIN32)
        if (!latch)
            return;
#else
        return;
#endif
    }

#if defined(WAIT_USE_EPOLL)
    WaitEventAdjustEpoll(set, event, EPOLL_CTL_MOD);
#elif defined(WAIT_USE_KQUEUE)
    WaitEventAdjustKqueue(set, event, old_events);
#elif defined(WAIT_USE_POLL)
    WaitEventAdjustPoll(set, event);
#elif defined(WAIT_USE_WIN32)
    WaitEventAdjustWin32(set, event);
#endif
}

#if defined(WAIT_USE_EPOLL)
/*
 * action can be one of EPOLL_CTL_ADD | EPOLL_CTL_MOD | EPOLL_CTL_DEL
 */
static void
WaitEventAdjustEpoll(WaitEventSet *set, WaitEvent *event, int action)
{
    struct epoll_event epoll_ev;
    int         rc;

    /* pointer to our event, returned by epoll_wait */
    epoll_ev.data.ptr = event;
    /* always wait for errors */
    epoll_ev.events = EPOLLERR | EPOLLHUP;

    /* prepare pollfd entry once */
    if (event->events == WL_LATCH_SET)
    {
        Assert(set->latch != NULL);
        epoll_ev.events |= EPOLLIN;
    }
    else if (event->events == WL_POSTMASTER_DEATH)
    {
        epoll_ev.events |= EPOLLIN;
    }
    else
    {
        Assert(event->fd != PGINVALID_SOCKET);
        Assert(event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE));

        if (event->events & WL_SOCKET_READABLE)
            epoll_ev.events |= EPOLLIN;
        if (event->events & WL_SOCKET_WRITEABLE)
            epoll_ev.events |= EPOLLOUT;
    }

    /*
     * Even though unused, we also pass epoll_ev as the data argument if
     * EPOLL_CTL_DEL is passed as action.  There used to be an epoll bug
     * requiring that, and actually it makes the code simpler...
     */
    rc = epoll_ctl(set->epoll_fd, action, event->fd, &epoll_ev);

    if (rc < 0)
        ereport(ERROR,
                (errcode_for_socket_access(),
                 errmsg("%s() failed: %m",
                        "epoll_ctl")));
}
#endif

#if defined(WAIT_USE_POLL)
static void
WaitEventAdjustPoll(WaitEventSet *set, WaitEvent *event)
{
    struct pollfd *pollfd = &set->pollfds[event->pos];

    pollfd->revents = 0;
    pollfd->fd = event->fd;

    /* prepare pollfd entry once */
    if (event->events == WL_LATCH_SET)
    {
        Assert(set->latch != NULL);
        pollfd->events = POLLIN;
    }
    else if (event->events == WL_POSTMASTER_DEATH)
    {
        pollfd->events = POLLIN;
    }
    else
    {
        Assert(event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE));
        pollfd->events = 0;
        if (event->events & WL_SOCKET_READABLE)
            pollfd->events |= POLLIN;
        if (event->events & WL_SOCKET_WRITEABLE)
            pollfd->events |= POLLOUT;
    }

    Assert(event->fd != PGINVALID_SOCKET);
}
#endif

#if defined(WAIT_USE_KQUEUE)

/*
 * On most BSD family systems, the udata member of struct kevent is of type
 * void *, so we could directly convert to/from WaitEvent *.  Unfortunately,
 * NetBSD has it as intptr_t, so here we wallpaper over that difference with
 * an lvalue cast.
 */
#define AccessWaitEvent(k_ev) (*((WaitEvent **)(&(k_ev)->udata)))

static inline void
WaitEventAdjustKqueueAdd(struct kevent *k_ev, int filter, int action,
                         WaitEvent *event)
{
    k_ev->ident = event->fd;
    k_ev->filter = filter;
    k_ev->flags = action;
    k_ev->fflags = 0;
    k_ev->data = 0;
    AccessWaitEvent(k_ev) = event;
}

static inline void
WaitEventAdjustKqueueAddPostmaster(struct kevent *k_ev, WaitEvent *event)
{
    /* For now postmaster death can only be added, not removed. */
    k_ev->ident = PostmasterPid;
    k_ev->filter = EVFILT_PROC;
    k_ev->flags = EV_ADD;
    k_ev->fflags = NOTE_EXIT;
    k_ev->data = 0;
    AccessWaitEvent(k_ev) = event;
}

static inline void
WaitEventAdjustKqueueAddLatch(struct kevent *k_ev, WaitEvent *event)
{
    /* For now latch can only be added, not removed. */
    k_ev->ident = SIGURG;
    k_ev->filter = EVFILT_SIGNAL;
    k_ev->flags = EV_ADD;
    k_ev->fflags = 0;
    k_ev->data = 0;
    AccessWaitEvent(k_ev) = event;
}

/*
 * old_events is the previous event mask, used to compute what has changed.
 */
static void
WaitEventAdjustKqueue(WaitEventSet *set, WaitEvent *event, int old_events)
{
    int         rc;
    struct kevent k_ev[2];
    int         count = 0;
    bool        new_filt_read = false;
    bool        old_filt_read = false;
    bool        new_filt_write = false;
    bool        old_filt_write = false;

    if (old_events == event->events)
        return;

    Assert(event->events != WL_LATCH_SET || set->latch != NULL);
    Assert(event->events == WL_LATCH_SET ||
           event->events == WL_POSTMASTER_DEATH ||
           (event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)));

    if (event->events == WL_POSTMASTER_DEATH)
    {
        /*
         * Unlike all the other implementations, we detect postmaster death
         * using process notification instead of waiting on the postmaster
         * alive pipe.
         */
        WaitEventAdjustKqueueAddPostmaster(&k_ev[count++], event);
    }
    else if (event->events == WL_LATCH_SET)
    {
        /* We detect latch wakeup using a signal event. */
        WaitEventAdjustKqueueAddLatch(&k_ev[count++], event);
    }
    else
    {
        /*
         * We need to compute the adds and deletes required to get from the
         * old event mask to the new event mask, since kevent treats readable
         * and writable as separate events.
         */
        if (old_events & WL_SOCKET_READABLE)
            old_filt_read = true;
        if (event->events & WL_SOCKET_READABLE)
            new_filt_read = true;
        if (old_events & WL_SOCKET_WRITEABLE)
            old_filt_write = true;
        if (event->events & WL_SOCKET_WRITEABLE)
            new_filt_write = true;
        if (old_filt_read && !new_filt_read)
            WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_READ, EV_DELETE,
                                     event);
        else if (!old_filt_read && new_filt_read)
            WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_READ, EV_ADD,
                                     event);
        if (old_filt_write && !new_filt_write)
            WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_WRITE, EV_DELETE,
                                     event);
        else if (!old_filt_write && new_filt_write)
            WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_WRITE, EV_ADD,
                                     event);
    }

    Assert(count > 0);
    Assert(count <= 2);

    rc = kevent(set->kqueue_fd, &k_ev[0], count, NULL, 0, NULL);

    /*
     * When adding the postmaster's pid, we have to consider that it might
     * already have exited and perhaps even been replaced by another process
     * with the same pid.  If so, we have to defer reporting this as an event
     * until the next call to WaitEventSetWaitBlock().
     */

    if (rc < 0)
    {
        if (event->events == WL_POSTMASTER_DEATH &&
            (errno == ESRCH || errno == EACCES))
            set->report_postmaster_not_running = true;
        else
            ereport(ERROR,
                    (errcode_for_socket_access(),
                     errmsg("%s() failed: %m",
                            "kevent")));
    }
    else if (event->events == WL_POSTMASTER_DEATH &&
             PostmasterPid != getppid() &&
             !PostmasterIsAlive())
    {
        /*
         * The extra PostmasterIsAliveInternal() check prevents false alarms
         * on systems that give a different value for getppid() while being
         * traced by a debugger.
         */
        set->report_postmaster_not_running = true;
    }
}

#endif

#if defined(WAIT_USE_WIN32)
static void
WaitEventAdjustWin32(WaitEventSet *set, WaitEvent *event)
{
    HANDLE     *handle = &set->handles[event->pos + 1];

    if (event->events == WL_LATCH_SET)
    {
        Assert(set->latch != NULL);
        *handle = set->latch->event;
    }
    else if (event->events == WL_POSTMASTER_DEATH)
    {
        *handle = PostmasterHandle;
    }
    else
    {
        int         flags = FD_CLOSE;   /* always check for errors/EOF */

        if (event->events & WL_SOCKET_READABLE)
            flags |= FD_READ;
        if (event->events & WL_SOCKET_WRITEABLE)
            flags |= FD_WRITE;
        if (event->events & WL_SOCKET_CONNECTED)
            flags |= FD_CONNECT;

        if (*handle == WSA_INVALID_EVENT)
        {
            *handle = WSACreateEvent();
            if (*handle == WSA_INVALID_EVENT)
                elog(ERROR, "failed to create event for socket: error code %d",
                     WSAGetLastError());
        }
        if (WSAEventSelect(event->fd, *handle, flags) != 0)
            elog(ERROR, "failed to set up event for socket: error code %d",
                 WSAGetLastError());

        Assert(event->fd != PGINVALID_SOCKET);
    }
}
#endif

/*
 * Wait for events added to the set to happen, or until the timeout is
 * reached.  At most nevents occurred events are returned.
 *
 * If timeout = -1, block until an event occurs; if 0, check sockets for
 * readiness, but don't block; if > 0, block for at most timeout milliseconds.
 *
 * Returns the number of events occurred, or 0 if the timeout was reached.
 *
 * Returned events will have the fd, pos, user_data fields set to the
 * values associated with the registered event.
 */
int
WaitEventSetWait(WaitEventSet *set, long timeout,
                 WaitEvent *occurred_events, int nevents,
                 uint32 wait_event_info)
{
    int         returned_events = 0;
    instr_time  start_time;
    instr_time  cur_time;
    long        cur_timeout = -1;

    Assert(nevents > 0);

    /*
     * Initialize timeout if requested.  We must record the current time so
     * that we can determine the remaining timeout if interrupted.
     */
    if (timeout >= 0)
    {
        INSTR_TIME_SET_CURRENT(start_time);
        Assert(timeout >= 0 && timeout <= INT_MAX);
        cur_timeout = timeout;
    }

    pgstat_report_wait_start(wait_event_info);

#ifndef WIN32
    waiting = true;
#else
    /* Ensure that signals are serviced even if latch is already set */
    pgwin32_dispatch_queued_signals();
#endif
    while (returned_events == 0)
    {
        int         rc;

        /*
         * Check if the latch is set already. If so, leave the loop
         * immediately, avoid blocking again. We don't attempt to report any
         * other events that might also be satisfied.
         *
         * If someone sets the latch between this and the
         * WaitEventSetWaitBlock() below, the setter will write a byte to the
         * pipe (or signal us and the signal handler will do that), and the
         * readiness routine will return immediately.
         *
         * On unix, If there's a pending byte in the self pipe, we'll notice
         * whenever blocking. Only clearing the pipe in that case avoids
         * having to drain it every time WaitLatchOrSocket() is used. Should
         * the pipe-buffer fill up we're still ok, because the pipe is in
         * nonblocking mode. It's unlikely for that to happen, because the
         * self pipe isn't filled unless we're blocking (waiting = true), or
         * from inside a signal handler in latch_sigurg_handler().
         *
         * On windows, we'll also notice if there's a pending event for the
         * latch when blocking, but there's no danger of anything filling up,
         * as "Setting an event that is already set has no effect.".
         *
         * Note: we assume that the kernel calls involved in latch management
         * will provide adequate synchronization on machines with weak memory
         * ordering, so that we cannot miss seeing is_set if a notification
         * has already been queued.
         */
        if (set->latch && !set->latch->is_set)
        {
            /* about to sleep on a latch */
            set->latch->maybe_sleeping = true;
            pg_memory_barrier();
            /* and recheck */
        }

        if (set->latch && set->latch->is_set)
        {
            occurred_events->fd = PGINVALID_SOCKET;
            occurred_events->pos = set->latch_pos;
            occurred_events->user_data =
                set->events[set->latch_pos].user_data;
            occurred_events->events = WL_LATCH_SET;
            occurred_events++;
            returned_events++;

            /* could have been set above */
            set->latch->maybe_sleeping = false;

            break;
        }

        /*
         * Wait for events using the readiness primitive chosen at the top of
         * this file. If -1 is returned, a timeout has occurred, if 0 we have
         * to retry, everything >= 1 is the number of returned events.
         */
        rc = WaitEventSetWaitBlock(set, cur_timeout,
                                   occurred_events, nevents);

        if (set->latch)
        {
            Assert(set->latch->maybe_sleeping);
            set->latch->maybe_sleeping = false;
        }

        if (rc == -1)
            break;              /* timeout occurred */
        else
            returned_events = rc;

        /* If we're not done, update cur_timeout for next iteration */
        if (returned_events == 0 && timeout >= 0)
        {
            INSTR_TIME_SET_CURRENT(cur_time);
            INSTR_TIME_SUBTRACT(cur_time, start_time);
            cur_timeout = timeout - (long) INSTR_TIME_GET_MILLISEC(cur_time);
            if (cur_timeout <= 0)
                break;
        }
    }
#ifndef WIN32
    waiting = false;
#endif

    pgstat_report_wait_end();

    return returned_events;
}


#if defined(WAIT_USE_EPOLL)

/*
 * Wait using linux's epoll_wait(2).
 *
 * This is the preferable wait method, as several readiness notifications are
 * delivered, without having to iterate through all of set->events. The return
 * epoll_event struct contain a pointer to our events, making association
 * easy.
 */
static inline int
WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
                      WaitEvent *occurred_events, int nevents)
{
    int         returned_events = 0;
    int         rc;
    WaitEvent  *cur_event;
    struct epoll_event *cur_epoll_event;

    /* Sleep */
    rc = epoll_wait(set->epoll_fd, set->epoll_ret_events,
                    nevents, cur_timeout);

    /* Check return code */
    if (rc < 0)
    {
        /* EINTR is okay, otherwise complain */
        if (errno != EINTR)
        {
            waiting = false;
            ereport(ERROR,
                    (errcode_for_socket_access(),
                     errmsg("%s() failed: %m",
                            "epoll_wait")));
        }
        return 0;
    }
    else if (rc == 0)
    {
        /* timeout exceeded */
        return -1;
    }

    /*
     * At least one event occurred, iterate over the returned epoll events
     * until they're either all processed, or we've returned all the events
     * the caller desired.
     */
    for (cur_epoll_event = set->epoll_ret_events;
         cur_epoll_event < (set->epoll_ret_events + rc) &&
         returned_events < nevents;
         cur_epoll_event++)
    {
        /* epoll's data pointer is set to the associated WaitEvent */
        cur_event = (WaitEvent *) cur_epoll_event->data.ptr;

        occurred_events->pos = cur_event->pos;
        occurred_events->user_data = cur_event->user_data;
        occurred_events->events = 0;

        if (cur_event->events == WL_LATCH_SET &&
            cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP))
        {
            /* Drain the signalfd. */
            drain();

            if (set->latch && set->latch->is_set)
            {
                occurred_events->fd = PGINVALID_SOCKET;
                occurred_events->events = WL_LATCH_SET;
                occurred_events++;
                returned_events++;
            }
        }
        else if (cur_event->events == WL_POSTMASTER_DEATH &&
                 cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP))
        {
            /*
             * We expect an EPOLLHUP when the remote end is closed, but
             * because we don't expect the pipe to become readable or to have
             * any errors either, treat those cases as postmaster death, too.
             *
             * Be paranoid about a spurious event signaling the postmaster as
             * being dead.  There have been reports about that happening with
             * older primitives (select(2) to be specific), and a spurious
             * WL_POSTMASTER_DEATH event would be painful. Re-checking doesn't
             * cost much.
             */
            if (!PostmasterIsAliveInternal())
            {
                if (set->exit_on_postmaster_death)
                    proc_exit(1);
                occurred_events->fd = PGINVALID_SOCKET;
                occurred_events->events = WL_POSTMASTER_DEATH;
                occurred_events++;
                returned_events++;
            }
        }
        else if (cur_event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
        {
            Assert(cur_event->fd != PGINVALID_SOCKET);

            if ((cur_event->events & WL_SOCKET_READABLE) &&
                (cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP)))
            {
                /* data available in socket, or EOF */
                occurred_events->events |= WL_SOCKET_READABLE;
            }

            if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
                (cur_epoll_event->events & (EPOLLOUT | EPOLLERR | EPOLLHUP)))
            {
                /* writable, or EOF */
                occurred_events->events |= WL_SOCKET_WRITEABLE;
            }

            if (occurred_events->events != 0)
            {
                occurred_events->fd = cur_event->fd;
                occurred_events++;
                returned_events++;
            }
        }
    }

    return returned_events;
}

#elif defined(WAIT_USE_KQUEUE)

/*
 * Wait using kevent(2) on BSD-family systems and macOS.
 *
 * For now this mirrors the epoll code, but in future it could modify the fd
 * set in the same call to kevent as it uses for waiting instead of doing that
 * with separate system calls.
 */
static int
WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
                      WaitEvent *occurred_events, int nevents)
{
    int         returned_events = 0;
    int         rc;
    WaitEvent  *cur_event;
    struct kevent *cur_kqueue_event;
    struct timespec timeout;
    struct timespec *timeout_p;

    if (cur_timeout < 0)
        timeout_p = NULL;
    else
    {
        timeout.tv_sec = cur_timeout / 1000;
        timeout.tv_nsec = (cur_timeout % 1000) * 1000000;
        timeout_p = &timeout;
    }

    /*
     * Report postmaster events discovered by WaitEventAdjustKqueue() or an
     * earlier call to WaitEventSetWait().
     */
    if (unlikely(set->report_postmaster_not_running))
    {
        if (set->exit_on_postmaster_death)
            proc_exit(1);
        occurred_events->fd = PGINVALID_SOCKET;
        occurred_events->events = WL_POSTMASTER_DEATH;
        return 1;
    }

    /* Sleep */
    rc = kevent(set->kqueue_fd, NULL, 0,
                set->kqueue_ret_events, nevents,
                timeout_p);

    /* Check return code */
    if (rc < 0)
    {
        /* EINTR is okay, otherwise complain */
        if (errno != EINTR)
        {
            waiting = false;
            ereport(ERROR,
                    (errcode_for_socket_access(),
                     errmsg("%s() failed: %m",
                            "kevent")));
        }
        return 0;
    }
    else if (rc == 0)
    {
        /* timeout exceeded */
        return -1;
    }

    /*
     * At least one event occurred, iterate over the returned kqueue events
     * until they're either all processed, or we've returned all the events
     * the caller desired.
     */
    for (cur_kqueue_event = set->kqueue_ret_events;
         cur_kqueue_event < (set->kqueue_ret_events + rc) &&
         returned_events < nevents;
         cur_kqueue_event++)
    {
        /* kevent's udata points to the associated WaitEvent */
        cur_event = AccessWaitEvent(cur_kqueue_event);

        occurred_events->pos = cur_event->pos;
        occurred_events->user_data = cur_event->user_data;
        occurred_events->events = 0;

        if (cur_event->events == WL_LATCH_SET &&
            cur_kqueue_event->filter == EVFILT_SIGNAL)
        {
            if (set->latch && set->latch->is_set)
            {
                occurred_events->fd = PGINVALID_SOCKET;
                occurred_events->events = WL_LATCH_SET;
                occurred_events++;
                returned_events++;
            }
        }
        else if (cur_event->events == WL_POSTMASTER_DEATH &&
                 cur_kqueue_event->filter == EVFILT_PROC &&
                 (cur_kqueue_event->fflags & NOTE_EXIT) != 0)
        {
            /*
             * The kernel will tell this kqueue object only once about the
             * exit of the postmaster, so let's remember that for next time so
             * that we provide level-triggered semantics.
             */
            set->report_postmaster_not_running = true;

            if (set->exit_on_postmaster_death)
                proc_exit(1);
            occurred_events->fd = PGINVALID_SOCKET;
            occurred_events->events = WL_POSTMASTER_DEATH;
            occurred_events++;
            returned_events++;
        }
        else if (cur_event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
        {
            Assert(cur_event->fd >= 0);

            if ((cur_event->events & WL_SOCKET_READABLE) &&
                (cur_kqueue_event->filter == EVFILT_READ))
            {
                /* readable, or EOF */
                occurred_events->events |= WL_SOCKET_READABLE;
            }

            if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
                (cur_kqueue_event->filter == EVFILT_WRITE))
            {
                /* writable, or EOF */
                occurred_events->events |= WL_SOCKET_WRITEABLE;
            }

            if (occurred_events->events != 0)
            {
                occurred_events->fd = cur_event->fd;
                occurred_events++;
                returned_events++;
            }
        }
    }

    return returned_events;
}

#elif defined(WAIT_USE_POLL)

/*
 * Wait using poll(2).
 *
 * This allows to receive readiness notifications for several events at once,
 * but requires iterating through all of set->pollfds.
 */
static inline int
WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
                      WaitEvent *occurred_events, int nevents)
{
    int         returned_events = 0;
    int         rc;
    WaitEvent  *cur_event;
    struct pollfd *cur_pollfd;

    /* Sleep */
    rc = poll(set->pollfds, set->nevents, (int) cur_timeout);

    /* Check return code */
    if (rc < 0)
    {
        /* EINTR is okay, otherwise complain */
        if (errno != EINTR)
        {
            waiting = false;
            ereport(ERROR,
                    (errcode_for_socket_access(),
                     errmsg("%s() failed: %m",
                            "poll")));
        }
        return 0;
    }
    else if (rc == 0)
    {
        /* timeout exceeded */
        return -1;
    }

    for (cur_event = set->events, cur_pollfd = set->pollfds;
         cur_event < (set->events + set->nevents) &&
         returned_events < nevents;
         cur_event++, cur_pollfd++)
    {
        /* no activity on this FD, skip */
        if (cur_pollfd->revents == 0)
            continue;

        occurred_events->pos = cur_event->pos;
        occurred_events->user_data = cur_event->user_data;
        occurred_events->events = 0;

        if (cur_event->events == WL_LATCH_SET &&
            (cur_pollfd->revents & (POLLIN | POLLHUP | POLLERR | POLLNVAL)))
        {
            /* There's data in the self-pipe, clear it. */
            drain();

            if (set->latch && set->latch->is_set)
            {
                occurred_events->fd = PGINVALID_SOCKET;
                occurred_events->events = WL_LATCH_SET;
                occurred_events++;
                returned_events++;
            }
        }
        else if (cur_event->events == WL_POSTMASTER_DEATH &&
                 (cur_pollfd->revents & (POLLIN | POLLHUP | POLLERR | POLLNVAL)))
        {
            /*
             * We expect an POLLHUP when the remote end is closed, but because
             * we don't expect the pipe to become readable or to have any
             * errors either, treat those cases as postmaster death, too.
             *
             * Be paranoid about a spurious event signaling the postmaster as
             * being dead.  There have been reports about that happening with
             * older primitives (select(2) to be specific), and a spurious
             * WL_POSTMASTER_DEATH event would be painful. Re-checking doesn't
             * cost much.
             */
            if (!PostmasterIsAliveInternal())
            {
                if (set->exit_on_postmaster_death)
                    proc_exit(1);
                occurred_events->fd = PGINVALID_SOCKET;
                occurred_events->events = WL_POSTMASTER_DEATH;
                occurred_events++;
                returned_events++;
            }
        }
        else if (cur_event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
        {
            int         errflags = POLLHUP | POLLERR | POLLNVAL;

            Assert(cur_event->fd >= PGINVALID_SOCKET);

            if ((cur_event->events & WL_SOCKET_READABLE) &&
                (cur_pollfd->revents & (POLLIN | errflags)))
            {
                /* data available in socket, or EOF */
                occurred_events->events |= WL_SOCKET_READABLE;
            }

            if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
                (cur_pollfd->revents & (POLLOUT | errflags)))
            {
                /* writeable, or EOF */
                occurred_events->events |= WL_SOCKET_WRITEABLE;
            }

            if (occurred_events->events != 0)
            {
                occurred_events->fd = cur_event->fd;
                occurred_events++;
                returned_events++;
            }
        }
    }
    return returned_events;
}

#elif defined(WAIT_USE_WIN32)

/*
 * Wait using Windows' WaitForMultipleObjects().
 *
 * Unfortunately this will only ever return a single readiness notification at
 * a time.  Note that while the official documentation for
 * WaitForMultipleObjects is ambiguous about multiple events being "consumed"
 * with a single bWaitAll = FALSE call,
 * https://blogs.msdn.microsoft.com/oldnewthing/20150409-00/?p=44273 confirms
 * that only one event is "consumed".
 */
static inline int
WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
                      WaitEvent *occurred_events, int nevents)
{
    int         returned_events = 0;
    DWORD       rc;
    WaitEvent  *cur_event;

    /* Reset any wait events that need it */
    for (cur_event = set->events;
         cur_event < (set->events + set->nevents);
         cur_event++)
    {
        if (cur_event->reset)
        {
            WaitEventAdjustWin32(set, cur_event);
            cur_event->reset = false;
        }

        /*
         * Windows does not guarantee to log an FD_WRITE network event
         * indicating that more data can be sent unless the previous send()
         * failed with WSAEWOULDBLOCK.  While our caller might well have made
         * such a call, we cannot assume that here.  Therefore, if waiting for
         * write-ready, force the issue by doing a dummy send().  If the dummy
         * send() succeeds, assume that the socket is in fact write-ready, and
         * return immediately.  Also, if it fails with something other than
         * WSAEWOULDBLOCK, return a write-ready indication to let our caller
         * deal with the error condition.
         */
        if (cur_event->events & WL_SOCKET_WRITEABLE)
        {
            char        c;
            WSABUF      buf;
            DWORD       sent;
            int         r;

            buf.buf = &c;
            buf.len = 0;

            r = WSASend(cur_event->fd, &buf, 1, &sent, 0, NULL, NULL);
            if (r == 0 || WSAGetLastError() != WSAEWOULDBLOCK)
            {
                occurred_events->pos = cur_event->pos;
                occurred_events->user_data = cur_event->user_data;
                occurred_events->events = WL_SOCKET_WRITEABLE;
                occurred_events->fd = cur_event->fd;
                return 1;
            }
        }
    }

    /*
     * Sleep.
     *
     * Need to wait for ->nevents + 1, because signal handle is in [0].
     */
    rc = WaitForMultipleObjects(set->nevents + 1, set->handles, FALSE,
                                cur_timeout);

    /* Check return code */
    if (rc == WAIT_FAILED)
        elog(ERROR, "WaitForMultipleObjects() failed: error code %lu",
             GetLastError());
    else if (rc == WAIT_TIMEOUT)
    {
        /* timeout exceeded */
        return -1;
    }

    if (rc == WAIT_OBJECT_0)
    {
        /* Service newly-arrived signals */
        pgwin32_dispatch_queued_signals();
        return 0;               /* retry */
    }

    /*
     * With an offset of one, due to the always present pgwin32_signal_event,
     * the handle offset directly corresponds to a wait event.
     */
    cur_event = (WaitEvent *) &set->events[rc - WAIT_OBJECT_0 - 1];

    occurred_events->pos = cur_event->pos;
    occurred_events->user_data = cur_event->user_data;
    occurred_events->events = 0;

    if (cur_event->events == WL_LATCH_SET)
    {
        /*
         * We cannot use set->latch->event to reset the fired event if we
         * aren't waiting on this latch now.
         */
        if (!ResetEvent(set->handles[cur_event->pos + 1]))
            elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());

        if (set->latch && set->latch->is_set)
        {
            occurred_events->fd = PGINVALID_SOCKET;
            occurred_events->events = WL_LATCH_SET;
            occurred_events++;
            returned_events++;
        }
    }
    else if (cur_event->events == WL_POSTMASTER_DEATH)
    {
        /*
         * Postmaster apparently died.  Since the consequences of falsely
         * returning WL_POSTMASTER_DEATH could be pretty unpleasant, we take
         * the trouble to positively verify this with PostmasterIsAlive(),
         * even though there is no known reason to think that the event could
         * be falsely set on Windows.
         */
        if (!PostmasterIsAliveInternal())
        {
            if (set->exit_on_postmaster_death)
                proc_exit(1);
            occurred_events->fd = PGINVALID_SOCKET;
            occurred_events->events = WL_POSTMASTER_DEATH;
            occurred_events++;
            returned_events++;
        }
    }
    else if (cur_event->events & WL_SOCKET_MASK)
    {
        WSANETWORKEVENTS resEvents;
        HANDLE      handle = set->handles[cur_event->pos + 1];

        Assert(cur_event->fd);

        occurred_events->fd = cur_event->fd;

        ZeroMemory(&resEvents, sizeof(resEvents));
        if (WSAEnumNetworkEvents(cur_event->fd, handle, &resEvents) != 0)
            elog(ERROR, "failed to enumerate network events: error code %d",
                 WSAGetLastError());
        if ((cur_event->events & WL_SOCKET_READABLE) &&
            (resEvents.lNetworkEvents & FD_READ))
        {
            /* data available in socket */
            occurred_events->events |= WL_SOCKET_READABLE;

            /*------
             * WaitForMultipleObjects doesn't guarantee that a read event will
             * be returned if the latch is set at the same time.  Even if it
             * did, the caller might drop that event expecting it to reoccur
             * on next call.  So, we must force the event to be reset if this
             * WaitEventSet is used again in order to avoid an indefinite
             * hang.  Refer https://msdn.microsoft.com/en-us/library/windows/desktop/ms741576(v=vs.85).aspx
             * for the behavior of socket events.
             *------
             */
            cur_event->reset = true;
        }
        if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
            (resEvents.lNetworkEvents & FD_WRITE))
        {
            /* writeable */
            occurred_events->events |= WL_SOCKET_WRITEABLE;
        }
        if ((cur_event->events & WL_SOCKET_CONNECTED) &&
            (resEvents.lNetworkEvents & FD_CONNECT))
        {
            /* connected */
            occurred_events->events |= WL_SOCKET_CONNECTED;
        }
        if (resEvents.lNetworkEvents & FD_CLOSE)
        {
            /* EOF/error, so signal all caller-requested socket flags */
            occurred_events->events |= (cur_event->events & WL_SOCKET_MASK);
        }

        if (occurred_events->events != 0)
        {
            occurred_events++;
            returned_events++;
        }
    }

    return returned_events;
}
#endif

/*
 * Get the number of wait events registered in a given WaitEventSet.
 */
int
GetNumRegisteredWaitEvents(WaitEventSet *set)
{
    return set->nevents;
}

#if defined(WAIT_USE_POLL)

/*
 * SetLatch uses SIGURG to wake up the process waiting on the latch.
 *
 * Wake up WaitLatch, if we're waiting.
 */
static void
latch_sigurg_handler(SIGNAL_ARGS)
{
    int         save_errno = errno;

    if (waiting)
        sendSelfPipeByte();

    errno = save_errno;
}

/* Send one byte to the self-pipe, to wake up WaitLatch */
static void
sendSelfPipeByte(void)
{
    int         rc;
    char        dummy = 0;

retry:
    rc = write(selfpipe_writefd, &dummy, 1);
    if (rc < 0)
    {
        /* If interrupted by signal, just retry */
        if (errno == EINTR)
            goto retry;

        /*
         * If the pipe is full, we don't need to retry, the data that's there
         * already is enough to wake up WaitLatch.
         */
        if (errno == EAGAIN || errno == EWOULDBLOCK)
            return;

        /*
         * Oops, the write() failed for some other reason. We might be in a
         * signal handler, so it's not safe to elog(). We have no choice but
         * silently ignore the error.
         */
        return;
    }
}

#endif

#if defined(WAIT_USE_POLL) || defined(WAIT_USE_EPOLL)

/*
 * Read all available data from self-pipe or signalfd.
 *
 * Note: this is only called when waiting = true.  If it fails and doesn't
 * return, it must reset that flag first (though ideally, this will never
 * happen).
 */
static void
drain(void)
{
    char        buf[1024];
    int         rc;
    int         fd;

#ifdef WAIT_USE_POLL
    fd = selfpipe_readfd;
#else
    fd = signal_fd;
#endif

    for (;;)
    {
        rc = read(fd, buf, sizeof(buf));
        if (rc < 0)
        {
            if (errno == EAGAIN || errno == EWOULDBLOCK)
                break;          /* the descriptor is empty */
            else if (errno == EINTR)
                continue;       /* retry */
            else
            {
                waiting = false;
#ifdef WAIT_USE_POLL
                elog(ERROR, "read() on self-pipe failed: %m");
#else
                elog(ERROR, "read() on signalfd failed: %m");
#endif
            }
        }
        else if (rc == 0)
        {
            waiting = false;
#ifdef WAIT_USE_POLL
            elog(ERROR, "unexpected EOF on self-pipe");
#else
            elog(ERROR, "unexpected EOF on signalfd");
#endif
        }
        else if (rc < sizeof(buf))
        {
            /* we successfully drained the pipe; no need to read() again */
            break;
        }
        /* else buffer wasn't big enough, so read again */
    }
}

#endif