Use caller's memory context for radix tree iteration state

[pgsql.git] / src / backend / storage / ipc / latch.c

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2025, 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_SYS_SIGNALFD_H
#include <sys/signalfd.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 "utils/memutils.h"
#include "utils/resowner.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

/*
 * By default, we use a self-pipe with poll() and a signalfd with epoll(), if
 * available.  For testing the choice can also be manually specified.
 */
#if defined(WAIT_USE_POLL) || defined(WAIT_USE_EPOLL)
#if defined(WAIT_USE_SELF_PIPE) || defined(WAIT_USE_SIGNALFD)
/* don't overwrite manual choice */
#elif defined(WAIT_USE_EPOLL) && defined(HAVE_SYS_SIGNALFD_H)
#define WAIT_USE_SIGNALFD
#else
#define WAIT_USE_SELF_PIPE
#endif
#endif

/* typedef in latch.h */
struct WaitEventSet
{
        ResourceOwner owner;

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

#ifdef WAIT_USE_SELF_PIPE
/* 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_SELF_PIPE) || defined(WAIT_USE_SIGNALFD)
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);

/* ResourceOwner support to hold WaitEventSets */
static void ResOwnerReleaseWaitEventSet(Datum res);

static const ResourceOwnerDesc wait_event_set_resowner_desc =
{
        .name = "WaitEventSet",
        .release_phase = RESOURCE_RELEASE_AFTER_LOCKS,
        .release_priority = RELEASE_PRIO_WAITEVENTSETS,
        .ReleaseResource = ResOwnerReleaseWaitEventSet,
        .DebugPrint = NULL
};

/* Convenience wrappers over ResourceOwnerRemember/Forget */
static inline void
ResourceOwnerRememberWaitEventSet(ResourceOwner owner, WaitEventSet *set)
{
        ResourceOwnerRemember(owner, PointerGetDatum(set), &wait_event_set_resowner_desc);
}
static inline void
ResourceOwnerForgetWaitEventSet(ResourceOwner owner, WaitEventSet *set)
{
        ResourceOwnerForget(owner, PointerGetDatum(set), &wait_event_set_resowner_desc);
}


/*
 * 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_SELF_PIPE)
        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_SIGNALFD
        sigset_t        signalfd_mask;

        if (IsUnderPostmaster)
        {
                /*
                 * It would probably be safe to re-use the inherited signalfd since
                 * signalfds only see the current process's pending signals, but it
                 * seems less surprising to close it and create our own.
                 */
                if (signal_fd != -1)
                {
                        /* Release postmaster's signal FD; ignore any error */
                        (void) close(signal_fd);
                        signal_fd = -1;
                        ReleaseExternalFD();
                }
        }

        /* 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(NULL, 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_SELF_PIPE)
        close(selfpipe_readfd);
        close(selfpipe_writefd);
        selfpipe_readfd = -1;
        selfpipe_writefd = -1;
        selfpipe_owner_pid = InvalidPid;
#endif

#if defined(WAIT_USE_SIGNALFD)
        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_SELF_PIPE)
        /* Assert InitializeLatchSupport has been called in this process */
        Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
#elif defined(WAIT_USE_SIGNALFD)
        /* Assert InitializeLatchSupport has been called in this process */
        Assert(signal_fd >= 0);
#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)
{
        int                     owner_pid;

        /* Sanity checks */
        Assert(latch->is_shared);

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

        owner_pid = latch->owner_pid;
        if (owner_pid != 0)
                elog(PANIC, "latch already owned by PID %d", owner_pid);

        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(CurrentResourceOwner, 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_SELF_PIPE)
                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().
 *
 * The WaitEventSet is tracked by the given 'resowner'.  Use NULL for session
 * lifetime.
 */
WaitEventSet *
CreateWaitEventSet(ResourceOwner resowner, 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 be 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

        if (resowner != NULL)
                ResourceOwnerEnlarge(resowner);

        data = (char *) MemoryContextAllocZero(TopMemoryContext, 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 (resowner != NULL)
        {
                ResourceOwnerRememberWaitEventSet(resowner, set);
                set->owner = resowner;
        }

#if defined(WAIT_USE_EPOLL)
        if (!AcquireExternalFD())
                elog(ERROR, "AcquireExternalFD, for 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())
                elog(ERROR, "AcquireExternalFD, for 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 (set->owner)
        {
                ResourceOwnerForgetWaitEventSet(set->owner, set);
                set->owner = NULL;
        }

#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)
        for (WaitEvent *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);
}

/*
 * Free a previously created WaitEventSet in a child process after a fork().
 */
void
FreeWaitEventSetAfterFork(WaitEventSet *set)
{
#if defined(WAIT_USE_EPOLL)
        close(set->epoll_fd);
        ReleaseExternalFD();
#elif defined(WAIT_USE_KQUEUE)
        /* kqueues are not normally inherited by child processes */
        ReleaseExternalFD();
#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_SOCKET_ACCEPT: Wait for new connection to a server socket,
 *       can be combined with other WL_SOCKET_* events (on non-Windows
 *       platforms, this is the same as WL_SOCKET_READABLE)
 * - WL_SOCKET_CLOSED: Wait for socket to be closed by remote peer.
 * - 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/ACCEPT 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_SELF_PIPE)
                event->fd = selfpipe_readfd;
#elif defined(WAIT_USE_SIGNALFD)
                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 |
                                                                WL_SOCKET_CLOSED));

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

        /*
         * 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 |
                                                                WL_SOCKET_CLOSED));
                pollfd->events = 0;
                if (event->events & WL_SOCKET_READABLE)
                        pollfd->events |= POLLIN;
                if (event->events & WL_SOCKET_WRITEABLE)
                        pollfd->events |= POLLOUT;
#ifdef POLLRDHUP
                if (event->events & WL_SOCKET_CLOSED)
                        pollfd->events |= POLLRDHUP;
#endif
        }

        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 |
                                                         WL_SOCKET_CLOSED)));

        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 | WL_SOCKET_CLOSED))
                        old_filt_read = true;
                if (event->events & (WL_SOCKET_READABLE | WL_SOCKET_CLOSED))
                        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);
        }

        /* For WL_SOCKET_READ -> WL_SOCKET_CLOSED, no change needed. */
        if (count == 0)
                return;

        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 (event->events & WL_SOCKET_ACCEPT)
                        flags |= FD_ACCEPT;

                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;
        }
        else
                INSTR_TIME_SET_ZERO(start_time);

        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,
                                        Min(nevents, set->nevents_space), 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 |
                                                                          WL_SOCKET_CLOSED))
                {
                        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 ((cur_event->events & WL_SOCKET_CLOSED) &&
                                (cur_epoll_event->events & (EPOLLRDHUP | EPOLLERR | EPOLLHUP)))
                        {
                                /* remote peer shut down, or error */
                                occurred_events->events |= WL_SOCKET_CLOSED;
                        }

                        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,
                                Min(nevents, set->nevents_space),
                                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 |
                                                                          WL_SOCKET_CLOSED))
                {
                        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_CLOSED) &&
                                (cur_kqueue_event->filter == EVFILT_READ) &&
                                (cur_kqueue_event->flags & EV_EOF))
                        {
                                /* the remote peer has shut down */
                                occurred_events->events |= WL_SOCKET_CLOSED;
                        }

                        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 |
                                                                          WL_SOCKET_CLOSED))
                {
                        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;
                        }

#ifdef POLLRDHUP
                        if ((cur_event->events & WL_SOCKET_CLOSED) &&
                                (cur_pollfd->revents & (POLLRDHUP | errflags)))
                        {
                                /* remote peer closed, or error */
                                occurred_events->events |= WL_SOCKET_CLOSED;
                        }
#endif

                        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().  Each call only "consumes" one
 * event, so we keep calling until we've filled up our output buffer to match
 * the behavior of the other implementations.
 *
 * https://blogs.msdn.microsoft.com/oldnewthing/20150409-00/?p=44273
 */
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;
                }

                /*
                 * We associate the socket with a new event handle for each
                 * WaitEventSet.  FD_CLOSE is only generated once if the other end
                 * closes gracefully.  Therefore we might miss the FD_CLOSE
                 * notification, if it was delivered to another event after we stopped
                 * waiting for it.  Close that race by peeking for EOF after setting
                 * up this handle to receive notifications, and before entering the
                 * sleep.
                 *
                 * XXX If we had one event handle for the lifetime of a socket, we
                 * wouldn't need this.
                 */
                if (cur_event->events & WL_SOCKET_READABLE)
                {
                        char            c;
                        WSABUF          buf;
                        DWORD           received;
                        DWORD           flags;

                        buf.buf = &c;
                        buf.len = 1;
                        flags = MSG_PEEK;
                        if (WSARecv(cur_event->fd, &buf, 1, &received, &flags, NULL, NULL) == 0)
                        {
                                occurred_events->pos = cur_event->pos;
                                occurred_events->user_data = cur_event->user_data;
                                occurred_events->events = WL_SOCKET_READABLE;
                                occurred_events->fd = cur_event->fd;
                                return 1;
                        }
                }

                /*
                 * 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];

        for (;;)
        {
                int                     next_pos;
                int                     count;

                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 ((cur_event->events & WL_SOCKET_ACCEPT) &&
                                (resEvents.lNetworkEvents & FD_ACCEPT))
                        {
                                /* incoming connection could be accepted */
                                occurred_events->events |= WL_SOCKET_ACCEPT;
                        }
                        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++;
                        }
                }

                /* Is the output buffer full? */
                if (returned_events == nevents)
                        break;

                /* Have we run out of possible events? */
                next_pos = cur_event->pos + 1;
                if (next_pos == set->nevents)
                        break;

                /*
                 * Poll the rest of the event handles in the array starting at
                 * next_pos being careful to skip over the initial signal handle too.
                 * This time we use a zero timeout.
                 */
                count = set->nevents - next_pos;
                rc = WaitForMultipleObjects(count,
                                                                        set->handles + 1 + next_pos,
                                                                        false,
                                                                        0);

                /*
                 * We don't distinguish between errors and WAIT_TIMEOUT here because
                 * we already have events to report.
                 */
                if (rc < WAIT_OBJECT_0 || rc >= WAIT_OBJECT_0 + count)
                        break;

                /* We have another event to decode. */
                cur_event = &set->events[next_pos + (rc - WAIT_OBJECT_0)];
        }

        return returned_events;
}
#endif

/*
 * Return whether the current build options can report WL_SOCKET_CLOSED.
 */
bool
WaitEventSetCanReportClosed(void)
{
#if (defined(WAIT_USE_POLL) && defined(POLLRDHUP)) || \
        defined(WAIT_USE_EPOLL) || \
        defined(WAIT_USE_KQUEUE)
        return true;
#else
        return false;
#endif
}

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

#if defined(WAIT_USE_SELF_PIPE)

/*
 * 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)
{
        if (waiting)
                sendSelfPipeByte();
}

/* 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_SELF_PIPE) || defined(WAIT_USE_SIGNALFD)

/*
 * 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_SELF_PIPE
        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_SELF_PIPE
                                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_SELF_PIPE
                        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

static void
ResOwnerReleaseWaitEventSet(Datum res)
{
        WaitEventSet *set = (WaitEventSet *) DatumGetPointer(res);

        Assert(set->owner != NULL);
        set->owner = NULL;
        FreeWaitEventSet(set);
}