storage/ipc/latch.c

/*-------------------------------------------------------------------------
 *
 * latch.c
 *	  Routines for inter-process latches
 *
 * The Unix implementation uses the so-called self-pipe trick to overcome the
 * race condition involved with poll() (or epoll_wait() on linux) 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.
 *
 * When SetLatch is called from the same process that owns the latch,
 * SetLatch writes the byte directly to the pipe. If it's owned by another
 * process, SIGUSR1 is sent and the signal handler in the waiting process
 * writes the byte to the pipe on behalf of the signaling process.
 *
 * 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) 1996-2017, 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_POLL_H
#include <poll.h>
#endif

#include "miscadmin.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "portability/instr_time.h"
#include "postmaster/postmaster.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/shmem.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_WIN32)
/* don't overwrite manual choice */
#elif defined(HAVE_SYS_EPOLL_H)
#define WAIT_USE_EPOLL
#elif defined(HAVE_POLL)
#define WAIT_USE_POLL
#elif WIN32
#define WAIT_USE_WIN32
#else
#error "no wait set implementation available"
#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;

#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_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
};

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

/* 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 sendSelfPipeByte(void);
static void drainSelfPipe(void);
#endif							/* WIN32 */

#if defined(WAIT_USE_EPOLL)
static void WaitEventAdjustEpoll(WaitEventSet *set, WaitEvent *event, int action);
#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)
{
#ifndef WIN32
	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;
		}
		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.
			 */
			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;
#else
	/* currently, nothing to do here for Windows */
#endif
}

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

#ifndef WIN32
	/* Assert InitializeLatchSupport has been called in this process */
	Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
#else
	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(volatile 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->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.
 *
 * In any process that calls OwnLatch(), make sure that
 * latch_sigusr1_handler() is called from the SIGUSR1 signal handler,
 * as shared latches use SIGUSR1 for inter-process communication.
 */
void
OwnLatch(volatile Latch *latch)
{
	/* Sanity checks */
	Assert(latch->is_shared);

#ifndef WIN32
	/* 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(volatile 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(volatile Latch *latch, int wakeEvents, long timeout,
		  uint32 wait_event_info)
{
	return WaitLatchOrSocket(latch, wakeEvents, PGINVALID_SOCKET, timeout,
							 wait_event_info);
}

/*
 * 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.
 *
 * 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(volatile 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 *) latch, NULL);

	if (wakeEvents & WL_POSTMASTER_DEATH && IsUnderPostmaster)
		AddWaitEventToSet(set, WL_POSTMASTER_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(volatile 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;

#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 to wake up the
	 * poll()/epoll_wait() 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 (waiting)
			sendSelfPipeByte();
	}
	else
		kill(owner_pid, SIGUSR1);
#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(volatile Latch *latch)
{
	/* Only the owner should reset the latch */
	Assert(latch->owner_pid == MyProcPid);

	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_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_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;

#if defined(WAIT_USE_EPOLL)
#ifdef EPOLL_CLOEXEC
	set->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
	if (set->epoll_fd < 0)
		elog(ERROR, "epoll_create1 failed: %m");
#else
	/* cope with ancient glibc lacking epoll_create1 (e.g., RHEL5) */
	set->epoll_fd = epoll_create(nevents);
	if (set->epoll_fd < 0)
		elog(ERROR, "epoll_create failed: %m");
	if (fcntl(set->epoll_fd, F_SETFD, FD_CLOEXEC) == -1)
		elog(ERROR, "fcntl(F_SETFD) failed on epoll descriptor: %m");
#endif							/* EPOLL_CLOEXEC */
#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 FD_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);
#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)
 *
 * 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 (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;
#ifndef WIN32
		event->fd = selfpipe_readfd;
#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_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.
 *
 * 'pos' is the id returned by AddWaitEventToSet.
 */
void
ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch)
{
	WaitEvent  *event;

	Assert(pos < set->nevents);

	event = &set->events[pos];

	/*
	 * 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)
	{
		/* we could allow to disable latch events for a while */
		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)
	{
		set->latch = latch;
	}

#if defined(WAIT_USE_EPOLL)
	WaitEventAdjustEpoll(set, event, EPOLL_CTL_MOD);
#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("epoll_ctl() failed: %m")));
}
#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_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 %u",
					 WSAGetLastError());
		}
		if (WSAEventSelect(event->fd, *handle, flags) != 0)
			elog(ERROR, "failed to set up event for socket: error code %u",
				 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_sigusr1_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)
		{
			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++;

			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 (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 preferrable 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("epoll_wait() failed: %m")));
		}
		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))
		{
			/* There's data in the self-pipe, clear it. */
			drainSelfPipe();

			if (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 signalling 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 (!PostmasterIsAlive())
			{
				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_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("poll() failed: %m")));
		}
		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. */
			drainSelfPipe();

			if (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 signalling 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 (!PostmasterIsAlive())
			{
				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)
	{
		if (!ResetEvent(set->latch->event))
			elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());

		if (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 (!PostmasterIsAlive())
		{
			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 %u",
				 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

/*
 * SetLatch uses SIGUSR1 to wake up the process waiting on the latch.
 *
 * Wake up WaitLatch, if we're waiting.  (We might not be, since SIGUSR1 is
 * overloaded for multiple purposes; or we might not have reached WaitLatch
 * yet, in which case we don't need to fill the pipe either.)
 *
 * NB: when calling this in a signal handler, be sure to save and restore
 * errno around it.
 */
#ifndef WIN32
void
latch_sigusr1_handler(void)
{
	if (waiting)
		sendSelfPipeByte();
}
#endif							/* !WIN32 */

/* Send one byte to the self-pipe, to wake up WaitLatch */
#ifndef WIN32
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							/* !WIN32 */

/*
 * Read all available data from the self-pipe
 *
 * 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).
 */
#ifndef WIN32
static void
drainSelfPipe(void)
{
	/*
	 * There shouldn't normally be more than one byte in the pipe, or maybe a
	 * few bytes if multiple processes run SetLatch at the same instant.
	 */
	char		buf[16];
	int			rc;

	for (;;)
	{
		rc = read(selfpipe_readfd, buf, sizeof(buf));
		if (rc < 0)
		{
			if (errno == EAGAIN || errno == EWOULDBLOCK)
				break;			/* the pipe is empty */
			else if (errno == EINTR)
				continue;		/* retry */
			else
			{
				waiting = false;
				elog(ERROR, "read() on self-pipe failed: %m");
			}
		}
		else if (rc == 0)
		{
			waiting = false;
			elog(ERROR, "unexpected EOF on self-pipe");
		}
		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							/* !WIN32 */