Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2158 insertions, 0 deletions

diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
new file mode 100644
index 0000000..3427bcf
--- /dev/null
+++ b/src/backend/storage/ipc/latch.c
@@ -0,0 +1,2158 @@
+/*-------------------------------------------------------------------------
+ *
+ * latch.c
+ *	  Routines for inter-process latches
+ *
+ * The poll() implementation uses the so-called self-pipe trick to overcome the
+ * race condition involved with poll() and setting a global flag in the signal
+ * handler. When a latch is set and the current process is waiting for it, the
+ * signal handler wakes up the poll() in WaitLatch by writing a byte to a pipe.
+ * A signal by itself doesn't interrupt poll() on all platforms, and even on
+ * platforms where it does, a signal that arrives just before the poll() call
+ * does not prevent poll() from entering sleep. An incoming byte on a pipe
+ * however reliably interrupts the sleep, and causes poll() to return
+ * immediately even if the signal arrives before poll() begins.
+ *
+ * The epoll() implementation overcomes the race with a different technique: it
+ * keeps SIGURG blocked and consumes from a signalfd() descriptor instead.  We
+ * don't need to register a signal handler or create our own self-pipe.  We
+ * assume that any system that has Linux epoll() also has Linux signalfd().
+ *
+ * The kqueue() implementation waits for SIGURG with EVFILT_SIGNAL.
+ *
+ * The Windows implementation uses Windows events that are inherited by all
+ * postmaster child processes. There's no need for the self-pipe trick there.
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/ipc/latch.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <fcntl.h>
+#include <limits.h>
+#include <signal.h>
+#include <unistd.h>
+#ifdef HAVE_SYS_EPOLL_H
+#include <sys/epoll.h>
+#endif
+#ifdef HAVE_SYS_EVENT_H
+#include <sys/event.h>
+#endif
+#ifdef HAVE_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 "storage/shmem.h"
+#include "utils/memutils.h"
+
+/*
+ * Select the fd readiness primitive to use. Normally the "most modern"
+ * primitive supported by the OS will be used, but for testing it can be
+ * useful to manually specify the used primitive.  If desired, just add a
+ * define somewhere before this block.
+ */
+#if defined(WAIT_USE_EPOLL) || defined(WAIT_USE_POLL) || \
+	defined(WAIT_USE_KQUEUE) || defined(WAIT_USE_WIN32)
+/* don't overwrite manual choice */
+#elif defined(HAVE_SYS_EPOLL_H)
+#define WAIT_USE_EPOLL
+#elif defined(HAVE_KQUEUE)
+#define WAIT_USE_KQUEUE
+#elif defined(HAVE_POLL)
+#define WAIT_USE_POLL
+#elif WIN32
+#define WAIT_USE_WIN32
+#else
+#error "no wait set implementation available"
+#endif
+
+/*
+ * By default, we use a self-pipe with poll() and a signalfd with epoll(), if
+ * available.  We avoid signalfd on illumos for now based on problem reports.
+ * 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) && \
+	!defined(__illumos__)
+#define WAIT_USE_SIGNALFD
+#else
+#define WAIT_USE_SELF_PIPE
+#endif
+#endif
+
+/* typedef in latch.h */
+struct WaitEventSet
+{
+	int			nevents;		/* number of registered events */
+	int			nevents_space;	/* maximum number of events in this set */
+
+	/*
+	 * Array, of nevents_space length, storing the definition of events this
+	 * set is waiting for.
+	 */
+	WaitEvent  *events;
+
+	/*
+	 * If WL_LATCH_SET is specified in any wait event, latch is a pointer to
+	 * said latch, and latch_pos the offset in the ->events array. This is
+	 * useful because we check the state of the latch before performing doing
+	 * syscalls related to waiting.
+	 */
+	Latch	   *latch;
+	int			latch_pos;
+
+	/*
+	 * WL_EXIT_ON_PM_DEATH is converted to WL_POSTMASTER_DEATH, but this flag
+	 * is set so that we'll exit immediately if postmaster death is detected,
+	 * instead of returning.
+	 */
+	bool		exit_on_postmaster_death;
+
+#if defined(WAIT_USE_EPOLL)
+	int			epoll_fd;
+	/* epoll_wait returns events in a user provided arrays, allocate once */
+	struct epoll_event *epoll_ret_events;
+#elif defined(WAIT_USE_KQUEUE)
+	int			kqueue_fd;
+	/* kevent returns events in a user provided arrays, allocate once */
+	struct kevent *kqueue_ret_events;
+	bool		report_postmaster_not_running;
+#elif defined(WAIT_USE_POLL)
+	/* poll expects events to be waited on every poll() call, prepare once */
+	struct pollfd *pollfds;
+#elif defined(WAIT_USE_WIN32)
+
+	/*
+	 * Array of windows events. The first element always contains
+	 * pgwin32_signal_event, so the remaining elements are offset by one (i.e.
+	 * event->pos + 1).
+	 */
+	HANDLE	   *handles;
+#endif
+};
+
+/* A common WaitEventSet used to implement WatchLatch() */
+static WaitEventSet *LatchWaitSet;
+
+/* The position of the latch in LatchWaitSet. */
+#define LatchWaitSetLatchPos 0
+
+#ifndef WIN32
+/* Are we currently in WaitLatch? The signal handler would like to know. */
+static volatile sig_atomic_t waiting = false;
+#endif
+
+#ifdef WAIT_USE_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);
+
+/*
+ * 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;
+
+	/* Block SIGURG, because we'll receive it through a signalfd. */
+	sigaddset(&UnBlockSig, SIGURG);
+
+	/* Set up the signalfd to receive SIGURG notifications. */
+	sigemptyset(&signalfd_mask);
+	sigaddset(&signalfd_mask, SIGURG);
+	signal_fd = signalfd(-1, &signalfd_mask, SFD_NONBLOCK | SFD_CLOEXEC);
+	if (signal_fd < 0)
+		elog(FATAL, "signalfd() failed");
+	ReserveExternalFD();
+#endif
+
+#ifdef WAIT_USE_KQUEUE
+	/* Ignore SIGURG, because we'll receive it via kqueue. */
+	pqsignal(SIGURG, SIG_IGN);
+#endif
+}
+
+void
+InitializeLatchWaitSet(void)
+{
+	int			latch_pos PG_USED_FOR_ASSERTS_ONLY;
+
+	Assert(LatchWaitSet == NULL);
+
+	/* Set up the WaitEventSet used by WaitLatch(). */
+	LatchWaitSet = CreateWaitEventSet(TopMemoryContext, 2);
+	latch_pos = AddWaitEventToSet(LatchWaitSet, WL_LATCH_SET, PGINVALID_SOCKET,
+								  MyLatch, NULL);
+	if (IsUnderPostmaster)
+		AddWaitEventToSet(LatchWaitSet, WL_EXIT_ON_PM_DEATH,
+						  PGINVALID_SOCKET, NULL, NULL);
+
+	Assert(latch_pos == LatchWaitSetLatchPos);
+}
+
+void
+ShutdownLatchSupport(void)
+{
+#if defined(WAIT_USE_POLL)
+	pqsignal(SIGURG, SIG_IGN);
+#endif
+
+	if (LatchWaitSet)
+	{
+		FreeWaitEventSet(LatchWaitSet);
+		LatchWaitSet = NULL;
+	}
+
+#if defined(WAIT_USE_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)
+{
+	/* 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
+
+	if (latch->owner_pid != 0)
+		elog(ERROR, "latch already owned");
+
+	latch->owner_pid = MyProcPid;
+}
+
+/*
+ * Disown a shared latch currently owned by the current process.
+ */
+void
+DisownLatch(Latch *latch)
+{
+	Assert(latch->is_shared);
+	Assert(latch->owner_pid == MyProcPid);
+
+	latch->owner_pid = 0;
+}
+
+/*
+ * Wait for a given latch to be set, or for postmaster death, or until timeout
+ * is exceeded. 'wakeEvents' is a bitmask that specifies which of those events
+ * to wait for. If the latch is already set (and WL_LATCH_SET is given), the
+ * function returns immediately.
+ *
+ * The "timeout" is given in milliseconds. It must be >= 0 if WL_TIMEOUT flag
+ * is given.  Although it is declared as "long", we don't actually support
+ * timeouts longer than INT_MAX milliseconds.  Note that some extra overhead
+ * is incurred when WL_TIMEOUT is given, so avoid using a timeout if possible.
+ *
+ * The latch must be owned by the current process, ie. it must be a
+ * process-local latch initialized with InitLatch, or a shared latch
+ * associated with the current process by calling OwnLatch.
+ *
+ * Returns bit mask indicating which condition(s) caused the wake-up. Note
+ * that if multiple wake-up conditions are true, there is no guarantee that
+ * we return all of them in one call, but we will return at least one.
+ */
+int
+WaitLatch(Latch *latch, int wakeEvents, long timeout,
+		  uint32 wait_event_info)
+{
+	WaitEvent	event;
+
+	/* Postmaster-managed callers must handle postmaster death somehow. */
+	Assert(!IsUnderPostmaster ||
+		   (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
+		   (wakeEvents & WL_POSTMASTER_DEATH));
+
+	/*
+	 * Some callers may have a latch other than MyLatch, or no latch at all,
+	 * or want to handle postmaster death differently.  It's cheap to assign
+	 * those, so just do it every time.
+	 */
+	if (!(wakeEvents & WL_LATCH_SET))
+		latch = NULL;
+	ModifyWaitEvent(LatchWaitSet, LatchWaitSetLatchPos, WL_LATCH_SET, latch);
+	LatchWaitSet->exit_on_postmaster_death =
+		((wakeEvents & WL_EXIT_ON_PM_DEATH) != 0);
+
+	if (WaitEventSetWait(LatchWaitSet,
+						 (wakeEvents & WL_TIMEOUT) ? timeout : -1,
+						 &event, 1,
+						 wait_event_info) == 0)
+		return WL_TIMEOUT;
+	else
+		return event.events;
+}
+
+/*
+ * Like WaitLatch, but with an extra socket argument for WL_SOCKET_*
+ * conditions.
+ *
+ * When waiting on a socket, EOF and error conditions always cause the socket
+ * to be reported as readable/writable/connected, so that the caller can deal
+ * with the condition.
+ *
+ * wakeEvents must include either WL_EXIT_ON_PM_DEATH for automatic exit
+ * if the postmaster dies or WL_POSTMASTER_DEATH for a flag set in the
+ * return value if the postmaster dies.  The latter is useful for rare cases
+ * where some behavior other than immediate exit is needed.
+ *
+ * NB: These days this is just a wrapper around the WaitEventSet API. When
+ * using a latch very frequently, consider creating a longer living
+ * WaitEventSet instead; that's more efficient.
+ */
+int
+WaitLatchOrSocket(Latch *latch, int wakeEvents, pgsocket sock,
+				  long timeout, uint32 wait_event_info)
+{
+	int			ret = 0;
+	int			rc;
+	WaitEvent	event;
+	WaitEventSet *set = CreateWaitEventSet(CurrentMemoryContext, 3);
+
+	if (wakeEvents & WL_TIMEOUT)
+		Assert(timeout >= 0);
+	else
+		timeout = -1;
+
+	if (wakeEvents & WL_LATCH_SET)
+		AddWaitEventToSet(set, WL_LATCH_SET, PGINVALID_SOCKET,
+						  latch, NULL);
+
+	/* Postmaster-managed callers must handle postmaster death somehow. */
+	Assert(!IsUnderPostmaster ||
+		   (wakeEvents & WL_EXIT_ON_PM_DEATH) ||
+		   (wakeEvents & WL_POSTMASTER_DEATH));
+
+	if ((wakeEvents & WL_POSTMASTER_DEATH) && IsUnderPostmaster)
+		AddWaitEventToSet(set, WL_POSTMASTER_DEATH, PGINVALID_SOCKET,
+						  NULL, NULL);
+
+	if ((wakeEvents & WL_EXIT_ON_PM_DEATH) && IsUnderPostmaster)
+		AddWaitEventToSet(set, WL_EXIT_ON_PM_DEATH, PGINVALID_SOCKET,
+						  NULL, NULL);
+
+	if (wakeEvents & WL_SOCKET_MASK)
+	{
+		int			ev;
+
+		ev = wakeEvents & WL_SOCKET_MASK;
+		AddWaitEventToSet(set, ev, sock, NULL, NULL);
+	}
+
+	rc = WaitEventSetWait(set, timeout, &event, 1, wait_event_info);
+
+	if (rc == 0)
+		ret |= WL_TIMEOUT;
+	else
+	{
+		ret |= event.events & (WL_LATCH_SET |
+							   WL_POSTMASTER_DEATH |
+							   WL_SOCKET_MASK);
+	}
+
+	FreeWaitEventSet(set);
+
+	return ret;
+}
+
+/*
+ * Sets a latch and wakes up anyone waiting on it.
+ *
+ * This is cheap if the latch is already set, otherwise not so much.
+ *
+ * NB: when calling this in a signal handler, be sure to save and restore
+ * errno around it.  (That's standard practice in most signal handlers, of
+ * course, but we used to omit it in handlers that only set a flag.)
+ *
+ * NB: this function is called from critical sections and signal handlers so
+ * throwing an error is not a good idea.
+ */
+void
+SetLatch(Latch *latch)
+{
+#ifndef WIN32
+	pid_t		owner_pid;
+#else
+	HANDLE		handle;
+#endif
+
+	/*
+	 * The memory barrier has to be placed here to ensure that any flag
+	 * variables possibly changed by this process have been flushed to main
+	 * memory, before we check/set is_set.
+	 */
+	pg_memory_barrier();
+
+	/* Quick exit if already set */
+	if (latch->is_set)
+		return;
+
+	latch->is_set = true;
+
+	pg_memory_barrier();
+	if (!latch->maybe_sleeping)
+		return;
+
+#ifndef WIN32
+
+	/*
+	 * See if anyone's waiting for the latch. It can be the current process if
+	 * we're in a signal handler. We use the self-pipe or SIGURG to ourselves
+	 * to wake up WaitEventSetWaitBlock() without races in that case. If it's
+	 * another process, send a signal.
+	 *
+	 * Fetch owner_pid only once, in case the latch is concurrently getting
+	 * owned or disowned. XXX: This assumes that pid_t is atomic, which isn't
+	 * guaranteed to be true! In practice, the effective range of pid_t fits
+	 * in a 32 bit integer, and so should be atomic. In the worst case, we
+	 * might end up signaling the wrong process. Even then, you're very
+	 * unlucky if a process with that bogus pid exists and belongs to
+	 * Postgres; and PG database processes should handle excess SIGUSR1
+	 * interrupts without a problem anyhow.
+	 *
+	 * Another sort of race condition that's possible here is for a new
+	 * process to own the latch immediately after we look, so we don't signal
+	 * it. This is okay so long as all callers of ResetLatch/WaitLatch follow
+	 * the standard coding convention of waiting at the bottom of their loops,
+	 * not the top, so that they'll correctly process latch-setting events
+	 * that happen before they enter the loop.
+	 */
+	owner_pid = latch->owner_pid;
+	if (owner_pid == 0)
+		return;
+	else if (owner_pid == MyProcPid)
+	{
+#if defined(WAIT_USE_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().
+ */
+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 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
+
+	data = (char *) MemoryContextAllocZero(context, sz);
+
+	set = (WaitEventSet *) data;
+	data += MAXALIGN(sizeof(WaitEventSet));
+
+	set->events = (WaitEvent *) data;
+	data += MAXALIGN(sizeof(WaitEvent) * nevents);
+
+#if defined(WAIT_USE_EPOLL)
+	set->epoll_ret_events = (struct epoll_event *) data;
+	data += MAXALIGN(sizeof(struct epoll_event) * nevents);
+#elif defined(WAIT_USE_KQUEUE)
+	set->kqueue_ret_events = (struct kevent *) data;
+	data += MAXALIGN(sizeof(struct kevent) * nevents);
+#elif defined(WAIT_USE_POLL)
+	set->pollfds = (struct pollfd *) data;
+	data += MAXALIGN(sizeof(struct pollfd) * nevents);
+#elif defined(WAIT_USE_WIN32)
+	set->handles = (HANDLE) data;
+	data += MAXALIGN(sizeof(HANDLE) * nevents);
+#endif
+
+	set->latch = NULL;
+	set->nevents_space = nevents;
+	set->exit_on_postmaster_death = false;
+
+#if defined(WAIT_USE_EPOLL)
+	if (!AcquireExternalFD())
+	{
+		/* treat this as though epoll_create1 itself returned EMFILE */
+		elog(ERROR, "epoll_create1 failed: %m");
+	}
+	set->epoll_fd = epoll_create1(EPOLL_CLOEXEC);
+	if (set->epoll_fd < 0)
+	{
+		ReleaseExternalFD();
+		elog(ERROR, "epoll_create1 failed: %m");
+	}
+#elif defined(WAIT_USE_KQUEUE)
+	if (!AcquireExternalFD())
+	{
+		/* treat this as though kqueue itself returned EMFILE */
+		elog(ERROR, "kqueue failed: %m");
+	}
+	set->kqueue_fd = kqueue();
+	if (set->kqueue_fd < 0)
+	{
+		ReleaseExternalFD();
+		elog(ERROR, "kqueue failed: %m");
+	}
+	if (fcntl(set->kqueue_fd, F_SETFD, FD_CLOEXEC) == -1)
+	{
+		int			save_errno = errno;
+
+		close(set->kqueue_fd);
+		ReleaseExternalFD();
+		errno = save_errno;
+		elog(ERROR, "fcntl(F_SETFD) failed on kqueue descriptor: %m");
+	}
+	set->report_postmaster_not_running = false;
+#elif defined(WAIT_USE_WIN32)
+
+	/*
+	 * To handle signals while waiting, we need to add a win32 specific event.
+	 * We accounted for the additional event at the top of this routine. See
+	 * port/win32/signal.c for more details.
+	 *
+	 * Note: pgwin32_signal_event should be first to ensure that it will be
+	 * reported when multiple events are set.  We want to guarantee that
+	 * pending signals are serviced.
+	 */
+	set->handles[0] = pgwin32_signal_event;
+	StaticAssertStmt(WSA_INVALID_EVENT == NULL, "");
+#endif
+
+	return set;
+}
+
+/*
+ * Free a previously created WaitEventSet.
+ *
+ * Note: preferably, this shouldn't have to free any resources that could be
+ * inherited across an exec().  If it did, we'd likely leak those resources in
+ * many scenarios.  For the epoll case, we ensure that by setting EPOLL_CLOEXEC
+ * when the FD is created.  For the Windows case, we assume that the handles
+ * involved are non-inheritable.
+ */
+void
+FreeWaitEventSet(WaitEventSet *set)
+{
+#if defined(WAIT_USE_EPOLL)
+	close(set->epoll_fd);
+	ReleaseExternalFD();
+#elif defined(WAIT_USE_KQUEUE)
+	close(set->kqueue_fd);
+	ReleaseExternalFD();
+#elif defined(WAIT_USE_WIN32)
+	WaitEvent  *cur_event;
+
+	for (cur_event = set->events;
+		 cur_event < (set->events + set->nevents);
+		 cur_event++)
+	{
+		if (cur_event->events & WL_LATCH_SET)
+		{
+			/* uses the latch's HANDLE */
+		}
+		else if (cur_event->events & WL_POSTMASTER_DEATH)
+		{
+			/* uses PostmasterHandle */
+		}
+		else
+		{
+			/* Clean up the event object we created for the socket */
+			WSAEventSelect(cur_event->fd, NULL, 0);
+			WSACloseEvent(set->handles[cur_event->pos + 1]);
+		}
+	}
+#endif
+
+	pfree(set);
+}
+
+/* ---
+ * Add an event to the set. Possible events are:
+ * - WL_LATCH_SET: Wait for the latch to be set
+ * - WL_POSTMASTER_DEATH: Wait for postmaster to die
+ * - WL_SOCKET_READABLE: Wait for socket to become readable,
+ *	 can be combined in one event with other WL_SOCKET_* events
+ * - WL_SOCKET_WRITEABLE: Wait for socket to become writeable,
+ *	 can be combined with other WL_SOCKET_* events
+ * - WL_SOCKET_CONNECTED: Wait for socket connection to be established,
+ *	 can be combined with other WL_SOCKET_* events (on non-Windows
+ *	 platforms, this is the same as WL_SOCKET_WRITEABLE)
+ * - WL_EXIT_ON_PM_DEATH: Exit immediately if the postmaster dies
+ *
+ * Returns the offset in WaitEventSet->events (starting from 0), which can be
+ * used to modify previously added wait events using ModifyWaitEvent().
+ *
+ * In the WL_LATCH_SET case the latch must be owned by the current process,
+ * i.e. it must be a process-local latch initialized with InitLatch, or a
+ * shared latch associated with the current process by calling OwnLatch.
+ *
+ * In the WL_SOCKET_READABLE/WRITEABLE/CONNECTED cases, EOF and error
+ * conditions cause the socket to be reported as readable/writable/connected,
+ * so that the caller can deal with the condition.
+ *
+ * The user_data pointer specified here will be set for the events returned
+ * by WaitEventSetWait(), allowing to easily associate additional data with
+ * events.
+ */
+int
+AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd, Latch *latch,
+				  void *user_data)
+{
+	WaitEvent  *event;
+
+	/* not enough space */
+	Assert(set->nevents < set->nevents_space);
+
+	if (events == WL_EXIT_ON_PM_DEATH)
+	{
+		events = WL_POSTMASTER_DEATH;
+		set->exit_on_postmaster_death = true;
+	}
+
+	if (latch)
+	{
+		if (latch->owner_pid != MyProcPid)
+			elog(ERROR, "cannot wait on a latch owned by another process");
+		if (set->latch)
+			elog(ERROR, "cannot wait on more than one latch");
+		if ((events & WL_LATCH_SET) != WL_LATCH_SET)
+			elog(ERROR, "latch events only support being set");
+	}
+	else
+	{
+		if (events & WL_LATCH_SET)
+			elog(ERROR, "cannot wait on latch without a specified latch");
+	}
+
+	/* waiting for socket readiness without a socket indicates a bug */
+	if (fd == PGINVALID_SOCKET && (events & WL_SOCKET_MASK))
+		elog(ERROR, "cannot wait on socket event without a socket");
+
+	event = &set->events[set->nevents];
+	event->pos = set->nevents++;
+	event->fd = fd;
+	event->events = events;
+	event->user_data = user_data;
+#ifdef WIN32
+	event->reset = false;
+#endif
+
+	if (events == WL_LATCH_SET)
+	{
+		set->latch = latch;
+		set->latch_pos = event->pos;
+#if defined(WAIT_USE_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));
+
+		if (event->events & WL_SOCKET_READABLE)
+			epoll_ev.events |= EPOLLIN;
+		if (event->events & WL_SOCKET_WRITEABLE)
+			epoll_ev.events |= EPOLLOUT;
+	}
+
+	/*
+	 * Even though unused, we also pass epoll_ev as the data argument if
+	 * EPOLL_CTL_DEL is passed as action.  There used to be an epoll bug
+	 * requiring that, and actually it makes the code simpler...
+	 */
+	rc = epoll_ctl(set->epoll_fd, action, event->fd, &epoll_ev);
+
+	if (rc < 0)
+		ereport(ERROR,
+				(errcode_for_socket_access(),
+				 errmsg("%s() failed: %m",
+						"epoll_ctl")));
+}
+#endif
+
+#if defined(WAIT_USE_POLL)
+static void
+WaitEventAdjustPoll(WaitEventSet *set, WaitEvent *event)
+{
+	struct pollfd *pollfd = &set->pollfds[event->pos];
+
+	pollfd->revents = 0;
+	pollfd->fd = event->fd;
+
+	/* prepare pollfd entry once */
+	if (event->events == WL_LATCH_SET)
+	{
+		Assert(set->latch != NULL);
+		pollfd->events = POLLIN;
+	}
+	else if (event->events == WL_POSTMASTER_DEATH)
+	{
+		pollfd->events = POLLIN;
+	}
+	else
+	{
+		Assert(event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE));
+		pollfd->events = 0;
+		if (event->events & WL_SOCKET_READABLE)
+			pollfd->events |= POLLIN;
+		if (event->events & WL_SOCKET_WRITEABLE)
+			pollfd->events |= POLLOUT;
+	}
+
+	Assert(event->fd != PGINVALID_SOCKET);
+}
+#endif
+
+#if defined(WAIT_USE_KQUEUE)
+
+/*
+ * On most BSD family systems, the udata member of struct kevent is of type
+ * void *, so we could directly convert to/from WaitEvent *.  Unfortunately,
+ * NetBSD has it as intptr_t, so here we wallpaper over that difference with
+ * an lvalue cast.
+ */
+#define AccessWaitEvent(k_ev) (*((WaitEvent **)(&(k_ev)->udata)))
+
+static inline void
+WaitEventAdjustKqueueAdd(struct kevent *k_ev, int filter, int action,
+						 WaitEvent *event)
+{
+	k_ev->ident = event->fd;
+	k_ev->filter = filter;
+	k_ev->flags = action;
+	k_ev->fflags = 0;
+	k_ev->data = 0;
+	AccessWaitEvent(k_ev) = event;
+}
+
+static inline void
+WaitEventAdjustKqueueAddPostmaster(struct kevent *k_ev, WaitEvent *event)
+{
+	/* For now postmaster death can only be added, not removed. */
+	k_ev->ident = PostmasterPid;
+	k_ev->filter = EVFILT_PROC;
+	k_ev->flags = EV_ADD;
+	k_ev->fflags = NOTE_EXIT;
+	k_ev->data = 0;
+	AccessWaitEvent(k_ev) = event;
+}
+
+static inline void
+WaitEventAdjustKqueueAddLatch(struct kevent *k_ev, WaitEvent *event)
+{
+	/* For now latch can only be added, not removed. */
+	k_ev->ident = SIGURG;
+	k_ev->filter = EVFILT_SIGNAL;
+	k_ev->flags = EV_ADD;
+	k_ev->fflags = 0;
+	k_ev->data = 0;
+	AccessWaitEvent(k_ev) = event;
+}
+
+/*
+ * old_events is the previous event mask, used to compute what has changed.
+ */
+static void
+WaitEventAdjustKqueue(WaitEventSet *set, WaitEvent *event, int old_events)
+{
+	int			rc;
+	struct kevent k_ev[2];
+	int			count = 0;
+	bool		new_filt_read = false;
+	bool		old_filt_read = false;
+	bool		new_filt_write = false;
+	bool		old_filt_write = false;
+
+	if (old_events == event->events)
+		return;
+
+	Assert(event->events != WL_LATCH_SET || set->latch != NULL);
+	Assert(event->events == WL_LATCH_SET ||
+		   event->events == WL_POSTMASTER_DEATH ||
+		   (event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)));
+
+	if (event->events == WL_POSTMASTER_DEATH)
+	{
+		/*
+		 * Unlike all the other implementations, we detect postmaster death
+		 * using process notification instead of waiting on the postmaster
+		 * alive pipe.
+		 */
+		WaitEventAdjustKqueueAddPostmaster(&k_ev[count++], event);
+	}
+	else if (event->events == WL_LATCH_SET)
+	{
+		/* We detect latch wakeup using a signal event. */
+		WaitEventAdjustKqueueAddLatch(&k_ev[count++], event);
+	}
+	else
+	{
+		/*
+		 * We need to compute the adds and deletes required to get from the
+		 * old event mask to the new event mask, since kevent treats readable
+		 * and writable as separate events.
+		 */
+		if (old_events & WL_SOCKET_READABLE)
+			old_filt_read = true;
+		if (event->events & WL_SOCKET_READABLE)
+			new_filt_read = true;
+		if (old_events & WL_SOCKET_WRITEABLE)
+			old_filt_write = true;
+		if (event->events & WL_SOCKET_WRITEABLE)
+			new_filt_write = true;
+		if (old_filt_read && !new_filt_read)
+			WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_READ, EV_DELETE,
+									 event);
+		else if (!old_filt_read && new_filt_read)
+			WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_READ, EV_ADD,
+									 event);
+		if (old_filt_write && !new_filt_write)
+			WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_WRITE, EV_DELETE,
+									 event);
+		else if (!old_filt_write && new_filt_write)
+			WaitEventAdjustKqueueAdd(&k_ev[count++], EVFILT_WRITE, EV_ADD,
+									 event);
+	}
+
+	Assert(count > 0);
+	Assert(count <= 2);
+
+	rc = kevent(set->kqueue_fd, &k_ev[0], count, NULL, 0, NULL);
+
+	/*
+	 * When adding the postmaster's pid, we have to consider that it might
+	 * already have exited and perhaps even been replaced by another process
+	 * with the same pid.  If so, we have to defer reporting this as an event
+	 * until the next call to WaitEventSetWaitBlock().
+	 */
+
+	if (rc < 0)
+	{
+		if (event->events == WL_POSTMASTER_DEATH &&
+			(errno == ESRCH || errno == EACCES))
+			set->report_postmaster_not_running = true;
+		else
+			ereport(ERROR,
+					(errcode_for_socket_access(),
+					 errmsg("%s() failed: %m",
+							"kevent")));
+	}
+	else if (event->events == WL_POSTMASTER_DEATH &&
+			 PostmasterPid != getppid() &&
+			 !PostmasterIsAlive())
+	{
+		/*
+		 * The extra PostmasterIsAliveInternal() check prevents false alarms
+		 * on systems that give a different value for getppid() while being
+		 * traced by a debugger.
+		 */
+		set->report_postmaster_not_running = true;
+	}
+}
+
+#endif
+
+#if defined(WAIT_USE_WIN32)
+static void
+WaitEventAdjustWin32(WaitEventSet *set, WaitEvent *event)
+{
+	HANDLE	   *handle = &set->handles[event->pos + 1];
+
+	if (event->events == WL_LATCH_SET)
+	{
+		Assert(set->latch != NULL);
+		*handle = set->latch->event;
+	}
+	else if (event->events == WL_POSTMASTER_DEATH)
+	{
+		*handle = PostmasterHandle;
+	}
+	else
+	{
+		int			flags = FD_CLOSE;	/* always check for errors/EOF */
+
+		if (event->events & WL_SOCKET_READABLE)
+			flags |= FD_READ;
+		if (event->events & WL_SOCKET_WRITEABLE)
+			flags |= FD_WRITE;
+		if (event->events & WL_SOCKET_CONNECTED)
+			flags |= FD_CONNECT;
+
+		if (*handle == WSA_INVALID_EVENT)
+		{
+			*handle = WSACreateEvent();
+			if (*handle == WSA_INVALID_EVENT)
+				elog(ERROR, "failed to create event for socket: error code %d",
+					 WSAGetLastError());
+		}
+		if (WSAEventSelect(event->fd, *handle, flags) != 0)
+			elog(ERROR, "failed to set up event for socket: error code %d",
+				 WSAGetLastError());
+
+		Assert(event->fd != PGINVALID_SOCKET);
+	}
+}
+#endif
+
+/*
+ * Wait for events added to the set to happen, or until the timeout is
+ * reached.  At most nevents occurred events are returned.
+ *
+ * If timeout = -1, block until an event occurs; if 0, check sockets for
+ * readiness, but don't block; if > 0, block for at most timeout milliseconds.
+ *
+ * Returns the number of events occurred, or 0 if the timeout was reached.
+ *
+ * Returned events will have the fd, pos, user_data fields set to the
+ * values associated with the registered event.
+ */
+int
+WaitEventSetWait(WaitEventSet *set, long timeout,
+				 WaitEvent *occurred_events, int nevents,
+				 uint32 wait_event_info)
+{
+	int			returned_events = 0;
+	instr_time	start_time;
+	instr_time	cur_time;
+	long		cur_timeout = -1;
+
+	Assert(nevents > 0);
+
+	/*
+	 * Initialize timeout if requested.  We must record the current time so
+	 * that we can determine the remaining timeout if interrupted.
+	 */
+	if (timeout >= 0)
+	{
+		INSTR_TIME_SET_CURRENT(start_time);
+		Assert(timeout >= 0 && timeout <= INT_MAX);
+		cur_timeout = timeout;
+	}
+
+	pgstat_report_wait_start(wait_event_info);
+
+#ifndef WIN32
+	waiting = true;
+#else
+	/* Ensure that signals are serviced even if latch is already set */
+	pgwin32_dispatch_queued_signals();
+#endif
+	while (returned_events == 0)
+	{
+		int			rc;
+
+		/*
+		 * Check if the latch is set already. If so, leave the loop
+		 * immediately, avoid blocking again. We don't attempt to report any
+		 * other events that might also be satisfied.
+		 *
+		 * If someone sets the latch between this and the
+		 * WaitEventSetWaitBlock() below, the setter will write a byte to the
+		 * pipe (or signal us and the signal handler will do that), and the
+		 * readiness routine will return immediately.
+		 *
+		 * On unix, If there's a pending byte in the self pipe, we'll notice
+		 * whenever blocking. Only clearing the pipe in that case avoids
+		 * having to drain it every time WaitLatchOrSocket() is used. Should
+		 * the pipe-buffer fill up we're still ok, because the pipe is in
+		 * nonblocking mode. It's unlikely for that to happen, because the
+		 * self pipe isn't filled unless we're blocking (waiting = true), or
+		 * from inside a signal handler in latch_sigurg_handler().
+		 *
+		 * On windows, we'll also notice if there's a pending event for the
+		 * latch when blocking, but there's no danger of anything filling up,
+		 * as "Setting an event that is already set has no effect.".
+		 *
+		 * Note: we assume that the kernel calls involved in latch management
+		 * will provide adequate synchronization on machines with weak memory
+		 * ordering, so that we cannot miss seeing is_set if a notification
+		 * has already been queued.
+		 */
+		if (set->latch && !set->latch->is_set)
+		{
+			/* about to sleep on a latch */
+			set->latch->maybe_sleeping = true;
+			pg_memory_barrier();
+			/* and recheck */
+		}
+
+		if (set->latch && set->latch->is_set)
+		{
+			occurred_events->fd = PGINVALID_SOCKET;
+			occurred_events->pos = set->latch_pos;
+			occurred_events->user_data =
+				set->events[set->latch_pos].user_data;
+			occurred_events->events = WL_LATCH_SET;
+			occurred_events++;
+			returned_events++;
+
+			/* could have been set above */
+			set->latch->maybe_sleeping = false;
+
+			break;
+		}
+
+		/*
+		 * Wait for events using the readiness primitive chosen at the top of
+		 * this file. If -1 is returned, a timeout has occurred, if 0 we have
+		 * to retry, everything >= 1 is the number of returned events.
+		 */
+		rc = WaitEventSetWaitBlock(set, cur_timeout,
+								   occurred_events, nevents);
+
+		if (set->latch)
+		{
+			Assert(set->latch->maybe_sleeping);
+			set->latch->maybe_sleeping = false;
+		}
+
+		if (rc == -1)
+			break;				/* timeout occurred */
+		else
+			returned_events = rc;
+
+		/* If we're not done, update cur_timeout for next iteration */
+		if (returned_events == 0 && timeout >= 0)
+		{
+			INSTR_TIME_SET_CURRENT(cur_time);
+			INSTR_TIME_SUBTRACT(cur_time, start_time);
+			cur_timeout = timeout - (long) INSTR_TIME_GET_MILLISEC(cur_time);
+			if (cur_timeout <= 0)
+				break;
+		}
+	}
+#ifndef WIN32
+	waiting = false;
+#endif
+
+	pgstat_report_wait_end();
+
+	return returned_events;
+}
+
+
+#if defined(WAIT_USE_EPOLL)
+
+/*
+ * Wait using linux's epoll_wait(2).
+ *
+ * This is the preferable wait method, as several readiness notifications are
+ * delivered, without having to iterate through all of set->events. The return
+ * epoll_event struct contain a pointer to our events, making association
+ * easy.
+ */
+static inline int
+WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
+					  WaitEvent *occurred_events, int nevents)
+{
+	int			returned_events = 0;
+	int			rc;
+	WaitEvent  *cur_event;
+	struct epoll_event *cur_epoll_event;
+
+	/* Sleep */
+	rc = epoll_wait(set->epoll_fd, set->epoll_ret_events,
+					nevents, cur_timeout);
+
+	/* Check return code */
+	if (rc < 0)
+	{
+		/* EINTR is okay, otherwise complain */
+		if (errno != EINTR)
+		{
+			waiting = false;
+			ereport(ERROR,
+					(errcode_for_socket_access(),
+					 errmsg("%s() failed: %m",
+							"epoll_wait")));
+		}
+		return 0;
+	}
+	else if (rc == 0)
+	{
+		/* timeout exceeded */
+		return -1;
+	}
+
+	/*
+	 * At least one event occurred, iterate over the returned epoll events
+	 * until they're either all processed, or we've returned all the events
+	 * the caller desired.
+	 */
+	for (cur_epoll_event = set->epoll_ret_events;
+		 cur_epoll_event < (set->epoll_ret_events + rc) &&
+		 returned_events < nevents;
+		 cur_epoll_event++)
+	{
+		/* epoll's data pointer is set to the associated WaitEvent */
+		cur_event = (WaitEvent *) cur_epoll_event->data.ptr;
+
+		occurred_events->pos = cur_event->pos;
+		occurred_events->user_data = cur_event->user_data;
+		occurred_events->events = 0;
+
+		if (cur_event->events == WL_LATCH_SET &&
+			cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP))
+		{
+			/* Drain the signalfd. */
+			drain();
+
+			if (set->latch && set->latch->is_set)
+			{
+				occurred_events->fd = PGINVALID_SOCKET;
+				occurred_events->events = WL_LATCH_SET;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+		else if (cur_event->events == WL_POSTMASTER_DEATH &&
+				 cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP))
+		{
+			/*
+			 * We expect an EPOLLHUP when the remote end is closed, but
+			 * because we don't expect the pipe to become readable or to have
+			 * any errors either, treat those cases as postmaster death, too.
+			 *
+			 * Be paranoid about a spurious event signaling the postmaster as
+			 * being dead.  There have been reports about that happening with
+			 * older primitives (select(2) to be specific), and a spurious
+			 * WL_POSTMASTER_DEATH event would be painful. Re-checking doesn't
+			 * cost much.
+			 */
+			if (!PostmasterIsAliveInternal())
+			{
+				if (set->exit_on_postmaster_death)
+					proc_exit(1);
+				occurred_events->fd = PGINVALID_SOCKET;
+				occurred_events->events = WL_POSTMASTER_DEATH;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+		else if (cur_event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
+		{
+			Assert(cur_event->fd != PGINVALID_SOCKET);
+
+			if ((cur_event->events & WL_SOCKET_READABLE) &&
+				(cur_epoll_event->events & (EPOLLIN | EPOLLERR | EPOLLHUP)))
+			{
+				/* data available in socket, or EOF */
+				occurred_events->events |= WL_SOCKET_READABLE;
+			}
+
+			if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
+				(cur_epoll_event->events & (EPOLLOUT | EPOLLERR | EPOLLHUP)))
+			{
+				/* writable, or EOF */
+				occurred_events->events |= WL_SOCKET_WRITEABLE;
+			}
+
+			if (occurred_events->events != 0)
+			{
+				occurred_events->fd = cur_event->fd;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+	}
+
+	return returned_events;
+}
+
+#elif defined(WAIT_USE_KQUEUE)
+
+/*
+ * Wait using kevent(2) on BSD-family systems and macOS.
+ *
+ * For now this mirrors the epoll code, but in future it could modify the fd
+ * set in the same call to kevent as it uses for waiting instead of doing that
+ * with separate system calls.
+ */
+static int
+WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
+					  WaitEvent *occurred_events, int nevents)
+{
+	int			returned_events = 0;
+	int			rc;
+	WaitEvent  *cur_event;
+	struct kevent *cur_kqueue_event;
+	struct timespec timeout;
+	struct timespec *timeout_p;
+
+	if (cur_timeout < 0)
+		timeout_p = NULL;
+	else
+	{
+		timeout.tv_sec = cur_timeout / 1000;
+		timeout.tv_nsec = (cur_timeout % 1000) * 1000000;
+		timeout_p = &timeout;
+	}
+
+	/*
+	 * Report postmaster events discovered by WaitEventAdjustKqueue() or an
+	 * earlier call to WaitEventSetWait().
+	 */
+	if (unlikely(set->report_postmaster_not_running))
+	{
+		if (set->exit_on_postmaster_death)
+			proc_exit(1);
+		occurred_events->fd = PGINVALID_SOCKET;
+		occurred_events->events = WL_POSTMASTER_DEATH;
+		return 1;
+	}
+
+	/* Sleep */
+	rc = kevent(set->kqueue_fd, NULL, 0,
+				set->kqueue_ret_events, nevents,
+				timeout_p);
+
+	/* Check return code */
+	if (rc < 0)
+	{
+		/* EINTR is okay, otherwise complain */
+		if (errno != EINTR)
+		{
+			waiting = false;
+			ereport(ERROR,
+					(errcode_for_socket_access(),
+					 errmsg("%s() failed: %m",
+							"kevent")));
+		}
+		return 0;
+	}
+	else if (rc == 0)
+	{
+		/* timeout exceeded */
+		return -1;
+	}
+
+	/*
+	 * At least one event occurred, iterate over the returned kqueue events
+	 * until they're either all processed, or we've returned all the events
+	 * the caller desired.
+	 */
+	for (cur_kqueue_event = set->kqueue_ret_events;
+		 cur_kqueue_event < (set->kqueue_ret_events + rc) &&
+		 returned_events < nevents;
+		 cur_kqueue_event++)
+	{
+		/* kevent's udata points to the associated WaitEvent */
+		cur_event = AccessWaitEvent(cur_kqueue_event);
+
+		occurred_events->pos = cur_event->pos;
+		occurred_events->user_data = cur_event->user_data;
+		occurred_events->events = 0;
+
+		if (cur_event->events == WL_LATCH_SET &&
+			cur_kqueue_event->filter == EVFILT_SIGNAL)
+		{
+			if (set->latch && set->latch->is_set)
+			{
+				occurred_events->fd = PGINVALID_SOCKET;
+				occurred_events->events = WL_LATCH_SET;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+		else if (cur_event->events == WL_POSTMASTER_DEATH &&
+				 cur_kqueue_event->filter == EVFILT_PROC &&
+				 (cur_kqueue_event->fflags & NOTE_EXIT) != 0)
+		{
+			/*
+			 * The kernel will tell this kqueue object only once about the
+			 * exit of the postmaster, so let's remember that for next time so
+			 * that we provide level-triggered semantics.
+			 */
+			set->report_postmaster_not_running = true;
+
+			if (set->exit_on_postmaster_death)
+				proc_exit(1);
+			occurred_events->fd = PGINVALID_SOCKET;
+			occurred_events->events = WL_POSTMASTER_DEATH;
+			occurred_events++;
+			returned_events++;
+		}
+		else if (cur_event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
+		{
+			Assert(cur_event->fd >= 0);
+
+			if ((cur_event->events & WL_SOCKET_READABLE) &&
+				(cur_kqueue_event->filter == EVFILT_READ))
+			{
+				/* readable, or EOF */
+				occurred_events->events |= WL_SOCKET_READABLE;
+			}
+
+			if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
+				(cur_kqueue_event->filter == EVFILT_WRITE))
+			{
+				/* writable, or EOF */
+				occurred_events->events |= WL_SOCKET_WRITEABLE;
+			}
+
+			if (occurred_events->events != 0)
+			{
+				occurred_events->fd = cur_event->fd;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+	}
+
+	return returned_events;
+}
+
+#elif defined(WAIT_USE_POLL)
+
+/*
+ * Wait using poll(2).
+ *
+ * This allows to receive readiness notifications for several events at once,
+ * but requires iterating through all of set->pollfds.
+ */
+static inline int
+WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
+					  WaitEvent *occurred_events, int nevents)
+{
+	int			returned_events = 0;
+	int			rc;
+	WaitEvent  *cur_event;
+	struct pollfd *cur_pollfd;
+
+	/* Sleep */
+	rc = poll(set->pollfds, set->nevents, (int) cur_timeout);
+
+	/* Check return code */
+	if (rc < 0)
+	{
+		/* EINTR is okay, otherwise complain */
+		if (errno != EINTR)
+		{
+			waiting = false;
+			ereport(ERROR,
+					(errcode_for_socket_access(),
+					 errmsg("%s() failed: %m",
+							"poll")));
+		}
+		return 0;
+	}
+	else if (rc == 0)
+	{
+		/* timeout exceeded */
+		return -1;
+	}
+
+	for (cur_event = set->events, cur_pollfd = set->pollfds;
+		 cur_event < (set->events + set->nevents) &&
+		 returned_events < nevents;
+		 cur_event++, cur_pollfd++)
+	{
+		/* no activity on this FD, skip */
+		if (cur_pollfd->revents == 0)
+			continue;
+
+		occurred_events->pos = cur_event->pos;
+		occurred_events->user_data = cur_event->user_data;
+		occurred_events->events = 0;
+
+		if (cur_event->events == WL_LATCH_SET &&
+			(cur_pollfd->revents & (POLLIN | POLLHUP | POLLERR | POLLNVAL)))
+		{
+			/* There's data in the self-pipe, clear it. */
+			drain();
+
+			if (set->latch && set->latch->is_set)
+			{
+				occurred_events->fd = PGINVALID_SOCKET;
+				occurred_events->events = WL_LATCH_SET;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+		else if (cur_event->events == WL_POSTMASTER_DEATH &&
+				 (cur_pollfd->revents & (POLLIN | POLLHUP | POLLERR | POLLNVAL)))
+		{
+			/*
+			 * We expect an POLLHUP when the remote end is closed, but because
+			 * we don't expect the pipe to become readable or to have any
+			 * errors either, treat those cases as postmaster death, too.
+			 *
+			 * Be paranoid about a spurious event signaling the postmaster as
+			 * being dead.  There have been reports about that happening with
+			 * older primitives (select(2) to be specific), and a spurious
+			 * WL_POSTMASTER_DEATH event would be painful. Re-checking doesn't
+			 * cost much.
+			 */
+			if (!PostmasterIsAliveInternal())
+			{
+				if (set->exit_on_postmaster_death)
+					proc_exit(1);
+				occurred_events->fd = PGINVALID_SOCKET;
+				occurred_events->events = WL_POSTMASTER_DEATH;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+		else if (cur_event->events & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
+		{
+			int			errflags = POLLHUP | POLLERR | POLLNVAL;
+
+			Assert(cur_event->fd >= PGINVALID_SOCKET);
+
+			if ((cur_event->events & WL_SOCKET_READABLE) &&
+				(cur_pollfd->revents & (POLLIN | errflags)))
+			{
+				/* data available in socket, or EOF */
+				occurred_events->events |= WL_SOCKET_READABLE;
+			}
+
+			if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
+				(cur_pollfd->revents & (POLLOUT | errflags)))
+			{
+				/* writeable, or EOF */
+				occurred_events->events |= WL_SOCKET_WRITEABLE;
+			}
+
+			if (occurred_events->events != 0)
+			{
+				occurred_events->fd = cur_event->fd;
+				occurred_events++;
+				returned_events++;
+			}
+		}
+	}
+	return returned_events;
+}
+
+#elif defined(WAIT_USE_WIN32)
+
+/*
+ * Wait using Windows' WaitForMultipleObjects().
+ *
+ * Unfortunately this will only ever return a single readiness notification at
+ * a time.  Note that while the official documentation for
+ * WaitForMultipleObjects is ambiguous about multiple events being "consumed"
+ * with a single bWaitAll = FALSE call,
+ * https://blogs.msdn.microsoft.com/oldnewthing/20150409-00/?p=44273 confirms
+ * that only one event is "consumed".
+ */
+static inline int
+WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
+					  WaitEvent *occurred_events, int nevents)
+{
+	int			returned_events = 0;
+	DWORD		rc;
+	WaitEvent  *cur_event;
+
+	/* Reset any wait events that need it */
+	for (cur_event = set->events;
+		 cur_event < (set->events + set->nevents);
+		 cur_event++)
+	{
+		if (cur_event->reset)
+		{
+			WaitEventAdjustWin32(set, cur_event);
+			cur_event->reset = false;
+		}
+
+		/*
+		 * Windows does not guarantee to log an FD_WRITE network event
+		 * indicating that more data can be sent unless the previous send()
+		 * failed with WSAEWOULDBLOCK.  While our caller might well have made
+		 * such a call, we cannot assume that here.  Therefore, if waiting for
+		 * write-ready, force the issue by doing a dummy send().  If the dummy
+		 * send() succeeds, assume that the socket is in fact write-ready, and
+		 * return immediately.  Also, if it fails with something other than
+		 * WSAEWOULDBLOCK, return a write-ready indication to let our caller
+		 * deal with the error condition.
+		 */
+		if (cur_event->events & WL_SOCKET_WRITEABLE)
+		{
+			char		c;
+			WSABUF		buf;
+			DWORD		sent;
+			int			r;
+
+			buf.buf = &c;
+			buf.len = 0;
+
+			r = WSASend(cur_event->fd, &buf, 1, &sent, 0, NULL, NULL);
+			if (r == 0 || WSAGetLastError() != WSAEWOULDBLOCK)
+			{
+				occurred_events->pos = cur_event->pos;
+				occurred_events->user_data = cur_event->user_data;
+				occurred_events->events = WL_SOCKET_WRITEABLE;
+				occurred_events->fd = cur_event->fd;
+				return 1;
+			}
+		}
+	}
+
+	/*
+	 * Sleep.
+	 *
+	 * Need to wait for ->nevents + 1, because signal handle is in [0].
+	 */
+	rc = WaitForMultipleObjects(set->nevents + 1, set->handles, FALSE,
+								cur_timeout);
+
+	/* Check return code */
+	if (rc == WAIT_FAILED)
+		elog(ERROR, "WaitForMultipleObjects() failed: error code %lu",
+			 GetLastError());
+	else if (rc == WAIT_TIMEOUT)
+	{
+		/* timeout exceeded */
+		return -1;
+	}
+
+	if (rc == WAIT_OBJECT_0)
+	{
+		/* Service newly-arrived signals */
+		pgwin32_dispatch_queued_signals();
+		return 0;				/* retry */
+	}
+
+	/*
+	 * With an offset of one, due to the always present pgwin32_signal_event,
+	 * the handle offset directly corresponds to a wait event.
+	 */
+	cur_event = (WaitEvent *) &set->events[rc - WAIT_OBJECT_0 - 1];
+
+	occurred_events->pos = cur_event->pos;
+	occurred_events->user_data = cur_event->user_data;
+	occurred_events->events = 0;
+
+	if (cur_event->events == WL_LATCH_SET)
+	{
+		/*
+		 * We cannot use set->latch->event to reset the fired event if we
+		 * aren't waiting on this latch now.
+		 */
+		if (!ResetEvent(set->handles[cur_event->pos + 1]))
+			elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());
+
+		if (set->latch && set->latch->is_set)
+		{
+			occurred_events->fd = PGINVALID_SOCKET;
+			occurred_events->events = WL_LATCH_SET;
+			occurred_events++;
+			returned_events++;
+		}
+	}
+	else if (cur_event->events == WL_POSTMASTER_DEATH)
+	{
+		/*
+		 * Postmaster apparently died.  Since the consequences of falsely
+		 * returning WL_POSTMASTER_DEATH could be pretty unpleasant, we take
+		 * the trouble to positively verify this with PostmasterIsAlive(),
+		 * even though there is no known reason to think that the event could
+		 * be falsely set on Windows.
+		 */
+		if (!PostmasterIsAliveInternal())
+		{
+			if (set->exit_on_postmaster_death)
+				proc_exit(1);
+			occurred_events->fd = PGINVALID_SOCKET;
+			occurred_events->events = WL_POSTMASTER_DEATH;
+			occurred_events++;
+			returned_events++;
+		}
+	}
+	else if (cur_event->events & WL_SOCKET_MASK)
+	{
+		WSANETWORKEVENTS resEvents;
+		HANDLE		handle = set->handles[cur_event->pos + 1];
+
+		Assert(cur_event->fd);
+
+		occurred_events->fd = cur_event->fd;
+
+		ZeroMemory(&resEvents, sizeof(resEvents));
+		if (WSAEnumNetworkEvents(cur_event->fd, handle, &resEvents) != 0)
+			elog(ERROR, "failed to enumerate network events: error code %d",
+				 WSAGetLastError());
+		if ((cur_event->events & WL_SOCKET_READABLE) &&
+			(resEvents.lNetworkEvents & FD_READ))
+		{
+			/* data available in socket */
+			occurred_events->events |= WL_SOCKET_READABLE;
+
+			/*------
+			 * WaitForMultipleObjects doesn't guarantee that a read event will
+			 * be returned if the latch is set at the same time.  Even if it
+			 * did, the caller might drop that event expecting it to reoccur
+			 * on next call.  So, we must force the event to be reset if this
+			 * WaitEventSet is used again in order to avoid an indefinite
+			 * hang.  Refer https://msdn.microsoft.com/en-us/library/windows/desktop/ms741576(v=vs.85).aspx
+			 * for the behavior of socket events.
+			 *------
+			 */
+			cur_event->reset = true;
+		}
+		if ((cur_event->events & WL_SOCKET_WRITEABLE) &&
+			(resEvents.lNetworkEvents & FD_WRITE))
+		{
+			/* writeable */
+			occurred_events->events |= WL_SOCKET_WRITEABLE;
+		}
+		if ((cur_event->events & WL_SOCKET_CONNECTED) &&
+			(resEvents.lNetworkEvents & FD_CONNECT))
+		{
+			/* connected */
+			occurred_events->events |= WL_SOCKET_CONNECTED;
+		}
+		if (resEvents.lNetworkEvents & FD_CLOSE)
+		{
+			/* EOF/error, so signal all caller-requested socket flags */
+			occurred_events->events |= (cur_event->events & WL_SOCKET_MASK);
+		}
+
+		if (occurred_events->events != 0)
+		{
+			occurred_events++;
+			returned_events++;
+		}
+	}
+
+	return returned_events;
+}
+#endif
+
+/*
+ * Get the number of wait events registered in a given WaitEventSet.
+ */
+int
+GetNumRegisteredWaitEvents(WaitEventSet *set)
+{
+	return set->nevents;
+}
+
+#if defined(WAIT_USE_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)
+{
+	int			save_errno = errno;
+
+	if (waiting)
+		sendSelfPipeByte();
+
+	errno = save_errno;
+}
+
+/* Send one byte to the self-pipe, to wake up WaitLatch */
+static void
+sendSelfPipeByte(void)
+{
+	int			rc;
+	char		dummy = 0;
+
+retry:
+	rc = write(selfpipe_writefd, &dummy, 1);
+	if (rc < 0)
+	{
+		/* If interrupted by signal, just retry */
+		if (errno == EINTR)
+			goto retry;
+
+		/*
+		 * If the pipe is full, we don't need to retry, the data that's there
+		 * already is enough to wake up WaitLatch.
+		 */
+		if (errno == EAGAIN || errno == EWOULDBLOCK)
+			return;
+
+		/*
+		 * Oops, the write() failed for some other reason. We might be in a
+		 * signal handler, so it's not safe to elog(). We have no choice but
+		 * silently ignore the error.
+		 */
+		return;
+	}
+}
+
+#endif
+
+#if defined(WAIT_USE_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