postgres/src/backend/port/win32_latch.c

/*-------------------------------------------------------------------------
 *
 * win32_latch.c
 *	  Routines for inter-process latches
 *
 * See unix_latch.c for header comments for the exported functions;
 * the API presented here is supposed to be the same as there.
 *
 * The Windows implementation uses Windows events that are inherited by
 * all postmaster child processes.
 *
 * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  src/backend/port/win32_latch.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <fcntl.h>
#include <signal.h>
#include <unistd.h>

#include "miscadmin.h"
#include "postmaster/postmaster.h"
#include "storage/latch.h"
#include "storage/shmem.h"


void
InitLatch(volatile Latch *latch)
{
	latch->is_set = false;
	latch->owner_pid = MyProcPid;
	latch->is_shared = false;

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

void
InitSharedLatch(volatile Latch *latch)
{
	SECURITY_ATTRIBUTES sa;

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

	/*
	 * 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());
}

void
OwnLatch(volatile Latch *latch)
{
	/* Sanity checks */
	Assert(latch->is_shared);
	if (latch->owner_pid != 0)
		elog(ERROR, "latch already owned");

	latch->owner_pid = MyProcPid;
}

void
DisownLatch(volatile Latch *latch)
{
	Assert(latch->is_shared);
	Assert(latch->owner_pid == MyProcPid);

	latch->owner_pid = 0;
}

int
WaitLatch(volatile Latch *latch, int wakeEvents, long timeout)
{
	return WaitLatchOrSocket(latch, wakeEvents, PGINVALID_SOCKET, timeout);
}

int
WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
				  long timeout)
{
	DWORD		rc;
	HANDLE		events[4];
	HANDLE		latchevent;
	HANDLE		sockevent = WSA_INVALID_EVENT;
	int			numevents;
	int			result = 0;
	int			pmdeath_eventno = 0;

	/* Ignore WL_SOCKET_* events if no valid socket is given */
	if (sock == PGINVALID_SOCKET)
		wakeEvents &= ~(WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE);

	Assert(wakeEvents != 0);	/* must have at least one wake event */

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

	/* Convert timeout to form used by WaitForMultipleObjects() */
	if (wakeEvents & WL_TIMEOUT)
		Assert(timeout >= 0);
	else
		timeout = INFINITE;

	/* Construct an array of event handles for WaitforMultipleObjects() */
	latchevent = latch->event;

	events[0] = latchevent;
	events[1] = pgwin32_signal_event;
	numevents = 2;
	if ((wakeEvents & WL_SOCKET_READABLE) ||
		(wakeEvents & WL_SOCKET_WRITEABLE))
	{
		int			flags = 0;

		if (wakeEvents & WL_SOCKET_READABLE)
			flags |= FD_READ;
		if (wakeEvents & WL_SOCKET_WRITEABLE)
			flags |= FD_WRITE;

		sockevent = WSACreateEvent();
		WSAEventSelect(sock, sockevent, flags);
		events[numevents++] = sockevent;
	}
	if (wakeEvents & WL_POSTMASTER_DEATH)
	{
		pmdeath_eventno = numevents;
		events[numevents++] = PostmasterHandle;
	}

	do
	{
		/*
		 * Reset the event, and check if the latch is set already. If someone
		 * sets the latch between this and the WaitForMultipleObjects() call
		 * below, the setter will set the event and WaitForMultipleObjects()
		 * will return immediately.
		 */
		if (!ResetEvent(latchevent))
			elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());
		if ((wakeEvents & WL_LATCH_SET) && latch->is_set)
		{
			result |= WL_LATCH_SET;
			/*
			 * Leave loop immediately, avoid blocking again. We don't attempt
			 * to report any other events that might also be satisfied.
			 */
			break;
		}

		rc = WaitForMultipleObjects(numevents, events, FALSE, timeout);

		if (rc == WAIT_FAILED)
			elog(ERROR, "WaitForMultipleObjects() failed: error code %lu", GetLastError());

		/* Participate in Windows signal emulation */
		else if (rc == WAIT_OBJECT_0 + 1)
			pgwin32_dispatch_queued_signals();

		else if (rc == WAIT_TIMEOUT)
		{
			result |= WL_TIMEOUT;
		}
		else if ((wakeEvents & WL_POSTMASTER_DEATH) &&
				 rc == WAIT_OBJECT_0 + pmdeath_eventno)
		{
			/* Postmaster died */
			result |= WL_POSTMASTER_DEATH;
		}
		else if ((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) &&
				 rc == WAIT_OBJECT_0 + 2)	/* socket is at event slot 2 */
		{
			WSANETWORKEVENTS resEvents;

			ZeroMemory(&resEvents, sizeof(resEvents));
			if (WSAEnumNetworkEvents(sock, sockevent, &resEvents) == SOCKET_ERROR)
				ereport(FATAL,
						(errmsg_internal("failed to enumerate network events: error code %lu", GetLastError())));

			if ((wakeEvents & WL_SOCKET_READABLE) &&
				(resEvents.lNetworkEvents & FD_READ))
			{
				result |= WL_SOCKET_READABLE;
			}
			if ((wakeEvents & WL_SOCKET_WRITEABLE) &&
				(resEvents.lNetworkEvents & FD_WRITE))
			{
				result |= WL_SOCKET_WRITEABLE;
			}
		}
		/* Otherwise it must be the latch event */
		else if (rc != WAIT_OBJECT_0)
			elog(ERROR, "unexpected return code from WaitForMultipleObjects(): %lu", rc);
	}
	while (result == 0);

	/* Clean up the handle we created for the socket */
	if (sockevent != WSA_INVALID_EVENT)
	{
		WSAEventSelect(sock, sockevent, 0);
		WSACloseEvent(sockevent);
	}

	return result;
}

void
SetLatch(volatile Latch *latch)
{
	HANDLE		handle;

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

	latch->is_set = true;

	/*
	 * 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().
		 */
	}
}

void
ResetLatch(volatile Latch *latch)
{
	/* Only the owner should reset the latch */
	Assert(latch->owner_pid == MyProcPid);

	latch->is_set = false;
}