Introduce latches. A latch is a boolean variable, with the capability to

wait until it is set. Latches can be used to reliably wait until a signal
arrives, which is hard otherwise because signals don't interrupt select()
on some platforms, and even when they do, there's race conditions.

On Unix, latches use the so called self-pipe trick under the covers to
implement the sleep until the latch is set, without race conditions. On
Windows, Windows events are used.

Use the new latch abstraction to sleep in walsender, so that as soon as
a transaction finishes, walsender is woken up to immediately send the WAL
to the standby. This reduces the latency between master and standby, which
is good.

Preliminary work by Fujii Masao. The latch implementation is by me, with
helpful comments from many people.

configure

@@ -27773,6 +27773,13 @@ _ACEOF
   SHMEM_IMPLEMENTATION="src/backend/port/win32_shmem.c"
 fi
 
+# Select latch implementation type.
+if test "$PORTNAME" != "win32"; then
+  LATCH_IMPLEMENTATION="src/backend/port/unix_latch.c"
+else
+  LATCH_IMPLEMENTATION="src/backend/port/win32_latch.c"
+fi
+
 # If not set in template file, set bytes to use libc memset()
 if test x"$MEMSET_LOOP_LIMIT" = x"" ; then
   MEMSET_LOOP_LIMIT=1024
@@ -29098,7 +29105,7 @@ fi
 ac_config_files="$ac_config_files GNUmakefile src/Makefile.global"
 
 
-ac_config_links="$ac_config_links src/backend/port/dynloader.c:src/backend/port/dynloader/${template}.c src/backend/port/pg_sema.c:${SEMA_IMPLEMENTATION} src/backend/port/pg_shmem.c:${SHMEM_IMPLEMENTATION} src/include/dynloader.h:src/backend/port/dynloader/${template}.h src/include/pg_config_os.h:src/include/port/${template}.h src/Makefile.port:src/makefiles/Makefile.${template}"
+ac_config_links="$ac_config_links src/backend/port/dynloader.c:src/backend/port/dynloader/${template}.c src/backend/port/pg_sema.c:${SEMA_IMPLEMENTATION} src/backend/port/pg_shmem.c:${SHMEM_IMPLEMENTATION} src/backend/port/pg_latch.c:${LATCH_IMPLEMENTATION} src/include/dynloader.h:src/backend/port/dynloader/${template}.h src/include/pg_config_os.h:src/include/port/${template}.h src/Makefile.port:src/makefiles/Makefile.${template}"
 
 
 if test "$PORTNAME" = "win32"; then
@@ -29722,6 +29729,7 @@ do
     "src/backend/port/dynloader.c") CONFIG_LINKS="$CONFIG_LINKS src/backend/port/dynloader.c:src/backend/port/dynloader/${template}.c" ;;
     "src/backend/port/pg_sema.c") CONFIG_LINKS="$CONFIG_LINKS src/backend/port/pg_sema.c:${SEMA_IMPLEMENTATION}" ;;
     "src/backend/port/pg_shmem.c") CONFIG_LINKS="$CONFIG_LINKS src/backend/port/pg_shmem.c:${SHMEM_IMPLEMENTATION}" ;;
+    "src/backend/port/pg_latch.c") CONFIG_LINKS="$CONFIG_LINKS src/backend/port/pg_latch.c:${LATCH_IMPLEMENTATION}" ;;
     "src/include/dynloader.h") CONFIG_LINKS="$CONFIG_LINKS src/include/dynloader.h:src/backend/port/dynloader/${template}.h" ;;
     "src/include/pg_config_os.h") CONFIG_LINKS="$CONFIG_LINKS src/include/pg_config_os.h:src/include/port/${template}.h" ;;
     "src/Makefile.port") CONFIG_LINKS="$CONFIG_LINKS src/Makefile.port:src/makefiles/Makefile.${template}" ;;

configure.in

 dnl Process this file with autoconf to produce a configure script.
-dnl $PostgreSQL: pgsql/configure.in,v 1.633 2010/07/09 04:10:58 tgl Exp $
+dnl $PostgreSQL: pgsql/configure.in,v 1.634 2010/09/11 15:48:04 heikki Exp $
 dnl
 dnl Developers, please strive to achieve this order:
 dnl
@@ -1700,6 +1700,13 @@ else
   SHMEM_IMPLEMENTATION="src/backend/port/win32_shmem.c"
 fi
 
+# Select latch implementation type.
+if test "$PORTNAME" != "win32"; then
+  LATCH_IMPLEMENTATION="src/backend/port/unix_latch.c"
+else
+  LATCH_IMPLEMENTATION="src/backend/port/win32_latch.c"
+fi
+
 # If not set in template file, set bytes to use libc memset()
 if test x"$MEMSET_LOOP_LIMIT" = x"" ; then
   MEMSET_LOOP_LIMIT=1024
@@ -1841,6 +1848,7 @@ AC_CONFIG_LINKS([
   src/backend/port/dynloader.c:src/backend/port/dynloader/${template}.c
   src/backend/port/pg_sema.c:${SEMA_IMPLEMENTATION}
   src/backend/port/pg_shmem.c:${SHMEM_IMPLEMENTATION}
+  src/backend/port/pg_latch.c:${LATCH_IMPLEMENTATION}
   src/include/dynloader.h:src/backend/port/dynloader/${template}.h
   src/include/pg_config_os.h:src/include/port/${template}.h
   src/Makefile.port:src/makefiles/Makefile.${template}

src/backend/access/transam/twophase.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.63 2010/08/13 20:10:50 rhaas Exp $
+ *		$PostgreSQL: pgsql/src/backend/access/transam/twophase.c,v 1.64 2010/09/11 15:48:04 heikki Exp $
  *
  * NOTES
  *		Each global transaction is associated with a global transaction
@@ -55,6 +55,7 @@
 #include "miscadmin.h"
 #include "pg_trace.h"
 #include "pgstat.h"
+#include "replication/walsender.h"
 #include "storage/fd.h"
 #include "storage/procarray.h"
 #include "storage/sinvaladt.h"
@@ -1025,6 +1026,13 @@ EndPrepare(GlobalTransaction gxact)
 
 	/* If we crash now, we have prepared: WAL replay will fix things */
 
+	/*
+	 * Wake up all walsenders to send WAL up to the PREPARE record
+	 * immediately if replication is enabled
+	 */
+	if (max_wal_senders > 0)
+		WalSndWakeup();
+
 	/* write correct CRC and close file */
 	if ((write(fd, &statefile_crc, sizeof(pg_crc32))) != sizeof(pg_crc32))
 	{
@@ -2005,6 +2013,13 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	/* Flush XLOG to disk */
 	XLogFlush(recptr);
 
+	/*
+	 * Wake up all walsenders to send WAL up to the COMMIT PREPARED record
+	 * immediately if replication is enabled
+	 */
+	if (max_wal_senders > 0)
+		WalSndWakeup();
+
 	/* Mark the transaction committed in pg_clog */
 	TransactionIdCommitTree(xid, nchildren, children);
 
@@ -2077,6 +2092,13 @@ RecordTransactionAbortPrepared(TransactionId xid,
 	/* Always flush, since we're about to remove the 2PC state file */
 	XLogFlush(recptr);
 
+	/*
+	 * Wake up all walsenders to send WAL up to the ABORT PREPARED record
+	 * immediately if replication is enabled
+	 */
+	if (max_wal_senders > 0)
+		WalSndWakeup();
+
 	/*
 	 * Mark the transaction aborted in clog.  This is not absolutely necessary
 	 * but we may as well do it while we are here.

src/backend/access/transam/xact.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.298 2010/08/13 20:10:50 rhaas Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.299 2010/09/11 15:48:04 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -36,6 +36,7 @@
 #include "libpq/be-fsstubs.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "replication/walsender.h"
 #include "storage/bufmgr.h"
 #include "storage/fd.h"
 #include "storage/lmgr.h"
@@ -1067,6 +1068,13 @@ RecordTransactionCommit(void)
 
 		XLogFlush(XactLastRecEnd);
 
+		/*
+		 * Wake up all walsenders to send WAL up to the COMMIT record
+		 * immediately if replication is enabled
+		 */
+		if (max_wal_senders > 0)
+			WalSndWakeup();
+
 		/*
 		 * Now we may update the CLOG, if we wrote a COMMIT record above
 		 */

src/backend/port/Makefile

@@ -13,7 +13,7 @@
 # be converted to Method 2.  
 #
 # IDENTIFICATION
-#    $PostgreSQL: pgsql/src/backend/port/Makefile,v 1.28 2010/07/05 18:54:37 tgl Exp $
+#    $PostgreSQL: pgsql/src/backend/port/Makefile,v 1.29 2010/09/11 15:48:04 heikki Exp $
 #
 #-------------------------------------------------------------------------
 
@@ -21,7 +21,7 @@ subdir = src/backend/port
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
-OBJS = dynloader.o pg_sema.o pg_shmem.o $(TAS)
+OBJS = dynloader.o pg_sema.o pg_shmem.o pg_latch.o $(TAS)
 
 ifeq ($(PORTNAME), darwin)
 SUBDIRS += darwin

src/backend/port/win32_latch.c

+/*-------------------------------------------------------------------------
+ *
+ * win32_latch.c
+ *	  Windows implementation of latches.
+ *
+ * The Windows implementation uses Windows events. See unix_latch.c for
+ * information on usage.
+ *
+ * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  $PostgreSQL: pgsql/src/backend/port/win32_latch.c,v 1.1 2010/09/11 15:48:04 heikki Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <fcntl.h>
+#include <signal.h>
+#include <unistd.h>
+
+#include "miscadmin.h"
+#include "replication/walsender.h"
+#include "storage/latch.h"
+#include "storage/shmem.h"
+#include "storage/spin.h"
+
+/*
+ * Shared latches are implemented with Windows events that are shared by
+ * all postmaster child processes. At postmaster startup we create enough
+ * Event objects, and mark them as inheritable so that they are accessible
+ * in child processes. The handles are stored in sharedHandles.
+ */
+typedef struct
+{
+	slock_t		mutex;			/* protects all the other fields */
+
+	int			maxhandles;		/* number of shared handles created */
+	int			nfreehandles;	/* number of free handles in array */
+	HANDLE		handles[1];		/* free handles, variable length */
+} SharedEventHandles;
+
+static SharedEventHandles *sharedHandles;
+
+void
+InitLatch(volatile Latch *latch)
+{
+	latch->event = CreateEvent(NULL, TRUE, FALSE, NULL);
+	latch->is_shared = false;
+	latch->is_set = false;
+}
+
+void
+InitSharedLatch(volatile Latch *latch)
+{
+	latch->is_shared = true;
+	latch->is_set = false;
+	latch->event = NULL;
+}
+
+void
+OwnLatch(volatile Latch *latch)
+{
+	HANDLE event;
+
+	/* Sanity checks */
+	Assert(latch->is_shared);
+	if (latch->event != 0)
+		elog(ERROR, "latch already owned");
+
+	/* Reserve an event handle from the shared handles array */
+	SpinLockAcquire(&sharedHandles->mutex);
+	if (sharedHandles->nfreehandles <= 0)
+	{
+		SpinLockRelease(&sharedHandles->mutex);
+		elog(ERROR, "out of shared event objects");
+	}
+	sharedHandles->nfreehandles--;
+	event = sharedHandles->handles[sharedHandles->nfreehandles];
+	SpinLockRelease(&sharedHandles->mutex);
+
+	latch->event = event;
+}
+
+void
+DisownLatch(volatile Latch *latch)
+{
+	Assert(latch->is_shared);
+	Assert(latch->event != NULL);
+
+	/* Put the event handle back to the pool */
+	SpinLockAcquire(&sharedHandles->mutex);
+	if (sharedHandles->nfreehandles >= sharedHandles->maxhandles)
+	{
+		SpinLockRelease(&sharedHandles->mutex);
+		elog(PANIC, "too many free event handles");
+	}
+	sharedHandles->handles[sharedHandles->nfreehandles] = latch->event;
+	sharedHandles->nfreehandles++;
+	SpinLockRelease(&sharedHandles->mutex);
+
+	latch->event = NULL;
+}
+
+bool
+WaitLatch(volatile Latch *latch, long timeout)
+{
+	return WaitLatchOrSocket(latch, PGINVALID_SOCKET, timeout) > 0;
+}
+
+int
+WaitLatchOrSocket(volatile Latch *latch, SOCKET sock, long timeout)
+{
+	DWORD		rc;
+	HANDLE		events[3];
+	HANDLE		latchevent;
+	HANDLE		sockevent;
+	int			numevents;
+	int			result = 0;
+
+	latchevent = latch->event;
+
+	events[0] = latchevent;
+	events[1] = pgwin32_signal_event;
+	numevents = 2;
+	if (sock != PGINVALID_SOCKET)
+	{
+		sockevent = WSACreateEvent();
+		WSAEventSelect(sock, sockevent, FD_READ);
+		events[numevents++] = sockevent;
+	}
+
+	for (;;)
+	{
+		/*
+		 * 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 %d", (int) GetLastError());
+		if (latch->is_set)
+		{
+			result = 1;
+			break;
+		}
+
+		rc = WaitForMultipleObjects(numevents, events, FALSE,
+								(timeout >= 0) ? (timeout / 1000) : INFINITE);
+		if (rc == WAIT_FAILED)
+			elog(ERROR, "WaitForMultipleObjects() failed: error code %d", (int) GetLastError());
+		else if (rc == WAIT_TIMEOUT)
+		{
+			result = 0;
+			break;
+		}
+		else if (rc == WAIT_OBJECT_0 + 1)
+			pgwin32_dispatch_queued_signals();
+		else if (rc == WAIT_OBJECT_0 + 2)
+		{
+			Assert(sock != PGINVALID_SOCKET);
+			result = 2;
+			break;
+		}
+		else if (rc != WAIT_OBJECT_0)
+			elog(ERROR, "unexpected return code from WaitForMultipleObjects(): %d", rc);
+	}
+
+	/* Clean up the handle we created for the socket */
+		if (sock != PGINVALID_SOCKET)
+	{
+		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 in case the
+	 * latch is just disowned between the test and the SetEvent call, and
+	 * event field set to NULL.
+	 *
+	 * Fetch handle field only once, in case the owner simultaneously
+	 * disowns the latch and clears handle. This assumes that HANDLE is
+	 * atomic, which isn't guaranteed to be true! In practice, it should be,
+	 * and in the worst case we end up calling SetEvent with a bogus handle,
+	 * and SetEvent will return an error with no harm done.
+	 */
+	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)
+{
+	latch->is_set = false;
+}
+
+/*
+ * Number of shared latches, used to allocate the right number of shared
+ * Event handles at postmaster startup. You must update this if you
+ * introduce a new shared latch!
+ */
+static int
+NumSharedLatches(void)
+{
+	int numLatches = 0;
+
+	/* Each walsender needs one latch */
+	numLatches += max_wal_senders;
+
+	return numLatches;
+}
+
+/*
+ * LatchShmemSize
+ *		Compute space needed for latch's shared memory
+ */
+Size
+LatchShmemSize(void)
+{
+	return offsetof(SharedEventHandles, handles) +
+		NumSharedLatches() * sizeof(HANDLE);
+}
+
+/*
+ * LatchShmemInit
+ *		Allocate and initialize shared memory needed for latches
+ */
+void
+LatchShmemInit(void)
+{
+	Size		size = LatchShmemSize();
+	bool		found;
+
+	sharedHandles = ShmemInitStruct("SharedEventHandles", size, &found);
+
+	/* If we're first, initialize the struct and allocate handles */
+	if (!found)
+	{
+		int i;
+		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;
+
+		SpinLockInit(&sharedHandles->mutex);
+		sharedHandles->maxhandles = NumSharedLatches();
+		sharedHandles->nfreehandles = sharedHandles->maxhandles;
+		for (i = 0; i < sharedHandles->maxhandles; i++)
+		{
+			sharedHandles->handles[i] = CreateEvent(&sa, TRUE, FALSE, NULL);
+			if (sharedHandles->handles[i] == NULL)
+				elog(ERROR, "CreateEvent failed: error code %d", (int) GetLastError());
+		}
+	}
+}

src/backend/replication/walsender.c

@@ -28,12 +28,13 @@
  * Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/replication/walsender.c,v 1.29 2010/07/22 13:03:11 rhaas Exp $
+ *	  $PostgreSQL: pgsql/src/backend/replication/walsender.c,v 1.30 2010/09/11 15:48:04 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
+#include <signal.h>
 #include <unistd.h>
 
 #include "access/xlog_internal.h"
@@ -66,9 +67,6 @@ bool		am_walsender = false;		/* Am I a walsender process ? */
 int			max_wal_senders = 0;	/* the maximum number of concurrent walsenders */
 int			WalSndDelay = 200;	/* max sleep time between some actions */
 
-#define NAPTIME_PER_CYCLE 100000L		/* max sleep time between cycles
-										 * (100ms) */
-
 /*
  * These variables are used similarly to openLogFile/Id/Seg/Off,
  * but for walsender to read the XLOG.
@@ -93,6 +91,7 @@ static volatile sig_atomic_t ready_to_stop = false;
 static void WalSndSigHupHandler(SIGNAL_ARGS);
 static void WalSndShutdownHandler(SIGNAL_ARGS);
 static void WalSndQuickDieHandler(SIGNAL_ARGS);
+static void WalSndXLogSendHandler(SIGNAL_ARGS);
 static void WalSndLastCycleHandler(SIGNAL_ARGS);
 
 /* Prototypes for private functions */
@@ -144,6 +143,16 @@ WalSenderMain(void)
 	/* Handle handshake messages before streaming */
 	WalSndHandshake();
 
+	/* Initialize shared memory status */
+	{
+		/* use volatile pointer to prevent code rearrangement */
+		volatile WalSnd *walsnd = MyWalSnd;
+
+		SpinLockAcquire(&walsnd->mutex);
+		walsnd->sentPtr = sentPtr;
+		SpinLockRelease(&walsnd->mutex);
+	}
+
 	/* Main loop of walsender */
 	return WalSndLoop();
 }
@@ -380,8 +389,6 @@ WalSndLoop(void)
 	/* Loop forever, unless we get an error */
 	for (;;)
 	{
-		long		remain;		/* remaining time (us) */
-
 		/*
 		 * Emergency bailout if postmaster has died.  This is to avoid the
 		 * necessity for manual cleanup of all postmaster children.
@@ -421,32 +428,42 @@ WalSndLoop(void)
 		/*
 		 * If we had sent all accumulated WAL in last round, nap for the
 		 * configured time before retrying.
-		 *
-		 * On some platforms, signals won't interrupt the sleep.  To ensure we
-		 * respond reasonably promptly when someone signals us, break down the
-		 * sleep into NAPTIME_PER_CYCLE increments, and check for interrupts
-		 * after each nap.
 		 */
 		if (caughtup)
 		{
-			remain = WalSndDelay * 1000L;
-			while (remain > 0)
-			{
-				/* Check for interrupts */
-				if (got_SIGHUP || shutdown_requested || ready_to_stop)
-					break;
+			/*
+			 * Even if we wrote all the WAL that was available when we started
+			 * sending, more might have arrived while we were sending this
+			 * batch. We had the latch set while sending, so we have not
+			 * received any signals from that time. Let's arm the latch
+			 * again, and after that check that we're still up-to-date.
+			 */
+			ResetLatch(&MyWalSnd->latch);
 
-				/* Sleep and check that the connection is still alive */
-				pg_usleep(remain > NAPTIME_PER_CYCLE ? NAPTIME_PER_CYCLE : remain);
-				CheckClosedConnection();
+			if (!XLogSend(output_message, &caughtup))
+				break;
+			if (caughtup && !got_SIGHUP && !ready_to_stop && !shutdown_requested)
+			{
+				/*
+				 * XXX: We don't really need the periodic wakeups anymore,
+				 * WaitLatchOrSocket should reliably wake up as soon as
+				 * something interesting happens.
+				 */
 
-				remain -= NAPTIME_PER_CYCLE;
+				/* Sleep */
+				WaitLatchOrSocket(&MyWalSnd->latch, MyProcPort->sock,
+								  WalSndDelay);
 			}
-		}
 
-		/* Attempt to send the log once every loop */
-		if (!XLogSend(output_message, &caughtup))
-			break;
+			/* Check if the connection was closed */
+			CheckClosedConnection();
+		}
+		else
+		{
+			/* Attempt to send the log once every loop */
+			if (!XLogSend(output_message, &caughtup))
+				break;
+		}
 	}
 
 	/*
@@ -493,10 +510,15 @@ InitWalSnd(void)
 		}
 		else
 		{
-			/* found */
-			MyWalSnd = (WalSnd *) walsnd;
+			/*
+			 * Found a free slot. Take ownership of the latch and initialize
+			 * the other fields.
+			 */
+			OwnLatch((Latch *) &walsnd->latch);
 			walsnd->pid = MyProcPid;
-			MemSet(&MyWalSnd->sentPtr, 0, sizeof(XLogRecPtr));
+			MemSet(&walsnd->sentPtr, 0, sizeof(XLogRecPtr));
+			/* Set MyWalSnd only after it's fully initialized. */
+			MyWalSnd = (WalSnd *) walsnd;
 			SpinLockRelease(&walsnd->mutex);
 			break;
 		}
@@ -523,6 +545,7 @@ WalSndKill(int code, Datum arg)
 	 * for this.
 	 */
 	MyWalSnd->pid = 0;
+	DisownLatch(&MyWalSnd->latch);
 
 	/* WalSnd struct isn't mine anymore */
 	MyWalSnd = NULL;
@@ -787,6 +810,8 @@ static void
 WalSndSigHupHandler(SIGNAL_ARGS)
 {
 	got_SIGHUP = true;
+	if (MyWalSnd)
+		SetLatch(&MyWalSnd->latch);
 }
 
 /* SIGTERM: set flag to shut down */
@@ -794,6 +819,8 @@ static void
 WalSndShutdownHandler(SIGNAL_ARGS)
 {
 	shutdown_requested = true;
+	if (MyWalSnd)
+		SetLatch(&MyWalSnd->latch);
 }
 
 /*
@@ -828,11 +855,20 @@ WalSndQuickDieHandler(SIGNAL_ARGS)
 	exit(2);
 }
 
+/* SIGUSR1: set flag to send WAL records */
+static void
+WalSndXLogSendHandler(SIGNAL_ARGS)
+{
+	latch_sigusr1_handler();
+}
+
 /* SIGUSR2: set flag to do a last cycle and shut down afterwards */
 static void
 WalSndLastCycleHandler(SIGNAL_ARGS)
 {
 	ready_to_stop = true;
+	if (MyWalSnd)
+		SetLatch(&MyWalSnd->latch);
 }
 
 /* Set up signal handlers */
@@ -847,7 +883,7 @@ WalSndSignals(void)
 	pqsignal(SIGQUIT, WalSndQuickDieHandler);	/* hard crash time */
 	pqsignal(SIGALRM, SIG_IGN);
 	pqsignal(SIGPIPE, SIG_IGN);
-	pqsignal(SIGUSR1, SIG_IGN); /* not used */
+	pqsignal(SIGUSR1, WalSndXLogSendHandler);	/* request WAL sending */
 	pqsignal(SIGUSR2, WalSndLastCycleHandler);	/* request a last cycle and
 												 * shutdown */
 
@@ -891,10 +927,21 @@ WalSndShmemInit(void)
 			WalSnd	   *walsnd = &WalSndCtl->walsnds[i];
 
 			SpinLockInit(&walsnd->mutex);
+			InitSharedLatch(&walsnd->latch);
 		}
 	}
 }
 
+/* Wake up all walsenders */
+void
+WalSndWakeup(void)
+{
+	int		i;
+
+	for (i = 0; i < max_wal_senders; i++)
+		SetLatch(&WalSndCtl->walsnds[i].latch);
+}
+
 /*
  * This isn't currently used for anything. Monitoring tools might be
  * interested in the future, and we'll need something like this in the

src/backend/storage/ipc/ipci.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/ipc/ipci.c,v 1.104 2010/02/16 22:34:50 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/ipc/ipci.c,v 1.105 2010/09/11 15:48:04 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -30,6 +30,7 @@
 #include "replication/walsender.h"
 #include "storage/bufmgr.h"
 #include "storage/ipc.h"
+#include "storage/latch.h"
 #include "storage/pg_shmem.h"
 #include "storage/pmsignal.h"
 #include "storage/procarray.h"
@@ -117,6 +118,7 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
 		size = add_size(size, SInvalShmemSize());
 		size = add_size(size, PMSignalShmemSize());
 		size = add_size(size, ProcSignalShmemSize());
+		size = add_size(size, LatchShmemSize());
 		size = add_size(size, BgWriterShmemSize());
 		size = add_size(size, AutoVacuumShmemSize());
 		size = add_size(size, WalSndShmemSize());
@@ -217,6 +219,7 @@ CreateSharedMemoryAndSemaphores(bool makePrivate, int port)
 	 */
 	PMSignalShmemInit();
 	ProcSignalShmemInit();
+	LatchShmemInit();
 	BgWriterShmemInit();
 	AutoVacuumShmemInit();
 	WalSndShmemInit();

src/backend/storage/ipc/procsignal.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procsignal.c,v 1.7 2010/08/30 06:33:22 heikki Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procsignal.c,v 1.8 2010/09/11 15:48:04 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -21,6 +21,7 @@
 #include "commands/async.h"
 #include "miscadmin.h"
 #include "storage/ipc.h"
+#include "storage/latch.h"
 #include "storage/procsignal.h"
 #include "storage/shmem.h"
 #include "storage/sinval.h"
@@ -278,5 +279,7 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 	if (CheckProcSignal(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN))
 		RecoveryConflictInterrupt(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
 
+	latch_sigusr1_handler();
+
 	errno = save_errno;
 }

src/include/replication/walsender.h

@@ -5,7 +5,7 @@
  *
  * Portions Copyright (c) 2010-2010, PostgreSQL Global Development Group
  *
- * $PostgreSQL: pgsql/src/include/replication/walsender.h,v 1.4 2010/06/17 00:06:34 itagaki Exp $
+ * $PostgreSQL: pgsql/src/include/replication/walsender.h,v 1.5 2010/09/11 15:48:04 heikki Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -13,6 +13,7 @@
 #define _WALSENDER_H
 
 #include "access/xlog.h"
+#include "storage/latch.h"
 #include "storage/spin.h"
 
 /*
@@ -24,6 +25,12 @@ typedef struct WalSnd
 	XLogRecPtr	sentPtr;		/* WAL has been sent up to this point */
 
 	slock_t		mutex;			/* locks shared variables shown above */
+
+	/*
+	 * Latch used by backends to wake up this walsender when it has work
+	 * to do.
+	 */
+	Latch		latch;
 } WalSnd;
 
 /* There is one WalSndCtl struct for the whole database cluster */
@@ -45,5 +52,6 @@ extern int	WalSenderMain(void);
 extern void WalSndSignals(void);
 extern Size WalSndShmemSize(void);
 extern void WalSndShmemInit(void);
+extern void WalSndWakeup(void);
 
 #endif   /* _WALSENDER_H */

src/include/storage/latch.h

+/*-------------------------------------------------------------------------
+ *
+ * latch.h
+ *	  Routines for interprocess latches
+ *
+ *
+ * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/storage/latch.h,v 1.1 2010/09/11 15:48:04 heikki Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef LATCH_H
+#define LATCH_H
+
+#include <signal.h>
+
+/*
+ * Latch structure should be treated as opaque and only accessed through
+ * the public functions. It is defined here to allow embedding Latches as
+ * part of bigger structs.
+ */
+typedef struct
+{
+	sig_atomic_t	is_set;
+	bool			is_shared;
+#ifndef WIN32
+	int				owner_pid;
+#else
+	HANDLE			event;
+#endif
+} Latch;
+
+/*
+ * prototypes for functions in latch.c
+ */
+extern void InitLatch(volatile Latch *latch);
+extern void InitSharedLatch(volatile Latch *latch);
+extern void OwnLatch(volatile Latch *latch);
+extern void DisownLatch(volatile Latch *latch);
+extern bool WaitLatch(volatile Latch *latch, long timeout);
+extern int	WaitLatchOrSocket(volatile Latch *latch, pgsocket sock,
+				  long timeout);
+extern void SetLatch(volatile Latch *latch);
+extern void ResetLatch(volatile Latch *latch);
+#define TestLatch(latch) (((volatile Latch *) latch)->is_set)
+
+extern Size LatchShmemSize(void);
+extern void LatchShmemInit(void);
+
+/*
+ * Unix implementation uses SIGUSR1 for inter-process signaling, Win32 doesn't
+ * need this.
+ */
+#ifndef WIN32
+extern void latch_sigusr1_handler(void);
+#else
+#define latch_sigusr1_handler()
+#endif
+
+#endif   /* LATCH_H */

src/tools/msvc/Mkvcbuild.pm

@@ -3,7 +3,7 @@ package Mkvcbuild;
 #
 # Package that generates build files for msvc build
 #
-# $PostgreSQL: pgsql/src/tools/msvc/Mkvcbuild.pm,v 1.59 2010/07/02 23:25:27 adunstan Exp $
+# $PostgreSQL: pgsql/src/tools/msvc/Mkvcbuild.pm,v 1.60 2010/09/11 15:48:04 heikki Exp $
 #
 use Carp;
 use Win32;
@@ -64,6 +64,7 @@ sub mkvcbuild
     $postgres->ReplaceFile('src\backend\port\dynloader.c','src\backend\port\dynloader\win32.c');
     $postgres->ReplaceFile('src\backend\port\pg_sema.c','src\backend\port\win32_sema.c');
     $postgres->ReplaceFile('src\backend\port\pg_shmem.c','src\backend\port\win32_shmem.c');
+    $postgres->ReplaceFile('src\backend\port\pg_latch.c','src\backend\port\win32_latch.c');
     $postgres->AddFiles('src\port',@pgportfiles);
     $postgres->AddDir('src\timezone');
     $postgres->AddFiles('src\backend\parser','scan.l','gram.y');