0001-Allow-latches-to-wait-for-socket-writability-without.patch

>From ac617c2e94f8e875c72b0f0f443b206126de9102 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Thu, 8 Jan 2015 14:09:48 +0100
Subject: [PATCH 1/6] Allow latches to wait for socket writability without
 waiting for readability.

So far WaitLatchOrSocket() required to pass in WL_SOCKET_READABLE as
that solely was used to indicate error conditions, like EOF. Waiting
for WL_SOCKET_WRITEABLE would have meant to busy wait upon socket
errors.

Adjust the API to signal errors by returning the socket as readable,
writable or both, depending on WL_SOCKET_READABLE/WL_SOCKET_WRITEABLE
being specified.  It would arguably be nicer to return WL_SOCKET_ERROR
but that's not possible on platforms and would probably also result in
more complex callsites.

This previously had explicitly been forbidden in e42a21b9e6c9, as
there was no use case at that point. We now are looking into making
FE/BE communication use latches, so it

Discussion: 20140927191243.GD5423@alap3.anarazel.de
Reviewed-By: Heikki Linnakangas
---
 src/backend/port/unix_latch.c  | 20 ++++++++++++++------
 src/backend/port/win32_latch.c | 16 +++++++++++-----
 2 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/src/backend/port/unix_latch.c b/src/backend/port/unix_latch.c
index ecf01bd..e18a2e7 100644
--- a/src/backend/port/unix_latch.c
+++ b/src/backend/port/unix_latch.c
@@ -200,9 +200,9 @@ WaitLatch(volatile Latch *latch, int wakeEvents, long timeout)
  * Like WaitLatch, but with an extra socket argument for WL_SOCKET_*
  * conditions.
  *
- * When waiting on a socket, WL_SOCKET_READABLE *must* be included in
- * 'wakeEvents'; WL_SOCKET_WRITEABLE is optional.  The reason for this is
- * that EOF and error conditions are reported only via WL_SOCKET_READABLE.
+ * When waiting on a socket, EOF and error conditions are reported by
+ * returning the socket as readable/writable or both, depending on
+ * WL_SOCKET_READABLE/WL_SOCKET_WRITEABLE being specified.
  */
 int
 WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
@@ -230,8 +230,6 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		wakeEvents &= ~(WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE);
 
 	Assert(wakeEvents != 0);	/* must have at least one wake event */
-	/* Cannot specify WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE */
-	Assert((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) != WL_SOCKET_WRITEABLE);
 
 	if ((wakeEvents & WL_LATCH_SET) && latch->owner_pid != MyProcPid)
 		elog(ERROR, "cannot wait on a latch owned by another process");
@@ -346,7 +344,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		{
 			/* at least one event occurred, so check revents values */
 			if ((wakeEvents & WL_SOCKET_READABLE) &&
-				(pfds[0].revents & (POLLIN | POLLHUP | POLLERR | POLLNVAL)))
+				(pfds[0].revents & POLLIN))
 			{
 				/* data available in socket, or EOF/error condition */
 				result |= WL_SOCKET_READABLE;
@@ -354,8 +352,17 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 			if ((wakeEvents & WL_SOCKET_WRITEABLE) &&
 				(pfds[0].revents & POLLOUT))
 			{
+				/* socket is writable */
 				result |= WL_SOCKET_WRITEABLE;
 			}
+			if (pfds[0].revents & (POLLHUP | POLLERR | POLLNVAL))
+			{
+				/* EOF/error condition */
+				if (wakeEvents & WL_SOCKET_READABLE)
+					result |= WL_SOCKET_READABLE;
+				if (wakeEvents & WL_SOCKET_WRITEABLE)
+					result |= WL_SOCKET_WRITEABLE;
+			}
 
 			/*
 			 * We expect a POLLHUP when the remote end is closed, but because
@@ -439,6 +446,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 			}
 			if ((wakeEvents & WL_SOCKET_WRITEABLE) && FD_ISSET(sock, &output_mask))
 			{
+				/* socket is writable, or EOF */
 				result |= WL_SOCKET_WRITEABLE;
 			}
 			if ((wakeEvents & WL_POSTMASTER_DEATH) &&
diff --git a/src/backend/port/win32_latch.c b/src/backend/port/win32_latch.c
index 112e60e..fe650b0 100644
--- a/src/backend/port/win32_latch.c
+++ b/src/backend/port/win32_latch.c
@@ -117,8 +117,6 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 		wakeEvents &= ~(WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE);
 
 	Assert(wakeEvents != 0);	/* must have at least one wake event */
-	/* Cannot specify WL_SOCKET_WRITEABLE without WL_SOCKET_READABLE */
-	Assert((wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE)) != WL_SOCKET_WRITEABLE);
 
 	if ((wakeEvents & WL_LATCH_SET) && latch->owner_pid != MyProcPid)
 		elog(ERROR, "cannot wait on a latch owned by another process");
@@ -152,10 +150,10 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 	if (wakeEvents & (WL_SOCKET_READABLE | WL_SOCKET_WRITEABLE))
 	{
 		/* Need an event object to represent events on the socket */
-		int			flags = 0;
+		int			flags = FD_CLOSE; /* always check for errors/EOF */
 
 		if (wakeEvents & WL_SOCKET_READABLE)
-			flags |= (FD_READ | FD_CLOSE);
+			flags |= FD_READ;
 		if (wakeEvents & WL_SOCKET_WRITEABLE)
 			flags |= FD_WRITE;
 
@@ -232,7 +230,7 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 				elog(ERROR, "failed to enumerate network events: error code %u",
 					 WSAGetLastError());
 			if ((wakeEvents & WL_SOCKET_READABLE) &&
-				(resEvents.lNetworkEvents & (FD_READ | FD_CLOSE)))
+				(resEvents.lNetworkEvents & FD_READ))
 			{
 				result |= WL_SOCKET_READABLE;
 			}
@@ -241,6 +239,14 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 			{
 				result |= WL_SOCKET_WRITEABLE;
 			}
+			if (resEvents.lNetworkEvents & FD_CLOSE)
+			{
+				if (wakeEvents & WL_SOCKET_READABLE)
+					result |= WL_SOCKET_READABLE;
+				if (wakeEvents & WL_SOCKET_WRITEABLE)
+					result |= WL_SOCKET_WRITEABLE;
+			}
+
 		}
 		else if ((wakeEvents & WL_POSTMASTER_DEATH) &&
 				 rc == WAIT_OBJECT_0 + pmdeath_eventno)
-- 
2.2.1.212.gc5b9256