From a2cf2b016afc03d384a1682cd02aab760273d6fb Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Thu, 13 Jan 2022 15:48:14 +1300
Subject: [PATCH] Fix handling of socket FD_CLOSE events on Windows.

In some situations we could hang waiting for an FD_CLOSE event that has
already been reported.  Fix this by adding some state to WaitEvent to
remember that we've already been told the socket is closed.  To handle
client code that creates and destroys multiple temporary WaitEventSet
objects and thus would lose that state, check for EOF explicitly the
first time through.
---
 src/backend/storage/ipc/latch.c | 41 +++++++++++++++++++++++++++++++++
 src/include/storage/latch.h     |  1 +
 2 files changed, 42 insertions(+)

diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
index 61c876beff..86b86e71c4 100644
--- a/src/backend/storage/ipc/latch.c
+++ b/src/backend/storage/ipc/latch.c
@@ -899,6 +899,7 @@ AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd, Latch *latch,
 	event->user_data = user_data;
 #ifdef WIN32
 	event->reset = false;
+	event->eof = -1;			/* unknown */
 #endif
 
 	if (events == WL_LATCH_SET)
@@ -1851,6 +1852,45 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 			cur_event->reset = false;
 		}
 
+		/*
+		 * Windows does not report FD_CLOSE repeatedly, and does not report
+		 * FD_READ on EOF.  Therefore we need to remember if we've seen
+		 * FD_CLOSE, to defend against callers that wait twice in a row
+		 * without a recv() that fails with WSAEWOULDBLOCK in between.
+		 */
+		if (cur_event->events & WL_SOCKET_READABLE)
+		{
+			/*
+			 * Check for EOF explicitly the first time through, to bridge the
+			 * gap between temporary WaitEventSet objects (such as those
+			 * created by WaitLatchOrSocket()).
+			 */
+			if (cur_event->eof == -1)
+			{
+				WSAPOLLFD	pollfd;
+				int			rc;
+
+				pollfd.fd = cur_event->fd;
+				pollfd.events = POLLRDNORM;
+				pollfd.revents = 0;
+				rc = WSAPoll(&pollfd, 1, 0);
+				if (rc == 1 && pollfd.revents & POLLHUP)
+					cur_event->eof = 1; /* EOF */
+				else
+					cur_event->eof = 0; /* data or error available */
+			}
+
+			/* If we've seen EOF or FD_CLOSE, keep reporting readiness. */
+			if (cur_event->eof == 1)
+			{
+				occurred_events->pos = cur_event->pos;
+				occurred_events->user_data = cur_event->user_data;
+				occurred_events->events = WL_SOCKET_READABLE;
+				occurred_events->fd = cur_event->fd;
+				return 1;
+			}
+		}
+
 		/*
 		 * Windows does not guarantee to log an FD_WRITE network event
 		 * indicating that more data can be sent unless the previous send()
@@ -2002,6 +2042,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 		{
 			/* EOF/error, so signal all caller-requested socket flags */
 			occurred_events->events |= (cur_event->events & WL_SOCKET_MASK);
+			cur_event->eof = 1;
 		}
 
 		if (occurred_events->events != 0)
diff --git a/src/include/storage/latch.h b/src/include/storage/latch.h
index 3aa7b33834..41a857c0c3 100644
--- a/src/include/storage/latch.h
+++ b/src/include/storage/latch.h
@@ -147,6 +147,7 @@ typedef struct WaitEvent
 	void	   *user_data;		/* pointer provided in AddWaitEventToSet */
 #ifdef WIN32
 	bool		reset;			/* Is reset of the event required? */
+	int8		eof;			/* Has EOF been reached on a socket? */
 #endif
 } WaitEvent;
 
-- 
2.33.1