From 8b08f138a3286503e339b546d758ef683514a929 Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Fri, 20 Jan 2023 05:50:39 +1300
Subject: [PATCH] Process pending postmaster work before connections.

Modify the new event loop code from commit 7389aad6 so that it checks
for work requested by signal handlers even if it doesn't see a latch
event yet.

This gives priority to shutdown and reload requests where the latch will
be reported later in the event array, or in a later call to
WaitEventSetWait(), due to scheduling details.  In particular, this
guarantees that a SIGHUP-then-connect sequence (as seen in
authentication tests) causes the postmaster to process the reload before
accepting the connection.  If the WaitEventSetWait() call saw the socket
as ready, and the reload signal was generated before the connection,
then the latest time the signal handler should be able to run is after
poll/epoll_wait/kevent returns but before we check the
pending_pm_reload_request flag.

Reported-by: Hou Zhijie <houzj.fnst@fujitsu.com>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/OS0PR01MB57163D3BF2AB42ECAA94E5C394C29%40OS0PR01MB5716.jpnprd01.prod.outlook.com

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 9cedc1b9f0..cec3fe8dc5 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -1739,20 +1739,24 @@ ServerLoop(void)
 		for (int i = 0; i < nevents; i++)
 		{
 			if (events[i].events & WL_LATCH_SET)
-			{
 				ResetLatch(MyLatch);
 
-				/* Process work requested via signal handlers. */
-				if (pending_pm_shutdown_request)
-					process_pm_shutdown_request();
-				if (pending_pm_child_exit)
-					process_pm_child_exit();
-				if (pending_pm_reload_request)
-					process_pm_reload_request();
-				if (pending_pm_pmsignal)
-					process_pm_pmsignal();
-			}
-			else if (events[i].events & WL_SOCKET_ACCEPT)
+			/*
+			 * The following is done unconditionally, even if we didn't see
+			 * WL_LATCH_SET.  This gives high priority to shutdown and reload
+			 * requests where the latch happens to appear later in events[] or
+			 * will be reported by a later call to WaitEventSetWait().
+			 */
+			if (pending_pm_shutdown_request)
+				process_pm_shutdown_request();
+			if (pending_pm_child_exit)
+				process_pm_child_exit();
+			if (pending_pm_reload_request)
+				process_pm_reload_request();
+			if (pending_pm_pmsignal)
+				process_pm_pmsignal();
+
+			if (events[i].events & WL_SOCKET_ACCEPT)
 			{
 				Port	   *port;
 
-- 
2.35.1