From bcd5b4bcbefc8b99be3678cf9c6c990a0c28f68e Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 23 Jan 2024 14:20:14 -0600
Subject: [PATCH] Fix crash in autoprewarm.
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Commit abb0b4fc03 moved the shared state for autoprewarm to a
dynamic shared memory (DSM) segment, but it left apw_detach_shmem()
in the on_shmem_exit callback list for the autoprewarm leader
process.  This is a problem because shmem_exit() detaches all the
DSM segments prior to calling the on_shmem_exit callbacks, thus
producing segfaults in the exit path for the autoprewarm leader
process.

To fix, move apw_detach_shmem() to the before_shmem_exit callback
list.  This commit also adds a check to pg_prewarm's test that the
server shut down normally.  It might be worth making this a common
check for all shutdowns in TAP tests, but that is left as a future
exercise.

Reported-by: Andres Freund
Reviewed-by: Andres Freund, Álvaro Herrera
Discussion: https://postgr.es/m/20240122204117.swton324xcoodnyi%40awork3.anarazel.de
---
 contrib/pg_prewarm/autoprewarm.c  | 10 ++++++++--
 contrib/pg_prewarm/t/001_basic.pl |  6 ++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
index 9ea6c2252a..06ee21d496 100644
--- a/contrib/pg_prewarm/autoprewarm.c
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -164,8 +164,14 @@ autoprewarm_main(Datum main_arg)
 	if (apw_init_shmem())
 		first_time = false;
 
-	/* Set on-detach hook so that our PID will be cleared on exit. */
-	on_shmem_exit(apw_detach_shmem, 0);
+	/*
+	 * Set on-detach hook so that our PID will be cleared on exit.
+	 *
+	 * NB: Autoprewarm's state is stored in a DSM segment, and DSM segments
+	 * are detached before calling the on_shmem_exit callbacks, so we must put
+	 * apw_detach_shmem in the before_shmem_exit callback list.
+	 */
+	before_shmem_exit(apw_detach_shmem, 0);
 
 	/*
 	 * Store our PID in the shared memory area --- unless there's already
diff --git a/contrib/pg_prewarm/t/001_basic.pl b/contrib/pg_prewarm/t/001_basic.pl
index bcd23a6914..825d3448ee 100644
--- a/contrib/pg_prewarm/t/001_basic.pl
+++ b/contrib/pg_prewarm/t/001_basic.pl
@@ -55,4 +55,10 @@ $node->wait_for_log(
 
 $node->stop;
 
+# control file should indicate normal shut down
+command_like(
+	[ 'pg_controldata', $node->data_dir() ],
+	qr/Database cluster state:\s*shut down/,
+	'cluster shut down normally');
+
 done_testing();