elog.c query_id support vs shutdown

Hi,

In the course of [1] I again looked at all pgstat_* callsites and noticed the
new calls to pgstat_get_my_query_id() in elog.c.

As currently implemented those pgstat_get_my_query_id() calls are not
safe. It's fine during backend startup because MyBEEntry is not set, but
during shutdown that's not ok, because we never unset MyBEEntry.

andres(at)awork3:~/src/postgresql$ /home/andres/build/postgres/dev-assert/vpath/src/backend/postgres --single postgres -D /srv/dev/pgdev-dev/ -c 'log_line_prefix=%Q' -c log_min_messages=debug1
0NOTICE: database system was shut down at 2021-08-07 16:42:26 PDT
0DEBUG: checkpoint record is at 1/1C000508
0DEBUG: redo record is at 1/1C000508; shutdown true
0DEBUG: next transaction ID: 213067; next OID: 16434
0DEBUG: next MultiXactId: 1; next MultiXactOffset: 0
0DEBUG: oldest unfrozen transaction ID: 701, in database 1
0DEBUG: oldest MultiXactId: 1, in database 1
0DEBUG: commit timestamp Xid oldest/newest: 0/0
0DEBUG: transaction ID wrap limit is 2147484348, limited by database with OID 1
0DEBUG: MultiXactId wrap limit is 2147483648, limited by database with OID 1
0DEBUG: starting up replication slots
0DEBUG: MultiXactId wrap limit is 2147483648, limited by database with OID 1
0DEBUG: MultiXact member stop limit is now 4294914944 based on MultiXact 1

PostgreSQL stand-alone backend 15devel
backend> 0NOTICE: shutting down
0DEBUG: performing replication slot checkpoint
Segmentation fault

#0 0x000055cb9b871632 in pgstat_get_my_query_id () at /home/andres/src/postgresql/src/backend/utils/activity/backend_status.c:1042
#1 0x000055cb9ba00398 in log_line_prefix (buf=0x7ffc751092e0, edata=0x55cb9bd6a900 <errordata>)
at /home/andres/src/postgresql/src/backend/utils/error/elog.c:2723
#2 0x000055cb9ba00ee6 in send_message_to_server_log (edata=0x55cb9bd6a900 <errordata>) at /home/andres/src/postgresql/src/backend/utils/error/elog.c:3017
#3 0x000055cb9b9fe1b3 in EmitErrorReport () at /home/andres/src/postgresql/src/backend/utils/error/elog.c:1542
#4 0x000055cb9b9fb600 in errfinish (filename=0x55cb9bc1a0eb "miscinit.c", lineno=974, funcname=0x55cb9bc1aea0 <__func__.5> "UnlinkLockFiles")
at /home/andres/src/postgresql/src/backend/utils/error/elog.c:597
#5 0x000055cb9ba0f2b4 in UnlinkLockFiles (status=0, arg=0) at /home/andres/src/postgresql/src/backend/utils/init/miscinit.c:974
#6 0x000055cb9b8148da in proc_exit_prepare (code=0) at /home/andres/src/postgresql/src/backend/storage/ipc/ipc.c:209
#7 0x000055cb9b814792 in proc_exit (code=0) at /home/andres/src/postgresql/src/backend/storage/ipc/ipc.c:107
#8 0x000055cb9b851568 in PostgresMain (argc=9, argv=0x55cb9daddec0, dbname=0x55cb9dac4f70 "postgres", username=0x55cb9dac3580 "andres")
at /home/andres/src/postgresql/src/backend/tcop/postgres.c:4702
#9 0x000055cb9b672c5b in main (argc=9, argv=0x55cb9daddec0) at /home/andres/src/postgresql/src/backend/main/main.c:195

I suspect that to make the elog.c usage safe, we'll have to clear MyBEEntry in
pgstat_beshutdown_hook().

Greetings,

Andres Freund

[1] https://www.postgresql.org/message-id/20210807210349.bby5ta2xrbnte6ht%40alap3.anarazel.de