BUG #18404: Select from pg_stat_activity in a plpgsql block can lead to a global locking issue

The following bug has been logged on the website:

Bug reference: 18404
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 16.2
Operating system: Ubuntu 22.04
Description:

The following script:
createuser u

cat << 'EOF' | psql -U u &
DO '
BEGIN
PERFORM count(*) FROM pg_stat_activity;
RAISE NOTICE ''sleeping...'';
PERFORM pg_sleep(1800);
END';
EOF
sleep 0.5

cat << EOF | psql &
VACUUM;
REINDEX SYSTEM;
EOF
sleep 0.5

cat << EOF | psql -U u
SELECT 1
EOF

ends with the last session stuck on startup:
law 3318525 3318511 0 15:00 ? 00:00:00 postgres: u regression
[local] DO
law 3318530 3318511 0 15:00 ? 00:00:00 postgres: law regression
[local] REINDEX waiting
law 3318533 3318511 0 15:00 ? 00:00:00 postgres: u regression
[local] startup waiting
law 3318628 3318511 0 15:01 ? 00:00:00 postgres: autovacuum
worker waiting
law 3318654 3318511 0 15:01 ? 00:00:00 postgres: autovacuum
worker waiting
law 3318676 3318511 0 15:02 ? 00:00:00 postgres: autovacuum
worker waiting

The backtrace of the last session backend is:
(gdb) bt
#0 0x00007fa5514ecf9a in epoll_wait (epfd=11, events=0x55970cae3d38,
maxevents=1, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
#1 0x000055970b04064b in WaitEventSetWaitBlock (set=0x55970cae3cd8,
cur_timeout=-1, occurred_events=0x7ffff6af57f0, nevents=1) at latch.c:1529
#2 0x000055970b040535 in WaitEventSetWait (set=0x55970cae3cd8, timeout=-1,
occurred_events=0x7ffff6af57f0, nevents=1, wait_event_info=50331648) at
latch.c:1475
#3 0x000055970b03f7d2 in WaitLatch (latch=0x7fa54e361734, wakeEvents=33,
timeout=0, wait_event_info=50331648) at latch.c:513
#4 0x000055970b06d8ea in ProcSleep (locallock=0x55970cb107d0,
lockMethodTable=0x55970b5e88e0 <default_lockmethod>) at proc.c:1294
#5 0x000055970b059f62 in WaitOnLock (locallock=0x55970cb107d0,
owner=0x55970cb73988) at lock.c:1818
#6 0x000055970b058a82 in LockAcquireExtended (locktag=0x7ffff6af5ba0,
lockmode=1, sessionLock=false, dontWait=false, reportMemoryError=true,
locallockp=0x7ffff6af5b98) at lock.c:1082
#7 0x000055970b055724 in LockRelationOid (relid=2676, lockmode=1) at
lmgr.c:117
#8 0x000055970b25319d in load_critical_index (indexoid=2676, heapoid=1260)
at relcache.c:4335
#9 0x000055970b252cee in RelationCacheInitializePhase3 () at
relcache.c:4150
#10 0x000055970b27d9be in InitPostgres (in_dbname=0x55970cb21978
"regression", dboid=16384, username=0x55970cae57c8 "u", useroid=0,
load_session_libraries=true, override_allow_connections=false,
out_dbname=0x0) at postinit.c:1180
#11 0x000055970b07dca4 in PostgresMain (dbname=0x55970cb21978 "regression",
username=0x55970cae57c8 "u") at postgres.c:4195
#12 0x000055970af9f014 in BackendRun (port=0x55970cb11c90) at
postmaster.c:4464
#13 0x000055970af9e8a0 in BackendStartup (port=0x55970cb11c90) at
postmaster.c:4192

The backtrace of the REINDEXing backend is:
(gdb) bt
#0 0x00007fa5514ecf9a in epoll_wait (epfd=11, events=0x55970cae3d38,
maxevents=1, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
#1 0x000055970b04064b in WaitEventSetWaitBlock (set=0x55970cae3cd8,
cur_timeout=-1, occurred_events=0x7ffff6af5090, nevents=1) at latch.c:1529
#2 0x000055970b040535 in WaitEventSetWait (set=0x55970cae3cd8, timeout=-1,
occurred_events=0x7ffff6af5090, nevents=1, wait_event_info=50331648) at
latch.c:1475
#3 0x000055970b03f7d2 in WaitLatch (latch=0x7fa54e361aa4, wakeEvents=33,
timeout=0, wait_event_info=50331648) at latch.c:513
#4 0x000055970b06d8ea in ProcSleep (locallock=0x55970cb105c0,
lockMethodTable=0x55970b5e88e0 <default_lockmethod>) at proc.c:1294
#5 0x000055970b059f62 in WaitOnLock (locallock=0x55970cb105c0,
owner=0x55970cb77a68) at lock.c:1818
#6 0x000055970b058a82 in LockAcquireExtended (locktag=0x7ffff6af5440,
lockmode=8, sessionLock=false, dontWait=false, reportMemoryError=true,
locallockp=0x7ffff6af5438) at lock.c:1082
#7 0x000055970b055724 in LockRelationOid (relid=2676, lockmode=8) at
lmgr.c:117
#8 0x000055970aaef8de in try_relation_open (relationId=2676, lockmode=8) at
relation.c:97
#9 0x000055970ab64a85 in try_index_open (relationId=2676, lockmode=8) at
indexam.c:161
#10 0x000055970ac1b146 in reindex_index (indexId=2676,
skip_constraint_checks=false, persistence=112 'p', params=0x7ffff6af56d8) at
index.c:3637
#11 0x000055970ac1bbe5 in reindex_relation (relid=1260, flags=5,
params=0x7ffff6af56d8) at index.c:3989
#12 0x000055970ad29ec6 in ReindexMultipleInternal (relids=0x55970cb17ef8,
params=0x7ffff6af5838) at indexcmds.c:3341
#13 0x000055970ad29923 in ReindexMultipleTables (objectName=0x0,
objectKind=REINDEX_OBJECT_SYSTEM, params=0x7ffff6af5838) at
indexcmds.c:3138
#14 0x000055970ad28e4e in ExecReindex (pstate=0x55970cb13958,
stmt=0x55970cae9ee0, isTopLevel=true) at indexcmds.c:2734
#15 0x000055970b0832c8 in standard_ProcessUtility (pstmt=0x55970cae9f90,
queryString=0x55970cae9518 "REINDEX SYSTEM;", readOnlyTree=false,
context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0,
dest=0x55970caea250,
qc=0x7ffff6af5ce0) at utility.c:964
#16 0x000055970b0825c9 in ProcessUtility (pstmt=0x55970cae9f90,
queryString=0x55970cae9518 "REINDEX SYSTEM;", readOnlyTree=false,
context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0,
dest=0x55970caea250, qc=0x7ffff6af5ce0)
at utility.c:530
#17 0x000055970b080e9b in PortalRunUtility (portal=0x55970cb63a18,
pstmt=0x55970cae9f90, isTopLevel=true, setHoldSnapshot=false,
dest=0x55970caea250, qc=0x7ffff6af5ce0) at pquery.c:1158
#18 0x000055970b081112 in PortalRunMulti (portal=0x55970cb63a18,
isTopLevel=true, setHoldSnapshot=false, dest=0x55970caea250,
altdest=0x55970caea250, qc=0x7ffff6af5ce0) at pquery.c:1315
#19 0x000055970b08055c in PortalRun (portal=0x55970cb63a18,
count=9223372036854775807, isTopLevel=true, run_once=true,
dest=0x55970caea250, altdest=0x55970caea250, qc=0x7ffff6af5ce0) at
pquery.c:791
#20 0x000055970b07926b in exec_simple_query (query_string=0x55970cae9518
"REINDEX SYSTEM;") at postgres.c:1274
#21 0x000055970b07e2d9 in PostgresMain (dbname=0x55970cb21978 "regression",
username=0x55970cae57c8 "law") at postgres.c:4637
#22 0x000055970af9f014 in BackendRun (port=0x55970cb11a90) at
postmaster.c:4464
#23 0x000055970af9e8a0 in BackendStartup (port=0x55970cb11a90) at
postmaster.c:4192

Without VACUUM in session 2, session 3 fails on connection due to
authentication timeout, but with VACUUM it hangs until the plpgsql
block in session 1 ends.