BUG #18127: Assertion HaveRegisteredOrActiveSnapshot failed on REINDEX CONCURRENTLY when blocksize=1

The following bug has been logged on the website:

Bug reference: 18127
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 16.0
Operating system: Ubuntu 22.04
Description:

A server compiled --with-blocksize=1 produces an assertion failure on
`make check`. More specifically, the failure triggered on a query like:
CREATE TABLE concur_reindex_tab (c1 int PRIMARY KEY);
CREATE TABLE concur_reindex_tab2 (c1 int REFERENCES concur_reindex_tab);
REINDEX INDEX CONCURRENTLY concur_reindex_tab_pkey;

Core was generated by `postgres: law regression [local] REINDEX
'.
Program terminated with signal SIGABRT, Aborted.

warning: Section `.reg-xstate/3738467' in core file too small.
#0 __pthread_kill_implementation (no_tid=0, signo=6,
threadid=139868701853504)
at ./nptl/pthread_kill.c:44
44 ./nptl/pthread_kill.c: No such file or directory.
(gdb) bt
#0 __pthread_kill_implementation (no_tid=0, signo=6,
threadid=139868701853504) at ./nptl/pthread_kill.c:44
#1 __pthread_kill_internal (signo=6, threadid=139868701853504) at
./nptl/pthread_kill.c:78
#2 __GI___pthread_kill (threadid=139868701853504, signo=signo(at)entry=6) at
./nptl/pthread_kill.c:89
#3 0x00007f35b7af0476 in __GI_raise (sig=sig(at)entry=6) at
../sysdeps/posix/raise.c:26
#4 0x00007f35b7ad67f3 in __GI_abort () at ./stdlib/abort.c:79
#5 0x0000559cd6118131 in ExceptionalCondition (conditionName=0x559cd61a5470
"HaveRegisteredOrActiveSnapshot()", fileName=0x559cd61a50c0
"toast_internals.c", lineNumber=670) at assert.c:66
#6 0x0000559cd59ad7ff in init_toast_snapshot
(toast_snapshot=0x7ffdf91b4d00) at toast_internals.c:670
#7 0x0000559cd59ad22c in toast_delete_datum (rel=0x7f35b8466c58,
value=139868524341189, is_speculative=false) at toast_internals.c:429
#8 0x0000559cd5a625e0 in toast_tuple_cleanup (ttc=0x7ffdf91b4e50) at
toast_helper.c:309
#9 0x0000559cd5a0c31c in heap_toast_insert_or_update (rel=0x7f35b8466c58,
newtup=0x559cd69dae18, oldtup=0x7ffdf91c2440, options=0) at
heaptoast.c:333
#10 0x0000559cd59f7418 in heap_update (relation=0x7f35b8466c58,
otid=0x559cd69dae1c, newtup=0x559cd69dae18, cid=0, crosscheck=0x0,
wait=true, tmfd=0x7ffdf91c24d0, lockmode=0x7ffdf91c24c8,
update_indexes=0x7ffdf91c252c) at heapam.c:3595
#11 0x0000559cd59f8109 in simple_heap_update (relation=0x7f35b8466c58,
otid=0x559cd69dae1c, tup=0x559cd69dae18, update_indexes=0x7ffdf91c252c) at
heapam.c:4053
#12 0x0000559cd5ad3b45 in CatalogTupleUpdate (heapRel=0x7f35b8466c58,
otid=0x559cd69dae1c, tup=0x559cd69dae18) at indexing.c:322
#13 0x0000559cd5aceda6 in index_concurrently_swap (newIndexId=20330,
oldIndexId=20299, oldName=0x559cd69dc368 "concur_reindex_ind1_ccold") at
index.c:1659
#14 0x0000559cd5be2573 in ReindexRelationConcurrently (relationOid=20299,
params=0x7ffdf91c2b68) at indexcmds.c:4088
#15 0x0000559cd5bdfe3a in ReindexIndex (indexRelation=0x559cd691a100,
params=0x7ffdf91c2b68, isTopLevel=true) at indexcmds.c:2809
#16 0x0000559cd5bdfc7a in ExecReindex (pstate=0x559cd6c3ae28,
stmt=0x559cd691a150, isTopLevel=true) at indexcmds.c:2739
#17 0x0000559cd5f38b7c in standard_ProcessUtility (pstmt=0x559cd691a2a0,
queryString=0x559cd69196e8 "REINDEX INDEX CONCURRENTLY
concur_reindex_ind1;", readOnlyTree=false, context=PROCESS_UTILITY_TOPLEVEL,
params=0x0, queryEnv=0x0,
dest=0x559cd691a560, qc=0x7ffdf91c3010) at utility.c:964
#18 0x0000559cd5f37e7d in ProcessUtility (pstmt=0x559cd691a2a0,
queryString=0x559cd69196e8 "REINDEX INDEX CONCURRENTLY
concur_reindex_ind1;", readOnlyTree=false, context=PROCESS_UTILITY_TOPLEVEL,
params=0x0, queryEnv=0x0,
dest=0x559cd691a560, qc=0x7ffdf91c3010) at utility.c:530
#19 0x0000559cd5f3674f in PortalRunUtility (portal=0x559cd698c8f8,
pstmt=0x559cd691a2a0, isTopLevel=true, setHoldSnapshot=false,
dest=0x559cd691a560, qc=0x7ffdf91c3010) at pquery.c:1158
#20 0x0000559cd5f369c6 in PortalRunMulti (portal=0x559cd698c8f8,
isTopLevel=true, setHoldSnapshot=false, dest=0x559cd691a560,
altdest=0x559cd691a560, qc=0x7ffdf91c3010) at pquery.c:1315
#21 0x0000559cd5f35e10 in PortalRun (portal=0x559cd698c8f8,
count=9223372036854775807, isTopLevel=true, run_once=true,
dest=0x559cd691a560, altdest=0x559cd691a560, qc=0x7ffdf91c3010) at
pquery.c:791
#22 0x0000559cd5f2eb1f in exec_simple_query (query_string=0x559cd69196e8
"REINDEX INDEX CONCURRENTLY concur_reindex_ind1;") at postgres.c:1274
#23 0x0000559cd5f33b8d in PostgresMain (dbname=0x559cd6950ac8 "regression",
username=0x559cd69159c8 "law") at postgres.c:4637
#24 0x0000559cd5e54bf3 in BackendRun (port=0x559cd69452f0) at
postmaster.c:4464
#25 0x0000559cd5e5447f in BackendStartup (port=0x559cd69452f0) at
postmaster.c:4192
#26 0x0000559cd5e507c4 in ServerLoop () at postmaster.c:1782
#27 0x0000559cd5e5006e in PostmasterMain (argc=8, argv=0x559cd6913850) at
postmaster.c:1466
#28 0x0000559cd5d046f9 in main (argc=8, argv=0x559cd6913850) at main.c:198

(gdb) frame 10
#10 0x0000559cd59f7418 in heap_update (relation=0x7f35b8466c58,
otid=0x559cd69dae1c,
newtup=0x559cd69dae18, cid=0, crosscheck=0x0, wait=true,
tmfd=0x7ffdf91c24d0,
lockmode=0x7ffdf91c24c8, update_indexes=0x7ffdf91c252c) at
heapam.c:3595
3595 heaptup =
heap_toast_insert_or_update(relation, newtup, &oldtup, 0);
(gdb) p need_toast
$1 = true
(gdb) p newtupsize
$2 = 256

(gdb) frame 6
#6 0x0000559cd59ad7ff in init_toast_snapshot
(toast_snapshot=0x7ffdf91b4d00) at toast_internals.c:670
670 Assert(HaveRegisteredOrActiveSnapshot());
(gdb) p RegisteredSnapshots
$4 = {ph_compare = 0x559cd61775e0 <xmin_cmp>, ph_arg = 0x0,
ph_root = 0x559cd64e35a8 <CatalogSnapshotData+72>}
(gdb) p *RegisteredSnapshots.ph_root
$5 = {first_child = 0x0, next_sibling = 0x0, prev_or_parent = 0x0}
(gdb) p ActiveSnapshot
$6 = (ActiveSnapshotElt *) 0x0

I think that the blocksize matters here just because it allows to reach
toast_delete_datum() inside index_concurrently_swap(). Perhaps the same
effect could be seen with the default block size, but with larger tuples
in pg_constraint (I haven't tried to construct such tuples yet).

This issue is not the same as [1], because in this case I really see no
registered or active snapshots.

Reproduced on REL_15_STABLE .. master.

[1]
https://www.postgresql.org/message-id/dc9dd229-ed30-6c62-4c41-d733ffff776b%40xs4all.nl