Truncation of mapped catalogs (whether local or shared) leads to server crash

Hi everyone,

I've noticed that truncating mapped catalogs causes the server to
crash due to an assertion failure. Here are the details:

Executing below commands:

-- set allow_system_table_mods TO on;
-- truncate table pg_type;

Results into a server crash with below backtrace:

...
#2  0x000055736767537d in ExceptionalCondition
(conditionName=0x5573678c5760 "relation->rd_rel->relkind ==
RELKIND_INDEX", fileName=0x5573678c4b28 "relcache.c",
    lineNumber=3896) at assert.c:66
#3  0x0000557367664e31 in RelationSetNewRelfilenumber
(relation=0x7f68240f1d58, persistence=112 'p') at relcache.c:3896
#4  0x000055736715b952 in ExecuteTruncateGuts
(explicit_rels=0x55736989e5b0, relids=0x55736989e600,
relids_logged=0x0, behavior=DROP_RESTRICT, restart_seqs=false,
    run_as_table_owner=false) at tablecmds.c:2146
#5  0x000055736715affa in ExecuteTruncate (stmt=0x55736989f950) at
tablecmds.c:1877
#6  0x0000557367493693 in standard_ProcessUtility
(pstmt=0x55736989fa00, queryString=0x55736989eed0 "truncate table
pg_type;", readOnlyTree=false,
    context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0,
dest=0x5573698a0330, qc=0x7ffe19367ac0) at utility.c:728

As seen from the backtrace above, the assertion failure occurs in
'RelationSetNewRelfilenumber()' at:

if (RelationIsMapped(relation))
{
    /* This case is only supported for indexes */
    Assert(relation->rd_rel->relkind == RELKIND_INDEX);
}

I would like to know why we are only expecting index tables here and
not the regular catalog tables. For instance, pg_type is a mapped
relation but not of index type, leading to the failure in this case.
Should we consider changing this Assert condition from RELKIND_INDEX
to (RELKIND_INDEX || RELKIND_RELATION)?

Additionally, is it advisable to restrict truncation of the pg_class
table? It's like a kind of circular dependency in case of pg_class
which is not applicable in case of other catalog tables.

--
With Regards,
Ashutosh Sharma.