Re: Failed Assert in pgstat_assoc_relation

On Mon, Nov 28, 2022 at 8:10 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> Hi,
>
> While reviewing/testing one of the patches I found the following Assert:
> #0 __pthread_kill_implementation (no_tid=0, signo=6,
> threadid=139624429171648) at ./nptl/pthread_kill.c:44
> #1 __pthread_kill_internal (signo=6, threadid=139624429171648) at
> ./nptl/pthread_kill.c:78
> #2 __GI___pthread_kill (threadid=139624429171648,
> signo=signo(at)entry=6) at ./nptl/pthread_kill.c:89
> #3 0x00007efcda6e3476 in __GI_raise (sig=sig(at)entry=6) at
> ../sysdeps/posix/raise.c:26
> #4 0x00007efcda6c97f3 in __GI_abort () at ./stdlib/abort.c:79
> #5 0x00005590bf283139 in ExceptionalCondition
> (conditionName=0x5590bf468170 "rel->pgstat_info->relation == NULL",
> fileName=0x5590bf46812b "pgstat_relation.c", lineNumber=143) at
> assert.c:66
> #6 0x00005590bf0ce5f8 in pgstat_assoc_relation (rel=0x7efcce996a48)
> at pgstat_relation.c:143
> #7 0x00005590beb83046 in initscan (scan=0x5590bfbf4af8, key=0x0,
> keep_startblock=false) at heapam.c:343
> #8 0x00005590beb8466f in heap_beginscan (relation=0x7efcce996a48,
> snapshot=0x5590bfb5a520, nkeys=0, key=0x0, parallel_scan=0x0,
> flags=449) at heapam.c:1223
> #9 0x00005590bf02af39 in table_beginscan (rel=0x7efcce996a48,
> snapshot=0x5590bfb5a520, nkeys=0, key=0x0) at
> ../../../src/include/access/tableam.h:891
> #10 0x00005590bf02bf8a in DefineQueryRewrite (rulename=0x5590bfb281d0
> "_RETURN", event_relid=16387, event_qual=0x0, event_type=CMD_SELECT,
> is_instead=true, replace=false, action=0x5590bfbf4aa8)
> at rewriteDefine.c:447
> #11 0x00005590bf02b5ab in DefineRule (stmt=0x5590bfb285c0,
> queryString=0x5590bfb277a8 "CREATE RULE \"_RETURN\" AS ON SELECT TO t
> DO INSTEAD SELECT * FROM t1;") at rewriteDefine.c:213
>
> I could reproduce this issue with the following steps:
> create table t1(c int);
> BEGIN;
> CREATE TABLE t (c int);
> SAVEPOINT q;
> CREATE RULE "_RETURN" AS ON SELECT TO t DO INSTEAD SELECT * FROM t1;
> select * from t;
> ROLLBACK TO q;
> CREATE RULE "_RETURN" AS ON SELECT TO t DO INSTEAD SELECT * FROM t1;
> ROLLBACK;
>
> Regards,
> Vignesh

I think what is happening here is that the previous relation is not
unlinked when pgstat_init_relation() is called
because the relation is now a view and for relations without storage
the relation is not unlinked in pgstat_init_relation()

void
pgstat_init_relation(Relation rel)
{
char relkind = rel->rd_rel->relkind;

/*
* We only count stats for relations with storage and partitioned tables
*/
if (!RELKIND_HAS_STORAGE(relkind) && relkind != RELKIND_PARTITIONED_TABLE)
{
rel->pgstat_enabled = false;
rel->pgstat_info = NULL;
return;
}

There is a logic in DefineQueryRewrite() which converts a relation to
a view when you create such a rule like the test case does.
So initially the relation had storage, the pgstat_info is linked,
then table is converted to a view, but in init, the previous
relation is not unlinked but when it tries to link a new relation, the
assert fails saying a previous relation is already linked to
pgstat_info

I have made a small patch with a fix, but I am not sure if this is the
right way to fix this.

regards,
Ajin Cherian
Fujitsu Australia