Re: Streaming decoding fails with "unexpected table_index_fetch_tuple call during logical decoding" when a relation has a TOASTed conbin (follow-up to BUG #18641)

Hi,

This is the reply for [1]. My mailer could not receive the original post due to
the company's policy, so I will put as the normal post. Sorry for inconvenience.

I confirmed this could happen on PG18, PG17 and PG14. Not tested, but expecting
for PG15 and 166 as well. This could not happen on PG19/HEAD because the
elog(ERROR) was removed by 87f7b824f20, but possible for all branches.

I think your analysis is correct. bsysscan tries to indicate that whether we're
scanning a system table, which was turned on at systable_beginscan* and turned
off at systable_endscan*. But if the systable scan is nested (i.e., pg_constraint.conbin),
the flag can be wrong reset. In PG18- the state is checked for every getnextslot,
which raised the ERROR. In PG19+ the check is unified at the beginning thus the
ERROR does not happen, but I guess the flag can be still wrong.

One idea is to track the depth of scans. Attached patch is for PG18, and I tried not
to modify the header as much as possible. It also had a test code based on your
reproducer. Can you see it's same as your expectation?

[1]: https://www.postgresql.org/message-id/CAF7a2M-OF%2BTjYUBvkSV2oyZNYuPm%3DEqTApcMDOC%2BaevBFPuWrg%40mail.gmail.com

Best regards,
Hayato Kuroda
FUJITSU LIMITED