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 Jiří,

Thanks for the test. I understood that no issues were found for now.

> One question while reading the patch, not a problem I could trigger: the depth
> is incremented in systable_beginscan* and decremented in systable_endscan* only
> if CheckXidAlive is valid, and that is evaluated separately at each end. If
> CheckXidAlive changed while a scan was open, the counter would be off by one.
> I could not find a path where that happens. Error paths look fine, since
> AbortTransaction/AbortSubTransaction call ResetLogicalStreamingState(). If a
> SysScanDesc field is acceptable despite the header concern, remembering in the
> scan whether it was counted would make the pairing explicit.

Hmm, I'm not excited to modify the exposed data structure yet, unless there is a
real issue. Per my analysis, SysScanDescData only contains pointers (8 bytes),
it does not have any paddings. This meant we need to modify a size of the data
structure, it might cause failures somewhere.
Also, the existing code has the same possibility while turning on/off bsysscan,
right? So I feel it's already accepted.
(Of course, we must fix if it causes a real failure)

Best regards,
Hayato Kuroda
FUJITSU LIMITED