Re: SSI: ON CONFLICT DO SELECT takes no predicate lock on the returned row

> Good catch.

Look like we also had a real-life serialization issue, this was
already mentioned and fixed differently by Andrey in another thread
[1]. That thread also has a different serialization-related patch I
was reviewing at the same time, and I didn't notice that Andrey sent
another email that day.

His patch also handles a similar issue in DO UPDATE / DO NOTHING which
my patch didn't address.

> I don't think this is quite right though. The details of how heapam uses SI
> locks is heapam specific and should live in the heapam code. I also think you
> actually need to acquire the predicate lock before ExecOnConflictLockRow()?

You are right, and after looking at Andrey's patch I think that it
also might have a similar but smaller race window, so I'll still
attach this v2 as it tries a different direction. This v2 still only
addresses the DO SELECT part.

Please also take a look at the patches in the other thread, both for
this issue and the other more corner-case issue we were discussing
there.

[1] https://www.postgresql.org/message-id/787936C5-4155-4CF9-939D-39DC0EC1C892@yandex-team.ru