Re: BUG #17385: "RESET transaction_isolation" inside serializable transaction causes Assert at the transaction end

On Thu, Mar 10, 2022 at 12:11 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Masahiko Sawada <sawada.mshk@gmail.com> writes:
> > On Wed, Feb 16, 2022 at 1:55 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> Yeah, I was considering that too.  A new GUC_NO_RESET flag would be
> >> cheaper than running the check hooks during RESET, and probably
> >> safer too.  On the other hand, we would lose the property that
> >> you can reset these settings as long as you've not yet taken a
> >> snapshot.  I wonder whether there is any code out there that
> >> depends on that ...
>
> > Indeed. I guess that it's relatively common that the transaction
> > isolation level is set after BEGIN TRANSACTION but I've not heard that
> > it's reset after BEGIN TRANSACTION with setting non-default
> > transaction isolation level.
>
> Yes, we certainly have to preserve the SET case, but using RESET
> in that way seems like it'd be pretty weird coding.  I'd have no
> hesitation about banning it in a HEAD-only change.  I'm slightly
> more nervous about doing so in a back-patched bug fix.  On the
> other hand, starting to call check hooks in a context that did
> not use them before is also scary to back-patch.

Agreed.

For back branches, it might be less scary to call check hooks for only
limited GUCs such as transaction_isolation.

> Do you want to draft up a patch that fixes this with a new
> GUC flag?

Yes, I'll update the patch accordingly.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/