Re: RE: Re:BUG #18369: logical decoding core on AssertTXNLsnOrder() - Mailing list pgsql-bugs

On Wed, Mar 6, 2024 at 9:04 AM ocean_li_996 <ocean_li_996@163.com> wrote:
>
> Thanks for your attention.
>
> At 2024-03-05 17:24:05, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com> wrote:
>
> >Dear Haiyang Li,
> >
> >...
> >## Found issue
> >
> >### Empty transaction is decoded on PG14 and PG15
> >
> >However, there is a room for generating ReorderBufferTxn for empty transactions,
> >which was introduced by 6b77048e5. Conditions are:
> >
> >1. There are sub transactions which modify only temp tables, and
> >2. the top transaction modifies the catalog.
> >
> >The call-stack toward the generation is below.
> >
> >```
> >ReorderBufferTXNByXid(create = true, create_as_top = true)
> >ReorderBufferXidSetCatalogChanges()    // for sub transactions
> >SnapBuildXidSetCatalogChanges()          // for top transaction
> >DecodeCommit()                                      // for top transaction
> >```
> >
> >The path has been introduced by 6b77048e5.
> >Previously, calling ReorderBufferXidSetCatalogChanges() for sub transactions
> >would be skipped, if they do not have catalog changes or they have not decoded yet.
> >However, the commit ensures sub transactions must be marked as containing
> >catalog changes, and this also enforces to decode transactions even if it is
> >empty.
> >
> >### Assertion failure
> >
> >The empty transactions would be created as top transactions. At that time,
> >AssertTXNLsnOrder() is called so that we ensured that first_lsn of top-transactions
> >must be strictly higher than previous. But they can be the same if there are more
> >than two empty transactions. It led an assertion failure.
> >
> Your analysis is correct for me.  Actually, I mentioned in [1] that I can reproduce this issue before 6b77048e5.
> After some attempts and analysis, I also believe that the issue will only occur after 6b77048e5.
>
> > ...
> >
> >## Possible solutions
> >
> >I think there are several solutions.
> >Note that I assumed here that fixes for all the versions should be almost the same.
> >
> >* Ease the condition in AssertTXNLsnOrder(). If the decoded transaction is empty,
> >  it can be allowed that the first_lsn is same as previous one.
> >  PSA file to see my consideration.
> LGFM.  For my observation, the most case failed on AsserTXNOrder is checking empty
> decoded transaction. Maybe we should pay more attention to review ReorderBufferTXNIsEmpty.
>
> >* Generate a ReorderBufferTXN as sub transaction when we are in this path.
> >  The approach has already been shared by you. However, note that this needs to
> >  extend the ReorderBufferXidSetCatalogChanges function, and breaks ABI
> >  compatibility [1].
> Yes, It breaks ABI compatibility.
>

One possibility is introducing ReorderBufferXidSetCatalogChangesEx, a
new API with a bool parameter. I don't know if this is a good idea but
I prefer not to tinker with asserts as proposed by Kuroda-San in
another approach.

--
With Regards,
Amit Kapila.