RE: Potential data loss due to race condition during logical replication slot creation

Dear Amit,

> I feel setting "needs_full_snapshot" to true for decoding means the
> snapshot will start tracking non-catalog committed xacts as well which
> is costly.

I think the approach was most conservative one which does not have to change
the version of the snapshot. However, I understood that you wanted to consider
the optimized solution for HEAD first.

> See SnapBuildCommitTxn(). Can we avoid this problem if we
> would have list of all running xacts when we serialize the snapshot by
> not decoding any xact whose xid lies in that list? If so, one idea to
> achieve could be that we maintain the highest_running_xid while
> serailizing the snapshot and then during restore if that
> highest_running_xid is <= builder->initial_xmin_horizon, then we
> ignore restoring the snapshot. We already have few such cases handled
> in SnapBuildRestore().

Based on the idea, I made a prototype. It can pass tests added by others and me.
How do other think?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/