Re: snapshot too old issues, first around wraparound and then more.

On Thu, Apr 2, 2020 at 11:28 AM Peter Geoghegan <pg@bowt.ie> wrote:
> In conclusion, I share Andres' concerns here. There are glaring
> problems with how we manipulate the data structure that controls the
> effective horizon for pruning. Maybe they can be fixed while leaving
> the code that manages the OldSnapshotControl circular buffer in
> something resembling its current form, but I doubt it. In my opinion,
> there is no approach to fixing "snapshot too old" that won't have some
> serious downside.

I'll add something that might be constructive: It would probably be a
good idea to introduce a function like syncrep.c's
SyncRepQueueIsOrderedByLSN() function, which is designed to be called
by assertions only. That would both clearly document and actually
verify the circular buffer/OldSnapshotControl data structure's
invariants.


--
Peter Geoghegan