On 17 March 2017 at 08:10, Stas Kelvich <s.kelv...@postgrespro.ru> wrote:
> While working on this i’ve spotted quite a nasty corner case with aborted > prepared > transaction. I have some not that great ideas how to fix it, but maybe i > blurred my > view and missed something. So want to ask here at first. > > Suppose we created a table, then in 2pc tx we are altering it and after that > aborting tx. > So pg_class will have something like this: > > xmin | xmax | relname > 100 | 200 | mytable > 200 | 0 | mytable > > After previous abort, tuple (100,200,mytable) becomes visible and if we will > alter table > again then xmax of first tuple will be set current xid, resulting in > following table: > > xmin | xmax | relname > 100 | 300 | mytable > 200 | 0 | mytable > 300 | 0 | mytable > > In that moment we’ve lost information that first tuple was deleted by our > prepared tx. Right. And while the prepared xact has aborted, we don't control when it aborts and when those overwrites can start happening. We can and should check if a 2pc xact is aborted before we start decoding it so we can skip decoding it if it's already aborted, but it could be aborted *while* we're decoding it, then have data needed for its snapshot clobbered. This hasn't mattered in the past because prepared xacts (and especially aborted 2pc xacts) have never needed snapshots, we've never needed to do something from the perspective of a prepared xact. I think we'll probably need to lock the 2PC xact so it cannot be aborted or committed while we're decoding it, until we finish decoding it. So we lock it, then check if it's already aborted/already committed/in progress. If it's aborted, treat it like any normal aborted xact. If it's committed, treat it like any normal committed xact. If it's in progress, keep the lock and decode it. People using logical decoding for 2PC will presumably want to control 2PC via logical decoding, so they're not so likely to mind such a lock. > * Try at first to scan catalog filtering out tuples with xmax bigger than > snapshot->xmax > as it was possibly deleted by our tx. Than if nothing found scan in a usual > way. I don't think that'll be at all viable with the syscache/relcache machinery. Way too intrusive. > * Do not decode such transaction at all. Yes, that's what I'd like to do, per above. -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
