On Fri, Nov 17, 2017 at 5:12 AM, Robert Haas <robertmh...@gmail.com> wrote: > On Thu, Nov 16, 2017 at 2:41 PM, Andres Freund <and...@anarazel.de> wrote: >>> To me, it seems like SnapBuildWaitSnapshot() is fundamentally >>> misdesigned >> >> Maybe I'm confused, but why is it fundamentally misdesigned? It's not >> such an absurd idea to wait for an xid in a WAL record. I get that >> there's a race condition here, which obviously bad, but I don't really >> see as evidence of the above claim. >> >> I actually think this code used to be safe because ProcArrayLock used to >> be held while generating and logging the running snapshots record. That >> was removed when fixing some other bug, but perhaps that shouldn't have >> been done... > > OK. Well, I might be overstating the case. My comment about > fundamental misdesign was really just based on the assumption that > XactLockTableWait() could be used to wait for an XID the instant it > was generated. That was never gonna work and there's no obvious clean > workaround for the problem. Getting snapshot building to work > properly seems to be Hard (TM).
The patches discussed here deserve tracking, so please note that I have added an entry in the CF app: https://commitfest.postgresql.org/16/1381/ -- Michael
