On Fri, Jul 21, 2017 at 7:17 PM, Kyotaro HORIGUCHI
<horiguchi.kyot...@lab.ntt.co.jp> wrote:
> In vac_truncate_clog, TruncateCLOG is called before
> SetTransactionIdLimit, which advances
> ShmemVariableCache->oldestXid. Given that the assertion in
> TruncateCLOG is valid, they should be called in reverse order. I
> suppose that CLOG files can be safely truncated after advancing
> XID limits.

If we keep the assertion by changing the order of changes to match the
comment like this, then don't we still have a problem if another
backend moves it backwards because of the data race I mentioned?  That
too could be fixed (perhaps by teaching SetTransactionIdLimit not to
overwrite higher values), but it sounds like the assertion might be a

Thomas Munro

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