Re: Fix slotsync worker busy loop causing repeated log messages

On Tue, Mar 3, 2026 at 1:12 PM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Saturday, February 28, 2026 1:03 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > On Fri, Feb 27, 2026 at 8:34 PM Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> > >
> > > Normally, the slotsync worker updates the standby slot using the
> > > primary's slot state. However, when confirmed_flush_lsn matches but
> > > restart_lsn does not, the worker does not actually update the standby
> > > slot. Despite that, the current code of update_local_synced_slot()
> > > appears to treat this situation as if an update occurred. As a result,
> > > the worker sleeps only for the minimum interval (200 ms) before
> > > retrying. In the next cycle, it again assumes an update happened, and
> > > continues looping with the short sleep interval, causing the repeated
> > > logical decoding log messages. Based on a quick analysis, this seems to be
> > the root cause.
> > >
> > > I think update_local_synced_slot() should return false (i.e., no
> > > update
> > > happened) when confirmed_flush_lsn is equal but restart_lsn differs
> > > between primary and standby.
> > >
> >
> > We expect that in such a case update_local_synced_slot() should advance
> > local_slot's 'restart_lsn' via LogicalSlotAdvanceAndCheckSnapState(),
> > otherwise, it won't go in the cheap code path next time. Normally, restart_lsn
> > advancement should happen when we process XLOG_RUNNING_XACTS and
> > call SnapBuildProcessRunningXacts(). In this particular case as both
> > restart_lsn and confirmed_flush_lsn are the same (0/03000140), the
> > machinery may not be processing XLOG_RUNNING_XACTS record. I have not
> > debugged the exact case yet but you can try by emitting some more records
> > on publisher, it should let the standby advance the slot. It is possible that we
> > can do something like you are proposing to silence the LOG messages but we
> > should know what is going on here.
>
> I reproduced and debugged this issue where a replication slot's restart_lsn
> fails to advance. In my environment, I found it only occurs when a synced
> slot first builds a consistent snapshot. The problematic code path is in
> SnapBuildProcessRunningXacts():
>
> if (builder->state < SNAPBUILD_CONSISTENT)
> {
> /* returns false if there's no point in performing cleanup just yet */
> if (!SnapBuildFindSnapshot(builder, lsn, running))
> return;
> }
>
> When a synced slot reaches consistency for the first time with no running
> transactions, SnapBuildFindSnapshot() returns false, causing the function to
> return without updating the candidate restart_lsn.
>
> So, an alternative approach is to improve this logic by updating the candidate
> restart_lsn in this case instead of returning early.
>

But why not return 'true' from SnapBuildFindSnapshot() in that case?
The comment atop SnapBuildFindSnapshot() says: "Returns true if there
is a point in performing internal maintenance/cleanup using the
xl_running_xacts record.". Doesn't updating restart_lsn fall under
that category?

However, I have a question that even if we haven't incremented it in
the first cycle, why is it not incrementing restart_lsn in consecutive
sync cycles.

--
With Regards,
Amit Kapila.