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Date: Tue, 25 Nov 2025 21:14:12 +0100
From: "Joel Jacobson" <joel@compiler.org>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: "Chao Li" <li.evan.chao@gmail.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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Subject: Re: Optimize LISTEN/NOTIFY
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On Thu, Nov 20, 2025, at 21:26, Tom Lane wrote:
> So I think there needs to be a serious effort made to move as
> much as we possibly can of the potentially-risky stuff into
> PreCommit_Notify.  In particular I think we ought to create
> the shared channel hash entry then, and even insert our PID
> into it.  We could expand the listenersArray entries to include
> both a PID and a boolean "is it REALLY listening?", and then
> during Exec_ListenCommit we'd only be required to find an
> entry we already added and set its boolean, so there's no OOM
> hazard.  Possibly do something similar with the local
> listenChannelsHash, so as to remove that possibility of OOM
> failure as well.
>
> (An alternative design could be to go ahead and insert our
> PID during PreCommit_Notify, and just tolerate the small
> risk of getting signaled when we didn't need to be.  But
> then we'd need some mechanism for cleaning out the bogus
> entry during AtAbort_Notify.)

[...back from a little detour with new insights...]

It looks to me like it would be best with two boolean fields; one
boolean to stage the updates during PreCommit_Notify, that each
pendingActions could flip back and forth, and another boolean that
represents the current value, which we would overwrite with the staged
value during AtCommit_Notify.

This way, cleanup for the rare edge-case when we did PreCommit_Notify
followed by AtAbort_Notify, seems simple; we just need to go through all
entires and delete those where current=false, since those entries were
newly added by PreCommit_Notify, i.e. we were not listening to those
channels since before.  Probably also setting a flag in
PreCommit_Notify, so that we only need to do cleanup in AtAbort_Notify
if we actually hit PreCommit_Notify.

I haven't implemented this yet, but I have a good feeling about this
approach.  Just wanted to share the plan before I start working, in case
anyone see any flaw with it, or see a better approach.

/Joel