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Date: Sat, 08 Nov 2025 13:59:21 +0100
From: "Joel Jacobson" <joel@compiler.org>
To: pgsql-hackers <pgsql-hackers@postgresql.org>
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Subject: Re: Optimize LISTEN/NOTIFY
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On Fri, Nov 7, 2025, at 19:59, Joel Jacobson wrote:
> * The logic in SignalBackends has been reworked and simplified,
>   thanks to the new isAdvancing and advancingPos fields.
>   I now think it's finally easy to reason about why each branch
>   in SignalBackends must be correct.

I was wrong. I wrongly assumed asyncQueueReadAllNotifications would read
up until head, which it might not actually do:

    * Process messages up to the stop position, end of page, or an
    * uncommitted message.

This in turn could cause a listening backend to remain behind, if there
would be no more notifies, so it unfortunately seems like we will always
need to signal when a backend isAdvancing, and therefore have no use of
the advancingPos field.

I will do more correctness and benchmark testing before posting a new
version. Just wanted to give you a heads up on the bug, so you don't
waste time reviewing.

/Joel