Re: almost-super-user problems that we haven't fixed yet

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: almost-super-user problems that we haven't fixed yet
Date: 2023-01-18 19:00:12
Message-ID: 20230118190012.GA3540486@nathanxps13
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On Wed, Jan 18, 2023 at 11:28:57AM -0500, Robert Haas wrote:
> In general, looks good. I think this will often call HaveNFreeProcs
> twice, though, and that would be better to avoid, e.g.

I should have thought of this. This is fixed in v2.

> In the common case where we hit neither limit, this only counts free
> connection slots once. We could do even better by making
> HaveNFreeProcs have an out parameter for the number of free procs
> actually found when it returns false, but that's probably not
> important.

Actually, I think it might be important. IIUC the separate calls to
HaveNFreeProcs might return different values for the same input, which
could result in incorrect error messages (e.g., you might get the
reserved_connections message despite setting reserved_connections to 0).
So, I made this change in v2, too.

> I don't think that we should default both the existing GUC and the new
> one to 3, because that raises the default limit in the case where the
> new feature is not used from 3 to 6. I think we should default one of
> them to 0 and the other one to 3. Not sure which one should get which
> value.

I chose to set reserved_connections to 0 since it is new and doesn't have a
pre-existing default value.

Nathan Bossart
Amazon Web Services:

Attachment Content-Type Size
v2-0001-Rename-ReservedBackends-to-SuperuserReservedBacke.patch text/x-diff 4.0 KB
v2-0002-Introduce-reserved_connections-and-pg_use_reserve.patch text/x-diff 12.9 KB

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