Date: Fri, 15 Nov 2024 09:18:50 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
	Pavan Deolasee <pavan.deolasee@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Noah Misch <noah@leadboat.com>, Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: Potential ABI breakage in upcoming minor releases
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Hi,

On Thu, Nov 14, 2024 at 11:35:25AM -0500, Peter Geoghegan wrote:
> On Thu, Nov 14, 2024 at 11:29 AM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> > ISTM that we have spare bytes where we could place that boolean without
> > breaking ABI.  In x86_64 there's a couple of 4-byte holes, but those
> > won't be there on x86, so not great candidates.  Fortunately there are
> > 3-byte and 7-byte holes also, which we can use safely.  We can move the
> > new boolean to those location.
> 
> Wasn't this part of the official guidelines?

Do you mean in [1] (Maintaining ABI compatibility while backpatching section)?

If so, it looks like it's not currently mentioned.

What about?

"
You cannot modify any struct definition in src/include/*. If any new members must
be added to a struct, put them in the padding holes (if any) in backbranches (or
at the end of the struct if there is no padding holes available).
"

Also we may need a decision table to ensure 32-bit portability based on the
hole size on 64-bit, something like?

64-bit hole size | use on 32-bit?
-----------------|---------------
<=3 bytes        | safe to use
4 bytes          | don't use
5-7 bytes        | use first (hole_size - 4) bytes only

Does that make sense?

Regards,

[1]: https://wiki.postgresql.org/wiki/Committing_checklist

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com