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Subject: Re: Return pg_control from pg_backup_stop().
To: Michael Paquier <michael@paquier.xyz>
Cc: Haibo Yan <tristan.yim@gmail.com>,
 Pg Hackers <pgsql-hackers@postgresql.org>,
 Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>,
 Andres Freund <andres@anarazel.de>, Fujii Masao <masao.fujii@gmail.com>
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From: David Steele <david@pgbackrest.org>
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Date: Wed, 18 Mar 2026 12:26:48 +0000 (UTC)
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On 3/18/26 15:26, Michael Paquier wrote:
> On Wed, Mar 18, 2026 at 07:35:47AM +0000, David Steele wrote:
>> You are correct -- the copy of pg_control needs to happen before
>> do_pg_backup_stop(). An older version of this patch saved pg_control in
>> backup_state which made the prior location safe. However, I missed moving
>> this code when I moved pg_control out of backup_state. Code review to the
>> rescue.
> 
> Right.  I am wondering also if the final result would not be better
> without 0002, actually, focusing only on the "simpler" base backup
> case through the replication protocol, and you are making a good case
> in mentioning it as not absolutely mandatory for base backups that are
> taken through the SQL functions.  One could always tweak the flag
> manually in the control file based on the contents taken from the data
> folder.  That's more hairy than writing the entire file, for sure,
> still possible.

Getting even 01 into PG19 would be a great outcome. This would solve the 
problem of torn pg_control and deleted backup labels for any backups 
made with pg_basebackup and that's going to cover a *lot* of cases.

Established third-party backup solutions that are not based on 
pg_basebackup are generally able to manipulate pg_control so that's not 
as much of a concern, perhaps. It does raise the barrier of entry for 
new backup software if they need to learn to read and validate 
pg_control to avoid a torn copy and set the flag. Patch 02 solves that 
problem in a general way so I still think it adds value for the 
ecosystem -- but we could always discuss that in the PG20 cycle.

Whatever gets committed for PG19 I'll write a followup patch to describe 
the hazards of reading pg_control and generally how to get a good copy. 
However, this will be complicated enough that the best answer will 
likely be to use pg_basebackup or some other reputable backup software. 
I don't love this -- I feel like the low-level interface should be 
usable with such hazards.

Regards,
-David