Message-ID: <c6778430-e5be-4eb8-a05c-f6914b092f0c@iki.fi>
Date: Sun, 5 Apr 2026 21:35:06 +0300
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: Better shared data structure management and resizable shared data
 structures
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
 Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>,
 pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
References: 
 <CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com>
 <CAExHW5v5FVZbsO9sLzztMZ11C3hgGStE=HkkV2bQkCyncess4w@mail.gmail.com>
 <b2451225-0a10-4ded-abef-4e9efbde7a8e@iki.fi>
 <CAExHW5tCC0T1ky=Jnq-AvMxa67Adaw7aQ4iQAO=BSdHcbSNBVg@mail.gmail.com>
 <791c3f18-f4de-4d84-ac6b-c7ccc074dd38@iki.fi>
 <CAExHW5tS7GncN90oJWOSzW_3F1EHL9xwe59L7Req3nUVgmObUw@mail.gmail.com>
 <9d919bd9-94dd-4bda-8ccf-ebced4178c53@iki.fi>
 <CAEze2WhMOHVgH2Xeyzx=VEk-Ta_YnQUqT+TdBiv5Lx8ESn2WZA@mail.gmail.com>
 <CAExHW5s6h=c_q2m72Nvyj1ghMEhPkOBkeN5Htn7YR=1BrNN-Sw@mail.gmail.com>
 <7d3ba240-9350-4dfc-bbe1-be6584aee236@iki.fi>
 <CAEze2WjQZff3znd6CtG-OBzYZMMqy5TyQSoAo=QTFT38tDndeQ@mail.gmail.com>
 <1c3a07a7-158d-4800-927c-2641c73277d8@iki.fi>
 <CAExHW5twYLhVjtApX3osdaz-4b0Tjvjj_Zr20orUb61qgiJmUw@mail.gmail.com>
 <CAExHW5sYaeZqzjUrG2V9U5ZFnZHW+X-Q2Zb9L62CRbeuAwC4EA@mail.gmail.com>
 <CAExHW5uoKOZKmmCb=4azZ5Lx0uT+CrmievXdSweAYHbo9HXMnQ@mail.gmail.com>
Content-Language: en-US
From: Heikki Linnakangas <hlinnaka@iki.fi>
In-Reply-To: 
 <CAExHW5uoKOZKmmCb=4azZ5Lx0uT+CrmievXdSweAYHbo9HXMnQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Archived-At: 
 <https://www.postgresql.org/message-id/c6778430-e5be-4eb8-a05c-f6914b092f0c%40iki.fi>
Precedence: bulk

On 05/04/2026 19:23, Ashutosh Bapat wrote:
>> Both MultiXactShmemInit and MultiXactShmemAttach set
>> OldestMemberMXactId, OldestVisibleMXactId. In future if we add another
>> global variable to point to the shared memory, somebody needs to
>> remember to initialize it in both these functions. Maybe deduplicate
>> it with something like attached? Similarly for PredicateLock related
>> changes.
> 
> Sorry, I attached the wrong patch. Here's the right patch.

Gotcha, yeah I've thought about that too. I even considered making 
ShmemInitRequested() automatically call all the attach callbacks after 
initialization, even in !EXEC_BACKEND builds. That way, you could put 
the backend-private steps only in the attach function, and have 
automatically be called in the postmaster too. I decided against it, 
because only few subsystems need the attach callback at all, and many of 
them need to do the "local" steps earlier in the init callback anyway. 
For example, XLOGShmemInit() uses the WALInsertLocks variable inside the 
function already.

In the end I decided it's OK as it is. I'm not too worried about the 
duplicated code in multixact.c and predicate.c, they fit in the same 
screen in an editor so it's pretty easy to see that they are duplicated 
for a reason. With more complicated logic it would be a different story.

- Heikki