Message-ID: <791c3f18-f4de-4d84-ac6b-c7ccc074dd38@iki.fi>
Date: Mon, 30 Mar 2026 23:15:44 +0300
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Subject: Re: Better shared data structure management and resizable shared data
 structures
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: 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>
 <CA+TgmoboxAaS6MCRf__LRhf8Ji3B0O9_=d7KzRS12QhOuocnTQ@mail.gmail.com>
 <c7b4dafc-7a6c-43e4-8b9e-20807396590b@iki.fi>
 <CA+Tgmob7E3CGDoEBB3mwfk+v1CgcOG8yL0VHje2gy9Ts=e1Zxw@mail.gmail.com>
 <f8fc6f77-292d-4225-9f54-382c39da5ebc@iki.fi>
 <CA+TgmobqwCHp1w6xkTJmZiq4C8Pr0O3tNUvSFZcAgYLkvF45bg@mail.gmail.com>
 <8a6799be-bd42-49fb-8914-856c97bb1977@iki.fi>
 <CA+Tgmob8ot7qam7a650nx_4jyEoFLs8s_kPMc1+EB7-dr56=WA@mail.gmail.com>
 <cc3ff976-c303-45cd-9a8d-a90483170e37@iki.fi>
 <CA+TgmoauqYXm8iA3FGRAVKxYShUxWBiS_MSLmQpTrmO7wNHamw@mail.gmail.com>
 <113724ab-0028-493f-9605-6e8570f0939f@iki.fi>
 <CAExHW5uTNWOSxJDWQAUnS0tZawob2_J3dRAtc67NHNZ98X4_xA@mail.gmail.com>
 <CAExHW5t439y61YD9bc7d5wZWHp6J=M43Qu3eEZOBPguZML7o2A@mail.gmail.com>
 <CAExHW5v5FVZbsO9sLzztMZ11C3hgGStE=HkkV2bQkCyncess4w@mail.gmail.com>
 <b2451225-0a10-4ded-abef-4e9efbde7a8e@iki.fi>
 <CAExHW5tCC0T1ky=Jnq-AvMxa67Adaw7aQ4iQAO=BSdHcbSNBVg@mail.gmail.com>
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From: Heikki Linnakangas <hlinnaka@iki.fi>
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On 30/03/2026 07:50, Ashutosh Bapat wrote:
> On Sat, Mar 28, 2026 at 4:47 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> On 27/03/2026 09:01, Ashutosh Bapat wrote:
>>> /* Restore basic shared memory pointers */
>>> if (UsedShmemSegAddr != NULL)
>>> + {
>>> InitShmemAllocator(UsedShmemSegAddr);
>>> + ShmemCallRequestCallbacks();
>>>
>>> It's not clear how we keep the list of registered callbacks across the
>>> backends and also after restart in-sync. How do we make sure that the
>>> callbacks registered at this time are the same callbacks registered
>>> before creating the shared memory? How do we make sure that the
>>> callbacks registered after the startup are also registered after
>>> restart?
>>
>> On Unix systems, the registered callbacks are inherited by fork(), and
>> also survive over crash restart. With EXEC_BACKEND, the assumption is
>> that calling a library's _PG_init() function will register the same
>> callbacks every time. We make the same assumption today with the
>> shmem_startup hook.
> 
> RegisterShmemCallbacks() may be called after the startup, and it will
> add new areas to the shared memory. How are those registries synced
> across the backends? From your answer below, those registries are not
> synced across backends. They will be wiped out by the restart and
> won't be registered again. Is that right? I think we need to document
> this fact and also the need to call RegisterShmemCallbacks() from all
> the backends where the new areas are required after the startup.

Correct. Ok, I'll add a note to comment on RegisterShmemCallbacks() to 
call that out more explicitly, hope it helps.

- Heikki