Message-ID: <34d1acd9-0aff-4d59-ada3-a583e1173043@iki.fi>
Date: Sun, 5 Apr 2026 18:50:11 +0300
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: Better shared data structure management and resizable shared data
 structures
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@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>
 <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>
 <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>
 <CAEze2WjgCROMMXY0+j8FFdm3iFcr7By-+6Mwiz=PgGSEydiW3A@mail.gmail.com>
Content-Language: en-US
From: Heikki Linnakangas <hlinnaka@iki.fi>
In-Reply-To: 
 <CAEze2WjgCROMMXY0+j8FFdm3iFcr7By-+6Mwiz=PgGSEydiW3A@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Archived-At: 
 <https://www.postgresql.org/message-id/34d1acd9-0aff-4d59-ada3-a583e1173043%40iki.fi>
Precedence: bulk

On 05/04/2026 02:17, Matthias van de Meent wrote:
>> +    pgss->extent = 0;
>> +    pgss->n_writers = 0;
>> +    pgss->gc_count = 0;
>> +    pgss->stats.dealloc = 0;
> 
> Shmem is said to be zero-initialized, should we remove the manual
> zero-initialization?

Yeah, perhaps. We already had initialization like this in many places, 
while others relied on the implicit initialization. Some places even do 
just this:

void
LogicalDecodingCtlShmemInit(void)
{
	bool		found;

	LogicalDecodingCtl = ShmemInitStruct("Logical decoding control",
										 LogicalDecodingCtlShmemSize(),
										 &found);

	if (!found)
		MemSet(LogicalDecodingCtl, 0, LogicalDecodingCtlShmemSize());
}

I think there are two directions we could go here:

1. Document that the memory is zeroed, and you can rely on it. Remove 
silly initializations like that in LogicalDecodingCtlShmemInit(). In 
other places the explicitly zero-initialization might have documentation 
value though.

2. Require the init functions to explicitly zero the memory. Document it 
and add valgrind checks.

I'm inclined to go with 1. But in the name of avoiding scope creep, not 
as part of these patches.

- Heikki