Feedback-ID: id4a34324:Fastmail
Date: Wed, 11 Mar 2026 19:09:26 -0400
From: Andres Freund <andres@anarazel.de>
To: Antonin Houska <ah@cybertec.at>
Cc: Kirill Reshke <reshkekirill@gmail.com>,
	Heikki Linnakangas <hlinnaka@iki.fi>,
 Melanie Plageman <melanieplageman@gmail.com>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
 pgsql-hackers@postgresql.org,
	Thomas Munro <thomas.munro@gmail.com>, Noah Misch <noah@leadboat.com>,
	Robert Haas <robertmhaas@gmail.com>,
 Michael Paquier <michael.paquier@gmail.com>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
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Hi,

On 2026-02-12 11:36:08 +0100, Antonin Houska wrote:
> Andres Freund <andres@anarazel.de> wrote:
>
> > For something committable, I think we should probably split IsMVCCSnapshot
> > into IsMVCCSnapshot(), just accepting SNAPSHOT_MVCC, and IsMVCCLikeSnapshot()
> > accepting both SNAPSHOT_MVCC and SNAPSHOT_HISTORIC_MVCC. And then go through
> > all the existing callers of IsMVCCSnapshot() - only about half should stay
> > as-is, I think.
>
> The attached patch tries to do that.

Thanks!


> From dcdbaf3095e632a1f7f65f3abc43eccff0249d4c Mon Sep 17 00:00:00 2001
> From: Antonin Houska <ah@cybertec.at>
> Date: Thu, 12 Feb 2026 11:14:00 +0100
> Subject: [PATCH] Refine checking of snapshot type.
>
> It appears to be confusing if IsMVCCSnapshot() evaluates to true for both
> "regular" and "historic" MVCC snapshot. This patch restricts the meaning of
> the macro to the "regular" MVCC snapshot, and introduces a new macro
> IsMVCCLikeSnapshot() to recognize both types.
>
> IsMVCCLikeSnapshot() is only used in functions that can (supposedly) be called
> during logical decoding.

I think I agree with where you selected IsMVCCSnapshot() and where you
selected IsMVCCLikeSnapshot().


> diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
> index b8c01a291a1..dd5aaae6953 100644
> --- a/src/include/utils/snapmgr.h
> +++ b/src/include/utils/snapmgr.h
> @@ -53,12 +53,14 @@ extern PGDLLIMPORT SnapshotData SnapshotToastData;
>
>  /* This macro encodes the knowledge of which snapshots are MVCC-safe */
>  #define IsMVCCSnapshot(snapshot)  \
> -	((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
> -	 (snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
> +	((snapshot)->snapshot_type == SNAPSHOT_MVCC)
>
>  #define IsHistoricMVCCSnapshot(snapshot)  \
>  	((snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
>
> +#define IsMVCCLikeSnapshot(snapshot)  \
> +	(IsMVCCSnapshot(snapshot) || IsHistoricMVCCSnapshot(snapshot))
> +
>  extern Snapshot GetTransactionSnapshot(void);
>  extern Snapshot GetLatestSnapshot(void);
>  extern void SnapshotSetCommandId(CommandId curcid);

Probably need to update the comments a bit.  What about something like


/*
 * Is the snapshot implemented as an MVCC snapshot (i.e. it uses
 * SNAPSHOT_MVCC).  If so, there will be at most be one visible row in a chain
 * of updated tuples, and each visible tuple will be seen exactly once.
 */
#define IsMVCCSnapshot(snapshot)  \
...

/*
 * Is the snapshot either an MVCC snapshot or has equivalent visibility
 * semantics (see IsMVCCSnapshot()).
 */
#define IsMVCCLikeSnapshot(snapshot)  \


Greetings,

Andres Freund