Feedback-ID: id4a34324:Fastmail
Date: Wed, 14 Dec 2022 09:35:52 -0800
From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	Ibrar Ahmed <ibrar.ahmad@gmail.com>,
	Amit Khandekar <amitdkhan.pg@gmail.com>, fabriziomello@gmail.com,
	tushar <tushar.ahuja@enterprisedb.com>,
	Rahila Syed <rahila.syed@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Minimal logical decoding on standbys
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Hi,

On 2022-12-14 10:55:31 -0500, Robert Haas wrote:
> I read through 0001 again and I noticed this:
> 
>  typedef struct xl_heap_prune
>  {
>      TransactionId snapshotConflictHorizon;
>      uint16      nredirected;
>      uint16      ndead;
> +    bool        onCatalogAccessibleInLogicalDecoding;
>      /* OFFSET NUMBERS are in the block reference 0 */
>  } xl_heap_prune;
> 
> I think this is unsafe on alignment-picky machines. I think it will
> cause the offset numbers to be aligned at an odd address.
> heap_xlog_prune() doesn't copy the data into aligned memory, so I
> think this will result in a misaligned pointer being passed down to
> heap_page_prune_execute.

I think the offset numbers are stored separately from the record, even
though it doesn't quite look like that in the above due to the way the
'OFFSET NUMBERS' is embedded in the struct. As they're stored with the
block reference 0, the added boolean shouldn't make a difference
alignment wise?

Or am I misunderstanding your point?

Greetings,

Andres Freund