Re: [PATCH] Compressed TOAST data corruption with REPACK CONCURRENTLY

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From: SATYANARAYANA NARLAPURAM <[email protected]>
To: Antonin Houska <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: [email protected]
Subject: Re: [PATCH] Compressed TOAST data corruption with REPACK CONCURRENTLY
Date: Fri, 17 Apr 2026 10:40:39 -0700
Message-ID: <CAHg+QDf+yGGizLHAOm_q8Y9SR-tuDa4vWYp+riJ1QWnWxeeLQw@mail.gmail.com> (raw)
In-Reply-To: <52301.1776440752@localhost>
References: <CAHg+QDeXb9HM2VGKXQedyCp52GzajJK5KOUdNi6oLjsS0nerQw@mail.gmail.com>
	<52301.1776440752@localhost>

Hi

On Fri, Apr 17, 2026 at 8:45 AM Antonin Houska <[email protected]> wrote:

> SATYANARAYANA NARLAPURAM <[email protected]> wrote:
>
> > restore_tuple() in repack.c uses SET_VARSIZE() to reconstruct the
> varlena header when
> > reading back external attributes from the spill file. In this process,
> looks like the flag
> > SET_VARSIZE_COMPRESSED is silently lost. Because of this, when REPACK
> CONCURRENTLY
> > run  any concurrently updated column whose value was TOAST-compressed
> ends up with raw
> > compressed bytes behind an "uncompressed" header returning garbled data
> on subsequent reads.
> > It appears that existing tests are using random chars which are
> uncompressable.
> >
> > Please find the attached
> 0001-Fix-restore_tuple-losing-varlena-compression-flag.patch to fix this.
> > Additionally I updated the existing repack_toast test to include the
> scenario I was talking about.
>
> Good catch, thanks!
>
> I'd slightly prefer to fix it w/o checking the varlena type, as
> attached. However, your test fails to reproduce the issue here, so I'm not
> able to verify the fix. I'll take a closer look early next week.
>

I started with that but tried to follow the existing code pattern. This
LGTM.
Please add a comment as well.


>
> --
> Antonin Houska
> Web: https://www.cybertec-postgresql.com
>
>

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