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 <acufieJk-t0euqJ1@msg.df7cb.de>
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From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Tue, 31 Mar 2026 15:10:36 +0300
Message-ID: 
 <CAN55FZ2-bKNKmMSRDx1xH3SyqwBVMZ8HFG2YNipQ7LCdKm7eKA@mail.gmail.com>
Subject: Re: pgsql: test_aio: Add basic tests for StartReadBuffers()
To: Christoph Berg <myon@debian.org>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@lists.postgresql.org,
	Melanie Plageman <melanieplageman@gmail.com>
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Hi,

On Tue, 31 Mar 2026 at 13:19, Christoph Berg <myon@debian.org> wrote:
>
> One of the AIO commits around this one (2026-03-28 00:00Z) broke the
> postgresql-19 builds on apt.pg.o. Only the Debian unstable (sid,
> assertions enabled) builds are working, everything else including the
> nearly identical testing (forky) is broken (where assertions are off):

Thank you for the report! I can reproduce this when I disable
assertions. The problem is that, in StartReadBuffersImpl() in
bufmgr.c:

```
#ifdef USE_ASSERT_CHECKING

                /*
                 * Initialize enough of ReadBuffersOperation to make
                 * CheckReadBuffersOperation() work. Outside of assertions
                 * that's not necessary when no IO is issued.
                 */
                operation->buffers = buffers;
                operation->blocknum = blockNum;
                operation->nblocks = 1;
                operation->nblocks_done = 1;
                CheckReadBuffersOperation(operation, true);
#endif
```

if (found) and if (i == 0) then we set operation->buffers and
operation->nblocks if the assertions are enabled but AIO tests expect
to read these values. So, read_buffers() in AIO tests read incorrect
values [1] when the assertions are disabled. Moving operation->buffers
and operation->nblocks outside of #ifdef USE_ASSERT_CHECKING like in
the attached fixes the problem.

[1]
read_buffers(PG_FUNCTION_ARGS)
{
    ...
    for (int nio = 0; nio < nios; nio++)
    {
        ReadBuffersOperation *operation = &operations[nio];
        int            nblocks_this_io = operation->nblocks;
        Datum        values[6] = {0};
        bool        nulls[6] = {0};
        ArrayType  *buffers_arr;

        /* convert buffer array to datum array */
        for (int i = 0; i < nblocks_this_io; i++)
        {
            Buffer        buf = operation->buffers[i];
    ...
    }


-- 
Regards,
Nazir Bilal Yavuz
Microsoft

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