Re: pgsql: test_aio: Add basic tests for StartReadBuffers()

Hi,

On Tue, 31 Mar 2026 at 13:19, Christoph Berg <myon(at)debian(dot)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