Re: [PATCH] Improve error message when trying to lock virtual tuple.

On Tue, 18 Jun 2024 at 09:32, Sven Klemm <sven(at)timescale(dot)com> wrote:
>
> On Mon, Jun 17, 2024 at 10:25 PM Matthias van de Meent
> <boekewurm+postgres(at)gmail(dot)com> wrote:
>
> > I think you're solving the wrong problem here, as I can't think of a
> > place where both virtual tuple slots and tuple locking are allowed at
> > the same time in core code.
> >
> > I mean, in which kind of situation could we get a Relation's table
> > slot which is not lockable by said relation's AM? Assuming the "could
> > not read block 0" error comes from the heap code, why does the
> > assertion in heapam_tuple_lock that checks for a
> > BufferHeapTupleTableSlot not fire before this `block 0` error? If the
> > error is not in the heapam code, could you show an example of the code
> > that breaks with that error code?
>
> In assertion enabled builds this will be stopped much earlier and not return
> the misleading error message. But most packaged postgres versions don't have
> assertions enabled and will produce the misleading `could not read block 0`
> error.
> I am aware that this not a postgres bug, but i think this error message
> is an improvement over the current situation.

Extensions shouldn't cause assertions to trigger, IMO, and I don't
think that this check in ExecLockRows is a good way to solve that
issue. In my opinion, authors should test their extension on
assert-enabled PostgreSQL, so that they're certain they're not doing

If you're dead-set on having users see less confusing error messages
when assertions should have triggered (but are not enabled, and thus
don't), I think the better place to add additional checks & error
messages is in the actual heapam_tuple_lock method, just after the
assertion, rather than in the AM-agnostic ExecLockRows: If or when a
tableAM decides that VirtualTableTupleSlot is the slot type they want
to use for passing tuples around, then that shouldn't be broken by
code in ExecLockRows that was put there to mimick an assert in the
heap AM.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)