Re: Fix bug with accessing to temporary tables of other sessions

Hi,

On Thu, Apr 9, 2026 at 9:35 PM Jim Jones <jim(dot)jones(at)uni-muenster(dot)de> wrote:
>
> On 09/04/2026 13:46, Daniil Davydov wrote:
> > 1) Right now, read stream seems like an appropriate place for this restriction.
> > But actually the StartReadBuffers is not "binded" to the read stream logic. I
> > mean that if someone calls it bypassing the "read_stream_begin_relation"
> > function (it is OK to do so), then our restriction will be violated again.
> > I think that it will be more reliable to add the restriction directly to the
> > StartReadBuffers. Also, we can add an assertion ("relation is not other temp
> > table") to the PinBufferForBlock. What do you think?> 2) If we decide to leave restriction in the "read_stream_begin_relation"
> > function, I would suggest adding a "rel != NULL" check here (read_stream.c):
> > + if (RELATION_IS_OTHER_TEMP(rel))
> > + ereport(ERROR,
> > + (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
> > + errmsg("cannot access temporary relations of other
> > sessions")));
> > 3) The "rel != NULL" checks may use the "RelationIsValid" macro, which seems
> > more pretty to me.
>
>
> Mm, not so sure...
> AFAICT moving the check to StartReadBuffersImpl would require an extra
> NULL guard that isn't needed in read_stream_begin_relation, as the
> callers already pass valid Relations. So, rel != NULL is not needed.

Hm. I see that read_stream_begin_relation immediately calls
read_stream_begin_impl, where we have a "rel != NULL" check (read_stream.c:787).
Anyway, I think that we shouldn't rely on the fact that a given Relation will
always be valid. Please, correct me if I am wrong.

I see that you don't really like the idea of moving this check. But since a
vectored variant of ReadBuffer() may be used by anyone, don't we need to take
it into account?

> Also, wouldn't it potentially make this check multiple times in a table
> scan?

Yep, it will. It is exactly the same logic as for ReadBuffer_common,
PrefetchBuffer and ReadBufferExtended (i.e. checking this constraint before
each buffer read). I don't see anything wrong with this approach. More
precisely, it would be good to avoid multiple checks, but I don't see a way to
do that.

--
Best regards,
Daniil Davydov