Re: can't drop table due to reference from orphaned temp function

Good day, Andres.

В Сб, 19/02/2022 в 13:31 -0800, Andres Freund пишет:
> Hi,
> 
> On 2022-02-19 10:00:02 -0800, Andres Freund wrote:
> > See backtrace below [1]. The same problem does *not* exist when starting to
> > use the same temp schema in a new session (which drops the old contents
> > first), which kind of explains why we've not previously noticed this.
> > 
> > But even so, I'm surprised we haven't noticed this before.
> 
> Ah, there's a reason for that. In many cases we'll have a catalog snapshot
> registered, which is enough for init_toast_snapshot(). But in Miles' example,
> the object dropped just prior ends with catalog invalidations.
> 
> Proposed bugfix and test attached.
> 
> I think it's ok to backpatch the test. There might be a slight change in
> output due to 618c16707a6d6e8f5c83ede2092975e4670201ad not being backpatched,
> but that's OK I think.
> 
> 
> I think it is dangerous that we return a cached catalog snapshot for things
> like GetOldestSnapshot() unless they're also registered or active - we can't
> rely on catalog snapshots to be present. Indeed, if we didn't, this bug would
> have been found before, as some added assertions confirm.
> 
> I don't think we can just ignore the catalog snapshot though, it can be
> registered in the future, so it actually is the oldest snapshot. But at least
> we should assert that there's some snapshot registered/active. In the attached
> patch I've added HaveRegisteredOrActiveSnapshot() and used that in
> init_toast_snapshot().

Reading your message and HaveRegisteredOrActiveSnapshot's body, I can't get its logic:
- if it is dangerous to have CatalogSnapshot alone in RegisteredSnapshots,
  then why we return 'false' if RegisteredSnapshots is NOT singular?

I believe, body of HaveRegisteredOrActiveSnapshot should look like:

    if (ActiveSnapshot != NULL)
        return true;

    if (pairingheap_is_empty(&RegisteredSnapshots))
        return false;

    /*
     * The catalog snapshot is in RegisteredSnapshots when valid, but can be
     * removed at any time due to invalidation processing. If explicitly
     * registered more than one snapshot has to be in RegisteredSnapshots.
     */    
    if (pairingheap_is_singular(&RegisteredSnapshots))
        return CatalogSnapshot == NULL;
    
    return true;

Am I wrong?

regards,

Yura