Re: sql_drop Event Trigger - Mailing list pgsql-hackers

On Wed, Feb 20, 2013 at 10:48 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> I have added some protections so that these do not fire on undesirable
> events (such as dropping an event trigger); also event triggers and
> other object types are filtered out of pg_dropped_objects, in case
> something like DROP OWNED happens.  (So for instance you could get an
> empty pg_dropped_objects if you did DROP OWNED and the mentioned user
> only owns an event trigger).  Maybe this needs to be reconsidered.

This is tricky.  The whole point of skipping the
ddl_command_start/ddl_command_end triggers when the command is one
that operates on event triggers is that we don't want a user to
install an event trigger that prevents said user from dropping that
event trigger afterwards.  We're currently safe from that, but with
the behavior you describe, we won't be: an event trigger that
unconditionally thows an error will block all drops, even of event
triggers.

I am inclined to suggest that we go back to an earlier suggestion I
made, which Dimitri didn't like, but which I still think might be the
best way forward: add a SUSET GUC that disables event triggers.  If we
do that, then our answer to that problem, for the current event
triggers and all we might add in the future, can be: well, if that
happens, set the disable_event_triggers GUC and retry the drop.  OTOH,
if we DON'T do that, then this problem is going to come back up every
time we add a new firing point, and it may be really tough to make
sure we're secure against this in all cases.

If you don't like that suggestion I'm open to other options, but I
think we should try hard to preserve the invariant that a superuser
can always drop an event trigger without interference from the event
trigger system itself.

> There's also code to re-obtain the list of objects to drop after the
> event trigger functions have run; the second list is compared to the
> first one, and if they differ, an error is raised.

This is definitely an improvement.  I am not 100% clear on whether
this is sufficient, but it sure seems a lot better than punting.

I think there might be a possible failure mode if somebody creates a
new object that depends on one of the objects to be dropped while the
event trigger is running.  Since system catalogs are normally read
with SnapshotNow, I think it might be possible to create a situation
where the first and second searches return different results even
though the trigger hasn't done anything naughty.  I believe that I
added some locking in 9.2 that will prevent this in the case where you
create a table that depends on a concurrently-dropped schema, but
there are other cases that have this problem, such as a function being
added to a concurrently-dropped schema.

Now, IMHO, that isn't necessarily a show-stopper for this patch,
because that same phenomenon can cause catalog corruption as things
stand.  So in the scenario where an event trigger causes a transaction
to fail because of this issue, it will actually be *preventing*
catalog corruption.  However, if this goes in, it might bump up the
priority of fixing some of those locking issues, since it may make
them more visible.

> It's rather annoying that performMultipleDeletions and performDeletion
> contain almost exactly the same code.  I think maybe it's time to morph
> the latter into a thin layer on top of the former.

Yeah, or at least factor out the code you need for this into its own function.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company