Re: ALTER TABLE lock strength reduction patch is unsafe

On Mon, Jun 20, 2011 at 10:56 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Sun, Jun 19, 2011 at 5:13 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>>> We scan pg_class in two ways: to rebuild a relcache entry based on a
>>> relation's oid (easy fix). We also scan pg_class to resolve the name
>>> to oid mapping. The name to oid mapping is performed *without* a lock
>>> on the relation, since we don't know which relation to lock. So the
>>> name lookup can fail if we are in the middle of a pg_class update.
>>> This is an existing potential bug in Postgres unrelated to my patch.
>
>> If this is a pre-existing bug, then it's not clear to me why we need
>> to do anything about it at all right now.
>
> Yeah.  This behavior has been there since day zero, and there have been
> very few complaints about it.  But note that there's only a risk for
> pg_class updates, not any other catalog, and there is exactly one kind
> of failure with very predictable consequences.

I agree we shouldn't do anything about the name lookups for 9.1
That is SearchCatCache using RELNAMENSP lookups, to be precise, as
well as triggers and few other similar call types.

> The ALTER TABLE patch
> has greatly expanded the scope of the issue, and that *is* a regression
> compared to prior releases.

I agree the scope for RELOID errors increased with my 9.1 patch. I'm
now happy with the locking patch (attached), which significantly
reduces the scope - back to the original error scope, in my testing.

I tried to solve both, but I think that's a step too far given the timing.

It seems likely that there will be objections to this patch. All I
would say is that issuing a stream of ALTER TABLEs against the same
table is not a common situation; if it were we would have seen more of
the pre-existing bug. ALTER TABLE command encompasses many subcommands
and we should evaluate each subcommand differently when we decide what
to do.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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