Re: Prevent concurrent DROP SCHEMA when certain objects are being initially created in the namespace

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Michael Paquier <michael(at)paquier(dot)xyz>
Cc: Jimmy Yih <jyih(at)pivotal(dot)io>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Prevent concurrent DROP SCHEMA when certain objects are being initially created in the namespace
Date: 2018-09-05 04:11:25
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Michael Paquier <michael(at)paquier(dot)xyz> writes:
> On Tue, Sep 04, 2018 at 03:09:21PM -0700, Jimmy Yih wrote:
>> When an empty namespace is being initially populated with certain objects,
>> it is possible for a DROP SCHEMA operation to come in and delete the
>> namespace without using CASCADE.

> It seems to me that we are missing some dependency tracking in some of
> those cases.

No, I think Jimmy is right: it's a race condition. The pg_depend entry
would produce the right result, except that it's not committed yet so
the DROP SCHEMA doesn't see it.

The bigger question is whether we want to do anything about this.
Historically we've not bothered with locking on database objects that
don't represent storage (ie, relations and databases). If we're going to
take this seriously, then we should for example also acquire lock on any
function that's referenced in a view definition, to ensure it doesn't go
away before the view is committed and its dependencies become visible.
Likewise for operators, opclasses, collations, text search objects, you
name it. And worse, we'd really need this sort of locking even in vanilla
DML queries, since objects could easily go away before the query is done.

I think that line of thought leads to an enormous increase in locking
overhead, for which we'd get little if any gain in usability. So my
inclination is to make an engineering judgment that we won't fix this.

It'd be interesting to know whether any other DBMSes make an effort
to prevent this kind of problem.

regards, tom lane

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