From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
cc: Jeremy Evans <code@jeremyevans.net>,
 Michael Paquier <michael@paquier.xyz>,
        pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #15367: Crash in pg_fe_scram_free when using foreign tables
In-reply-to: <3979.1536433320@sss.pgh.pa.us>
References: <153626613985.23143.4743626885618266803@wrigleys.postgresql.org>
 <20180906215836.GP2726@paquier.xyz>
 <20180906222843.GF17425@jeremyevans.local>
 <20180907165103.GH17425@jeremyevans.local>
 <20180907175518.GI17425@jeremyevans.local>
 <20180907181522.GB32058@paquier.xyz> <20016.1536351535@sss.pgh.pa.us>
 <20180907204403.GC32058@paquier.xyz> <16558.1536407783@sss.pgh.pa.us>
 <18398.1536410055@sss.pgh.pa.us> <87o9d8rv08.fsf@news-spur.riddles.org.uk>
 <3979.1536433320@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Sat, 08 Sep 2018 15:02:00 -0400"
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Date: Sat, 08 Sep 2018 15:11:15 -0400
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I wrote:
> I tried setting up a scram-auth-based postgres_fdw connection on
> my RHEL6 box and forcing it through the questionable code path
> by issuing a dummy ALTER USER MAPPING command from another session.
> It worked fine, so the problem doesn't seem to be present on Linux.

Oh, I meant to show the test I used, for the benefit of anyone who
wants to test on other platforms.  Do this (with adjustments to
taste) to set up:

set password_encryption = "scram-sha-256" ;
create user u1 password 'foobar';
create extension postgres_fdw;
create server loopback foreign data wrapper postgres_fdw
  options (host 'localhost', dbname 'postgres');
create user mapping for u1 server loopback
 options (user 'u1', password 'foobar');
grant usage on foreign server loopback to u1;
\c - u1
create table remote (f1 int);
insert into remote values(1),(2);
create foreign table localt (f1 int) server loopback
  options (table_name 'remote');

At this point you should be able to do

select * from localt;

as u1; if not, adjust pg_hba.conf to require a password on localhost
connections.  Once the select works, *leave that session open*, and
in another session do any no-op ALTER on the user mapping, such as

alter user mapping for u1 server loopback options (set user 'u1');

Now, in the first session, repeat

select * from localt;

It'll dump core with a backtrace similar to what Jeremy showed,
if the bug is present.

			regards, tom lane