Re: BUG #15367: Crash in pg_fe_scram_free when using foreign tables

On Sat, Sep 08, 2018 at 03:02:00PM -0400, Tom Lane 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.
>
> Repeating the same thing on a nearby OpenBSD 6.0 image ... kaboom!
> It fails exactly as Jeremy describes.  Furthermore, I can confirm
> that the attached patch fixes it.

That's not good!  I have tried the report on Linux only, with pgbench -C
doing many foreign scans through postgres_fdw which connects to the
remote using SCRAM authentication, and this was stable.  Anything newer
than ec136d19 is impacted.  I am surprised this has not been seen
earlier to be honest.

> I wonder which other platforms we have that behave like this?
> And how come we've not seen symptoms before?  It's hard to believe
> that src/common/saslprep.c is the only thing libpq imports
> that has FRONTEND-vs-not-FRONTEND behavior differences.

libpq is quite particular in the fact that it compiles directly with
stuff in src/common, src/port, etc.  I think that the same problem
exists for pg_md5_encrypt().  So I like the solution proposed.  I guess
you have made sure that postgres_fdw becomes stable this way?  Have you
tried to stress it more with pgbench -C doing only foreign scans and
dozens of clients running in parallel?
--
Michael