From: Tom Lane <tgl@sss.pgh.pa.us>
To: Eric Thinnes <e.thinnes@gmx.de>
cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>,
        pgsql-bugs@lists.postgresql.org
Subject: Re: Segmentation fault when calling BlessTupleDesc in a C function in
 parallel on PostgreSQL-(12.6, 12.7, 13.2, 13.3)
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Comments: In-reply-to Eric Thinnes <e.thinnes@gmx.de>
	message dated "Fri, 14 May 2021 14:26:52 +0200"
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Date: Fri, 14 May 2021 11:20:35 -0400
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Eric Thinnes <e.thinnes@gmx.de> writes:
> The function always delivers the same result with the same call
> parameters except for the determination of the result types and the
> generation of the TupleDesc, the function has no side effects.

BlessTupleDesc certainly has side effects; that's pretty much the
whole point of it.  If you're marking this thing as parallel safe,
I imagine that the problem boils down to "parallel worker locally
registers a blessed tupdesc, but the leader has never heard of it,
so fails to interpret the returned tuple".

Perhaps a workaround is possible by using a named composite type
instead of generating a rowtype on the fly?  But that won't work
if you want this to be polymorphic.

Also, I recall that there are provisions in typcache.c in recent
versions to support a shared pool of registered (i.e. blessed)
tuple descriptors.  That would likely solve your problem, but
I've never looked into how to use that code.  I doubt just
calling BlessTupleDesc is enough.  In particular, I imagine
you need to make sure the leader process creates the tupdesc
first, so it's available to all workers.

			regards, tom lane