ALTER TABLE error - Could not read block 0

Hi,

While testing SQLSmith, I see that executing the following SQL on the
regression database gives an unexpected error. It doesn't crash the engine,
nonetheless I thought reporting it doesn't hurt (This tool is new to me, so
do let me know if I am doing something wrong here).

Steps to reproduce:
==================

ubuntu(at)laptop:$ git log | head -3
commit 77c1537f512e6ac2513f8695c795dc94cbf207ee
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Date: Mon Aug 24 16:46:52 2020 +0900
ubuntu(at)laptop:$ dropdb --if-exists tempdel && createdb tempdel
ubuntu(at)laptop:$ ./pg_regress --use-existing --schedule=serial_schedule
--dbname=tempdel >/dev/null
ubuntu(at)laptop:$ psql -c "SELECT version();" postgres
version
--------------------------------------------------------------------------------------------------------
PostgreSQL 14devel on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit
(1 row)

ubuntu(at)laptop:$ psql -c "ALTER TABLE ONLY public.pline ALTER COLUMN
slotname SET DATA TYPE TEXT USING cast(null as bpchar), ALTER COLUMN
comment SET DATA TYPE TEXT USING
public.pslot_backlink_view(cast(public.pline.backlink as bpchar));" tempdel
ERROR: could not read block 0 in file "base/13550023/13569741": read only
0 of 8192 bytes
CONTEXT: SQL statement "select * from PLine where slotname =
"outer".rec.backlink"
PL/pgSQL function pslot_backlink_view(character) line 20 at SQL statement

-
robins tharakan