BUG #19687: ALTER SEQUENCE provokes error XX001 could not read blocks

The following bug has been logged on the website:

Bug reference: 19687
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 19beta3
Operating system: Ubuntu 24.04
Description:

The following script:
psql -c "CREATE SEQUENCE s AS bigint MAXVALUE 1000000;"
for ((i=1; i<=100; i++)); do
echo "iteration $i"
for ((i=1; i<=100; i++)); do echo "SELECT * FROM s;"; done | psql
>/dev/null &
for ((i=1; i<=100; i++)); do echo "ALTER SEQUENCE s AS int;"; done | psql
>/dev/null
grep -A1 "ERROR: could not read block" server.log && break;
wait
done

triggers:
iteration 3
ERROR: could not read blocks 0..0 in file "base/16384/16597": read only 0
of 8192 bytes
2026-09-14 07:53:16.554 EEST [2078499:4] psql XX001 ERROR: could not read
blocks 0..0 in file "base/16384/16597": read only 0 of 8192 bytes
2026-09-14 07:53:16.554 EEST [2078499:5] psql XX001 STATEMENT: SELECT *
FROM s;

This error is easily reproduced with io_workers, but it can be reproduced
starting from 3d79013b9, with increased number of iterations and SELECT
clients.