per-sesson errors after interrupting CLUSTER pg_attrdef

This was briefly scary but seems to have been limited to my psql session (no
other errors logged). Issue with catcache (?)

I realized that the backup job I'd kicked off was precluding the CLUSTER from
running, but that CLUSTER was still holding lock and stalling everything else
under the sun.

psql (10.0, server 9.6.5)
...

ts=# CLUSTER pg_attribute USING pg_attribute_relid_attnum_index ; ^CCancel request sent ERROR: canceling statement due to user request

ts=# \df qci_add*
ERROR: could not read block 8 in file "base/16400/999225102": read only 0 of 8192 bytes
ts=# \dt+ pg_att

ts=# \dt+ pg_attrdef
ERROR: could not read block 8 in file "base/16400/999225102": read only 0 of 8192 bytes
ts=# ^C
ts=# \q

postgres=# SELECT session_line ln, user_name, error_severity sev, left(message,66) , left(query,66) FROM postgres_log_2017_10_19_1700 WHERE session_id='59e93953.20c9' ORDER BY 1,2 DESC ;
ln | user_name | sev | left | left
----+-----------+-------+--------------------------------------------------------------------+--------------------------------------------------------------------
1 | pryzbyj | LOG | statement: CLUSTER pg_attribute USING pg_attribute_relid_attnum_in |
2 | pryzbyj | ERROR | canceling statement due to user request | CLUSTER pg_attribute USING pg_attribute_relid_attnum_index ;
3 | pryzbyj | LOG | statement: SELECT n.nspname as "Schema", +|
| | | p.proname as "Name", +|
| | | |
4 | pryzbyj | ERROR | could not read block 8 in file "base/16400/999225102": read only 0 | SELECT n.nspname as "Schema", +
| | | | p.proname as "Name", +
| | | | pg_catalog.
5 | pryzbyj | LOG | statement: SELECT pg_catalog.quote_ident(c.relname) FROM pg_catalo |
6 | pryzbyj | ERROR | could not read block 1 in file "base/16400/999225102": read only 0 | SELECT pg_catalog.quote_ident(c.relname) FROM pg_catalog.pg_class
7 | pryzbyj | LOG | statement: SELECT pg_catalog.quote_ident(c.relname) FROM pg_catalo |
8 | pryzbyj | ERROR | could not read block 1 in file "base/16400/999225102": read only 0 | SELECT pg_catalog.quote_ident(c.relname) FROM pg_catalog.pg_class
9 | pryzbyj | LOG | statement: SELECT pg_catalog.quote_ident(c.relname) FROM pg_catalo |
10 | pryzbyj | ERROR | could not read block 1 in file "base/16400/999225102": read only 0 | SELECT pg_catalog.quote_ident(c.relname) FROM pg_catalog.pg_class
11 | pryzbyj | LOG | statement: SELECT n.nspname as "Schema", +|
| | | c.relname as "Name", +|
| | | |
12 | pryzbyj | ERROR | could not read block 8 in file "base/16400/999225102": read only 0 | SELECT n.nspname as "Schema", +
| | | | c.relname as "Name", +
| | | | CASE c.relk
(12 rows)