unaccent(text) fails depending on search_path (WAS: pg_upgrade fails saying function unaccent(text) doesn't exist)

OK, I found the cause of the unaccent dictionary problem, and a workaround.

It's not the vacuumdb version, not the unaccent version, and it's not even
a pg_upgrade problem: I get this error also with PG 9.4.18 running on the
old cluster, with both the 10.5 vacuumdb and the 9.4.18 vacuumdb, and I get
the same error in both.

And it's not strictly a vacuumdb problem, though vacuumdb triggers it.

Here's a very minimal test case, unrelated to my DB, that you ought to be
able to reproduce:

SET search_path = "$user"; SELECT public.unaccent('fóö');
SET
ERROR: text search dictionary "unaccent" does not exist

and here's a workaround:

SET search_path = "$user"; SELECT public.unaccent(tsdict.oid, 'fóö')
FROM pg_catalog.pg_ts_dict tsdict WHERE dictname='unaccent';
SET
unaccent
----------
foo
(1 row)

The workaround avoids the OID lookup of the dictionary ... that lookup (in
the single-argument unaccent function) is done by unqualified name:

https://github.com/postgres/postgres/blob/fb8697b31aaeebe6170c572739867dcaa01053c6/contrib/unaccent/unaccent.c#L377

dictOid = get_ts_dict_oid(stringToQualifiedNameList("unaccent"),
false);

and that fails if the search path doesn't include public.

So it is indeed triggered by the security changes that Bruce mentioned;
those were backported into 9.4.17:
https://www.postgresql.org/docs/9.4/static/release-9-4-17.html ... and so
got pulled in by my Macports upgrades. So nothing to do with pg_upgrade.

So the workaround for my vacuumdb/function-index problem is to give
unaccent the OID of the text search dictionary, so that the search path
isn't in play:

CREATE OR REPLACE FUNCTION public.semantic_normalize(title text)
RETURNS text
LANGUAGE sql
IMMUTABLE STRICT
AS $function$
SELECT lower(public.unaccent(16603, btrim(regexp_replace($1, '\s+', ' ',
'g'), ' "')))
$function$;

and that makes vacuumdb -z work in both 9.4.18 and 10.5, and makes
./analyze_new_cluster.sh complete without problems.

The proper fix is, I suppose, to make the single-argument unaccent function
explicitly look up the dictionary in the same schema as the function itself
is in.

Cheers,
Gulli