Call gen_random_uuid directly from the public schema (#944)

Also fix 11.up -- otherwise users without public in their search path will not be able to
install it.  Likewise do nothing on 13.down -- so users downgrading will get to a version
*with* the public prefix, just as they would with the _new_ 11.up.

ddl/000011_parade.up.sql

@@ -57,7 +57,7 @@ LANGUAGE sql VOLATILE AS $$
     SET actor_id = owner_id,
         status_code = 'in-progress',
         num_tries = num_tries + 1,
-        performance_token = gen_random_uuid(),
+        performance_token = public.gen_random_uuid(),
         action_deadline = NOW() + max_duration -- NULL if max_duration IS NULL
     WHERE id IN (
         SELECT id

ddl/000013_parade_searchpath.down.sql

@@ -0,0 +1,3 @@
+-- Do nothing on DOWN.  "UP" exists only to fix a bug in a function defined in
+-- 000011_parade.up.sql, which looks up `gen_random_uuid' with no schema.  Now
+-- that version is gone, so always uses the schema.

ddl/000013_parade_searchpath.up.sql

@@ -0,0 +1,27 @@
+-- Marks up to `max_tasks' on one of `actions' as in-progress and
+-- belonging to `actor_id' and returns their ids and a "performance
+-- token".  Both must be returned to complete the task successfully.
+CREATE OR REPLACE FUNCTION own_tasks(
+    max_tasks INTEGER, actions VARCHAR ARRAY, owner_id VARCHAR, max_duration INTERVAL
+)
+RETURNS TABLE(task_id VARCHAR, token UUID, num_failures INTEGER, action VARCHAR, body TEXT)
+LANGUAGE sql VOLATILE AS $$
+    UPDATE tasks
+    SET actor_id = owner_id,
+        status_code = 'in-progress',
+        num_tries = num_tries + 1,
+        performance_token = public.gen_random_uuid(),
+        action_deadline = NOW() + max_duration -- NULL if max_duration IS NULL
+    WHERE id IN (
+        SELECT id
+        FROM tasks
+        WHERE can_allocate_task(id, status_code, action_deadline, num_signals, total_dependencies) AND
+            action = ANY(actions) AND
+            (max_tries IS NULL OR num_tries < max_tries)
+        -- maybe: AND not_before <= NOW()
+        -- maybe: ORDER BY priority (eventually)
+        ORDER BY random()
+        FOR UPDATE SKIP LOCKED
+        LIMIT max_tasks)
+    RETURNING id, performance_token, num_failures, action, body
+$$;