feat: isolation level for roles/functions

test/io/fixtures.sql

@@ -8,7 +8,13 @@ ALTER ROLE :USER SET pgrst.db_anon_role = 'postgrest_test_anonymous';
 
 CREATE ROLE postgrest_test_author;
 
-GRANT postgrest_test_anonymous, postgrest_test_author TO :USER;
+CREATE ROLE postgrest_test_serializable;
+alter role postgrest_test_serializable set default_transaction_isolation = 'serializable';
+
+CREATE ROLE postgrest_test_repeatable_read;
+alter role postgrest_test_repeatable_read set default_transaction_isolation = 'REPEATABLE READ';
+
+GRANT postgrest_test_anonymous, postgrest_test_author, postgrest_test_serializable, postgrest_test_repeatable_read TO :USER;
 
 CREATE SCHEMA v1;
 GRANT USAGE ON SCHEMA v1 TO postgrest_test_anonymous;
@@ -108,3 +114,31 @@ begin
     alter role current_user set statement_timeout = %L;
   $$, timeout);
 end $_$ volatile language plpgsql ;
+
+create table items as select x as id from generate_series(1,5) x;
+
+create view items_w_isolation_level as
+select
+  id,
+  current_setting('transaction_isolation', true) as isolation_level
+from items;
+
+grant all on items_w_isolation_level to postgrest_test_anonymous, postgrest_test_repeatable_read, postgrest_test_serializable;
+
+create function default_isolation_level()
+returns text as $$
+  select current_setting('transaction_isolation', true);
+$$
+language sql;
+
+create function serializable_isolation_level()
+returns text as $$
+  select current_setting('transaction_isolation', true);
+$$
+language sql set default_transaction_isolation = 'serializable';
+
+create function repeatable_read_isolation_level()
+returns text as $$
+  select current_setting('transaction_isolation', true);
+$$
+language sql set default_transaction_isolation = 'REPEATABLE READ';