Change find_user_role’s and login’s volatility to stable

According to PostgreSQL’s manual[0]:

  “STABLE indicates that the function cannot modify the database, and
   that within a single table scan it will consistently return the same
   result for the same argument values, but that its result could change
   across SQL statements.”

This definition matches both functions.  Moreover, find_user_role did
not need to be written in plpgsql, that i assume—but did not test—are
slower than sql functions.

[0]: https://www.postgresql.org/docs/14/sql-createfunction.html

deploy/find_user_role.sql

@@ -10,19 +10,13 @@ set search_path to auth, numerus, public;
 create or replace function find_user_role(email email, password text) returns name
 as
 $$
-declare
-	found_role name;
-begin
 	select role
-	into found_role
 	from auth."user"
 	where "user".email = find_user_role.email
 	and "user".password = crypt(find_user_role.password, "user".password);
-
-	return found_role;
-end;
 $$
-language plpgsql;
+language sql
+stable;
 
 comment on function find_user_role(email, text) is
 'Return the database role assigned to the user with the given email and password';

deploy/login.sql

@@ -22,6 +22,7 @@ begin
 end;
 $$
 language plpgsql
+stable
 security definer;
 
 comment on function login(email, text) is

test/find_user_role.sql

@@ -10,10 +10,10 @@ select plan(12);
 set search_path to auth, numerus, public;
 
 select has_function('find_user_role');
-select function_lang_is('find_user_role', array ['email', 'text'], 'plpgsql');
+select function_lang_is('find_user_role', array ['email', 'text'], 'sql');
 select function_returns('find_user_role', array ['email', 'text'], 'name');
 select isnt_definer('find_user_role', array ['email', 'text']);
-select volatility_is('find_user_role', array ['email', 'text'], 'volatile');
+select volatility_is('find_user_role', array ['email', 'text'], 'stable');
 select function_privs_are('find_user_role', array ['email', 'text'], 'guest', array []::text[]);
 select function_privs_are('find_user_role', array ['email', 'text'], 'invoicer', array []::text[]);
 select function_privs_are('find_user_role', array ['email', 'text'], 'admin', array []::text[]);

test/login.sql

@@ -13,7 +13,7 @@ select has_function('login');
 select function_lang_is('login', array ['email', 'text'], 'plpgsql');
 select function_returns('login', array ['email', 'text'], 'name');
 select is_definer('login', array ['email', 'text']);
-select volatility_is('login', array ['email', 'text'], 'volatile');
+select volatility_is('login', array ['email', 'text'], 'stable');
 select function_privs_are('login', array ['email', 'text'], 'guest', array ['EXECUTE']);
 select function_privs_are('login', array ['email', 'text'], 'invoicer', array []::text[]);
 select function_privs_are('login', array ['email', 'text'], 'admin', array []::text[]);