plpgsql: error for txn control inside exception-handling block is eager

[DrewKimball]

This is a tracking issue for a difference in behavior between CRDB and postgres. Postgres does not allow COMMIT or ROLLBACK statements inside a PL/pgSQL block that has an exception handler. However, postgres checks this condition lazily, so that the error is only thrown if the transaction control statement is actually executed:

CREATE PROCEDURE p(a INT) LANGUAGE PLpgSQL AS $$
  BEGIN
    IF a > 0 THEN
      COMMIT;
    END IF;
  EXCEPTION WHEN division_by_zero THEN
    RAISE NOTICE 'foo';
  END;
$$;

CALL p(0); -- Postgres does not raise an error.
CALL p(1); -- Postgres raises an error.

CRDB eagerly checks for this situation, and throws the error in response to the CREATE PROCEDURE statement instead. This behavior is unlikely to change because users are unlikely to want to write such a stored procedure.

Jira issue: CRDB-36295