gating migrations in SQL

up.sql

CREATE OR REPLACE FUNCTION raise_notice(text)
RETURNS void AS $$
BEGIN
  RAISE NOTICE '%', $1;
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION acquire_lock(bigint)
RETURNS bool AS $$
BEGIN
    IF (SELECT pg_try_advisory_lock($1) = 't') THEN
        PERFORM raise_notice('acquired lock, continuing.');
        RETURN 't';
    ELSE
        PERFORM raise_notice('failed to acquire lock.');
        PERFORM pg_advisory_lock($1);
        PERFORM raise_notice('lock finished, good bye');
        PERFORM pg_advisory_unlock_all();
        RAISE EXCEPTION 'refusing to perform migration';
        RETURN 'f';
    END IF;
END;
$$ LANGUAGE plpgsql;

\set ON_ERROR_STOP on
SELECT acquire_lock(12);
\set ON_ERROR_STOP off

SELECT raise_notice('running migration');
SELECT pg_sleep(15);
SELECT raise_notice('finished migration');

SELECT pg_advisory_unlock_all();