lock.js

var locks = {};

function lock(id, callback) {
    var lock = locks[id];

    if (lock) {
        lock.push(callback);
    } else {
        locks[id] = [];
        callback(unlock(id));
    }

    function unlock(id) {
        return function () {
            var lock = locks[id];
            if (lock.length) {
                var next = lock.shift();
                process.nextTick(function () {
                    next(unlock(id))
                });
            } else {
                locks[id] = undefined;
            }
        }
    }
}


lock(1, function (unlock) {
    console.log('lock1');
    setTimeout(unlock, 1000);
});

lock(2, function (unlock) {
    console.log('lock2');
    setTimeout(unlock, 1000);
});

lock(1, function (unlock) {
    console.log('lock3');
    setTimeout(unlock, 1000);
});


lock(2, function (unlock) {
    console.log('lock4');
    setTimeout(unlock, 1000);
});