function during(test, fn, callback) {
    callback = onlyOnce(callback || noop);

    function next(err) {
        if (err) return callback(err);
        test(check);
    }

    function check(err, truth) {
        if (err) return callback(err);
        if (!truth) return callback(null);
        fn(next);
    }

    test(check);
}
Like [`whilst`]{@link module:ControlFlow.whilst}, except the `test` is an asynchronous function that
is passed a callback in the form of `function (err, truth)`. If error is
passed to `test` or `fn`, the main callback is immediately called with the
value of the error.

@name during
@static
@memberOf module:ControlFlow
@method
@see [async.whilst]{@link module:ControlFlow.whilst}
@category Control Flow
@param {Function} test - asynchronous truth test to perform before each
execution of `fn`. Invoked with (callback).
@param {Function} fn - A function which is called each time `test` passes.
The function is passed a `callback(err)`, which must be called once it has
completed with an optional `err` argument. Invoked with (callback).
@param {Function} [callback] - A callback which is called after the test
function has failed and repeated execution of `fn` has stopped. `callback`
will be passed an error, if one occured, otherwise `null`.
@example

var count = 0;

async.during(
    function (callback) {
        return callback(null, count < 5);
    },
    function (callback) {
        count++;
        setTimeout(callback, 1000);
    },
    function (err) {
        // 5 seconds have passed
    }
);