Function decided_on 

pub fn decided_on<S: TaskSource + ?Sized, T>(
    source: &S,
    task: Option<&Task>,
    if_version: Option<&str>,
    when_priority: &[Priority],
) -> Option<BatchAnswer<T>>

Decide one batched item against its hold and its guard on task, the source’s own read of it — the decision TaskSource::update_tasks’ default makes, for an override that read many items together: Some answer when nothing is to be written.