Function in_trigger 

pub async fn in_trigger<T, F>(f: F) -> ServiceResult<T>
where
    F: Future<Output = ServiceResult<T>>,

Run f, the triggers of one hook, one level deeper than the run that encloses it. The generated write hooks wrap every trigger they run in this, so writes a trigger makes through other entities’ services (whose hooks wrap theirs the same way) count their nesting: the TRIGGER_MAX_DEPTHth level inside another is refused with the violation TRIGGER_DEPTH_EXCEEDED, which rolls the whole write back.