Function trigger_fetch_for_parent 

pub(super) fn trigger_fetch_for_parent<Types, S, P>(
    fetcher: &Arc<Fetcher<Types, S, P>>,
    leaf: &LeafQueryData<Types>,
)
where
    Types: NodeType,
    Header<Types>: QueryableHeader<Types>,
    Payload<Types>: QueryablePayload<Types>,
    S: VersionedDataSource + 'static,
    for<'a> S::Transaction<'a>: UpdateAvailabilityStorage<Types>,
    for<'a> S::ReadOnly<'a>: AvailabilityStorage<Types> + NodeStorage<Types> + PrunedHeightStorage,
    P: AvailabilityProvider<Types>,

Trigger a fetch of the parent of the given leaf, if it is missing.

This ensures that a passive fetch for a leaf will always resolve eventually, even if we miss the decide event for that leaf: we will eventually get a decide for a later leaf, and then fetch the chain backwards.