Function shard_source_fetch 

pub(crate) fn shard_source_fetch<'inner, K, V, T, D, TInner>(
    descs: StreamVec<'inner, TInner, (usize, ExchangeableBatchPart<T>)>,
    name: &str,
    client: impl Future<Output = PersistClient> + Send + 'static,
    shard_id: ShardId,
    key_schema: Arc<K::Schema>,
    val_schema: Arc<V::Schema>,
    is_transient: bool,
    error_handler: ErrorHandler,
) -> (StreamVec<'inner, TInner, FetchedBlob<K, V, T, D>>, StreamVec<'inner, TInner, Infallible>, PressOnDropButton)
where
    K: Debug + Codec,
    V: Debug + Codec,
    T: Timestamp + Lattice + Codec64 + Sync,
    D: Monoid + Codec64 + Send + Sync,
    TInner: Timestamp + Refines<T>,