Function build_update_stream 

fn build_update_stream<'scope, T>(
    bundle: &CollectionBundle<'scope, T>,
    as_of: Antichain<Timestamp>,
    source_key: Option<Vec<LirScalarExpr>>,
    source_relation: usize,
    initial_closure: JoinClosure,
) -> (VecCollection<'scope, T, Row, Diff>, VecCollection<'scope, T, DataflowErrorSer, Diff>)
where
    T: RenderTimestamp,

Builds the initial update stream of a delta path from a collection bundle.

With a source_key, demuxes over the two flavors of arrangement the bundle might hold for that key, dispatching to the generic build_update_stream_trace. Without a source_key, the source relation is consumed as a raw (unarranged) collection via build_update_stream_stream.

The returned error collection holds only the errors the initial closure produces. The errors bundle already carries are the caller’s to propagate, once, rather than once per delta path. See bundle_errs.