Function rebuild_candidates 

async fn rebuild_candidates<R, S>(
    resolver: &R,
    selector: &S,
    weighting: WeightingParams,
    me: OffchainPublicKey,
    src_key: OffchainPublicKey,
    dest_key: OffchainPublicKey,
    hops: usize,
    kind: &'static str,
) -> Result<Option<WeightedCollection<ValidatedPath>>>
where
    R: ChainKeyOperations + ChainReadChannelOperations + Send + Sync,
    S: PathSelector,

Rebuilds the weighted candidate collection for one (source, destination, hops) triple from whatever the graph currently says.

Shared by the lazy cache fill, the background sweep and the on-demand recompute, so all three necessarily agree: a divergence here would make a session’s weights depend on which of the three happened to run last.

Ok(None) means the selector offered nothing, or nothing survived validation. Callers decide what that means — a fill turns it into PathNotFound, a refresh leaves the existing entry alone. Err is reserved for a selector that actually failed.