Function collect_nar 

pub async fn collect_nar(
    stream: Pin<Box<dyn Stream<Item = Result<Bytes, StoreError>> + Send>>,
    limit: Option<usize>,
) -> Result<Vec<u8>, StoreError>

Drain a NarStream into one buffer.

This is the unbounded path. It exists for callers that genuinely need the whole NAR (the get_nar convenience verb, test doubles) and for capped tiers via limit. Every use is a deliberate decision to hold O(nar) bytes.

limit — when Some(max), collection refuses the moment the accumulated length would exceed max, returning StoreError::TooLarge. It never accumulates past the cap, so a capped caller’s peak is max + NAR_CHUNK_BYTES and not one byte more.

Errors

Propagates any error the stream yields, or StoreError::TooLarge when a limit is set and exceeded.