Function read_blob 

pub async fn read_blob(
    remote: &Remote,
    ref_name: &str,
    path: &str,
    cache: &PackIndexCache,
) -> Result<Bytes, PackchainError>

Read the contents of path at ref_name’s tip from a packchain remote.

Walks chain.json + path-index.json to resolve path → blob SHA, then consults each segment’s .idx newest-first for the blob’s pack entry. The matching entry’s bytes are fetched via a ranged GET, zlib-decompressed, and (when the entry is a delta) recursively resolved against its base. The entry’s eventual blob payload is returned as an owned Bytes.

cache amortises pack-index parsing across calls within the same process. Long-running consumers (CI agents, build systems) should keep one PackIndexCache for the lifetime of the process so the per-call cost is one or two API calls plus a zlib inflate; one-shot callers can pass &PackIndexCache::default() and discard.

Errors

• PackchainError::WrongEngine when the remote’s engine is not StorageEngine::Packchain.
• PackchainError::ChainAbsent when the branch is unknown to the bucket.
• PackchainError::PathIndexAbsent when chain.json exists but path-index.json does not (a partially crashed first push).
• PackchainError::TransientChainPathIndexMismatch when both exist but path_index.tip != chain.tip — a brief window during a concurrent push or compact where chain.json has been overwritten ahead of path-index.json. Retry-shaped: a subsequent read converges once the writer finishes.
• PackchainError::MalformedPath for .. segments, leading /, empty path, or empty segments (consecutive slashes).
• PackchainError::PathNotFound when the path does not exist in the resolved tree.
• PackchainError::PathNotABlob when the path resolves to a directory rather than a file.
• PackchainError::BlobNotInChain when the path-index named a blob SHA absent from every pack referenced by chain.json.
• PackchainError::DeltaTooDeep / PackchainError::MalformedDelta / PackchainError::MalformedPackEntry / PackchainError::Decompress for pack-corruption shapes.
• PackchainError::PackMissing, PackchainError::Store, or PackchainError::Io for transport / I/O failures.
• PackchainError::ConcurrentGcRetriesExhausted when a vigorous concurrent manage gc sweep kept deleting packs the reader had just discovered, exhausting the internal retry schedule. Callers should re-invoke read_blob after the compaction settles.