.nornir/vec-to-bytes-migration.md

holger `Vec<u8>` → `Bytes` migration plan (zero-copy fetch path)

Shovel-ready plan for the audit HIGH "core trait returns owned Vec<u8>". ** = not yet done.

RepositoryBackendTrait::fetch (traits/src/lib.rs:160) returns Option<Vec<u8>>; change to Option<bytes::Bytes>.

Bytes derefs to &[u8], so every existing &[u8]/as_deref() reader keeps working unchanged.
put(&self, &[u8]) stays — Bytes derefs in.
Also: fetch_many_with_upstreams → HashMap<ArtifactId, Bytes>; HolgerObject::fetch (traits:301) for symmetry. bytes is already in the tree via prost.

Hot path: grpc.rs stream_artifact (~391–434) currently does Bytes::from(data) then buf.slice(..) per chunk — with fetch→Bytes the conversion at L421 disappears (already slices Bytes). Unary fetch_artifact proto field is already prost::bytes::Bytes.
Proxy/cache + downstream clones become cheap ref-count moves.
Caveat: znippy extract_file still copies once at decompress (unavoidable) → Bytes::from(vec); the win is downstream zero-copy, not the extraction.

14 znippy backends (znippy-rust/npm/docker/python/maven/gem/go/deb/rpm/helm/nuget/composer/conda/package): wrap the existing extraction in Bytes::from(vec).
2 file (file-rust:368 fs::read, file-oci:294): Bytes::from(vec).
3 proxy/wrapper (proxy.rs:47–75 ProxyBackend + 2 test stubs): no logic change — moves transparent, put(&data) derefs.
1 upstream (RustRegistryUpstream), 6 test/mock.

Trait seam (fetch/fetch_many/HolgerObject::fetch → Bytes).
Hot path first: RustRepoZnippy::fetch → Bytes + drop the grpc.rs stream conversion; run tests.
Remaining 13 znippy backends.
file-rust + file-oci.
proxy + upstream + test mocks.
Callers (LocalHolger:99, UI data:270, fetch_many callers).
(later, separate) handle_http2_request body + ConnectorTrait::download_asset — not on the fetch hot path.

Build breaks: none expected (deref-compatible). One commit per phase; tests after each. Source: read-only scout 2026-06-28.