lanspread/crates/lanspread-peer/ARCHITECTURE.md

# lanspread-peer proposed protocol and architecture

This document proposes a tighter, more fault-tolerant protocol while keeping
the current idea: mDNS discovery, QUIC transport, on-demand metadata, and
chunked file transfers.

## Goals (unchanged)

- Local LAN discovery via mDNS.
- QUIC + JSON messages for control, raw streams for file data.
- UI drives operations through `PeerCommand`, peers remain headless.
- Peers can appear/disappear at any time without data loss.

## Peer lifecycle and message flow

### 1) Startup and advertise

- Start QUIC server.
- Advertise via mDNS with TXT records:
  - `peer_id` (stable ID, not tied to IP)
  - `proto_ver`
  - `library_rev` (monotonic local library revision)
  - optional `hostname`

### 2) Discovery and handshake

When a peer is discovered:

1. Connect and send `Hello { peer_id, proto_ver, listen_addr, library_rev,
   library_digest, features }`. `listen_addr` is mandatory; the QUIC source port
   is only a temporary transport port and must not be recorded as the peer's
   listener.
2. Receive `HelloAck { peer_id, proto_ver, listen_addr, library_rev,
   library_digest, features }`.
3. If the remote `peer_id` is already known but the address changed, update it.
4. If protocol versions are incompatible, drop the peer (and keep mDNS watching).
5. If library digests match, do nothing else.
6. If digests differ:
   - If we have a known `library_rev` for that peer, request `LibraryDelta`.
   - Otherwise request `LibrarySnapshot`.

### 3) Steady state

- Any message updates `last_seen`.
- Pings run only when idle (or on a longer interval), not every 5 seconds.
- Library updates are pushed as deltas, debounced and coalesced.

### 4) Shutdown

- Optional `Goodbye { peer_id }` lets others remove the peer quickly.
- If a peer vanishes without goodbye, stale timeout + ping removal handle it.
- Goodbye is a hint, never required for correctness.

## Library sync protocol

### Summary and snapshot

- `LibrarySummary { peer_id, summary: { library_rev, library_digest, game_count } }`
- `LibrarySnapshot { peer_id, snapshot: { library_rev, games: Vec<GameSummary> } }`

### Delta updates

- `LibraryDelta { peer_id, delta: { from_rev, to_rev, added, updated, removed } }`
- `removed` is a list of game IDs.
- Deltas are idempotent; ignore if `to_rev` <= known rev.

### GameSummary (concept)

- `id`, `name`, `eti_version`, `size`, `downloaded`, `installed`
- `manifest_hash` (hash of file list + sizes)
- `availability` (e.g., `ready`, `downloading`, `local_only`)

## When peers broadcast their game list

- Only on changes, not on a timer.
- Filesystem events are gated per game ID instead of time-debounced:
  - an active operation lock drops events for that game;
  - a rescan already running for the ID sets a rescan-pending flag;
  - the running rescan loops once more when that flag was set.
- Local library scans emit `LocalLibraryChanged` only for real library changes,
  except that accepted game-directory changes can force a UI snapshot for the
  new path without sending a peer delta.
- Active operation mutations emit `ActiveOperationsChanged` from the mutation
  path instead of riding on local library scans.
- Send `LibraryDelta` to known peers; send `LibrarySummary` on new connections.

## Local game scanning: fast and low cost

### Strategy

1. Maintain a persistent on-disk index (per game):
   - `manifest_hash`, total size, file list (optional), and a fingerprint
     (root-level `version.ini` mtime, root-level `.eti` mtime/size, and
     `local/` directory presence).
2. Use filesystem watchers to update only changed games.
3. Keep a 300-second fallback scan to recover from missed events.

### Fast-path scanning

- On startup, list only top-level game directories.
- For each game, read a cheap fingerprint:
  - root-level `.eti` file names, sizes, and mtimes
  - root-level `version.ini` mtime
  - presence of `local/` as a directory
- If fingerprint unchanged, reuse cached size and manifest hash.
- Only run a recursive scan for new or changed games.

## Local State and Recovery

Downloaded and installed are independent predicates:

- `downloaded` is true only when `<game_root>/version.ini` exists as a regular
  file. The sentinel is written last through `.version.ini.tmp` and atomic
  rename. An interrupted replacement leaves no restored old sentinel because
  archive bytes may already have changed.
- `installed` is true when `<game_root>/local/` is a directory. The contents of
  `local/` are user-owned and are skipped by manifests, fingerprints, and file
  serving.

Reserved per-game paths:

- `.version.ini.tmp` and `.version.ini.discarded` are download transaction
  scratch files and are swept during startup recovery.
- `.local.installing/` is extraction staging.
- `.local.backup/` holds the previous install while an update or uninstall is in
  flight.
- `.lanspread.json` is the atomic per-game intent log.
- `.lanspread_owned` inside `.local.*` directories proves Lanspread ownership
  when the current intent is `None`.

Downloaded-file removal is not an uninstall transaction. It removes the whole
game root only for a catalog ID that is a single direct child of the configured
game directory, has a regular root-level `version.ini`, and has no `local/`,
`.local.installing/`, or `.local.backup/` path.

Recovery reads `.lanspread.json` and combines the recorded intent with the
observed `local/`, `.local.installing/`, and `.local.backup/` state. Intent
states `Installing`, `Updating`, and `Uninstalling` prove ownership of the
corresponding reserved directories even if the marker was not flushed before a
crash. With intent `None`, markerless `.local.*` directories are left untouched.

### Result

Most scans become O(number of game dirs), with full recursion only when needed.

## File manifests and downloads

- Keep `GetGame`/manifest requests, but keyed by `manifest_hash` so repeated
  calls can be skipped when unchanged.
- Downloads remain chunked QUIC streams with the existing integrity checks.
- A game is transferable only when its ID is in the catalog, no operation is
  active for that ID, and the root-level `version.ini` sentinel exists.
- `local/` paths are never served, even if a stale or malicious manifest request
  asks for them.

## Fault tolerance rules

- Every peer is keyed by `peer_id`, not by IP address.
- Peer addresses are listener addresses from mDNS or `Hello`/`HelloAck`, never
  ephemeral QUIC source ports.
- `library_rev` is monotonic and guards against out-of-order updates.
- Any mismatch or missing delta falls back to `LibrarySnapshot`.
- Loss of goodbye is harmless; stale timeout is authoritative.

## Roadmap from current design to this one

1. Protocol updates in `lanspread-proto`:
   - Define `Hello`, `HelloAck`, `LibrarySummary`, `LibrarySnapshot`,
     `LibraryDelta`, and optional `Goodbye` messages.
   - Thread `peer_id`, `library_rev`, and `manifest_hash` through all
     library and manifest-bearing types.
   - Make `Hello` and `HelloAck` carry the sender's `listen_addr`,
     `library_rev`, and `library_digest` so both sides can record stable
     listener addresses and immediately select `LibraryDelta` vs
     `LibrarySnapshot`.
2. Peer identity:
   - Persist a stable `peer_id` (UUID) in the peer config and inject it into
     `PeerInfo` and `PeerGameDB` at startup.
   - Track `peer_id -> SocketAddr` in the discovery table and update the
     address on any incoming handshake or mDNS refresh.
3. Discovery handshake:
   - Publish `peer_id` and `library_rev` in mDNS TXT records to avoid
     immediate TCP/QUIC roundtrips when nothing changed.
   - Add a lightweight handshake in `run_peer_discovery` that exchanges
     `Hello`/`HelloAck` before any library sync.
   - Ignore peers that do not advertise the current protocol version.
4. Library revisioning:
   - Store a monotonic `library_rev` locally and increment only after a
     successful index refresh completes.
   - Apply `LibraryDelta` when `library_rev` matches; reject stale or future
     revisions and request `LibrarySnapshot` instead.
   - Cache the last accepted `manifest_hash` per peer to short-circuit
     manifest requests when unchanged.
5. Local index + scan optimizations:
   - Introduce a cached index file (e.g., `.lanspread/index.json`) that stores
     per-root fingerprints and computed manifests.
   - Use filesystem watchers with a debounce window to collect changes and
     incrementally update the cache.
   - Schedule a low-frequency full scan to reconcile missed watcher events.
6. Announce updates:
   - Broadcast `LibraryDelta` updates keyed by `library_rev`.
   - Send `LibrarySummary` on new connections to seed the delta flow.
7. File manifest caching:
   - Store per-game `manifest_hash` and only fetch details when changed.
8. Liveness:
   - Reduce ping frequency; update `last_seen` on any message.
   - Add optional `Goodbye` on shutdown paths.
9. Tests:
   - Delta apply/merge, rev ordering, manifest hashing, and scan cache behavior.