# lanspread-peer

`lanspread-peer` is the networking runtime that lets Lanspread nodes find each
other on the local network, exchange library metadata, and transfer game files.
It is designed to run headless – other crates (most notably
`lanspread-tauri-deno-ts`) embed it and drive it through a channel-based API.

## Runtime Overview

- `start_peer(game_dir, tx_events, peer_game_db)` boots the asynchronous runtime in the
  background and returns an `UnboundedSender<PeerCommand>` that the caller uses
  for control. The function immediately forwards the supplied game directory via
  `PeerCommand::SetGameDir` and keeps using the provided `PeerGameDB` so the UI
  layer can observe live peer metadata.
- `PeerCommand` represents the small control surface exposed to the UI layer:
  `ListGames`, `GetGame`, `DownloadGameFiles`, and `SetGameDir`.
- `PeerEvent` enumerates everything the peer runtime reports back to the UI:
  library snapshots, download lifecycle updates, and peer membership changes.
- `PeerGameDB` collects remote peer metadata. It aggregates discovered peers’
  `Game` definitions, tracks the latest ETI version per title, and keeps the
  last seen list of `GameFileDescription` entries for each peer.

Internally the peer runtime owns three long-lived tasks that run for the
lifetime of the process:

1. **Server component** (`run_server_component`) – listens for QUIC connections,
   advertises via mDNS, and serves `Request::ListGames`, `Request::GetGame`,
   `Request::GetGameFileData`, and `Request::GetGameFileChunk` by reading from
   the local game directory.
2. **Discovery loop** (`run_peer_discovery`) – uses the `lanspread-mdns`
   helper to discover other peers. The blocking mDNS work is executed on a
   dedicated thread via `tokio::task::spawn_blocking` so that the Tokio runtime
   remains responsive.
3. **Ping service** (`run_ping_service`) – periodically issues QUIC ping requests
   to keep peer liveness up to date and prunes stale entries from `PeerGameDB`.

`scan_local_library` maintains a lightweight on-disk index and produces both a
`GameDB` and protocol summaries. The resulting database is used to respond to
incoming metadata requests (`Request::ListGames` / `Request::GetGame`).

## Networking and File Transfer

- Transport is handled by [`s2n-quic`](https://github.com/aws/s2n-quic); TLS
  cert/key material is compiled in from the repository root.
- Protocol messages are JSON-encoded structures defined in
  `lanspread-proto::{Request, Response}`.
- File transfers stream raw bytes over dedicated bidirectional QUIC streams.
  `peer::send_game_file_data` sends entire files, while
  `peer::send_game_file_chunk` services ranged requests.

### Download Pipeline

When the UI asks to download a game:

1. The UI first issues `PeerCommand::GetGame`. Each peer that still reports the
   game is queried via `request_game_details_from_peer`, and their file
   manifests are merged inside `PeerGameDB`.
2. Once the UI receives `PeerEvent::GotGameFiles`, it forwards the selected file
   list back with `PeerCommand::DownloadGameFiles`.
3. `download_game_files` prepares the filesystem (creating directories and
   pre-sizing files where possible), emits `PeerEvent::DownloadGameFilesBegin`,
   and builds a per-peer plan (`build_peer_plans`) that round-robins file chunks
   across the available peers that advertise the latest version.
4. Each plan is executed in its own task (`download_from_peer`). Chunk requests
   use per-chunk QUIC streams and write into pre-created files. The chunk writer
   keeps existing data intact and only truncates when we intentionally fall back
  to a full file transfer, which prevents corruption when multiple peers fill
   different regions of the same file.
5. Failures are accumulated and retried (up to `MAX_RETRY_COUNT`) via
   `retry_failed_chunks`. If everything succeeds,
   `PeerEvent::DownloadGameFilesFinished` is emitted; otherwise the UI receives
   `PeerEvent::DownloadGameFilesFailed`.

## Integration with `lanspread-tauri-deno-ts`

The Tauri application embeds this crate in
`crates/lanspread-tauri-deno-ts/src-tauri/src/lib.rs`:

- `LanSpreadState` holds onto the peer control channel, the latest aggregated
  `GameDB`, per-game download state, and the user-selected game directory.
- The Tauri commands (`request_games`, `install_game`, `update_game`, and
  `update_game_directory`) translate UI actions into `PeerCommand`s. In
  particular, `update_game_directory` records the filesystem path, kicks off the
  peer runtime on first use, and mirrors the installed/uninstalled state into
  the UI-facing database.
- A background task consumes `PeerEvent`s and fans them out to the front-end via
  Tauri publish/subscribe events (`games-list-updated`, `game-download-*`,
  `peer-*`). Successful downloads trigger an `unrar` sidecar to unpack ETI
  archives and clean up the temporary backup folders that are created when
  updates begin.
- When downloads fail the Tauri layer restores the on-disk backup, keeping the
  previous installation consistent even after partial transfers.

## Security & Operational Notes

- All QUIC connections are TLS encrypted; the shipped certificates are suitable
  for local-network trust but should be rotated for production deployments.
- Peer discovery is restricted to the local link via mDNS.
- Long-running blocking mDNS calls are isolated on dedicated threads which keeps
  the async runtime responsive even when discovery takes a long time.
- File writes are chunk-safe: partial chunk downloads now open files without
  truncating existing data, avoiding the corruption that occurred previously
  when multiple peers collectively filled a file.

## Known Limitations

- `PeerGameDB` currently models the latest metadata that other peers advertise.
  If the UI needs to surface titles that only exist locally, additional merging
  with the locally scanned `GameDB` will be required.
- The download planner uses a simple round-robin and does not yet take per-peer
  throughput or failures into account when distributing work.

Refer to the source (particularly `src/lib.rs`) for the exact message shapes and
state machines.
