# Rust Client Detailed Design

## Purpose

Provide an async Rust client crate for MXAccess Gateway, plus a test CLI and
unit tests. The Rust client should use `tonic` and `tokio`.

Follow the [Rust Style Guide](./style-guides/RustStyleGuide.md) for handwritten
code and the [Protobuf Style Guide](./style-guides/ProtobufStyleGuide.md) for
generated contract inputs.

## Crate Layout

Recommended layout:

```text
clients/rust/
  Cargo.toml
  build.rs
  crates/
    mxgateway-client/
      src/lib.rs
      src/client.rs
      src/session.rs
      src/options.rs
      src/auth.rs
      src/value.rs
      src/error.rs
      src/generated/
    mxgw-cli/
      src/main.rs
  tests/
```

Expected dependencies:

- `tonic`
- `prost`
- `prost-types`
- `tokio`
- `tokio-stream`
- `thiserror`
- `clap`
- `serde`
- `serde_json`
- `tracing`

## Library API

Suggested API:

```rust
pub struct GatewayClient { /* tonic channel + generated client */ }

pub struct ClientOptions {
    pub endpoint: String,
    pub api_key: String,
    pub plaintext: bool,
    pub ca_file: Option<PathBuf>,
    pub server_name_override: Option<String>,
    pub connect_timeout: Duration,
    pub call_timeout: Duration,
}

impl GatewayClient {
    pub async fn connect(options: ClientOptions) -> Result<Self, Error>;
    pub async fn open_session(&self, options: OpenSessionOptions) -> Result<Session, Error>;
    pub async fn invoke(&self, request: MxCommandRequest) -> Result<MxCommandReply, Error>;
}
```

Session:

```rust
pub struct Session {
    pub id: String,
}

impl Session {
    pub async fn register(&self, client_name: &str) -> Result<i32, Error>;
    pub async fn add_item(&self, server_handle: i32, item: &str) -> Result<i32, Error>;
    pub async fn add_item2(&self, server_handle: i32, item: &str, context: &str) -> Result<i32, Error>;
    pub async fn advise(&self, server_handle: i32, item_handle: i32) -> Result<(), Error>;
    pub async fn write(&self, server_handle: i32, item_handle: i32, value: MxValue, user_id: i32) -> Result<(), Error>;
    pub async fn events(&self) -> Result<impl Stream<Item = Result<MxEvent, Error>>, Error>;
    pub async fn close(&self) -> Result<(), Error>;
}
```

## Authentication

Use a `tonic` interceptor or request extension layer to add:

```text
authorization: Bearer <api key>
```

Use `SecretString` or equivalent if a dependency is acceptable. Always redact
API keys in `Debug` output.

## TLS

Support:

- plaintext channel for local development,
- native or rustls TLS depending on project preference,
- custom CA file,
- domain override.

## Streaming

Expose event streams as a `Stream<Item = Result<MxEvent, Error>>`. Dropping the
stream should cancel the underlying gRPC stream.

Do not buffer unboundedly in the client. If a helper channel is used, make it
bounded.

## Error Handling

Use `thiserror`:

```rust
pub enum Error {
    Transport(tonic::transport::Error),
    Status(tonic::Status),
    Authentication(String),
    Authorization(String),
    Session(SessionError),
    Worker(WorkerError),
    Command(CommandError),
    MxAccess(MxAccessError),
    Timeout,
    Cancelled,
}
```

Preserve raw command replies in `CommandError` where applicable.

## Test CLI

Binary: `mxgw`.

Use `clap` derive.

Commands:

```text
mxgw version
mxgw smoke --endpoint http://localhost:5000 --api-key-env MXGATEWAY_API_KEY --item TestChildObject.TestInt
mxgw stream-events --session-id <id> --json
mxgw write --session-id <id> --server-handle 1 --item-handle 1 --type int32 --value 123
```

JSON output should use `serde_json`.

## Unit Tests

Use a fake `tonic` server started on a local ephemeral port, or abstract the
generated client behind a trait for unit tests.

Required tests:

- generated client compiles from proto,
- auth metadata injection,
- TLS/plaintext endpoint construction,
- value conversion,
- command request construction,
- error mapping from `tonic::Status`,
- event stream order,
- stream cancellation,
- CLI parsing,
- JSON redaction.

## Integration Tests

Skip unless:

```text
MXGATEWAY_INTEGRATION=1
```

Use `tokio::test`. Run bounded smoke flow and ensure `CloseSession` is attempted
with `drop` fallback docs, but do not rely on `Drop` for async close.