# M4a task 09: the WebSocket connection

**Branch:** `m4a` (run `git switch m4a`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `gatewayd: ws conn, messages, pings, closing and a dead peer`

## Goal

The connection that uses tasks 07 and 08: open it, send text, and wait for the next message while
keeping the connection alive (spec section 6):

- A ping from the server is answered with a pong carrying the same payload.
- We send a ping every `ping_every`. No bytes at all for `dead_after` means the peer is dead.
- A close frame is answered with a close frame, and the connection ends (`Closed`); so does the end
  of the stream.
- Any protocol error ends the connection. `gatewayd` then reconnects (task 14); nothing panics.

## Files

- Copy: `crates/gatewayd/tests/ws_conn.rs`, `crates/gatewayd/tests/support/ws_server.rs`, and the
  skeleton `crates/gatewayd/src/ws/conn.rs`
- Modify: `crates/gatewayd/src/ws/mod.rs` (`pub mod conn;`), `docs/implementer-log.md`

## The skeleton

```rust
pub const PATH: &str = "/api/v4/websocket";
pub struct Timing { pub ping_every: Duration, pub dead_after: Duration }   // Clone, Copy
pub struct Ws { stream, decoder, random, timing, last_heard, last_ping }

impl Ws {
    pub fn open(connector: &Connector, token: &str, timing: Timing,
                mut random: Box<dyn Read + Send>) -> Result<Ws, WsError>;
    fn send(&mut self, opcode: u8, payload: &[u8]) -> Result<(), WsError>;
    pub fn send_text(&mut self, text: &str) -> Result<(), WsError>;
    pub fn poll(&mut self, wait: Duration) -> Result<Option<String>, WsError>;
    pub fn close(mut self);
}
pub fn host_header(server: &ServerUrl) -> String;
```

All are `todo!()`, each with its steps above it. `poll` is the one with the most in it: it returns
the next text message, or `None` after about `wait` with none, and does the pinging, the pong
answers and the dead-peer check on the way. Its read timeout is always the time to the **next
thing it must do** (the end of `wait`, the next ping, or the dead-after limit), so a quiet
connection neither spins nor oversleeps. Write it as its comment says, step by step; a helper
function for step 1 is fine.

`random` gives the handshake key and a fresh 4-byte mask for every frame we send (in `gatewayd`,
`/dev/urandom`).

## Steps

- [ ] **1. Copy.** `git switch m4a`, then
  `cp docs/plans/M4a/files/crates/gatewayd/tests/ws_conn.rs crates/gatewayd/tests/ && cp docs/plans/M4a/files/crates/gatewayd/tests/support/ws_server.rs crates/gatewayd/tests/support/ && cp docs/plans/M4a/files/crates/gatewayd/src/ws/conn.rs crates/gatewayd/src/ws/`.
  Add `pub mod conn;` to `crates/gatewayd/src/ws/mod.rs`.
- [ ] **2. See it fail.** `cargo test -p gatewayd --test ws_conn`. Expected: it compiles and 10
  tests fail.
- [ ] **3. Fill `host_header`, `send`, `send_text`, `open`, `close`, then `poll`**,
  `cargo check -p gatewayd` after each.
- [ ] **4. See it pass.** `cargo test -p gatewayd --test ws_conn`, five times. Expected: 10 passed
  each time, in under a second.
- [ ] **5. Run the gate.** `cargo fmt --all`, then `make gate`. Expected last line: `gate: ok`.
- [ ] **6. Log and commit.** `git add crates/gatewayd docs/implementer-log.md Cargo.lock && git commit`

## Done when

- `cargo test -p gatewayd --test ws_conn` passes five times running; `make gate` prints `gate: ok`.

## Stop and report if

- A test passes only sometimes, or takes seconds.