# M4b task 06: the courier

**Branch:** `m4b` (run `git switch m4b`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `brokerd: courier, carrying pending approvals to gatewayd and answers back`

## Goal

When an `ask` call enters the pending table and `[gateway]` is set, a **courier** thread carries
it to `gatewayd` over `gateway.sock` (task 02's four frames) and brings the answer back (M4b spec,
section 5):

- It stops as soon as the table no longer holds the approval (answered in `bxctl`, expired, the
  requester gone), closing the connection; `gatewayd` then marks the post "no longer pending".
- If `gatewayd` cannot be reached or cannot post, it logs **once** for this approval, with the
  pointer `see docs/runbook.md#gateway-unreachable`, and tries again every 5 seconds; when a later
  try posts it, it logs `brokerd: approval <id> posted in Mattermost`.
- If `gatewayd` goes away after posting, it posts it again.
- On an answer, it takes the entry out of the table and answers it with `admin::answer`, exactly
  as `bxctl approve` and `refuse` do, with the Mattermost user and post (task 04); then sends what
  came of it. If the entry was already gone, the outcome is `Gone` and nothing is written.

The courier runs **beside** the connection thread that waits for the verdict: a scoped thread
(`std::thread::Builder::spawn_scoped`), stopped by a flag when the wait ends. So `ask`'s wait moves
into a function of its own, called inside `courier::alongside`.

## Files

- Copy: the skeleton `crates/brokerd/src/courier.rs`, `crates/brokerd/tests/courier.rs`,
  `crates/brokerd/tests/support/fake_gateway.rs`
- Append: `docs/plans/M4b/files/runbook-gateway-unreachable.md` to the end of `docs/runbook.md`
- Modify: `crates/brokerd/src/lib.rs` (`pub mod courier;` after `pub mod config;`),
  `crates/brokerd/src/broker.rs`, `docs/implementer-log.md`

## The skeleton

Written, because they are the protocol and its glue: `RUNBOOK`, `Timing` (5 s retry, 1 s tick),
`Attempt`, **`alongside`**, **`carry`**, **`attempt`**, **`exchange`** and `lost`. Read them first:
they call everything you write. To fill, each a few lines with the code in its comment:
`finished`, `pause`, `frames`, `answer`.

`frames` reads `gatewayd`'s frames whole on a thread of its own and hands them over a channel, so
the courier can wait with a timeout without losing half a frame (`UnixStream::peek` is not stable).

## `broker.rs`, exactly

1. `use std::sync::mpsc::RecvTimeoutError;` becomes
   `use std::sync::mpsc::{Receiver, RecvTimeoutError};`, and add `use crate::courier;` after
   `use crate::config::Config;`.
2. In `pending`, keep a copy of what the table lists, just before it is inserted:
   `let shown = info.clone();` on the line before `let verdict = broker.table().insert(info, ask);`.
3. Replace step 4 of `pending`, from `// 4. Wait for the verdict, or for the peer to leave.` down
   to the end of that `let outcome = loop { … };`, with:

```rust
    // 4. Wait for the verdict, or for the peer to leave, with a courier carrying the approval to
    //    Mattermost meanwhile when `[gateway]` is set.
    let waited = courier::alongside(broker, &shown, || {
        wait_for_verdict(broker, stream, &verdict, seq)
    });
    let outcome = match waited {
        Waited::Verdict(outcome) => outcome,
        Waited::PeerLeft => return None, // 4. the peer left while we waited
    };
```

4. Add, before `pub fn alive`, the old step 4 as a function (the same branches, with `return`s
   instead of `break`s):

```rust
/// What waiting for the verdict came to.
enum Waited {
    Verdict(Result<Verdict, DenyReason>),
    PeerLeft,
}

/// Wait for the verdict, or for the peer to leave. If the peer leaves and the entry is still in
/// the table, taking it ends the approval unanswered; if someone already took it, their verdict
/// is awaited.
fn wait_for_verdict(
    broker: &Broker,
    stream: &UnixStream,
    verdict: &Receiver<Verdict>,
    seq: u64,
) -> Waited {
    loop {
        match verdict.recv_timeout(Duration::from_secs(1)) {
            Ok(verdict) => return Waited::Verdict(Ok(verdict)),
            Err(RecvTimeoutError::Timeout) => {
                if alive(stream) {
                    continue;
                }
                match broker.table().take(seq) {
                    Some(_) => return Waited::PeerLeft,
                    None => match verdict.recv() {
                        Ok(verdict) => return Waited::Verdict(Ok(verdict)),
                        Err(_) => return Waited::Verdict(Err(DenyReason::AuditUnavailable)),
                    },
                }
            }
            // The taker dropped it unanswered.
            Err(RecvTimeoutError::Disconnected) => {
                return Waited::Verdict(Err(DenyReason::AuditUnavailable));
            }
        }
    }
}
```

## Steps

- [ ] **1. Copy.** `git switch m4b`, then
  `cp docs/plans/M4b/files/crates/brokerd/src/courier.rs crates/brokerd/src/ && cp docs/plans/M4b/files/crates/brokerd/tests/courier.rs crates/brokerd/tests/ && cp docs/plans/M4b/files/crates/brokerd/tests/support/fake_gateway.rs crates/brokerd/tests/support/`
  and `cat docs/plans/M4b/files/runbook-gateway-unreachable.md >> docs/runbook.md`. Add
  `pub mod courier;` to `lib.rs`.
- [ ] **2. Make the four `broker.rs` changes.** `cargo build -p brokerd` must succeed.
- [ ] **3. See it fail.** `cargo test -p brokerd --test courier`. Expected: 9 fail, 1 passes
  (`without_a_gateway_nothing_is_carried`), in about 10 s.
- [ ] **4. Fill `finished`, `pause`, `frames`, `answer`**, `cargo check -p brokerd` after each.
- [ ] **5. See it pass.** `cargo test -p brokerd --test courier`, three times: 10 passed each
  time, in about 10 s (two tests wait out the 5 s retry on purpose). Then `cargo test -p brokerd`:
  every suite passes.
- [ ] **6. Run the gate.** `cargo fmt --all`, then `make gate`. Expected last line: `gate: ok`.
- [ ] **7. Log and commit.** `git add crates/brokerd docs/runbook.md docs/implementer-log.md Cargo.lock && git commit`

## Done when

- The courier suite passes three times running; `make gate` prints `gate: ok`.

## Stop and report if

- A comment above a `todo!()` does not fit its function, or contradicts a test: quote it.
- A written function (`alongside`, `carry`, `attempt`, `exchange`) seems to need a change.
- A test hangs or takes much longer than stated.