Plan M4b: approvals over Mattermost in 15 tasks

Skeletons for new files only; existing files change by exact edits, so no task overwrites an
earlier one's work. Each task's tests were run against a reference built in order from master,
with the gate at each step and mutations checked; each skeleton compiles and fails its tests, and
every todo!() comment was read beside its signature. The reference is kept off this machine.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/plans/M4b/14-gatewayd-serve-approvals.md

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+# M4b task 14: the serve loop posts approvals and marks them
+
+**Branch:** `m4b` (run `git switch m4b`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `gatewayd: serve, posting approvals, marking them, and the listener`
+
+## Goal
+
+The serve loop does what `gateway.sock` connections ask (M4b spec, sections 6 and 8):
+
+- **Show**: place the approval (its session's turn in flight: that thread; otherwise the first
+  approver's direct messages, as a new thread), post its parts in order, record the posts in the
+  state file, register it for answers (task 11), and reply with the post ids, or why it could not
+  be posted.
+- **Ended**: forget it, and add how it ended to its last post, naming who answered.
+- At the **first connection after a start**, the approval posts the last run left behind are
+  marked: their last post is replaced by its title and the restart line.
+- `run` opens `gateway.sock` before connecting; a failure stops `gatewayd` at start
+  (`Stop::Start`, pointer `gatewayd-start-failed`).
+
+## Files
+
+- Copy: the skeleton `crates/gatewayd/src/serve/approvals.rs`,
+  `crates/gatewayd/tests/serve_approvals.rs`, `crates/gatewayd/tests/support/brokerd_client.rs`,
+  and a **new version** of the given `crates/gatewayd/tests/support/fake_mm.rs` (it gains post
+  edits, user names, and refusing posts)
+- Modify: `crates/gatewayd/src/serve/mod.rs`, `docs/implementer-log.md`
+
+## The skeleton
+
+Written: `Open`, **`gateway_requests`** (drain the channel, never blocking) and **`show`** (read
+both first). To fill, each with its steps in its comment: `place`, `ended`, `mark_left_behind`,
+`mark`.
+
+## `serve/mod.rs`, exactly
+
+1. `mod approvals;` before `mod handle;`. Imports: `use std::collections::HashMap;`,
+   `use crate::approvals::Registry;`, `use crate::gateway::{self, ToLoop};`.
+2. Three fields at the end of `struct Gateway`:
+
+```rust
+    /// The approvals posted here, shared with the turns (for the notice) and judged by commands.
+    registry: Arc<Registry>,
+    /// What `gateway.sock` connections ask of the loop.
+    gateway_rx: Receiver<ToLoop>,
+    /// The last post of each approval posted here and not yet ended.
+    open: HashMap<u64, approvals::Open>,
+```
+
+3. `Gateway::new` gains a last parameter `gateway_rx: Receiver<ToLoop>`, and sets
+   `registry: Arc::new(Registry::new()), gateway_rx, open: HashMap::new(),`.
+4. In `run`, right before `let client = Arc::new(Client::new(…));`:
+
+```rust
+    let listener = match gateway::listen(&config.gateway_socket()) {
+        Ok(listener) => listener,
+        Err(why) => return Stop::Start(why),
+    };
+    let (gateway_tx, gateway_rx) = mpsc::channel();
+    if let Err(e) = gateway::serve(
+        listener,
+        gateway_tx,
+        gateway::Timing::default(),
+        Arc::clone(&log),
+    ) {
+        return Stop::Start(format!("cannot start the gateway.sock thread: {e}"));
+    }
+```
+
+   and `Gateway::new(config, client, state, log)` becomes
+   `Gateway::new(config, client, state, log, gateway_rx)`.
+5. In `connected`, inside `if !self.restarted { … }`, after the `take_in_flight` loop:
+   `self.mark_left_behind()?;`
+6. In `event_loop`, after `self.finished()?;`: `self.gateway_requests()?;`
+
+## Steps
+
+- [ ] **1. Copy.** `git switch m4b`, then
+  `cp docs/plans/M4b/files/crates/gatewayd/src/serve/approvals.rs crates/gatewayd/src/serve/ && cp docs/plans/M4b/files/crates/gatewayd/tests/serve_approvals.rs crates/gatewayd/tests/ && cp docs/plans/M4b/files/crates/gatewayd/tests/support/brokerd_client.rs docs/plans/M4b/files/crates/gatewayd/tests/support/fake_mm.rs crates/gatewayd/tests/support/`
+- [ ] **2. Make the six `serve/mod.rs` changes.** `cargo build -p gatewayd` must succeed.
+- [ ] **3. See it fail.** `cargo test -p gatewayd --test serve_approvals`. Expected: 4 fail, 1
+  passes (the socket is there while `gatewayd` runs).
+- [ ] **4. Fill `mark`, `place`, `ended`, `mark_left_behind`**, `cargo check -p gatewayd` after
+  each.
+- [ ] **5. See it pass.** `cargo test -p gatewayd --test serve_approvals`, three times: 5 passed.
+  Then `cargo test -p gatewayd`: 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/gatewayd docs/implementer-log.md Cargo.lock && git commit`
+
+## Done when
+
+- `cargo test -p gatewayd` passes; `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.
+- `show` or `gateway_requests` seems to need a change: they are given.