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spec(slice-4): barge-in / VAD-driven playout kill on dedicated media thread

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Spearhead step 4 of 6. The FOB reflex loop: on brain speech_started
advisory, kill playout from the core-authoritative buffer; resume on
first fresh audio_out post-barge. Graduates the 20ms media loop off
the tokio pool onto a dedicated std::thread (ARCHITECTURE.md mandate,
deferred from slice-1 loop_driver.rs:18-23). Reflex<P: AudioPipe>
wrapper in rutster-media (FOB) decorates the pipe — loop_driver.rs +
rtc_session.rs stay byte-identical (the §8.5 #6 seam gate, restated).

Decisions (brainstorming 2026-07-01):
- Trigger: advisory-only (brain speech_started/stopped); local VAD deferred
- Resume: first fresh audio_out; SpeechStopped observational only
- Thread: single dedicated std::thread; per-session/threadpool deferred
- Approach C: B's thread model + reflex.rs module (per user selection)

Out-of-scope (§1.2): local VAD, per-session threads, min-mute floor,
brain-side input_audio_buffer.interrupt, TLS, authn, spend cap.

Depends on: slice-1 (media loop + seam), slice-2 (tap + playout buffer),
slice-3 (brain + speech_started/stopped advisory, S4 turn-ownership lock).
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# Rutster slice 4 — Barge-in: VAD-driven playout kill on a dedicated media thread
- **Status:** Draft (pending review)
- **Date:** 2026-07-01
- **Spearhead step:** 4 of 6 (vision-revision §10 / PORT_PLAN "Phasing")
- **Origin:** brainstorming session 2026-07-01
- **Depends on:** [slice 1 — WebRTC media loopback](2026-06-28-slice-1-webrtc-loopback-design.md),
[slice 2 — The agent tap](2026-06-28-slice-2-agent-tap-design.md), and slice-3's
OpenAI Realtime brain (merged as `c30a452` — `MockRealtimeBrain` + translator + the
`speech_started` / `speech_stopped` advisory events). All three must be landed and green.
- **Related:** [ADR-0002](../../adr/0002-north-star-and-fused-core.md) (fused vertical — the
hot-path hop invariant this slice re-affirms), [ADR-0008](../../adr/0008-fob-and-green-zone.md)
(FOB/green-zone doctrine — the reflex is a FOB member: hot-path, differentiating),
[ARCHITECTURE.md §"Biggest technical risk"](../../ARCHITECTURE.md) (the reflex loop *is*
the remaining long pole), [ARCHITECTURE.md §"Media plane"](../../ARCHITECTURE.md)
("Dedicated timing threads for the 20ms loop, **never the shared tokio pool**" — this
slice finally lands that mandate).
---
## TL;DR
Stand up spearhead step 4: the **FOB reflex loop**. Slice 3 pre-paved the advisory signals
(`speech_started` / `speech_stopped` from the brain) and locked the turn-ownership decision
(OpenAI Realtime server-side VAD disabled; the FOB owns turn-taking). Slice 4 **acts** on
those advisories: on `speech_started`, the FOB kills playout from the core-authoritative
buffer; playout resumes when the brain's *first fresh `audio_out` after the barge* arrives,
proving the brain has yielded and started a new response. No brain round-trip gates the
kill — the decision lives in the 20 ms media loop.
Slice 4 also **graduates the media loop off the tokio pool**: a single dedicated `std::thread`
owns all `RtcSession`s exclusively and drives the 20 ms tick via `std::thread::sleep`. This
honors ARCHITECTURE.md's "never the shared tokio pool" mandate, which slice-1 explicitly
deferred to "step 4 (barge-in)" (`loop_driver.rs:18-23`). The graduation is load-bearing:
the reflex is the differentiator and the long pole, and its timing discipline demands a
thread that doesn't compete with the axum runtime for scheduling.
The **seam slice 1→3 preserved** (`loop_driver.rs` + `rtc_session.rs` byte-identical) holds
for slice-4 as well: the reflex is a `Reflex<P>` wrapper decorating the `AudioPipe` trait,
invisibly to `loop_driver::drive`. Only the binary-side wiring (`session_map.rs` →
`media_thread.rs`) changes shape; the media crate's hot path stays untouched.
---
## 1. Scope
### 1.1 In scope
- Implementation of spearhead step 4: **barge-in / VAD-driven playout kill**, where "VAD"
for the MVP is the brain's `speech_started` / `speech_stopped` advisory (slice-3 pre-paved)
— no local DSP energy detector this slice (deferred).
- A **new `Reflex<P: AudioPipe>` wrapper** (`rutster-media/src/reflex.rs`) that decorates the
pipe the `RtcSession` holds. The reflex owns the mute state machine, the advisory channel
receiver, and the barge-in flush trigger. It is the concrete embodiment of ARCHITECTURE.md's
"local real-time reflexes" row for the barge-in case.
- A **new `barge_in_flush` method on the `AudioPipe` trait** (default impl delegates to
`clear_playout_ring`) — the seam object's "kill now" path: clear the playout ring AND drain
the brain-bound `rx_audio_out` channel of any frames queued before the barge so the first
`audio_out` observed post-barge is provably post-barge. `TapAudioPipe` overrides;
`EchoAudioPipe` uses the default.
- A **new `AdvisoryEvent` enum** (`SpeechStarted { at }`, `SpeechStopped { at }`) flowing
over a tokio mpsc from the TapEngine (tokio) to the Reflex (media thread). The engine
pushes the events it already decodes from the brain (slice-3 wired these as log+count;
slice-4 *forwards* them into the reflex).
- A **new dedicated media thread** (`rutster/src/media_thread.rs`) replacing the tokio
`spawn_poll_task`. One `std::thread::spawn` at binary startup owns
`HashMap<ChannelId, RtcSession>` exclusively; all access from axum is via a command
channel (`AcceptOffer`, `Delete`, `Shutdown`). The 20 ms tick is `std::thread::sleep`.
- **Rewired `session_map.rs`** (binary): `SessionEntry.rtc: Arc<Mutex<RtcSession>>` →
`cmd_tx: mpsc::Sender<MediaCmd>`. `create_session`, `post_offer`, `close`,
`spawn_poll_task` all route through the command channel. The async handlers are cold-path;
no cross-thread coordination happens on the 20 ms tick.
- **`MockRealtimeBrain` extension** (`rutster-brain-realtime/src/mock.rs`): gains the ability
to emit `speech_started` / `speech_stopped` on a programmable schedule (e.g. "after N
audio_in frames received, send `speech_started`; after M more, send `speech_stopped`").
- **Barge-in e2e integration test** (extends slice-3's
`crates/rutster/tests/realtime_integration.rs` harness): synthetic WebRTC peer →
MediaThread → TapEngine → MockRealtimeBrain; mock emits `speech_started`; assert playout
goes silent within ≤1 tick (20 ms); mock emits fresh `audio_out`; assert playout resumes.
- New `ReflexMetrics` (`barge_in_count`, `advisory_dropped`, `frames_suppressed`) mirroring
`TapMetrics` shape (atomics, snapshot fn). Threaded through the same `TapConn.metrics`
surface where reasonable, or a new side-car.
- Thorough learner-facing comments on the new std-thread / channel-bridge / wrapper-decorator
patterns (slice-1 §7 standard carries over).
### 1.2 Out of scope (with scheduled return)
| Deferred item | Returns in | Why deferred |
|---|---|---|
| Local VAD (energy/RMS detector in `on_pcm_frame`) | post-spearhead refinement | Advisory-only MVP per slice-4 brainstorming decision. Local VAD needs threshold tuning + DSP analysis worth its own slice; the `Reflex<P>` wrapper shape is designed so a local-VAD decorator composes as a second wrapper outside (or inside) the advisory one. |
| Per-session media threads / threadpool shard | later rung | Single thread covers spearhead scale (loopback dev + low-concurrency PSTN via slice-5). The command-channel seam between axum and the thread makes the graduation to a threadpool shard localized. |
| Trickle ICE | later | Unchanged from slice-1 deferral. |
| Min-mute floor / inter-word-gap debouncing | post-spearhead | `SpeechStopped` is a no-op for mute; a floor timer on resume would protect against brain-yield races (brain emits fresh `audio_out` before the caller's inter-word gap ends). Defer until observed in practice. |
| Brain-side `input_audio_buffer.interrupt` / `clear` on barge | slice-5 or brain-side | Whether the brain should clear its own input buffer on `speech_started` is a brain-UX decision, not a FOB one; the FOB only kills *playout* (its half-duplex gate). The advisory already tells the brain what happened; the brain's response is its own concern. |
| Half-duplex gating beyond playout kill | later rung | Barge-in is the first half-duplex reflex; full HD gating (mixing, jitter buffer interaction, multi-party) arrives with conferencing. |
| TLS on HTTP / WSS | slice-5 | Unchanged. |
| Authn / authz / multi-tenancy | slice-6 | Unchanged. |
| Spend cap / abuse gate | slice-6 | Unchanged. |
| Browser-based automated e2e (Playwright/Selenium) | post-spearhead | Unchanged. The synthetic-peer harness from slice-2/3 is the test vehicle. |
---
## 2. Architecture delta
### 2.1 The reflex wrapper
`Reflex<P: AudioPipe>` is a zero-cost-style decorator around any `AudioPipe`. It sits
between `RtcSession.pipe` (which `loop_driver::drive` calls via `session.pipe.next_pcm_frame()`)
and the concrete pipe (`TapAudioPipe` in production, `EchoAudioPipe` in slice-1's unit tests).
`loop_driver` is oblivious to the wrapper: it still calls `session.pipe.next_pcm_frame()`,
the dynamic dispatch through `Box<dyn AudioPipe>` lands in `Reflex::next_pcm_frame`, which
applies the state machine and delegates to `inner.next_pcm_frame()` per the table in §3.2.
The reflex owns three pieces of state:
- `advisory_rx: mpsc::Receiver<AdvisoryEvent>` — drained sync-non-blocking via `try_recv`
on the 20 ms tick before delegating to `inner`. Fed by the TapEngine task over tokio mpsc.
- `muted: bool` — the kill state. `next_pcm_frame` returns `None` while muted, *unless* the
inner returns `Some` (the resume condition — the first fresh `audio_out` clears mute).
- `barge_epoch: u64` — incremented on every `SpeechStarted`. Not strictly required for the
advisory-only MVP (the flush + drain makes the resume race-free), but it's the seam for a
future local-VAD wrapper that could race the advisory. Documented as forward-compatible.
### 2.2 The dedicated media thread
A single `std::thread::spawn` replaces the tokio `spawn_poll_task`. The thread owns
`HashMap<ChannelId, RtcSession>` **exclusively** — no `Arc<Mutex<RtcSession>>` shared with
axum. All access from the axum handlers is via a command channel:
```rust
enum MediaCmd {
AcceptOffer { id: ChannelId, sdp: String, reply: oneshot::Sender<Result<String, String>> },
Delete { id: ChannelId, reply: oneshot::Sender<()> },
Shutdown { reply: oneshot::Sender<()> },
}
```
The thread loop per 10 ms meta-tick:
1. Drain `cmd_rx` via `try_recv` loop — handle all pending commands before ticking.
2. For each session in the map: drain the per-session `flush_rx` side-channel (slice-2's
existing disconnect-flush signal) BEFORE `run_poll_once`, then call
`RtcSession::run_poll_once(now)` (the unchanged `loop_driver::drive`).
3. After `run_poll_once`, observe `channel.state`:
- `Connected && tap.is_none()` → spawn the TapEngine (tokio task via the
`tokio::runtime::Handle` captured at thread-start) + wire
`Reflex<TapAudioPipe>` as the session's pipe. Mirror of slice-2's spawn seam,
relocated from `session_map.rs::drive_all_sessions` to here.
- `Closed` → remove the entry + drop the session.
4. `std::thread::sleep(Duration::from_millis(10))` — 10 ms meta-tick. (Stable API:
`std::thread::sleep_until` is nightly-only; `sleep(dur)` is the stable path. The 20 ms
outbound encode tick is driven inside `loop_driver::drive` (unchanged); the 10 ms
meta-tick gives finer resolution so str0m's `Timeout` outputs are honored promptly.)
**The tokio ↔ std-thread bridge:** all channels are tokio mpsc/oneshot (constructable on
tokio, drainable via `try_recv`/`blocking_recv` from any thread). The `tokio::runtime::Handle`
captured at `MediaThread::spawn` time is used on the std thread to `handle.spawn(...)` the
TapEngine when the `Connected` transition fires. No async code runs on the std thread itself
— only sync channel ops + `RtcSession::run_poll_once`.
**Why a single thread, not per-session:** spearhead scale. One loopback peer at a time in
dev; even at low PSTN concurrency (slice-5) one thread drives dozens of sessions in 10 ms.
Per-session threads arrive when the threadpool shard model lands (deferred). The
command-channel seam between axum and the thread makes that graduation localized.
### 2.3 The hot-path audit (ADR-0002 honored)
ADR-0002's load-bearing rule: *"the control↔media gRPC hop on the per-call hot path is
removed."* Slice 4 does not re-introduce a hop:
- The reflex's kill decision happens **inside** `Reflex::next_pcm_frame` on the dedicated
thread — no channel send, no cross-thread coordination on the 20 ms tick. The advisory
arrives via a `try_recv` drain (sync, non-blocking).
- axum → media-thread is **cold-path only** (SDP accept, DELETE). None of it runs on the
20 ms tick.
- The brain WS ↔ TapEngine (tokio) path is unchanged from slice-3. The advisory channel
is a *third* mpsc alongside the existing `tx_pcm_in`/`rx_audio_out`/`flush_tx` — same
pattern, additive.
**The fused vertical stays fused.** ADR-0002 honored.
---
## 3. Component design
### 3.1 `AdvisoryEvent` enum
```rust
// crates/rutster-media/src/reflex.rs
/// A turn-event advisory from the brain. The brain decodes its own
/// speech-to-text / VAD results and forwards these; the FOB *owns*
/// turn-taking and acts on them (slice-3 §4.3 — OpenAI Realtime
/// server-side VAD is DISABLED; the FOB's reflex is authoritative).
///
/// Carried over a tokio mpsc from the TapEngine (tokio task) to the
/// `Reflex` wrapper (media thread). Drained sync via `try_recv` on the
/// 20 ms tick — the kill decision lives in the loop, not in a handler.
#[derive(Debug)]
pub enum AdvisoryEvent {
/// The brain detected caller speech. Trigger barge-in: kill playout.
SpeechStarted { at: Instant },
/// The brain detected caller speech ended. Observed + counted; does
/// NOT toggle mute (the resume condition is "first fresh audio_out
/// after the barge", not "speech_stopped" — see §3.2 state table).
SpeechStopped { at: Instant },
}
```
### 3.2 `Reflex<P>` state machine
| Current state | Event | Action | New state |
|---|---|---|---|
| Playing | `SpeechStarted` | `muted=true`; `epoch++`; `inner.barge_in_flush()` (clear ring + drain `rx_audio_out` so stale brain frames queued pre-barge are dropped); `metrics.barge_in_count++` | Muted |
| Muted | `SpeechStarted` (duplicate/re-barge) | `epoch++`; `barge_in_flush()` again (fresh barge resets the "fresh audio" clock); `barge_in_count++` | Muted |
| Muted | `SpeechStopped` | increment `advisory_observed_speech_stopped` counter; **no state change** | Muted |
| Playing | `SpeechStopped` | increment counter; **no state change** | Playing |
| Muted | inner `next_pcm_frame()` returns `Some(f)` (fresh brain audio arrived post-barge) | `muted=false`; return `Some(f)` | Playing |
| Muted | inner `next_pcm_frame()` returns `None` | return `None` (silence); `metrics.frames_suppressed++` | Muted |
**Why `SpeechStopped` is a no-op for mute:** per the resume-semantics decision (resume on
first fresh `audio_out`). The brain's `speech_stopped` is *observed* (counter) but doesn't
gate — this avoids the inter-word-gap problem (caller pauses, VAD fires stopped, brain
un-mutes too early, brain's audio overlaps caller's next word). The resume condition is
"the brain has yielded and started a new response," which is provably signaled by the first
`audio_out` frame after the barge — not by the caller's silence.
**Why `epoch`:** not strictly needed for advisory-only (MVP), but it's the seam for the
local-VAD backstop (deferred per §1.2). A future `LocalVadReflex` wrapper racing the
advisory would need to disambiguate "is this barge a re-barge of the same event or a new
one" — the epoch is the disambiguator. Forward-compatible.
### 3.3 `AudioPipe` trait extension
```rust
// crates/rutster-media/src/pcm.rs — additive method on `AudioPipe`
/// Barge-in flush: clear the playout ring AND drain the inbound brain
/// audio queue of any frames queued before the barge. Called by `Reflex`
/// on `SpeechStarted`. The drain of `rx_audio_out` is what makes the
/// resume condition race-free: the first `audio_out` observed post-barge
/// is provably post-barge (frames queued pre-barge are dropped here).
///
/// Default impl delegates to `clear_playout_ring` — sufficient for
/// pipes without an inbound queue to drain (like `EchoAudioPipe`).
fn barge_in_flush(&mut self) {
self.clear_playout_ring();
}
```
`TapAudioPipe` overrides:
```rust
// crates/rutster-tap/src/tap_audio_pipe.rs
fn barge_in_flush(&mut self) {
// Clear the playout ring (drops buffered brain-proposed frames).
self.playout_ring.clear();
// Drain rx_audio_out of any frames the engine task queued before
// the barge. Without this, a stale frame in the mpsc would un-mute
// immediately on the next tick — defeating the "first fresh audio_out"
// resume condition. Hot-path: try_recv loop, bounded, no blocking.
while self.rx_audio_out.try_recv().is_ok() {
self.metrics.barge_drained_inflight.fetch_add(1, Ordering::Relaxed);
}
}
```
### 3.4 `Reflex<P>` struct + impl
```rust
// crates/rutster-media/src/reflex.rs
pub struct Reflex<P: AudioPipe> {
inner: P,
advisory_rx: mpsc::Receiver<AdvisoryEvent>,
muted: bool,
barge_epoch: u64,
metrics: Arc<ReflexMetrics>,
}
impl<P: AudioPipe> Reflex<P> {
pub fn new(inner: P, advisory_rx: mpsc::Receiver<AdvisoryEvent>, metrics: Arc<ReflexMetrics>) -> Self {
Self { inner, advisory_rx, muted: false, barge_epoch: 0, metrics }
}
/// Drain all pending advisories + apply the state table. Called at
/// the top of `next_pcm_frame`. Hot-path: try_recv loop, bounded.
fn drain_advisories(&mut self) {
while let Ok(ev) = self.advisory_rx.try_recv() {
match ev {
AdvisoryEvent::SpeechStarted { at } => {
self.muted = true;
self.barge_epoch = self.barge_epoch.wrapping_add(1);
self.inner.barge_in_flush();
self.metrics.barge_in_count.fetch_add(1, Ordering::Relaxed);
tracing::info!(epoch = self.barge_epoch, ?at, "barge-in");
}
AdvisoryEvent::SpeechStopped { at: _ } => {
self.metrics.advisory_observed_speech_stopped.fetch_add(1, Ordering::Relaxed);
// No state change — see §3.2.
}
}
}
}
}
impl<P: AudioPipe> AudioPipe for Reflex<P> {
fn next_pcm_frame(&mut self) -> Option<PcmFrame> {
self.drain_advisories();
if self.muted {
// Muted: pull from inner. Some(f) = fresh brain audio arrived
// post-barge → un-mute + return. None = silence, stay muted.
match self.inner.next_pcm_frame() {
Some(f) => {
self.muted = false;
Some(f)
}
None => {
self.metrics.frames_suppressed.fetch_add(1, Ordering::Relaxed);
None
}
}
} else {
self.inner.next_pcm_frame()
}
}
fn on_pcm_frame(&mut self, frame: PcmFrame) {
// Inbound caller audio is NEVER gated by the reflex. The brain
// still hears the caller during barge — that's the point (the
// brain needs to know the caller interrupted; the FOB only kills
// its OWN playout, not the caller's path to the brain).
self.inner.on_pcm_frame(frame)
}
fn clear_playout_ring(&mut self) {
// The reconnect-flush path (slice-2 §5.3) still works through the
// wrapper. If it fires during mute, the ring stays empty and mute
// clears on the next post-reconnect audio_out.
self.inner.clear_playout_ring()
}
fn barge_in_flush(&mut self) {
// Allow outer wrappers (future local-VadReflex) to barge the inner.
self.inner.barge_in_flush()
}
}
```
### 3.5 `ReflexMetrics`
Mirror of `TapMetrics` shape (atomics + snapshot struct):
```rust
// crates/rutster-media/src/reflex.rs
#[derive(Default)]
pub struct ReflexMetrics {
pub barge_in_count: AtomicU64,
pub advisory_dropped: AtomicU64, // advisory channel full (e.g. 16-cap)
pub frames_suppressed: AtomicU64, // None returns while muted
pub advisory_observed_speech_stopped: AtomicU64,
}
pub struct ReflexMetricsSnapshot {
pub barge_in_count: u64,
pub advisory_dropped: u64,
pub frames_suppressed: u64,
pub advisory_observed_speech_stopped: u64,
}
// `barge_drained_inflight` lives on `TapMetrics` (in `rutster-tap`), not
// `ReflexMetrics`, because the drain happens inside `TapAudioPipe::barge_in_flush`,
// not inside `Reflex`. The path: `Reflex::drain_advisories` calls
// `inner.barge_in_flush()` which is `TapAudioPipe::barge_in_flush`, which is
// where the `rx_audio_out` drain + the counter increment happen.
```
---
## 4. The dedicated media thread
### 4.1 `MediaThread`
```rust
// crates/rutster/src/media_thread.rs
pub struct MediaThread {
cmd_tx: mpsc::Sender<MediaCmd>,
join: Option<std::thread::JoinHandle<()>>,
}
enum MediaCmd {
AcceptOffer { id: ChannelId, sdp: String, reply: oneshot::Sender<Result<String, String>> },
Delete { id: ChannelId, reply: oneshot::Sender<()> },
Shutdown { reply: oneshot::Sender<()> },
}
```
Spawned at binary startup (`main.rs`), before `axum::serve`. The thread captures a
`tokio::runtime::Handle` (to spawn TapEngine tasks when `Connected` transitions fire) and
owns `HashMap<ChannelId, RtcSession>` + (per-session, lazily) the TapConn / advisory_rx /
Reflex wrapper.
### 4.2 Thread loop (per 10 ms meta-tick)
1. `cmd_rx.try_recv()` loop — handle ALL pending commands before ticking. `AcceptOffer`
calls `RtcSession::accept_offer(sdp)` and replies via the oneshot. `Delete` fires
`close_tx` + bounded-await the engine task (750 ms cap via
`tokio::runtime::Handle::block_on(timeout(...))`) — the std thread briefly enters the
tokio runtime to await; cold-path, not the 20 ms tick. `Shutdown` drains + replies.
2. For each `RtcSession` in the map:
- Drain per-session `flush_rx` side-channel (slice-2's existing disconnect-flush) BEFORE
`run_poll_once`.
- Call `RtcSession::run_poll_once(now)` — the unchanged `loop_driver::drive`.
- Observe `channel.state`:
- `Connected && tap.is_none()` → `handle.spawn(spawn_tap_engine(...))` to bring up the
tokio task; construct `Reflex::new(TapAudioPipe::new(...), advisory_rx, metrics)`;
call `RtcSession::set_pipe(reflex)`. Mirror of slice-2's spawn seam.
- `Closed` → remove the entry (drops the `RtcSession` + its pipe + advisory ends).
3. `std::thread::sleep(Duration::from_millis(10))` — 10 ms meta-tick.
### 4.3 `session_map.rs` rewire
`SessionEntry` loses `rtc: Arc<Mutex<RtcSession>>`, gains `cmd_tx: mpsc::Sender<MediaCmd>`
(cloned per-entry; cheap). `tap_url` stays (the thread reads it when spawning the engine).
`tap_conn: Option<TapConn>` moves onto the media thread (the thread owns it after spawn).
- `AppState::create_session` → sends a `Register { tap_url, reply }` command to the media
thread; the **thread** constructs `RtcSession::new()` (saves a cross-thread move of the
struct + keeps all `RtcSession` construction on the thread that owns it). The thread
replies with `(id, cmd_tx_for_this_session)`; axum stores `SessionEntry { cmd_tx, tap_url,
tap_conn: None }`.
- `AppState::get(id)` (SDP path) → `cmd_tx.send(AcceptOffer { ... }).await` + `reply.await`.
Cold-path; the axum handler is async.
- `AppState::close(id)` → `cmd_tx.send(Delete { id, reply }).await` + `reply.await`. The
reply returns after the TapEngine teardown completes on the thread.
- `spawn_poll_task` → `spawn_media_thread`: constructs the channels, spawns the std thread,
stores `cmd_tx` + `join` in `AppState`. Same idempotent-guard pattern.
### 4.4 TapEngine extension
`spawn_tap_engine` returns a third channel end: `advisory_tx: mpsc::Sender<AdvisoryEvent>`.
The pump loop, on receiving `speech_started` / `speech_stopped` from the brain (slice-3
already decodes these in the tap protocol layer — `protocol_events.rs`), pushes the
corresponding `AdvisoryEvent` into `advisory_tx`. If the channel is full, drop + count
(hot-path "drop + observe" policy; an advisory is a hint, not a command). The `Reflex`
wrapper holds `advisory_rx`.
### 4.5 `MockRealtimeBrain` extension
`rutster-brain-realtime/src/mock.rs` gains a programmable advisory schedule: the test can
register "after N `audio_in` frames received, send `speech_started`" and "after M more,
send `speech_stopped`". The mock already asserts `turn_detection: null` on
`session.update` (slice-3's S4 lock); slice-4 keeps that assertion.
---
## 5. Data flow
### 5.1 Barge-in (the kill)
```
1. caller speaks into mic → peer RTP → str0m decode → on_pcm_frame → tx_pcm_in → TapClient → audio_in (WS) → brain
2. brain's VAD fires → brain sends speech_started back over WS (slice-3 already decodes this)
3. TapEngine pump loop → push AdvisoryEvent::SpeechStarted → advisory_tx (tokio mpsc, 16-cap)
4. media thread 20 ms tick → Reflex::next_pcm_frame → drain_advisories → SpeechStarted seen
→ muted=true; epoch++; inner.barge_in_flush() (ring cleared + rx_audio_out drained)
→ returns None (silence) for this + subsequent ticks while muted
5. loop_driver::drive pulls None from pipe → encodes Opus silence → peer hears silence
(the brain's in-flight audio_out frames are dropped; no overlap with caller's speech)
```
### 5.2 Resume (the un-mute)
```
1. brain decides to yield/respond → sends a fresh audio_out frame
(provably post-barge: barge_in_flush drained rx_audio_out)
2. TapClient → audio_out (WS) → TapEngine → tx_audio_out → rx_audio_out → playout ring
3. media thread 20 ms tick → Reflex::next_pcm_frame → drain_advisories (empty)
→ muted=true → inner.next_pcm_frame() returns Some(f) (fresh brain audio)
→ muted=false; return Some(f)
4. loop_driver encodes + writes → peer hears the brain's new response
```
### 5.3 Cold-path (axum ↔ media thread)
```
- POST /v1/sessions → AppState::create_session → MediaCmd::Register → thread constructs RtcSession → reply(id)
- POST /v1/sessions/{id}/offer → AppState::get + cmd_tx.send(AcceptOffer) → thread.lock(session).accept_offer(sdp) → reply(answer)
- DELETE /v1/sessions/{id} → AppState::close → cmd_tx.send(Delete) → thread: fire close_tx, bounded-await engine task teardown → reply
- graceful shutdown → cmd_tx.send(Shutdown) → thread drains + drops → reply → join
```
---
## 6. Why these decisions
### 6.1 Why advisory-only (no local VAD) for the MVP
- Matches slice-3's S4 turn-ownership posture: OpenAI Realtime's server-side VAD is disabled;
the FOB owns turn-taking. The brain already runs VAD (it has to, to do STT); forwarding its
result is the cheapest path to a working barge-in.
- Local VAD (energy/RMS detector in `on_pcm_frame`) is DSP work + threshold tuning — worth
its own slice. The `Reflex<P>` wrapper shape is designed so a `LocalVadReflex` decorator
composes outside (or inside) the advisory one when it arrives.
- YAGNI: prove the advisory→reflex→kill path end-to-end first; add the backstop if the
brain's VAD latency proves insufficient in practice.
### 6.2 Why resume on first fresh `audio_out` (not `speech_stopped`)
- The "the brain has yielded and started a new response" condition is provably signaled by
the first `audio_out` frame after the barge — not by the caller's silence. `speech_stopped`
fires between words; resuming on it un-mutes too early (inter-word-gap overlap).
- The `barge_in_flush` drain of `rx_audio_out` makes the resume race-free: the first
`audio_out` observed post-barge is provably post-barge (frames queued pre-barge are dropped
in the flush).
### 6.3 Why a single dedicated thread (not per-session)
- Spearhead scale: one loopback peer in dev; even at low PSTN concurrency (slice-5), one
thread drives dozens of sessions in 10 ms.
- The command-channel seam between axum and the thread makes the graduation to a threadpool
shard localized — when per-CPU-shard threading arrives, it's a fan-out of the
`cmd_rx`/`HashMap` shape, not a redesign.
- Per-session threads arrive when load demands; the spearhead's "shortest blocking path"
rule dislikes spawning work per session that may not need it (pre-ICE-connected sessions
would redundantly spin).
### 6.4 Why `Reflex<P>` as a wrapper (not inline in `TapAudioPipe`)
- Composition: a future `LocalVadReflex<P>` composes outside the advisory `Reflex<P>`, the
same way `Reflex<TapAudioPipe>` composes today. The pattern (decorator over `AudioPipe`) is
forward-compatible without restructuring.
- The seam: `loop_driver.rs` byte-identical (still calls `pipe.next_pcm_frame()`). If the
reflex lived inline in `TapAudioPipe`, the binary-side wiring would still change but the
`TapAudioPipe` module itself would grow the reflex state — less isolated.
- YAGNI caveated: the wrapper is the right abstraction for advisory-only because there's
exactly one reflex. When local VAD arrives, the wrapper pattern pays off; the spec does
not pre-empt that by collapsing the wrapper now.
### 6.5 Why `barge_in_flush` on `AudioPipe` (not just `clear_playout_ring`)
- `clear_playout_ring` (slice-2) clears the *ring*. `barge_in_flush` clears the ring AND
drains the *inbound brain queue* (`rx_audio_out`). The distinction matters: on a brain
disconnect (slice-2's case), the brain is gone — `rx_audio_out` will drain itself on the
next `Disconnected` `try_recv`. On a barge-in, the brain is alive and may have queued
frames pre-barge that would un-mute immediately if not drained here. Two different
"clear the playout path" semantics, two methods.
---
## 7. Done-criteria
1. `cargo test --all` passes (stable + 1.85, the CI matrix).
2. `cargo fmt --check` + `cargo clippy -- -D warnings` clean.
3. `loop_driver.rs` + `rtc_session.rs` **byte-identical** to slice-3 — CI-asserted via
`git diff --exit-code main -- crates/rutster-media/src/loop_driver.rs
crates/rutster-media/src/rtc_session.rs` (the §8.5 #6 seam gate, restated for slice-4).
4. Dedicated media thread drives sessions off the tokio pool; `MediaThread` integration test
passes (AcceptOffer / Delete / Shutdown).
5. `Reflex` state-machine unit tests all pass:
- `SpeechStarted` → next `next_pcm_frame` returns None even if ring has frames.
- `SpeechStarted` then `inner.next_pcm_frame()=Some` → un-mutes, returns the frame.
- `SpeechStopped` during Muted → stays Muted.
- `SpeechStopped` during Playing → no-op.
- Duplicate `SpeechStarted` re-flushes + stays Muted.
- Metrics counters (`barge_in_count`, `frames_suppressed`) increment correctly.
- `advisory_rx` full → `advisory_dropped` increments, no panic.
6. `barge_in_flush` unit tests pass (ring + `rx_audio_out` drain).
7. Barge-in e2e: `speech_started` → playout silent within ≤1 tick (20 ms); fresh `audio_out`
→ playout resumes. Extends slice-3's `realtime_integration.rs` harness.
8. S4 turn-ownership lock preserved: `MockRealtimeBrain` still asserts
`turn_detection: null` on `session.update` (slice-3's #7, unchanged).
9. `MockRealtimeBrain` extended to emit `speech_started`/`speech_stopped` on schedule.
10. `cargo doc --no-deps` renders the new `reflex.rs` + `media_thread.rs` module/item docs
cleanly (learner-facing comments present per AGENTS.md code style).
---
## 8. Open decisions
- ~~Trigger source~~ — decided: advisory-only (brain `speech_started`/`speech_stopped`).
- ~~Resume semantics~~ — decided: first fresh `audio_out` post-barge; `SpeechStopped`
observational only.
- ~~Thread model~~ — decided: single dedicated `std::thread`; per-session/threadpool deferred.
- **`MockRealtimeBrain` advisory schedule API shape** — landed in §4.5 as a programmable
"after N audio_in frames" schedule. Could alternatively be a free-form
`Vec<(trigger_frame_count, AdvisoryEvent)>` queue. The plan will pin the concrete API.
- **Thread shutdown ordering vs TapEngine teardown** — `Delete` command handler fires
`close_tx` + bounded-await the engine task (750 ms cap via
`tokio::runtime::Handle::block_on(timeout(...))`); the reply oneshot returns after
teardown. Cold-path, std thread briefly enters the tokio runtime to await. Documented as
an acceptable deviation (not the 20 ms tick).
---
## 9. Cross-references
- [slice-1 spec](2026-06-28-slice-1-webrtc-loopback-design.md) — the media loop + the seam
(`AudioSource`/`AudioSink` traits in `rutster-media`); slice-1 §8.5 #6 is the seam gate
this slice re-affirms.
- [slice-2 spec](2026-06-28-slice-2-agent-tap-design.md) — the tap interface, the
`TapAudioPipe`, the core-authoritative playout buffer (§4.1), the `flush_tx` side-channel
pattern that the `advisory_rx` mirrors.
- slice-3 (merged `c30a452`) — `MockRealtimeBrain`, the translator, the
`speech_started`/`speech_stopped` protocol events, the S4 turn-ownership lock.
- [ADR-0002](../../adr/0002-north-star-and-fused-core.md) — fused vertical; the hot-path
hop invariant this slice re-affirms (§2.3 audit).
- [ADR-0008](../../adr/0008-fob-and-green-zone.md) — FOB/green-zone doctrine; the reflex is
a FOB member (hot-path, security-constitutive for turn-taking, differentiating).
- [ARCHITECTURE.md](../../ARCHITECTURE.md) — §"Media plane" ("Dedicated timing threads
for the 20ms loop, never the shared tokio pool" — this slice lands it); §"Biggest
technical risk" (the reflex loop *is* the remaining long pole).
- [PORT_PLAN.md](../../PORT_PLAN.md) — §Phasing, step 4 = barge-in.