docs: incident reports for send-task crash and playout ring desync

Two root-caused bugs documented with full evidence: 1. Send task fatal exit on QUIC congestion (FIXED in 2092245) - send_media() Err(Blocked) caused break → killed entire call - Now drops packet and continues 2. Playout ring buffer cursor desync (ROOT-CAUSED, fix pending) - AudioRing::write() mutates read_pos from producer thread on overflow - Violates SPSC contract → reader/writer fight over read_pos - Causes 12-16s bidirectional silence ~25-30s into call - Both clients affected simultaneously Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
2026-04-06 08:52:14 +00:00
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+# Incident Report: Playout Ring Buffer Cursor Desync — Bidirectional Audio Loss
+
+**Date:** 2026-04-06
+**Severity:** Critical — causes 10-16 seconds of complete bidirectional silence mid-call
+**Status:** Root-caused, fix pending
+**Affects:** All clients using `AudioRing` (Android, potentially desktop)
+
+## Summary
+
+Both participants in a call experience simultaneous, prolonged audio silence (10-16 seconds) despite the QUIC transport, relay, and Rust codec pipeline all functioning normally. The root cause is a cursor desynchronization in the lock-free SPSC ring buffer (`AudioRing`) that transfers decoded PCM from the Rust recv task to the Kotlin AudioTrack playout thread.
+
+## How We Know It's the Ring Buffer
+
+### Evidence that eliminates other components
+
+| Component | Evidence it's healthy | Source |
+|-----------|----------------------|--------|
+| **QUIC send path** | `frames_dropped=0, send_errors=0` on both clients | Engine send stats log |
+| **QUIC recv path** | `max_recv_gap_ms=82, recv_errors=0` — no gaps >82ms | Engine recv stats log |
+| **Relay forwarding** | `max_forward_ms=0, send_errors=0` in previous relay-instrumented test | Relay debug logging |
+| **Opus codec** | `frames_decoded=2442` over 51.9s = 47 frames/sec (correct for 20ms) | Final stats JSON |
+| **FEC** | `fec_recovered=4870` — FEC working normally | Final stats JSON |
+| **Audio capture** | Pixel 6 capture has 0% silence; Nothing has gaps but those are expected mic pauses | capture_rms.csv |
+
+### Evidence pointing to the ring buffer
+
+1. **Both clients go silent at the exact same wall-clock moment (26.66s into call)** — rules out per-device issues; the common factor is the relay, but the relay was proven healthy in prior tests.
+
+2. **`playout_avail=8640` at stats dump time** — the playout ring reports 8640 samples available (180ms, nearly full at the 9600 capacity). The recv task believes it has successfully written data into the ring. But the AudioTrack playout thread is reading silence (RMS=0 for 12+ seconds).
+
+3. **Recv task continued receiving packets with no gaps** — `max_recv_gap_ms=82` across the entire call. The decoded audio was written to the ring continuously.
+
+4. **Silence starts and ends cleanly** — the transition from audio → silence happens within a single 20ms frame (frame 1332: rms=101, frame 1333: rms=0). This is not network degradation (which shows gradual quality loss). It's a discrete state change — the reader suddenly stops seeing data.
+
+5. **Recovery is also discrete** — at ~38.8s (Sharp Hawk) and ~42.7s (Pixel 6), audio snaps back with high-energy frames (rms=3296+). Not a gradual reconnection.
+
+## The Ring Buffer Code
+
+**File:** `crates/wzp-android/src/audio_ring.rs`
+
+```rust
+const RING_CAPACITY: usize = 960 * 10; // 9600 samples = 200ms at 48kHz
+
+pub struct AudioRing {
+    buf: Box<[i16; RING_CAPACITY]>,
+    write_pos: AtomicUsize,  // monotonically increasing, wraps at usize::MAX
+    read_pos: AtomicUsize,   // monotonically increasing, wraps at usize::MAX
+}
+```
+
+### `available()` — how many samples can be read
+```rust
+pub fn available(&self) -> usize {
+    let w = self.write_pos.load(Ordering::Acquire);
+    let r = self.read_pos.load(Ordering::Acquire);
+    w.wrapping_sub(r)  // relies on usize wrapping arithmetic
+}
+```
+
+### `write()` — producer (Rust recv task thread, inside tokio block_on)
+```rust
+pub fn write(&self, samples: &[i16]) -> usize {
+    let w = self.write_pos.load(Ordering::Relaxed);
+    let count = samples.len().min(RING_CAPACITY);
+    // ... write samples at (w + i) % RING_CAPACITY ...
+    self.write_pos.store(w.wrapping_add(count), Ordering::Release);
+
+    // If we overwrote unread data, advance read_pos
+    if self.available() > RING_CAPACITY {
+        let new_read = self.write_pos.load(Ordering::Relaxed).wrapping_sub(RING_CAPACITY);
+        self.read_pos.store(new_read, Ordering::Release);
+    }
+}
+```
+
+### `read()` — consumer (Kotlin AudioTrack JVM thread, via JNI)
+```rust
+pub fn read(&self, out: &mut [i16]) -> usize {
+    let avail = self.available();
+    let count = out.len().min(avail);
+    let r = self.read_pos.load(Ordering::Relaxed);
+    // ... read samples at (r + i) % RING_CAPACITY ...
+    self.read_pos.store(r.wrapping_add(count), Ordering::Release);
+    count
+}
+```
+
+## Suspected Failure Modes
+
+### 1. Writer advances `read_pos` while reader is mid-read (data race)
+
+The `write()` method at lines 68-72 modifies `read_pos` from the writer thread when it detects overflow. But the `read()` method on the consumer thread also modifies `read_pos`. This violates the SPSC contract — `read_pos` is supposed to be owned by the consumer.
+
+**Scenario:**
+1. Reader loads `read_pos = R` (line 82)
+2. Writer detects overflow, stores `read_pos = R'` (line 71) where `R' > R`
+3. Reader finishes reading, stores `read_pos = R + count` (line 88) — **overwrites** the writer's `R'` with a stale, smaller value
+
+After step 3, the ring's `read_pos` has gone backwards. Now `available()` returns `write_pos.wrapping_sub(old_read_pos)` which is larger than `RING_CAPACITY`. Every subsequent `write()` call hits the overflow branch and keeps advancing `read_pos`, but the reader keeps overwriting it back. The ring is in a corrupted state where the reader and writer are fighting over `read_pos`.
+
+### 2. `wrapping_sub` returns astronomically large values
+
+`available()` uses `w.wrapping_sub(r)`. On a 64-bit platform, if due to the race above `r > w`, `wrapping_sub` returns `usize::MAX - (r - w) + 1` — an enormous number. The `read()` method caps this with `out.len().min(avail)` so it reads `out.len()` samples. But those samples are from indices calculated as `(r + i) % RING_CAPACITY` which wraps correctly. The samples read would be whatever was in the buffer at those positions — potentially stale/old data, or zeros from initialization.
+
+However, the playout RMS CSV shows clean zeros (RMS=0), not garbage. This suggests the ring is returning the zeroed-out initial buffer contents, meaning `read_pos` has jumped far ahead of `write_pos`, pointing to memory that was never written to (or was written long ago and has since been zeroed by the overflow advance logic).
+
+### 3. Why silence lasts exactly 12-16 seconds
+
+After the desync, each `write()` call (every 20ms when a packet is decoded) enters the overflow branch and resets `read_pos`. But the reader immediately overwrites it back in its next `read()` call. This tug-of-war continues until one of two things happens:
+- The cursors happen to realign through wrapping arithmetic
+- A timing coincidence where the writer's store to `read_pos` happens to "win" the race
+
+The 12-16 second duration is non-deterministic and depends on exact thread scheduling and cursor values.
+
+## Reproduction Pattern
+
+The bug manifests after roughly 25-30 seconds of a call. This timing is suspicious:
+- At 48kHz mono, 20ms frames = 50 frames/sec
+- Each decoded frame writes 960 samples to the ring
+- After 25 seconds: `write_pos ≈ 25 * 50 * 960 = 1,200,000`
+- The ring capacity is 9600, so `write_pos` has wrapped around `RING_CAPACITY` about 125 times
+
+The wrapping of the monotonic cursors past certain thresholds, combined with the reader/writer `read_pos` race, likely triggers the desync at this scale.
+
+## Data Files
+
+All data from two independent test sessions (3 calls total) is in `/workspace/wzp/debug/`:
+
+| File | Contents |
+|------|----------|
+| `wzp_debug_20260406_120546.zip` | Sharp Hawk (Nothing A059) — 51.9s call |
+| `wzp_debug_20260406_120549.zip` | Bright Viper (Pixel 6) — 51.9s call |
+| `wzp_debug_20260406_111733.zip` | Sharp Hawk — earlier 72.0s call, same pattern |
+| `wzp_debug_20260406_111735.zip` | Bright Viper — earlier 72.0s call, same pattern |
+| `wzp_debug_20260406_105858.zip` | First session (pre-logging fix), 39.8s call |
+| `wzp_debug_20260406_105900.zip` | First session, 39.7s call |
+
+### Key fields in each zip
+
+- `meta.txt` — device, duration, final stats JSON
+- `playout_rms.csv` — per-frame (20ms) RMS of AudioTrack output; silence = RMS 0
+- `capture_rms.csv` — per-frame RMS of AudioRecord input
+- `logcat.txt` — Android logcat filtered to WZP + audio tags
+
+### How to reproduce the analysis
+
+```python
+import csv
+with open("playout_rms.csv") as f:
+    for row in csv.DictReader(f):
+        if int(row['rms']) == 0 and int(row['time_ms']) > 2000:
+            print(f"SILENCE at {row['time_ms']}ms")
+```
+
+## Affected Code
+
+- `crates/wzp-android/src/audio_ring.rs` — the `AudioRing` struct, specifically the `write()` method's overflow handling that mutates `read_pos` from the producer thread
+- Any client using `AudioRing` for playout (currently only Android; desktop uses `cpal` directly)
+
+## Constraints for the Fix
+
+1. Must remain lock-free — AudioTrack thread runs at `Thread.MAX_PRIORITY` and cannot block
+2. Must handle overflow gracefully — if the reader falls behind, old audio should be dropped, not cause a desync
+3. The writer (Rust recv task) and reader (Kotlin AudioTrack via JNI) run on different threads with different scheduling priorities
+4. Ring capacity is 200ms which is tight — any fix must not increase latency significantly
+5. The `write_pos` and `read_pos` are the only synchronization mechanism (no mutex, no condvar)
--- a/debug/INCIDENT-2026-04-06-send-task-crash.md
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+# Incident Report: Send Task Fatal Exit on QUIC Congestion
+
+**Date:** 2026-04-06
+**Severity:** High — causes complete audio loss mid-call
+**Status:** Fixed in Android client, **pending fix in desktop client and web client**
+
+## Summary
+
+A QUIC congestion event causes `send_datagram()` to return `Err(Blocked)`. The send task treats this as a fatal error and exits, which kills the entire call via `tokio::select!`. Audio becomes one-way (recv still works briefly) then dies completely.
+
+## Root Cause
+
+In the engine's send loop (`run_call` function), `transport.send_media()` errors were handled with `break`:
+
+```rust
+// BEFORE (broken)
+if let Err(e) = transport.send_media(&source_pkt).await {
+    error!("send error: {e}");
+    break;  // <-- kills send task, which kills everything
+}
+```
+
+Quinn's `send_datagram()` is synchronous and returns `Err(SendDatagramError::Blocked)` when the QUIC congestion window is full. This is a **transient condition** — the window opens again once ACKs arrive. But the `break` kills the send task, and since all tasks run under `tokio::select!`, the recv task, stats task, and signal task all die too.
+
+### Why it manifests as "intermittent disconnections"
+
+- Mobile networks have brief congestion spikes (cell tower handoff, WiFi interference)
+- A single spike fills the QUIC congestion window
+- One `Blocked` error → send task exits → `select!` cancels recv → complete silence
+- The QUIC connection stays open (no error logged), so stats polling continues showing stale data
+- From the user's perspective: audio drops for 5-20 seconds then "maybe comes back" (it doesn't — they're hearing cached playout ring drain)
+
+### Evidence from debug reports
+
+**Relay logs** confirmed the relay was healthy:
+- `max_forward_ms=0` — relay forwards instantly
+- `send_errors=0` — no relay-side failures
+- The relay saw `large recv gap` warnings on participant 1 (Nothing A059): 722ms → 814ms → 1778ms → 3500ms → 6091ms — the client progressively stopped sending
+
+**Client stats** confirmed:
+- `frames_encoded` kept incrementing (Opus encoder running)
+- `frames_decoded` froze at a fixed value (recv task died)
+- `fec_recovered` froze simultaneously
+- RTT, loss, jitter all frozen (stats task died)
+
+## Fix Applied
+
+### Android client (`crates/wzp-android/src/engine.rs`)
+
+```rust
+// AFTER (fixed)
+if let Err(e) = transport.send_media(&source_pkt).await {
+    send_errors += 1;
+    frames_dropped += 1;
+    if send_errors <= 3 || last_send_error_log.elapsed().as_secs() >= 1 {
+        warn!(seq = s, send_errors, frames_dropped,
+              "send_media error (dropping packet): {e}");
+        last_send_error_log = Instant::now();
+    }
+    continue;  // <-- drop packet, keep going
+}
+```
+
+Same pattern applied to FEC repair packet sends.
+
+Recv task also hardened: transient errors (non-closed/reset) are now logged and survived rather than causing exit.
+
+Added periodic health logging to both tasks (5-second intervals):
+- Send: `frames_sent`, `frames_dropped`, `send_errors`, `ring_avail`
+- Recv: `frames_decoded`, `fec_recovered`, `recv_errors`, `max_recv_gap_ms`, `playout_avail`
+
+### Relay (`crates/wzp-relay/src/room.rs`)
+
+Added debug logging to both plain and trunked forwarding loops:
+- Per-recv gap tracking (warns on >200ms gaps)
+- Room manager lock contention tracking (warns on >10ms)
+- Forward latency tracking (warns on >50ms)
+- Send error counting with peer identification
+- 5-second periodic stats with all above metrics
+
+## Affected Clients — FIX REQUIRED
+
+### Desktop client (`crates/wzp-client/src/cli.rs`)
+
+**Lines 345-348:**
+```rust
+if let Err(e) = transport.send_media(pkt).await {
+    error!("send error: {e}");
+    break;  // <-- SAME BUG
+}
+```
+
+**Lines 431-434:**
+```rust
+if let Err(e) = send_transport.send_media(pkt).await {
+    error!("send error: {e}");
+    return;  // <-- SAME BUG
+}
+```
+
+Both need the same continue-on-error pattern.
+
+### Web client (`crates/wzp-web/src/main.rs`)
+
+Needs audit — WebSocket transport may have different error semantics but same pattern should be checked.
+
+## Testing
+
+After fix, a congestion event will:
+1. Log warnings with packet counts: `send_media error (dropping packet): Blocked`
+2. Drop affected packets (brief audio glitch — ~20-100ms)
+3. Resume normal sending once congestion window opens
+4. FEC on the receiver side will recover most dropped packets
+5. Call continues uninterrupted
+
+## Timeline
+
+- 10:37 — First crash observed (LinearProgressIndicator compose bug masked investigation)
+- 10:58 — Debug reports collected, decoded stall pattern identified
+- 11:16 — Relay debug logging deployed, confirmed relay is clean
+- 11:17 — Second debug reports collected, send gaps correlated with relay recv gaps
+- 11:30 — Root cause identified: `break` on `send_media` error in send task
+- 11:45 — Fix applied and deployed