zebra-spaces: idempotent attach + drop stale stream cache (kill host double-audio)

Fox 2026-06-05 telemetry on host: "host is hearing two" — peer-joined
u=64b0 (fxhp-phone rejoin with new uuid, same pubkey) fired THREE
attach calls within 1 second for the SAME uuid:

  Call 1: pool=1 → fallback <audio> path, "tracks=0"
  Call 2: pool=1 → AudioContext success
  Call 3: pool=2 → AudioContext success (after mesh stream swap)

Three overlapping audio chains = "two streams" perceived.

Root causes:

1. flushSfuStreams used the cached sfuStreamsByPubHex entry, which still
   pointed at the OLD phone session's stream (uuid=91ab had left 5s
   earlier). Old stream's audio tracks were ended. attach received a
   zero-live-track stream and fell through to <audio> fallback.

2. Multiple paths (peer-joined → flushSfuStreams, sub PC ontrack,
   mid-spotlight supplant) each tried to attach the same uuid in the
   same tick. No idempotency check, so each call tore down and rebuilt.

Two fixes:

A. attachAudioStreamViaWorklet: reject streams with zero live audio
   tracks (skip silently); make same-uuid + same-stream calls a no-op.
   Repeated calls during a peer-joined burst stop racing.

B. handleRemoteSfuTrack: listen for the cached track's 'ended' event
   and drop the sfuStreamsByPubHex entry. Next flushSfuStreams won't
   hand a dead stream to attach.

Combined: the publisher's rejoin cycle now produces exactly ONE attach
chain — the one fed by the live ontrack — and ignores stale cache hits
+ duplicate attach calls.

web/zebra-spaces.html

@@ -3180,6 +3180,20 @@ function applyLipSyncForPub(pubHex){
  * applied only when role==='listener' — speakers don't need lock-
  * screen transport controls. */
 function attachAudioStreamViaWorklet(uuid, stream, targetSeconds){
+  /* Reject streams that have no live audio tracks. flushSfuStreams hands
+   * us whatever sfuStreamsByPubHex cached for the publisher's pubkey —
+   * after a publisher leaves and rejoins with the SAME pubkey, the old
+   * cached stream's tracks are ended even though the cache entry still
+   * exists. Without this guard, attach creates a silent MediaStreamSource
+   * → fallback to <audio> path → double-audio when the real ontrack
+   * arrives moments later. Fox 2026-06-05 telemetry: "host hearing two"
+   * after peer-joined u=64b0 fired three attaches in 1s. */
+  const liveAudio = (stream && stream.getAudioTracks ? stream.getAudioTracks() : [])
+    .filter(t => t.readyState === 'live');
+  if (liveAudio.length === 0){
+    logLine('', 'audio attach skip uuid='+uuid.slice(0,4)+' — no live audio tracks');
+    return false;
+  }
   if (!audioCtx){
     try { audioCtx = new (window.AudioContext || window.webkitAudioContext)(); }
     catch(e){ logLine('err','audioCtx create: '+e.message); return false; }
@@ -3189,6 +3203,15 @@ function attachAudioStreamViaWorklet(uuid, stream, targetSeconds){
   }
   logLine('', 'audio attach uuid='+uuid.slice(0,4)+' target='+targetSeconds+'s (current pool='+listenerAudioNodes.size+')');
   const existing = listenerAudioNodes.get(uuid);
+  /* IDEMPOTENT: if we already have a chain for this uuid AND it points at
+   * THIS exact stream object, treat as no-op. Multiple paths (peer-joined
+   * → flushSfuStreams, sub PC ontrack, mid-spotlight supplant) can each
+   * race to attach the same stream within the same tick — without this,
+   * each rebuilds the chain and two of them overlap audibly. */
+  if (existing && existing.stream === stream){
+    logLine('', 'audio attach no-op uuid='+uuid.slice(0,4)+' — same stream already attached');
+    return true;
+  }
   if (existing){
     try { existing.src.disconnect(); } catch(_){}
     try { if (existing.jbuf) existing.jbuf.disconnect(); } catch(_){}
@@ -4644,8 +4667,21 @@ function handleRemoteSfuTrack(ev){
   try { if (ev.receiver) ev.receiver.playoutDelayHint = playoutDelayForRole(myRole); } catch(_){}
   try { if (ev.receiver) ev.receiver.jitterBufferTarget = playoutDelayForRole(myRole) * 1000; } catch(_){}
   /* cache by full pubkey (already resolved above) so it survives the
-   * member's session uuid changing across leave/rejoin */
+   * member's session uuid changing across leave/rejoin. When the cached
+   * track ends (publisher leaves), drop the cache entry — flushSfuStreams
+   * was handing the dead stream to attachAudioStreamViaWorklet on the
+   * NEXT peer-joined with matching pubkey, racing the live ontrack.
+   * Fox 2026-06-05 telemetry: peer-joined u=64b0 fired "audio attach
+   * fresh=1 tracks=0" before the real attach. */
   sfuStreamsByPubHex.set(pubHex, ev.streams[0]);
+  try {
+    ev.track.addEventListener('ended', () => {
+      if (sfuStreamsByPubHex.get(pubHex) === ev.streams[0]){
+        sfuStreamsByPubHex.delete(pubHex);
+        logLine('', 'sfu cache drop pub='+pubHex.slice(0,4)+' — track ended');
+      }
+    }, { once: true });
+  } catch(_){}
   for (const [uuid, mm] of members){
     try {
       if (mm.pubkey && hex(unb64(mm.pubkey)) === pubHex){
@@ -7786,8 +7822,8 @@ logLine('', 'ready — pick a handle, type a rendezvous code, enter the space');
 
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