android/app/src/main/java/com/fabledsword/minstrel/diagnostics/DiagnosticsReporter.kt

@@ -18,6 +18,7 @@ import com.fabledsword.minstrel.connectivity.NetworkStatusController
 import com.fabledsword.minstrel.di.ApplicationScope
 import com.fabledsword.minstrel.player.PlayerController
 import com.fabledsword.minstrel.player.RemotePlayerState
+import com.fabledsword.minstrel.player.TransportObservation
 import com.fabledsword.minstrel.player.output.OutputPickerController
 import com.fabledsword.minstrel.player.output.OutputRoute
 import dagger.hilt.android.qualifiers.ApplicationContext
@@ -103,6 +104,7 @@ class DiagnosticsReporter @Inject constructor(
             launch { collectUpnpDrops() }
             launch { collectPlayerState() }
             launch { collectTrackChanges() }
+            launch { collectTransportFlap() }
             launch { collectRoutes() }
             launch { heartbeatLoop() }
         }
@@ -191,6 +193,67 @@ class DiagnosticsReporter @Inject constructor(
             }
     }
 
+    /**
+     * Catch the renderer rapidly leaving and re-entering PLAYING.
+     *
+     * The operator reports the Sonos "play pause play pause, like someone
+     * pressing it every half second", usually as a track starts, cleared by a
+     * manual pause or skip. Nothing here could see that: `player_state`
+     * carries source/loading/error but not playing, `track_change` needs the
+     * index to move, and the heartbeat samples once per 45s. The symptom fell
+     * through every existing collector, which is why it has only ever been
+     * described and never measured.
+     *
+     * Records every raw transport change (cheap — steady playback produces
+     * a couple per track) and, when they come in a burst, one summary event
+     * carrying the whole sequence. The summary is the useful artefact: it
+     * pairs the renderer's states with local-vs-Sonos track and position, so
+     * an episode says whether the app and the renderer disagreed about which
+     * track was playing, or agreed while the renderer rebuffered.
+     *
+     * See [TransportObservation] on the 1 Hz sampling limit.
+     */
+    private suspend fun collectTransportFlap() {
+        val detector = TransportFlapDetector()
+        playerController.transportEvents.collect { obs ->
+            record("upnp_sync", buildJsonObject {
+                put("event", "transport")
+                put("state", obs.state)
+                put("status_ok", obs.statusOk)
+                put("sonos_track", obs.trackNumber)
+                put("sonos_pos_ms", obs.positionMs)
+                put("play_intent", obs.playIntent)
+            })
+            detector.onChange(obs)?.let { recordFlapSummary(it) }
+        }
+    }
+
+    private suspend fun recordFlapSummary(recent: List<TransportObservation>) {
+        val ui = playerController.uiState.value
+        val casting = outputPicker.routesState.value.current.protocol !=
+            OutputRoute.Protocol.SYSTEM
+        val spanMs = recent.last().atElapsedMs - recent.first().atElapsedMs
+        record("upnp_sync", buildJsonObject {
+            put("event", "transport_flap")
+            put("changes", recent.size)
+            put("window_ms", spanMs)
+            // The sequence itself, e.g. "PLAYING>TRANSITIONING>STOPPED>PLAYING".
+            // Whether STOPPED appears at all is the first question to ask of a
+            // captured episode.
+            put("sequence", recent.joinToString(">") { it.state })
+            put("sonos_positions_ms", recent.joinToString(",") { it.positionMs.toString() })
+            put("sonos_tracks", recent.joinToString(",") { it.trackNumber.toString() })
+            put("local_index", ui.queueIndex)
+            put("local_track_id", ui.currentTrack?.id ?: "")
+            put("local_pos_ms", ui.positionMs)
+            putSonos(this, casting)
+            put("upnp_loading", ui.isUpnpLoading)
+            put("server_health", networkStatus.state.value.name)
+            put("route", outputPicker.routesState.value.current.name)
+            addPowerFields(this)
+        })
+    }
+
     private suspend fun collectRoutes() {
         // 'playback' — route changes happen for all outputs. This only ever
         // logs the ACTIVE route (routesState.current), so no "connected" flag.

android/app/src/main/java/com/fabledsword/minstrel/diagnostics/TransportFlapDetector.kt

@@ -0,0 +1,65 @@
+package com.fabledsword.minstrel.diagnostics
+
+import com.fabledsword.minstrel.player.TransportObservation
+
+/**
+ * Decides when a run of renderer transport changes is a *flap* — the renderer
+ * repeatedly failing to settle — rather than an ordinary track transition.
+ *
+ * The operator reports the Sonos "play pause play pause, like someone pressing
+ * it every half second", usually as a track starts. No diagnostic event could
+ * see it, so it has been described several times and measured never. This is
+ * the rule that decides when an episode is worth writing down.
+ *
+ * Pure decision state, like [com.fabledsword.minstrel.player.RemoteStallWatchdog]:
+ * the caller owns the flow and the recording, this only answers "is this an
+ * episode, and which readings make it up". Keeps the windowing and the
+ * one-episode-one-summary rule testable without a renderer or a clock.
+ */
+class TransportFlapDetector(
+    private val windowMs: Long = FLAP_WINDOW_MS,
+    private val minChanges: Int = FLAP_MIN_CHANGES,
+    private val summaryCooldownMs: Long = FLAP_SUMMARY_COOLDOWN_MS,
+) {
+    private val recent = ArrayDeque<TransportObservation>()
+    private var lastSummaryAtMs: Long? = null
+
+    /**
+     * Feed one transport change. Returns the readings making up an episode
+     * worth recording, or null when there is nothing to say.
+     *
+     * The returned list is a copy: the caller may hold it while more readings
+     * arrive.
+     */
+    fun onChange(observation: TransportObservation): List<TransportObservation>? {
+        recent.addLast(observation)
+        while (recent.isNotEmpty() &&
+            observation.atElapsedMs - recent.first().atElapsedMs > windowMs
+        ) {
+            recent.removeFirst()
+        }
+        if (recent.size < minChanges) return null
+        // One episode, one summary. A sustained fault would otherwise emit a
+        // summary per reading and bury the per-change events underneath them.
+        val since = lastSummaryAtMs
+        if (since != null && observation.atElapsedMs - since < summaryCooldownMs) return null
+        lastSummaryAtMs = observation.atElapsedMs
+        return recent.toList()
+    }
+
+    /** Forget everything — call when the route changes or casting ends. */
+    fun reset() {
+        recent.clear()
+        lastSummaryAtMs = null
+    }
+
+    companion object {
+        // Readings arrive at the 1 Hz poll cadence, and a normal track
+        // transition is 2-3 changes (PLAYING -> TRANSITIONING -> PLAYING).
+        // Four inside six seconds is not a track change, and it is not a
+        // person at the Sonos app either; it is the renderer not settling.
+        const val FLAP_WINDOW_MS = 6_000L
+        const val FLAP_MIN_CHANGES = 4
+        const val FLAP_SUMMARY_COOLDOWN_MS = 60_000L
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/player/MinstrelForwardingPlayer.kt

@@ -14,6 +14,7 @@ import com.fabledsword.minstrel.connectivity.NetworkStatusController
 import com.fabledsword.minstrel.connectivity.ServerHealth
 import com.fabledsword.minstrel.player.output.ActiveUpnp
 import com.fabledsword.minstrel.player.output.ActiveUpnpHolder
+import com.fabledsword.minstrel.player.output.upnp.PositionInfo
 import com.fabledsword.minstrel.player.output.upnp.SoapFaultException
 import com.fabledsword.minstrel.player.output.upnp.TransportInfo
 import com.fabledsword.minstrel.player.output.upnp.TransportState
@@ -87,6 +88,11 @@ class MinstrelForwardingPlayer(
         val onStalled: (trackId: String) -> Unit = {},
         /** The renderer's queue is short of ours and needs rebuilding. */
         val onQueueTruncated: () -> Unit = {},
+        /**
+         * A raw poll reading, emitted only when it differs from the previous
+         * one. Diagnostics-only; see [TransportObservation].
+         */
+        val onTransport: (TransportObservation) -> Unit = {},
     )
 
     private val scope = CoroutineScope(SupervisorJob() + Dispatchers.IO)
@@ -117,6 +123,10 @@ class MinstrelForwardingPlayer(
     @Volatile private var cachedNrTracks: Int = 0
     @Volatile private var lastMediaInfoAtMs: Long = 0L
 
+    // Previous raw transport reading, so [TransportObservation]s are emitted
+    // on change rather than once a second forever. Null until the first poll.
+    @Volatile private var lastObservedTransport: Pair<TransportState, Boolean>? = null
+
     // Wake channel for the poll loop. requestImmediatePoll() trySend's a Unit;
     // pollLoop's select{} races the delay against this channel so the next
     // pollOnce can fire immediately instead of waiting up to POLL_INTERVAL_MS.
@@ -508,17 +518,35 @@ class MinstrelForwardingPlayer(
             // without this the radio power-saves on a locked screen and the
             // poll below starves -- see [CastNetworkLock].
             castNetworkLock.acquire()
-            // Pause the wrapped ExoPlayer so we are not playing local audio
-            // simultaneously with the remote renderer. handler.post targets the
-            // application looper, so this runs on the same thread that processes
-            // our override calls -- no race with the pause() override branching
-            // to SOAP (holder.active is already non-null by the time this post
-            // fires, but delegate.pause() bypasses the override entirely).
-            handler.post { delegate.pause() }
+            // STOP the wrapped ExoPlayer -- not pause. pause() is only
+            // playWhenReady=false: ExoPlayer's LoadControl keeps loading, so a
+            // paused-but-prepared player goes on downloading the current track
+            // (~50s of buffer). During a cast that means the phone pulls the
+            // same file the renderer is streaming, over the same WiFi, and
+            // re-arms on every track change via syncLocalCursorToRemote's
+            // seekTo. At FLAC bitrates that is a second full-rate download
+            // competing with the speaker for air, starting exactly when a new
+            // track does. stop() ends the loading; Media3 keeps the media
+            // items, the current index and the position, so cursor sync and
+            // the handoff back are unaffected, and getPlaybackState() already
+            // reports STATE_READY while remote so no external reader sees IDLE.
+            //
+            // handler.post targets the application looper, so this runs on the
+            // same thread that processes our override calls -- no race with the
+            // pause() override branching to SOAP (holder.active is already
+            // non-null by the time this post fires, but delegate.stop()
+            // bypasses the override entirely).
+            handler.post { delegate.stop() }
             pollJob = scope.launch { pollLoop(active) }
         } else {
             castNetworkLock.release()
             remoteState.reset()
+            lastObservedTransport = null
+            // The delegate was stopped for the cast, so it is IDLE and would
+            // ignore a play(). Re-prepare it for local playback. Safe when the
+            // queue is empty, and it does not start playback on its own --
+            // playWhenReady is still false until something calls play().
+            handler.post { delegate.prepare() }
             // The next cast starts with a clean attempt budget; a stall on the
             // route we just left says nothing about the next one.
             stallWatchdog.reset()
@@ -614,6 +642,7 @@ class MinstrelForwardingPlayer(
             }
             TransportState.TRANSITIONING, TransportState.UNKNOWN -> Unit
         }
+        observeTransport(transport, info)
         checkForStall(active, info.trackUri, transport)
         notifyRemoteStateChanged()
     }
@@ -685,6 +714,31 @@ class MinstrelForwardingPlayer(
         }
     }
 
+    /**
+     * Publish this poll's raw transport reading if it differs from the last.
+     *
+     * Change-gated on purpose: steady playback is one reading repeated once a
+     * second, which is worth nothing and would fill the ring buffer. What is
+     * worth capturing is the renderer LEAVING a state — which during normal
+     * playback happens a couple of times per track, and during the fault the
+     * operator describes should happen repeatedly within a few seconds.
+     */
+    private fun observeTransport(transport: TransportInfo, info: PositionInfo) {
+        val key = transport.state to transport.statusOk
+        if (key == lastObservedTransport) return
+        lastObservedTransport = key
+        events.onTransport(
+            TransportObservation(
+                state = transport.state.name,
+                statusOk = transport.statusOk,
+                trackNumber = info.track,
+                positionMs = info.relTimeMs,
+                playIntent = remoteState.lastPlayIntent,
+                atElapsedMs = SystemClock.elapsedRealtime(),
+            ),
+        )
+    }
+
     /**
      * How the renderer's queue compares to ours, for [RemoteStallWatchdog].
      *

android/app/src/main/java/com/fabledsword/minstrel/player/PlayerController.kt

@@ -77,6 +77,13 @@ class PlayerController @Inject constructor(
      * during UPnP playback shows "Disconnected from <name>" to the user.
      */
     val dropEvents: SharedFlow<String> = playerFactory.dropEvents
+
+    /**
+     * Raw UPnP transport readings from [PlayerFactory.transportEvents], for
+     * the diagnostics reporter. Read-only tap — nothing in the playback path
+     * consumes it.
+     */
+    val transportEvents: SharedFlow<TransportObservation> = playerFactory.transportEvents
     private val sessionToken =
         SessionToken(context, ComponentName(context, MinstrelPlayerService::class.java))
 

android/app/src/main/java/com/fabledsword/minstrel/player/PlayerFactory.kt

@@ -95,6 +95,18 @@ class PlayerFactory @Inject constructor(
     )
     val queueRepairEvents: SharedFlow<Unit> = queueRepairInternal.asSharedFlow()
 
+    // Raw renderer transport readings, change-gated. Unlike the flows above
+    // this one carries a SEQUENCE — the diagnostics flap detector needs
+    // several readings in a row to tell oscillation from a normal track
+    // transition — so it buffers more than one and drops oldest under
+    // pressure rather than collapsing to the latest.
+    private val transportInternal = MutableSharedFlow<TransportObservation>(
+        replay = 0,
+        extraBufferCapacity = TRANSPORT_EVENT_BUFFER,
+        onBufferOverflow = BufferOverflow.DROP_OLDEST,
+    )
+    val transportEvents: SharedFlow<TransportObservation> = transportInternal.asSharedFlow()
+
     fun build(): Player {
         val exo = buildExoPlayer()
         return MinstrelForwardingPlayer(
@@ -107,6 +119,7 @@ class PlayerFactory @Inject constructor(
                 onDrop = { name -> emitDrop(name) },
                 onStalled = { trackId -> stallEventsInternal.tryEmit(trackId) },
                 onQueueTruncated = { queueRepairInternal.tryEmit(Unit) },
+                onTransport = { transportInternal.tryEmit(it) },
             ),
         )
     }
@@ -160,6 +173,12 @@ class PlayerFactory @Inject constructor(
                 .build(),
         )
 
+    private companion object {
+        // Enough readings to hold a whole flap episode plus the normal
+        // transitions around it; the detector's window is only a few seconds.
+        const val TRANSPORT_EVENT_BUFFER = 32
+    }
+
     private fun emitDrop(routeName: String) {
         dropEventsInternal.tryEmit(routeName)
     }

android/app/src/main/java/com/fabledsword/minstrel/player/TransportObservation.kt

@@ -0,0 +1,38 @@
+package com.fabledsword.minstrel.player
+
+/**
+ * One reading of what a UPnP renderer says it is doing, taken by the poll
+ * loop and emitted only when it differs from the previous reading.
+ *
+ * Exists for diagnostics. The operator reports the Sonos rapidly
+ * play-pause-play-pausing at the start of a track, and nothing in the
+ * diagnostics could see it: `player_state` records source / loading / error
+ * but not whether we are playing, `track_change` needs the queue index to
+ * move, and the heartbeat samples once every 45 seconds. A symptom that
+ * lasts a few seconds and changes no index fell straight through all three.
+ *
+ * This is the closest observation point we have to the renderer's own truth
+ * — the raw GetTransportInfo reading, before the two-poll confirmation and
+ * the UI's smoothing have had a chance to hide the wobble.
+ *
+ * **It samples at the poll cadence (1 Hz).** If the real oscillation is
+ * faster than that, what lands here is an aliased jagged sequence rather
+ * than the true waveform. That still answers the question that matters —
+ * whether the renderer is steadily PLAYING or repeatedly leaving that state
+ * — but it cannot measure the true period. If a captured episode comes back
+ * looking clean, the next instrument is burst sampling, not this one.
+ */
+data class TransportObservation(
+    /** [com.fabledsword.minstrel.player.output.upnp.TransportState] name. */
+    val state: String,
+    /** CurrentTransportStatus: false means the renderer reports an error. */
+    val statusOk: Boolean,
+    /** The renderer's 1-based queue position at this reading. */
+    val trackNumber: Int,
+    /** The renderer's reported position within the track. */
+    val positionMs: Long,
+    /** Whether the operator's last intent was to be playing. */
+    val playIntent: Boolean,
+    /** Monotonic stamp, so a consumer can measure gaps between readings. */
+    val atElapsedMs: Long,
+)

android/app/src/test/java/com/fabledsword/minstrel/diagnostics/TransportFlapDetectorTest.kt

@@ -0,0 +1,128 @@
+package com.fabledsword.minstrel.diagnostics
+
+import com.fabledsword.minstrel.player.TransportObservation
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertNull
+
+/**
+ * The rule deciding when renderer transport changes are worth recording as an
+ * episode. Worth pinning because both failure directions are costly: too eager
+ * and every track transition writes a summary that buries the real one, too
+ * shy and the operator's stutter goes unmeasured for another month.
+ */
+class TransportFlapDetectorTest {
+
+    private fun obs(state: String, atMs: Long, track: Int = 1, posMs: Long = 0L) =
+        TransportObservation(
+            state = state,
+            statusOk = true,
+            trackNumber = track,
+            positionMs = posMs,
+            playIntent = true,
+            atElapsedMs = atMs,
+        )
+
+    /**
+     * PLAYING -> TRANSITIONING -> PLAYING is what a queue advance looks like.
+     * It happens on every single track and must never be recorded as a fault.
+     */
+    @Test
+    fun `an ordinary track transition is not an episode`() {
+        val d = TransportFlapDetector()
+        assertNull(d.onChange(obs("PLAYING", 0)))
+        assertNull(d.onChange(obs("TRANSITIONING", 1_000)))
+        assertNull(d.onChange(obs("PLAYING", 2_000)))
+    }
+
+    @Test
+    fun `four changes inside the window is an episode`() {
+        val d = TransportFlapDetector()
+        d.onChange(obs("PLAYING", 0))
+        d.onChange(obs("STOPPED", 500))
+        d.onChange(obs("PLAYING", 1_000))
+        // assertNotNull returns the value, so the asserts below need no cast.
+        val episode = assertNotNull(d.onChange(obs("STOPPED", 1_500)))
+        assertEquals(4, episode.size)
+        assertEquals(
+            listOf("PLAYING", "STOPPED", "PLAYING", "STOPPED"),
+            episode.map { it.state },
+        )
+    }
+
+    /**
+     * Changes spread thinly are normal listening — a few track advances over
+     * a couple of minutes must not accumulate into a false episode.
+     */
+    @Test
+    fun `changes spread beyond the window never accumulate`() {
+        val d = TransportFlapDetector()
+        repeat(20) { i ->
+            assertNull(d.onChange(obs("PLAYING", i * 10_000L)))
+        }
+    }
+
+    /** The window slides: old readings age out rather than counting forever. */
+    @Test
+    fun `readings older than the window are dropped`() {
+        val d = TransportFlapDetector()
+        d.onChange(obs("PLAYING", 0))
+        d.onChange(obs("STOPPED", 1_000))
+        // Long gap — the two above are now stale.
+        assertNull(d.onChange(obs("PLAYING", 30_000)))
+        assertNull(d.onChange(obs("STOPPED", 30_500)))
+        // Only three fresh readings so far.
+        assertNull(d.onChange(obs("PLAYING", 31_000)))
+        assertNotNull(d.onChange(obs("STOPPED", 31_500)))
+    }
+
+    /**
+     * A fault that persists produces a change every poll. Without the cooldown
+     * every one of them would write a summary, which is exactly the noise that
+     * makes a diagnostics dump unreadable.
+     */
+    @Test
+    fun `a sustained fault reports one episode, not one per reading`() {
+        val d = TransportFlapDetector()
+        var episodes = 0
+        repeat(40) { i ->
+            if (d.onChange(obs(if (i % 2 == 0) "PLAYING" else "STOPPED", i * 500L)) != null) {
+                episodes++
+            }
+        }
+        assertEquals(1, episodes)
+    }
+
+    /** Past the cooldown, a fresh episode is worth recording again. */
+    @Test
+    fun `a later episode reports again once the cooldown has passed`() {
+        val d = TransportFlapDetector()
+        repeat(4) { d.onChange(obs("PLAYING", it * 500L)) }
+        val second = (0 until 4).map { d.onChange(obs("STOPPED", 90_000 + it * 500L)) }
+        assertEquals(1, second.count { it != null })
+    }
+
+    @Test
+    fun `reset forgets the window and the cooldown`() {
+        val d = TransportFlapDetector()
+        repeat(4) { d.onChange(obs("PLAYING", it * 500L)) }
+        d.reset()
+        repeat(3) { d.onChange(obs("PLAYING", 3_000 + it * 500L)) }
+        // A 4th change after reset is a new episode, cooldown notwithstanding.
+        assertNotNull(d.onChange(obs("STOPPED", 5_000)))
+    }
+
+    /** The episode is a snapshot — later readings must not mutate it. */
+    @Test
+    fun `a returned episode is not mutated by later readings`() {
+        val d = TransportFlapDetector()
+        d.onChange(obs("PLAYING", 0))
+        d.onChange(obs("STOPPED", 500))
+        d.onChange(obs("PLAYING", 1_000))
+        val episode = assertNotNull(d.onChange(obs("STOPPED", 1_500)))
+        val sizeAtCapture = episode.size
+        repeat(5) { d.onChange(obs("PLAYING", 2_000 + it * 500L)) }
+        assertEquals(sizeAtCapture, episode.size)
+    }
+}