refactor(player): split Sonos queue loading out of the picker — #2728

detekt flagged OutputPickerController as LargeClass once the verify
path landed. Extracting rather than suppressing: how the renderer's
queue is shaped is a different concern from which route is selected,
and it had grown big enough to hide a bug — every write in here is a
SOAP call that can fail on its own, and nothing ever read the result
back.

SonosQueueLoader now owns load / extend / verify / append and the
incremental diff. The picker keeps route selection and asks it for
queue work. No behaviour change.

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

@@ -10,11 +10,9 @@ import com.fabledsword.minstrel.models.TrackRef
 import com.fabledsword.minstrel.player.PlayerController
 import com.fabledsword.minstrel.player.PlayerFactory
 import com.fabledsword.minstrel.player.RemotePlayerState
-import com.fabledsword.minstrel.player.StreamTokenProvider
 import com.fabledsword.minstrel.player.output.upnp.AVTransportClient
 import com.fabledsword.minstrel.player.output.upnp.RenderingControlClient
 import com.fabledsword.minstrel.player.output.upnp.SoapClient
-import com.fabledsword.minstrel.player.output.upnp.SoapFaultException
 import com.fabledsword.minstrel.player.output.upnp.TransportState
 import com.fabledsword.minstrel.player.output.upnp.UpnpDiscoveryController
 import com.fabledsword.minstrel.player.output.upnp.bareUdn
@@ -61,7 +59,7 @@ data class RouteSnapshot(
  *  - [OutputRoute.Protocol.SYSTEM] — MediaRouter.selectRoute (built-in,
  *    wired, Bluetooth)
  *  - [OutputRoute.Protocol.UPNP] — mint a signed stream token via
- *    [StreamTokenProvider.mint], drive the discovered renderer with
+ *    [SonosQueueLoader], drive the discovered renderer with
  *    AVTransport.SetAVTransportURI + Play, pause local playback so
  *    audio yields to the network speaker
  *  - [OutputRoute.Protocol.CAST] / [OutputRoute.Protocol.SONOS] —
@@ -79,7 +77,7 @@ class OutputPickerController @Inject constructor(
     private val upnpDiscovery: UpnpDiscoveryController,
     private val playerController: PlayerController,
     private val playerFactory: PlayerFactory,
-    private val streamTokens: StreamTokenProvider,
+    private val sonosQueue: SonosQueueLoader,
     private val activeUpnpHolder: ActiveUpnpHolder,
     private val remoteState: RemotePlayerState,
     private val okHttp: OkHttpClient,
@@ -256,7 +254,7 @@ class OutputPickerController @Inject constructor(
      * setMediaItems override clears holder.active + sets target so the
      * imminent play() call drops (drops via isLoadingUpnp() = true). Then
      * this collector observes the uiState.queue change and re-runs
-     * loadQueueOnSonos to push the new tracks to Sonos.
+     * SonosQueueLoader.load to push the new tracks to Sonos.
      *
      * Discrimination: selectUpnp's initial-load path doesn't change
      * uiState.queue (the queue was already populated before route
@@ -291,8 +289,8 @@ class OutputPickerController @Inject constructor(
      * Rebuild the renderer's queue when playback stopped because the renderer
      * ran off the end of a queue shorter than ours.
      *
-     * [extendQueueOnSonos] tolerates individual AddURIToQueue failures and
+     * [SonosQueueLoader] tolerates individual AddURIToQueue failures and
-     * gives up entirely after [EXTEND_ABORT_AFTER_FAILURES] consecutive ones
+     * gives up appending after a few consecutive ones
      * -- Sonos rate-limits burst adds. Until this existed that left a short
      * queue on the renderer and nothing to notice it: the renderer played what
      * it had and stopped, and the app went on believing there were forty
@@ -301,7 +299,7 @@ class OutputPickerController @Inject constructor(
      *
      * A full reload, not an incremental diff: the renderer's copy is known to
      * be wrong, and the diff path reasons from what we *think* it holds, which
-     * is exactly the assumption that failed. loadQueueOnSonos re-seeks to the
+     * is exactly the assumption that failed. The load re-seeks to the
      * current track and plays, so recovery lands where the listener was.
      */
     private suspend fun observeQueueRepairRequests() {
@@ -343,7 +341,7 @@ class OutputPickerController @Inject constructor(
             queue.size, outputRoute.name, currentIndex,
         )
         runCatching {
-            loadQueueOnSonos(transport, outputRoute, queue, currentIndex)
+            sonosQueue.load(transport, outputRoute, queue, currentIndex)
         }.onFailure { e ->
             // Leave the route active: the renderer is reachable enough to have
             // told us its queue length, so dropping to local would be a harsher
@@ -379,7 +377,7 @@ class OutputPickerController @Inject constructor(
             return@withLock
         }
         val handledIncrementally = runCatching {
-            tryIncrementalResync(transport, oldIds, newQueue)
+            sonosQueue.tryIncrementalResync(transport, oldIds, newQueue)
         }.getOrElse { e ->
             Timber.w(e, "Sonos incremental resync errored; falling back to full reload")
             false
@@ -403,7 +401,7 @@ class OutputPickerController @Inject constructor(
         val rendering = renderingClientFor(routeId)
         Timber.w("Sonos resync: full reload of %d tracks on %s", newQueue.size, outputRoute.name)
         runCatching {
-            loadQueueOnSonos(transport, outputRoute, newQueue, newCurrentIndex)
+            sonosQueue.load(transport, outputRoute, newQueue, newCurrentIndex)
             activeUpnpHolder.set(
                 ActiveUpnp(
                     routeId = routeId,
@@ -420,98 +418,6 @@ class OutputPickerController @Inject constructor(
         }
     }
 
-    /**
-     * Diff-based incremental Sonos queue sync. Returns true when the new
-     * queue can be produced from the old one with a remove-then-insert at
-     * the same middle slice -- the common-prefix and common-suffix portions
-     * stay untouched, and the current Sonos track must lie in the preserved
-     * prefix (otherwise the diff would orphan playback). Returns false to
-     * signal the caller to fall back to a full reload.
-     */
-    private suspend fun tryIncrementalResync(
-        transport: AVTransportClient,
-        oldIds: List<String>,
-        newQueue: List<TrackRef>,
-    ): Boolean {
-        val newIds = newQueue.map { it.id }
-        if (oldIds == newIds) return true
-        val prefixLen = commonPrefixLength(oldIds, newIds)
-        val suffixLen = commonSuffixLength(
-            oldIds.subList(prefixLen, oldIds.size),
-            newIds.subList(prefixLen, newIds.size),
-        )
-        val removedCount = oldIds.size - prefixLen - suffixLen
-        val addedCount = newIds.size - prefixLen - suffixLen
-        // Sonos's current track number is 1-based; compare against the
-        // preserved-prefix range as 0-based. If the current track is in
-        // the removed slice, incremental can't preserve playback -- caller
-        // falls back to full rebuild.
-        val currentSonosIdx0 = remoteState.trackNumber - 1
-        val canApply = currentSonosIdx0 in 0 until prefixLen
-        if (canApply) {
-            applyQueueDiff(transport, newQueue, prefixLen, removedCount, addedCount)
-        } else {
-            Timber.w(
-                "Sonos incremental: current track %d not in preserved prefix [0,%d); full rebuild",
-                currentSonosIdx0,
-                prefixLen,
-            )
-        }
-        return canApply
-    }
-
-    private suspend fun applyQueueDiff(
-        transport: AVTransportClient,
-        newQueue: List<TrackRef>,
-        prefixLen: Int,
-        removedCount: Int,
-        addedCount: Int,
-    ) {
-        if (removedCount > 0) {
-            Timber.w(
-                "Sonos incremental: RemoveTrackRangeFromQueue start=%d count=%d",
-                prefixLen + 1,
-                removedCount,
-            )
-            transport.removeTrackRangeFromQueue(
-                startingIndex = prefixLen + 1,
-                numberOfTracks = removedCount,
-            )
-        }
-        if (addedCount == 0) return
-        Timber.w(
-            "Sonos incremental: AddURIToQueue x%d starting at position %d",
-            addedCount,
-            prefixLen + 1,
-        )
-        for (i in 0 until addedCount) {
-            val ref = newQueue[prefixLen + i]
-            val token = streamTokens.mint(ref.id)
-            transport.addURIToQueue(
-                uri = token.url,
-                mime = token.mime,
-                title = token.title,
-                enqueuedURIPosition = prefixLen + i + 1,
-            )
-            if (i > 0) delay(EXTEND_THROTTLE_MS)
-        }
-    }
-
-    private fun commonPrefixLength(a: List<String>, b: List<String>): Int {
-        val limit = minOf(a.size, b.size)
-        for (i in 0 until limit) {
-            if (a[i] != b[i]) return i
-        }
-        return limit
-    }
-
-    private fun commonSuffixLength(a: List<String>, b: List<String>): Int {
-        val limit = minOf(a.size, b.size)
-        for (i in 0 until limit) {
-            if (a[a.size - 1 - i] != b[b.size - 1 - i]) return i
-        }
-        return limit
-    }
 
     /**
      * Called when the active UPnP route drops unexpectedly (the poll loop's
@@ -605,7 +511,7 @@ class OutputPickerController @Inject constructor(
      *  1. Pause local so the user doesn't keep hearing local audio.
      *  2. Set target early so ForwardingPlayer drops transport taps
      *     while the 17-second queue load is in progress.
-     *  3. Wire active LAST (after loadQueueOnSonos) so SOAP commands
+     *  3. Wire active LAST (after the queue load) so SOAP commands
      *     are never routed to a half-loaded Sonos queue.
      */
     private suspend fun selectUpnp(route: OutputRoute) = selectUpnpMutex.withLock {
@@ -646,7 +552,7 @@ class OutputPickerController @Inject constructor(
         // taps don't hit Sonos's stale state from a prior session.
         activeUpnpHolder.setTarget(effectiveRoute.id)
         runCatching {
-            loadQueueOnSonos(transport, effectiveRoute, uiState.queue, uiState.queueIndex)
+            sonosQueue.load(transport, effectiveRoute, uiState.queue, uiState.queueIndex)
             // Wire active LAST -- SOAP path is now safe to use.
             activeUpnpHolder.set(
                 ActiveUpnp(
@@ -763,179 +669,6 @@ class OutputPickerController @Inject constructor(
         return if (i >= 0) segments.getOrNull(i + 1) else null
     }
 
-    private suspend fun loadQueueOnSonos(
-        transport: AVTransportClient,
-        route: OutputRoute,
-        queue: List<TrackRef>,
-        currentIndex: Int,
-    ) {
-        Timber.w("UPnP select: clear queue on %s", route.name)
-        transport.removeAllTracksFromQueue()
-        val initialEnd = (currentIndex + 1).coerceAtMost(queue.size)
-        val initialBatch = queue.subList(0, initialEnd)
-        Timber.w(
-            "UPnP select: add %d initial tracks (currentIndex=%d, totalQueue=%d)",
-            initialBatch.size, currentIndex, queue.size,
-        )
-        initialBatch.forEachIndexed { idx, ref ->
-            val token = streamTokens.mint(ref.id)
-            transport.addURIToQueue(
-                uri = token.url,
-                mime = token.mime,
-                title = token.title,
-                enqueuedURIPosition = idx + 1,
-            )
-        }
-        val coordinatorUdn = route.id.bareUdn()
-        val queueUri = "x-rincon-queue:$coordinatorUdn#0"
-        Timber.w("UPnP select: SetAVTransportURI %s", queueUri)
-        transport.setAVTransportURI(queueUri, "")
-        Timber.w("UPnP select: Seek to track %d", currentIndex + 1)
-        transport.seekToTrack(currentIndex + 1)
-        Timber.w("UPnP select: Play")
-        transport.play()
-        Timber.w("UPnP select: initial done; backgrounding remainder")
-        val remaining = queue.drop(initialEnd)
-        // Verify even when there is no tail to append: the initial batch is
-        // sent the same way and can be dropped the same way.
-        scope.launch {
-            if (remaining.isNotEmpty()) {
-                extendQueueOnSonos(transport, route, remaining, initialEnd)
-            }
-            verifyQueueLength(transport, route, queue)
-        }
-    }
-
-    /**
-     * Background-append tracks after activation. Runs concurrently with Sonos
-     * playback. Cancels if the user disconnects from this route (active.routeId
-     * changes or becomes null). Correctness of the result is [verifyQueueLength]'s
-     * job, not this function's.
-     */
-    private suspend fun extendQueueOnSonos(
-        transport: AVTransportClient,
-        route: OutputRoute,
-        tracks: List<TrackRef>,
-        startPosition: Int,
-    ) {
-        Timber.w(
-            "UPnP extend: appending %d tracks starting at position %d",
-            tracks.size, startPosition + 1,
-        )
-        val succeeded = appendTracksToQueue(transport, route, tracks, startPosition)
-        Timber.w("UPnP extend: done (%d / %d appended)", succeeded, tracks.size)
-    }
-
-    /**
-     * Confirm the renderer holds as many tracks as we sent, and append the
-     * tail it dropped.
-     *
-     * [appendTracksToQueue] tolerates individual failures and gives up after
-     * [EXTEND_ABORT_AFTER_FAILURES] consecutive ones, because Sonos rate-limits
-     * burst adds (logcat 2026-06-04: 33 consecutive failures once offset 39 was
-     * reached). That is the right call in the moment — some tracks loaded beats
-     * none — but it used to be the end of the story, and the renderer was left
-     * holding a queue shorter than ours with nothing aware of it. It then
-     * played what it had and stopped, which looked exactly like playback dying
-     * for no reason.
-     *
-     * Sonos appends sequentially, so a short queue means a missing tail: taking
-     * `fullQueue.drop(nrTracks)` is the gap. Bounded at [VERIFY_ROUNDS] passes
-     * so a renderer that refuses to grow can't spin here forever.
-     */
-    @Suppress("ReturnCount") // each bail-out is a distinct reason to stop verifying
-    private suspend fun verifyQueueLength(
-        transport: AVTransportClient,
-        route: OutputRoute,
-        fullQueue: List<TrackRef>,
-    ) {
-        repeat(VERIFY_ROUNDS) { round ->
-            if (activeUpnpHolder.active.value?.routeId != route.id) return
-            val nrTracks = runCatching { transport.getMediaInfo().nrTracks }
-                .getOrElse { e ->
-                    Timber.w(e, "UPnP verify: GetMediaInfo failed on %s", route.name)
-                    return
-                }
-            // 0 means the renderer told us nothing usable, not that its queue
-            // is empty. Guessing "empty" here would re-send the whole queue to
-            // a renderer that is playing it perfectly well.
-            if (nrTracks <= 0) {
-                Timber.w("UPnP verify: no usable NrTracks from %s; skipping", route.name)
-                return
-            }
-            if (nrTracks >= fullQueue.size) {
-                Timber.w("UPnP verify: renderer holds %d tracks, queue intact", nrTracks)
-                return
-            }
-            val missing = fullQueue.drop(nrTracks)
-            Timber.w(
-                "UPnP verify: %s holds %d of %d tracks; appending %d missing (round %d)",
-                route.name, nrTracks, fullQueue.size, missing.size, round + 1,
-            )
-            appendTracksToQueue(transport, route, missing, nrTracks)
-        }
-        Timber.w("UPnP verify: gave up repairing queue length on %s", route.name)
-    }
-
-    /**
-     * Append [tracks] at [startPosition] (0-based), returning how many landed.
-     * Tolerates individual AddURIToQueue failures — log and continue so some
-     * tracks loaded is better than zero tracks loaded — and stops early after
-     * [EXTEND_ABORT_AFTER_FAILURES] consecutive ones.
-     */
-    private suspend fun appendTracksToQueue(
-        transport: AVTransportClient,
-        route: OutputRoute,
-        tracks: List<TrackRef>,
-        startPosition: Int,
-    ): Int {
-        var consecutiveFailures = 0
-        var succeeded = 0
-        var aborted = false
-        for ((i, ref) in tracks.withIndex()) {
-            if (aborted) break
-            if (activeUpnpHolder.active.value?.routeId != route.id) {
-                Timber.w("UPnP extend: cancelled at offset %d (route changed)", i)
-                aborted = true
-            } else {
-                val outcome = runCatching {
-                    val token = streamTokens.mint(ref.id)
-                    transport.addURIToQueue(
-                        uri = token.url,
-                        mime = token.mime,
-                        title = token.title,
-                        enqueuedURIPosition = startPosition + i + 1,
-                    )
-                }
-                if (outcome.isSuccess) {
-                    consecutiveFailures = 0
-                    succeeded += 1
-                    // Throttle the burst so we don't tickle Sonos's burst-add
-                    // rejection -- logcat 2026-06-04 showed 33 consecutive
-                    // failures clustered at ~10ms intervals once offset 39 was
-                    // reached, which looks like a rate-limit kicking in. The
-                    // delay is small enough that extending 100 tracks adds
-                    // only ~5s to background work that's already async.
-                    delay(EXTEND_THROTTLE_MS)
-                } else {
-                    consecutiveFailures += 1
-                    val e = outcome.exceptionOrNull()
-                    val detail = (e as? SoapFaultException)?.let {
-                        "code=${it.code} desc=${it.description}"
-                    } ?: e?.message
-                    Timber.w(e, "UPnP extend: append failed at offset %d -- %s", i, detail)
-                    if (consecutiveFailures >= EXTEND_ABORT_AFTER_FAILURES) {
-                        Timber.w(
-                            "UPnP extend: aborting after %d consecutive failures",
-                            consecutiveFailures,
-                        )
-                        aborted = true
-                    }
-                }
-            }
-        }
-        return succeeded
-    }
 
     private fun renderingClientFor(routeId: String): RenderingControlClient? {
         val rcUrl = upnpDiscovery.routes.value
@@ -967,16 +700,6 @@ class OutputPickerController @Inject constructor(
     }
 
     private companion object {
-        const val EXTEND_ABORT_AFTER_FAILURES = 3
-        const val EXTEND_THROTTLE_MS = 50L
-
-        // Verify/repair passes after a queue load. Two: one to catch the
-        // common case (a rate-limit burst dropped a chunk), one to catch a
-        // repair that itself got rate-limited. Beyond that the renderer is
-        // refusing for a reason retrying won't fix, and the stall watchdog
-        // becomes the backstop.
-        const val VERIFY_ROUNDS = 2
-
         // 5 minutes of continuous non-playing on a UPnP route before we
         // revert to the phone speaker, so a stale Sonos selection can't make
         // a later "tap play" do nothing.

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

@@ -0,0 +1,316 @@
+package com.fabledsword.minstrel.player.output
+
+import com.fabledsword.minstrel.di.ApplicationScope
+import com.fabledsword.minstrel.models.TrackRef
+import com.fabledsword.minstrel.player.RemotePlayerState
+import com.fabledsword.minstrel.player.StreamTokenProvider
+import com.fabledsword.minstrel.player.output.upnp.AVTransportClient
+import com.fabledsword.minstrel.player.output.upnp.SoapFaultException
+import com.fabledsword.minstrel.player.output.upnp.bareUdn
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+import timber.log.Timber
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Owns the shape of a Sonos renderer's native queue: loading it, growing it,
+ * diffing it against local queue mutations, and — the part that was missing —
+ * confirming the renderer actually took what we sent.
+ *
+ * Split out of [OutputPickerController], which is about *which route is
+ * selected*. How many tracks the renderer is holding is a separate concern
+ * with its own failure modes, and it had grown large enough to hide one:
+ * every write here is a SOAP call that can fail individually, and until
+ * [verifyQueueLength] nothing ever read the result back.
+ */
+@Singleton
+class SonosQueueLoader @Inject constructor(
+    @ApplicationScope private val scope: CoroutineScope,
+    private val streamTokens: StreamTokenProvider,
+    private val activeUpnpHolder: ActiveUpnpHolder,
+    private val remoteState: RemotePlayerState,
+) {
+    suspend fun load(
+        transport: AVTransportClient,
+        route: OutputRoute,
+        queue: List<TrackRef>,
+        currentIndex: Int,
+    ) {
+        Timber.w("UPnP select: clear queue on %s", route.name)
+        transport.removeAllTracksFromQueue()
+        val initialEnd = (currentIndex + 1).coerceAtMost(queue.size)
+        val initialBatch = queue.subList(0, initialEnd)
+        Timber.w(
+            "UPnP select: add %d initial tracks (currentIndex=%d, totalQueue=%d)",
+            initialBatch.size, currentIndex, queue.size,
+        )
+        initialBatch.forEachIndexed { idx, ref ->
+            val token = streamTokens.mint(ref.id)
+            transport.addURIToQueue(
+                uri = token.url,
+                mime = token.mime,
+                title = token.title,
+                enqueuedURIPosition = idx + 1,
+            )
+        }
+        val coordinatorUdn = route.id.bareUdn()
+        val queueUri = "x-rincon-queue:$coordinatorUdn#0"
+        Timber.w("UPnP select: SetAVTransportURI %s", queueUri)
+        transport.setAVTransportURI(queueUri, "")
+        Timber.w("UPnP select: Seek to track %d", currentIndex + 1)
+        transport.seekToTrack(currentIndex + 1)
+        Timber.w("UPnP select: Play")
+        transport.play()
+        Timber.w("UPnP select: initial done; backgrounding remainder")
+        val remaining = queue.drop(initialEnd)
+        // Verify even when there is no tail to append: the initial batch is
+        // sent the same way and can be dropped the same way.
+        scope.launch {
+            if (remaining.isNotEmpty()) {
+                extendQueueOnSonos(transport, route, remaining, initialEnd)
+            }
+            verifyQueueLength(transport, route, queue)
+        }
+    }
+
+    /**
+     * Background-append tracks after activation. Runs concurrently with Sonos
+     * playback. Cancels if the user disconnects from this route (active.routeId
+     * changes or becomes null). Correctness of the result is [verifyQueueLength]'s
+     * job, not this function's.
+     */
+    private suspend fun extendQueueOnSonos(
+        transport: AVTransportClient,
+        route: OutputRoute,
+        tracks: List<TrackRef>,
+        startPosition: Int,
+    ) {
+        Timber.w(
+            "UPnP extend: appending %d tracks starting at position %d",
+            tracks.size, startPosition + 1,
+        )
+        val succeeded = appendTracksToQueue(transport, route, tracks, startPosition)
+        Timber.w("UPnP extend: done (%d / %d appended)", succeeded, tracks.size)
+    }
+
+    /**
+     * Confirm the renderer holds as many tracks as we sent, and append the
+     * tail it dropped.
+     *
+     * [appendTracksToQueue] tolerates individual failures and gives up after
+     * [EXTEND_ABORT_AFTER_FAILURES] consecutive ones, because Sonos rate-limits
+     * burst adds (logcat 2026-06-04: 33 consecutive failures once offset 39 was
+     * reached). That is the right call in the moment — some tracks loaded beats
+     * none — but it used to be the end of the story, and the renderer was left
+     * holding a queue shorter than ours with nothing aware of it. It then
+     * played what it had and stopped, which looked exactly like playback dying
+     * for no reason.
+     *
+     * Sonos appends sequentially, so a short queue means a missing tail: taking
+     * `fullQueue.drop(nrTracks)` is the gap. Bounded at [VERIFY_ROUNDS] passes
+     * so a renderer that refuses to grow can't spin here forever.
+     */
+    @Suppress("ReturnCount") // each bail-out is a distinct reason to stop verifying
+    private suspend fun verifyQueueLength(
+        transport: AVTransportClient,
+        route: OutputRoute,
+        fullQueue: List<TrackRef>,
+    ) {
+        repeat(VERIFY_ROUNDS) { round ->
+            if (activeUpnpHolder.active.value?.routeId != route.id) return
+            val nrTracks = runCatching { transport.getMediaInfo().nrTracks }
+                .getOrElse { e ->
+                    Timber.w(e, "UPnP verify: GetMediaInfo failed on %s", route.name)
+                    return
+                }
+            // 0 means the renderer told us nothing usable, not that its queue
+            // is empty. Guessing "empty" here would re-send the whole queue to
+            // a renderer that is playing it perfectly well.
+            if (nrTracks <= 0) {
+                Timber.w("UPnP verify: no usable NrTracks from %s; skipping", route.name)
+                return
+            }
+            if (nrTracks >= fullQueue.size) {
+                Timber.w("UPnP verify: renderer holds %d tracks, queue intact", nrTracks)
+                return
+            }
+            val missing = fullQueue.drop(nrTracks)
+            Timber.w(
+                "UPnP verify: %s holds %d of %d tracks; appending %d missing (round %d)",
+                route.name, nrTracks, fullQueue.size, missing.size, round + 1,
+            )
+            appendTracksToQueue(transport, route, missing, nrTracks)
+        }
+        Timber.w("UPnP verify: gave up repairing queue length on %s", route.name)
+    }
+
+    /**
+     * Append [tracks] at [startPosition] (0-based), returning how many landed.
+     * Tolerates individual AddURIToQueue failures — log and continue so some
+     * tracks loaded is better than zero tracks loaded — and stops early after
+     * [EXTEND_ABORT_AFTER_FAILURES] consecutive ones.
+     */
+    private suspend fun appendTracksToQueue(
+        transport: AVTransportClient,
+        route: OutputRoute,
+        tracks: List<TrackRef>,
+        startPosition: Int,
+    ): Int {
+        var consecutiveFailures = 0
+        var succeeded = 0
+        var aborted = false
+        for ((i, ref) in tracks.withIndex()) {
+            if (aborted) break
+            if (activeUpnpHolder.active.value?.routeId != route.id) {
+                Timber.w("UPnP extend: cancelled at offset %d (route changed)", i)
+                aborted = true
+            } else {
+                val outcome = runCatching {
+                    val token = streamTokens.mint(ref.id)
+                    transport.addURIToQueue(
+                        uri = token.url,
+                        mime = token.mime,
+                        title = token.title,
+                        enqueuedURIPosition = startPosition + i + 1,
+                    )
+                }
+                if (outcome.isSuccess) {
+                    consecutiveFailures = 0
+                    succeeded += 1
+                    // Throttle the burst so we don't tickle Sonos's burst-add
+                    // rejection -- logcat 2026-06-04 showed 33 consecutive
+                    // failures clustered at ~10ms intervals once offset 39 was
+                    // reached, which looks like a rate-limit kicking in. The
+                    // delay is small enough that extending 100 tracks adds
+                    // only ~5s to background work that's already async.
+                    delay(EXTEND_THROTTLE_MS)
+                } else {
+                    consecutiveFailures += 1
+                    val e = outcome.exceptionOrNull()
+                    val detail = (e as? SoapFaultException)?.let {
+                        "code=${it.code} desc=${it.description}"
+                    } ?: e?.message
+                    Timber.w(e, "UPnP extend: append failed at offset %d -- %s", i, detail)
+                    if (consecutiveFailures >= EXTEND_ABORT_AFTER_FAILURES) {
+                        Timber.w(
+                            "UPnP extend: aborting after %d consecutive failures",
+                            consecutiveFailures,
+                        )
+                        aborted = true
+                    }
+                }
+            }
+        }
+        return succeeded
+    }
+
+    /**
+     * Diff-based incremental Sonos queue sync. Returns true when the new
+     * queue can be produced from the old one with a remove-then-insert at
+     * the same middle slice -- the common-prefix and common-suffix portions
+     * stay untouched, and the current Sonos track must lie in the preserved
+     * prefix (otherwise the diff would orphan playback). Returns false to
+     * signal the caller to fall back to a full reload.
+     */
+    suspend fun tryIncrementalResync(
+        transport: AVTransportClient,
+        oldIds: List<String>,
+        newQueue: List<TrackRef>,
+    ): Boolean {
+        val newIds = newQueue.map { it.id }
+        if (oldIds == newIds) return true
+        val prefixLen = commonPrefixLength(oldIds, newIds)
+        val suffixLen = commonSuffixLength(
+            oldIds.subList(prefixLen, oldIds.size),
+            newIds.subList(prefixLen, newIds.size),
+        )
+        val removedCount = oldIds.size - prefixLen - suffixLen
+        val addedCount = newIds.size - prefixLen - suffixLen
+        // Sonos's current track number is 1-based; compare against the
+        // preserved-prefix range as 0-based. If the current track is in
+        // the removed slice, incremental can't preserve playback -- caller
+        // falls back to full rebuild.
+        val currentSonosIdx0 = remoteState.trackNumber - 1
+        val canApply = currentSonosIdx0 in 0 until prefixLen
+        if (canApply) {
+            applyQueueDiff(transport, newQueue, prefixLen, removedCount, addedCount)
+        } else {
+            Timber.w(
+                "Sonos incremental: current track %d not in preserved prefix [0,%d); full rebuild",
+                currentSonosIdx0,
+                prefixLen,
+            )
+        }
+        return canApply
+    }
+
+    private suspend fun applyQueueDiff(
+        transport: AVTransportClient,
+        newQueue: List<TrackRef>,
+        prefixLen: Int,
+        removedCount: Int,
+        addedCount: Int,
+    ) {
+        if (removedCount > 0) {
+            Timber.w(
+                "Sonos incremental: RemoveTrackRangeFromQueue start=%d count=%d",
+                prefixLen + 1,
+                removedCount,
+            )
+            transport.removeTrackRangeFromQueue(
+                startingIndex = prefixLen + 1,
+                numberOfTracks = removedCount,
+            )
+        }
+        if (addedCount == 0) return
+        Timber.w(
+            "Sonos incremental: AddURIToQueue x%d starting at position %d",
+            addedCount,
+            prefixLen + 1,
+        )
+        for (i in 0 until addedCount) {
+            val ref = newQueue[prefixLen + i]
+            val token = streamTokens.mint(ref.id)
+            transport.addURIToQueue(
+                uri = token.url,
+                mime = token.mime,
+                title = token.title,
+                enqueuedURIPosition = prefixLen + i + 1,
+            )
+            if (i > 0) delay(EXTEND_THROTTLE_MS)
+        }
+    }
+
+    private fun commonPrefixLength(a: List<String>, b: List<String>): Int {
+        val limit = minOf(a.size, b.size)
+        for (i in 0 until limit) {
+            if (a[i] != b[i]) return i
+        }
+        return limit
+    }
+
+    private fun commonSuffixLength(a: List<String>, b: List<String>): Int {
+        val limit = minOf(a.size, b.size)
+        for (i in 0 until limit) {
+            if (a[a.size - 1 - i] != b[b.size - 1 - i]) return i
+        }
+        return limit
+    }
+
+    private companion object {
+        // Abort the append loop after this many consecutive AddURIToQueue
+        // failures; Sonos rate-limits burst adds and a wall of failures means
+        // it has stopped accepting, not that the next one might land.
+        const val EXTEND_ABORT_AFTER_FAILURES = 3
+        const val EXTEND_THROTTLE_MS = 50L
+
+        // Verify/repair passes after a queue load. Two: one to catch the
+        // common case (a rate-limit burst dropped a chunk), one to catch a
+        // repair that itself got rate-limited. Beyond that the renderer is
+        // refusing for a reason retrying won't fix, and the stall watchdog
+        // becomes the backstop.
+        const val VERIFY_ROUNDS = 2
+    }
+}