address the second offline review: cancel guard, bounded reconcile

app/src/test/kotlin/com/subtracks/data/download/FakeDownloadEngine.kt

@@ -9,6 +9,7 @@ class FakeDownloadEngine : DownloadEngine {
     var cancelCalls = 0
     var enqueueDelayMs = 0L
     var failEnqueueFor: String? = null
+    var failCancelOnEmpty = false
 
     override fun enqueue(request: EngineRequest): Long {
         if (enqueueDelayMs > 0) Thread.sleep(enqueueDelayMs)
@@ -23,6 +24,7 @@ class FakeDownloadEngine : DownloadEngine {
     override fun download(id: Long): EngineDownload? = downloads[id]
 
     override fun cancel(ids: List<Long>) {
+        if (ids.isEmpty() && failCancelOnEmpty) throw IllegalArgumentException("no ids")
         cancelCalls++
         cancelled += ids
     }

app/src/test/kotlin/com/subtracks/data/repo/DownloadRepositoryTest.kt

@@ -362,6 +362,23 @@ class DownloadRepositoryTest {
             assertTrue(engine.cancelled.isNotEmpty())
         }
 
+    @Test
+    fun cancellingQueuedRowsWithoutEngineIdsLeavesTheEngineAlone() =
+        runTest {
+            seedLibrary()
+            runBlocking {
+                db.downloadDao().upsert(SongDownload(1, "s1", DownloadStatus.Queued))
+                db.downloadDao().upsert(SongDownload(1, "s2", DownloadStatus.Queued))
+            }
+            // DownloadManager.remove throws on an empty id list; a queue of only waiting rows has
+            // no engine ids, so cancelling must not call the engine at all.
+            engine.failCancelOnEmpty = true
+
+            runBlocking { repository.cancelActive() }
+
+            assertTrue(allRows().isEmpty())
+        }
+
     @Test
     fun theDownloadStartsBeforeItsArtworkIsFetched() =
         runTest {

app/src/test/kotlin/com/subtracks/data/sync/SyncManagerTest.kt

@@ -224,6 +224,7 @@ class SyncManagerTest {
                 "<song id=\"s1\" title=\"Song One\" albumId=\"al1\" artistId=\"ar1\" track=\"1\"/>" +
                 "</searchResult3></subsonic-response>"
     }
+
     @Test
     fun aSyncRequestWhileOfflineIsRefusedWithoutTouchingTheNetwork() =
         runBlocking {