stream sync with SAX and write the library per batch

The DOM-based sync materialised the whole library in memory before
writing: SubsonicSource accumulated every paged response into one list
and SubsonicClient built a full DOM per response, so peak memory grew
with the library size (hundreds of MB for large libraries).

Replace it with a streaming pipeline:

- SubsonicXml is now a SAX reader that maps entities straight from the
  stream. Album disc titles are accumulated from child elements and
  attached when the album closes.
- SubsonicClient exposes stream()/check() instead of returning a DOM.
- MusicSource returns batches as Flow<List<T>>; SubsonicSource emits one
  batch per response page with backpressure so only one batch is held.
- SyncService syncs each entity sequentially, commits one transaction
  per batch, and prunes with an existing-minus-seen stale set. Playlist
  song tails are pruned once the whole flow has been counted, and
  orphaned playlist_songs rows are removed with a single delete.

Sync is no longer atomic: on failure the committed batches remain and
the queue's library cache is now invalidated on failure too.

app/src/integrationTest/kotlin/com/subtracks/data/source/subsonic/SubsonicSourceIntegrationTest.kt

@@ -1,5 +1,7 @@
 package com.subtracks.data.source.subsonic
 
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.runBlocking
 import okhttp3.HttpUrl.Companion.toHttpUrl
 import okhttp3.OkHttpClient
@@ -40,7 +42,7 @@ class SubsonicSourceIntegrationTest(
     @Test
     fun artists() =
         runBlocking {
-            val artists = source.getArtists()
+            val artists = source.artists().collectAll()
 
             assertEquals(2, artists.size)
             assertNotNull(artists.first { it.name == "Ugress" }.starred)
@@ -50,7 +52,7 @@ class SubsonicSourceIntegrationTest(
     @Test
     fun albums() =
         runBlocking {
-            val albums = source.getAlbums()
+            val albums = source.albums().collectAll()
 
             assertEquals(3, albums.size)
 
@@ -72,23 +74,23 @@ class SubsonicSourceIntegrationTest(
     @Test
     fun songs() =
         runBlocking {
-            assertEquals(20, source.getSongs().size)
+            assertEquals(20, source.songs().collectAll().size)
         }
 
     @Test
     fun playlists() =
         runBlocking {
-            val playlists = source.getPlaylists()
+            val playlists = source.playlists().collectAll()
 
             assertEquals(1, playlists.size)
-            assertEquals(7, source.getPlaylistSongs(playlists).size)
+            assertEquals(7, source.playlistSongs(playlists.map { it.id }).collectAll().size)
         }
 
     @Test
     fun albumArtistRelation() =
         runBlocking {
-            val artists = source.getArtists()
-            val albums = source.getAlbums()
+            val artists = source.artists().collectAll()
+            val albums = source.albums().collectAll()
 
             val ugressAlbums =
                 albums
@@ -104,6 +106,8 @@ class SubsonicSourceIntegrationTest(
             assertEquals(listOf("I Don't Know What I'm Doing"), bradAlbums)
         }
 
+    private suspend fun <T> Flow<List<T>>.collectAll(): List<T> = toList().flatten()
+
     data class Server(
         val name: String,
         val baseUrl: String,

app/src/main/kotlin/com/subtracks/data/db/LibraryDao.kt

@@ -115,6 +115,12 @@ interface LibraryDao {
         position: Long,
     )
 
+    @Query(
+        "DELETE FROM playlist_songs WHERE sourceId = :sourceId " +
+            "AND playlistId NOT IN (SELECT id FROM playlists WHERE sourceId = :sourceId)",
+    )
+    suspend fun deleteOrphanPlaylistSongs(sourceId: Long)
+
     @Query(
         "SELECT * FROM albums WHERE sourceId = :sourceId " +
             "AND (:starredFilter = 0 OR (:starredFilter = 1 AND starred IS NOT NULL) OR (:starredFilter = 2 AND starred IS NULL)) " +

app/src/main/kotlin/com/subtracks/data/repo/SourceRepository.kt

@@ -112,7 +112,7 @@ class SourceRepository(
         useTokenAuth: Boolean,
     ): Result<Unit> =
         withContext(Dispatchers.IO) {
-            runCatching { client(address, username, password, useTokenAuth).get("ping") }.map { }
+            runCatching { client(address, username, password, useTokenAuth).check("ping") }
         }
 
     suspend fun sync(): Result<Unit> =

app/src/main/kotlin/com/subtracks/data/source/MusicSource.kt

@@ -5,19 +5,20 @@ import com.subtracks.data.model.Artist
 import com.subtracks.data.model.Playlist
 import com.subtracks.data.model.PlaylistSong
 import com.subtracks.data.model.Song
+import kotlinx.coroutines.flow.Flow
 
 interface MusicSource {
     val id: Long
 
     suspend fun ping()
 
-    suspend fun getArtists(): List<Artist>
+    fun artists(): Flow<List<Artist>>
 
-    suspend fun getAlbums(): List<Album>
+    fun albums(): Flow<List<Album>>
 
-    suspend fun getSongs(): List<Song>
+    fun songs(): Flow<List<Song>>
 
-    suspend fun getPlaylists(): List<Playlist>
+    fun playlists(): Flow<List<Playlist>>
 
-    suspend fun getPlaylistSongs(playlists: List<Playlist>): List<PlaylistSong>
+    fun playlistSongs(playlistIds: List<String>): Flow<List<PlaylistSong>>
 }

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicClient.kt

@@ -3,11 +3,8 @@ package com.subtracks.data.source.subsonic
 import okhttp3.HttpUrl
 import okhttp3.OkHttpClient
 import okhttp3.Request
-import org.w3c.dom.Document
-import org.w3c.dom.Element
+import java.io.InputStream
 import java.security.MessageDigest
-import javax.xml.parsers.DocumentBuilderFactory
-import javax.xml.parsers.ParserConfigurationException
 
 class SubsonicException(
     val code: Int,
@@ -41,10 +38,18 @@ class SubsonicClient(
         return builder.build()
     }
 
-    fun get(
+    fun check(
         method: String,
         params: Map<String, String> = emptyMap(),
-    ): Document {
+    ) {
+        stream(method, params) { SubsonicXml.readStatus(it) }
+    }
+
+    fun <T> stream(
+        method: String,
+        params: Map<String, String> = emptyMap(),
+        body: (InputStream) -> T,
+    ): T {
         val request =
             Request
                 .Builder()
@@ -55,20 +60,7 @@ class SubsonicClient(
             if (!response.isSuccessful) {
                 throw SubsonicException(-1, "HTTP ${response.code}")
             }
-            val stream = response.body.byteStream()
-            val document = secureDocumentBuilderFactory().newDocumentBuilder().parse(stream)
-            val root = document.documentElement
-            if (root == null || root.tagName != "subsonic-response") {
-                throw SubsonicException(-1, "Unexpected response from the server")
-            }
-            if (root.getAttribute("status") == "failed") {
-                val error = root.getElementsByTagName("error").item(0) as? Element
-                throw SubsonicException(
-                    error?.getAttribute("code")?.toIntOrNull() ?: -1,
-                    error?.getAttribute("message") ?: "Unknown error",
-                )
-            }
-            return document
+            return body(response.body.byteStream())
         }
     }
 
@@ -76,27 +68,6 @@ class SubsonicClient(
         const val API_VERSION = "1.13.0"
         const val CLIENT = "subtracks"
 
-        private fun secureDocumentBuilderFactory(): DocumentBuilderFactory =
-            DocumentBuilderFactory.newInstance().apply {
-                isNamespaceAware = false
-                isExpandEntityReferences = false
-                setFeatureQuietly("http://apache.org/xml/features/disallow-doctype-decl", true)
-                setFeatureQuietly("http://xml.org/sax/features/external-general-entities", false)
-                setFeatureQuietly("http://xml.org/sax/features/external-parameter-entities", false)
-                setFeatureQuietly("http://apache.org/xml/features/nonvalidating/load-external-dtd", false)
-            }
-
-        private fun DocumentBuilderFactory.setFeatureQuietly(
-            name: String,
-            enabled: Boolean,
-        ) {
-            try {
-                setFeature(name, enabled)
-            } catch (_: ParserConfigurationException) {
-                // Android's parser does not implement every hardening flag; the root-tag check in get() is the backstop.
-            }
-        }
-
         private fun randomSalt(): String = (1..4).map { ('a'..'z').random() }.joinToString("")
 
         private fun md5Hex(input: String): String =

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicSource.kt

@@ -6,9 +6,20 @@ import com.subtracks.data.model.Playlist
 import com.subtracks.data.model.PlaylistSong
 import com.subtracks.data.model.Song
 import com.subtracks.data.source.MusicSource
+import kotlinx.coroutines.CancellationException
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.channels.trySendBlocking
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.buffer
+import kotlinx.coroutines.flow.channelFlow
+import kotlinx.coroutines.flow.emitAll
+import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.flow.flowOn
 import kotlinx.coroutines.withContext
+import org.xml.sax.Attributes
 
+@OptIn(ExperimentalCoroutinesApi::class)
 class SubsonicSource(
     override val id: Long,
     private val client: SubsonicClient,
@@ -18,53 +29,77 @@ class SubsonicSource(
     private var emptyQuerySearchSupported: Boolean? = null
 
     override suspend fun ping() {
-        withContext(Dispatchers.IO) { client.get("ping") }
+        withContext(Dispatchers.IO) { client.check("ping") }
     }
 
-    override suspend fun getArtists(): List<Artist> =
-        withContext(Dispatchers.IO) {
-            SubsonicXml.artists(id, client.get("getArtists"))
-        }
+    override fun artists(): Flow<List<Artist>> =
+        entityBatches(
+            method = "getArtists",
+            tag = "artist",
+            create = { SubsonicXml.artist(id, it) },
+            accept = { it.id.isNotEmpty() },
+        )
 
-    override suspend fun getAlbums(): List<Album> =
-        withContext(Dispatchers.IO) {
+    override fun albums(): Flow<List<Album>> =
+        flow {
             val (frequent, recent) = fetchRanks()
-            val albums = mutableListOf<Album>()
             var offset = 0
             var pages = 0
             while (pages < MAX_PAGES) {
-                val document = client.get("getAlbumList2", page("newest", offset))
-                val batch = SubsonicXml.albums(id, document)
-                albums +=
-                    batch.map {
-                        it.copy(frequentRank = frequent[it.id], recentRank = recent[it.id])
-                    }
-                if (batch.size < PAGE_SIZE) break
+                var count = 0
+                entityBatches(
+                    method = "getAlbumList2",
+                    params = page("newest", offset),
+                    tag = "album",
+                    create = { SubsonicXml.albumDraft(id, it) },
+                    onChild = { draft, name, attrs -> if (name == "discTitles") draft.addDiscTitle(attrs) },
+                    accept = { it.id.isNotEmpty() },
+                ).collect { batch ->
+                    count += batch.size
+                    emit(
+                        batch.map {
+                            it.toAlbum().copy(
+                                frequentRank = frequent[it.id],
+                                recentRank = recent[it.id],
+                            )
+                        },
+                    )
+                }
+                if (count < PAGE_SIZE) break
                 offset += PAGE_SIZE
                 pages++
             }
-            albums
-        }
+        }.flowOn(Dispatchers.IO)
 
-    override suspend fun getSongs(): List<Song> =
-        withContext(Dispatchers.IO) {
-            if (supportsEmptyQuerySearch()) searchSongs() else albumSongs()
-        }
-
-    override suspend fun getPlaylists(): List<Playlist> =
-        withContext(Dispatchers.IO) {
-            SubsonicXml.playlists(id, client.get("getPlaylists"))
-        }
-
-    override suspend fun getPlaylistSongs(playlists: List<Playlist>): List<PlaylistSong> =
-        withContext(Dispatchers.IO) {
-            val result = mutableListOf<PlaylistSong>()
-            for (playlist in playlists) {
-                val document = client.get("getPlaylist", mapOf("id" to playlist.id))
-                result += SubsonicXml.playlistSongs(id, playlist.id, document)
+    override fun songs(): Flow<List<Song>> =
+        flow {
+            if (supportsEmptyQuerySearch()) {
+                emitAll(searchSongs())
+            } else {
+                emitAll(albumSongs())
             }
-            result
-        }
+        }.flowOn(Dispatchers.IO)
+
+    override fun playlists(): Flow<List<Playlist>> =
+        entityBatches(
+            method = "getPlaylists",
+            tag = "playlist",
+            create = { SubsonicXml.playlist(id, it) },
+            accept = { it.id.isNotEmpty() },
+        )
+
+    override fun playlistSongs(playlistIds: List<String>): Flow<List<PlaylistSong>> =
+        flow {
+            for (playlistId in playlistIds) {
+                var position = 0L
+                entityBatches(
+                    method = "getPlaylist",
+                    params = mapOf("id" to playlistId),
+                    tag = "entry",
+                    create = { attrs -> SubsonicXml.playlistSong(id, playlistId, position++, attrs) },
+                ).collect { emit(it) }
+            }
+        }.flowOn(Dispatchers.IO)
 
     fun streamUri(songId: String) =
         client.uri(
@@ -93,16 +128,84 @@ class SubsonicSource(
         )
     }
 
-    private fun fetchRanks(): Pair<Map<String, Long>, Map<String, Long>> = fetchRank("frequent") to fetchRank("recent")
+    private fun searchSongs(): Flow<List<Song>> =
+        flow {
+            var offset = 0
+            var pages = 0
+            while (pages < MAX_PAGES) {
+                var count = 0
+                entityBatches(
+                    method = "search3",
+                    params =
+                        mapOf(
+                            "query" to "\"\"",
+                            "songCount" to PAGE_SIZE.toString(),
+                            "songOffset" to offset.toString(),
+                            "artistCount" to "0",
+                            "albumCount" to "0",
+                        ),
+                    tag = "song",
+                    create = { SubsonicXml.song(id, it) },
+                    accept = { it.id.isNotEmpty() },
+                ).collect { batch ->
+                    count += batch.size
+                    emit(batch)
+                }
+                if (count < PAGE_SIZE) break
+                offset += PAGE_SIZE
+                pages++
+            }
+        }
 
-    private fun fetchRank(type: String): Map<String, Long> {
-        val ranks = mutableMapOf<String, Long>()
+    private fun albumSongs(): Flow<List<Song>> =
+        flow {
+            var offset = 0
+            var pages = 0
+            while (pages < MAX_PAGES) {
+                val albums = ArrayList<String>(PAGE_SIZE)
+                entityBatches(
+                    method = "getAlbumList2",
+                    params = page("alphabeticalByName", offset),
+                    tag = "album",
+                    create = { it.attr("id") },
+                    accept = { it.isNotEmpty() },
+                ).collect { albums += it }
+                for (albumId in albums) {
+                    entityBatches(
+                        method = "getAlbum",
+                        params = mapOf("id" to albumId),
+                        tag = "song",
+                        create = { SubsonicXml.song(id, it) },
+                        accept = { it.id.isNotEmpty() },
+                    ).collect { emit(it) }
+                }
+                if (albums.size < PAGE_SIZE) break
+                offset += PAGE_SIZE
+                pages++
+            }
+        }
+
+    private suspend fun fetchRanks(): Pair<Map<String, Long>, Map<String, Long>> = fetchRank("frequent") to fetchRank("recent")
+
+    private suspend fun fetchRank(type: String): Map<String, Long> {
+        val ranks = HashMap<String, Long>()
         var offset = 0
         var pages = 0
         while (pages < MAX_PAGES) {
-            val batch = SubsonicXml.albums(id, client.get("getAlbumList2", page(type, offset)))
-            batch.forEachIndexed { index, album -> ranks[album.id] = (offset + index).toLong() }
-            if (batch.size < PAGE_SIZE) break
+            var index = 0
+            entityBatches(
+                method = "getAlbumList2",
+                params = page(type, offset),
+                tag = "album",
+                create = { it.attr("id") },
+                accept = { it.isNotEmpty() },
+            ).collect { batch ->
+                batch.forEach { albumId ->
+                    ranks[albumId] = (offset + index).toLong()
+                    index++
+                }
+            }
+            if (index < PAGE_SIZE) break
             offset += PAGE_SIZE
             pages++
         }
@@ -113,12 +216,10 @@ class SubsonicSource(
         emptyQuerySearchSupported?.let { return it }
         val supported =
             try {
-                val document =
-                    client.get(
-                        "search3",
-                        mapOf("query" to "\"\"", "songCount" to "1", "artistCount" to "0", "albumCount" to "0"),
-                    )
-                document.getElementsByTagName("song").length > 0
+                client.stream(
+                    "search3",
+                    mapOf("query" to "\"\"", "songCount" to "1", "artistCount" to "0", "albumCount" to "0"),
+                ) { input -> SubsonicXml.containsEntity(input, "song") }
             } catch (_: SubsonicException) {
                 false
             }
@@ -126,48 +227,41 @@ class SubsonicSource(
         return supported
     }
 
-    private fun searchSongs(): List<Song> {
-        val songs = mutableListOf<Song>()
-        var offset = 0
-        var pages = 0
-        while (pages < MAX_PAGES) {
-            val document =
-                client.get(
-                    "search3",
-                    mapOf(
-                        "query" to "\"\"",
-                        "songCount" to PAGE_SIZE.toString(),
-                        "songOffset" to offset.toString(),
-                        "artistCount" to "0",
-                        "albumCount" to "0",
-                    ),
-                )
-            val batch = SubsonicXml.songs(id, document)
-            songs += batch
-            if (batch.size < PAGE_SIZE) break
-            offset += PAGE_SIZE
-            pages++
-        }
-        return songs
-    }
+    private fun <T> entityBatches(
+        method: String,
+        params: Map<String, String> = emptyMap(),
+        tag: String,
+        create: (Attributes) -> T,
+        onChild: (T, String, Attributes) -> Unit = { _, _, _ -> },
+        accept: (T) -> Boolean = { true },
+    ): Flow<List<T>> =
+        channelFlow {
+            val buffer = ArrayList<T>(PAGE_SIZE)
 
-    private fun albumSongs(): List<Song> {
-        val songs = mutableListOf<Song>()
-        var offset = 0
-        var pages = 0
-        while (pages < MAX_PAGES) {
-            val document = client.get("getAlbumList2", page("alphabeticalByName", offset))
-            val albums = SubsonicXml.albums(id, document)
-            for (album in albums) {
-                val albumDocument = client.get("getAlbum", mapOf("id" to album.id))
-                songs += SubsonicXml.songs(id, albumDocument)
+            fun flush() {
+                if (buffer.isEmpty()) return
+                if (trySendBlocking(buffer.toList()).isFailure) {
+                    throw CancellationException("Library sync cancelled")
+                }
+                buffer.clear()
             }
-            if (albums.size < PAGE_SIZE) break
-            offset += PAGE_SIZE
-            pages++
-        }
-        return songs
-    }
+
+            client.stream(method, params) { input ->
+                SubsonicXml.readEntities(
+                    input = input,
+                    entityTag = tag,
+                    create = create,
+                    onChild = onChild,
+                    onEnd = { entity ->
+                        if (accept(entity)) {
+                            buffer += entity
+                            if (buffer.size >= PAGE_SIZE) flush()
+                        }
+                    },
+                )
+            }
+            flush()
+        }.flowOn(Dispatchers.IO).buffer(1)
 
     private fun page(
         type: String,

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicXml.kt

@@ -5,147 +5,305 @@ import com.subtracks.data.model.Artist
 import com.subtracks.data.model.Playlist
 import com.subtracks.data.model.PlaylistSong
 import com.subtracks.data.model.Song
-import org.w3c.dom.Document
-import org.w3c.dom.Element
+import org.xml.sax.Attributes
+import org.xml.sax.SAXException
+import org.xml.sax.helpers.DefaultHandler
+import java.io.InputStream
 import java.time.LocalDate
 import java.time.OffsetDateTime
 import java.time.ZoneOffset
 import java.time.format.DateTimeParseException
+import javax.xml.parsers.ParserConfigurationException
+import javax.xml.parsers.SAXParserFactory
 
 object SubsonicXml {
-    fun artists(
+    fun artist(
         sourceId: Long,
-        document: Document,
-    ): List<Artist> =
-        document
-            .elements("artist")
-            .map { element ->
-                Artist(
-                    sourceId = sourceId,
-                    id = element.attr("id"),
-                    name = element.attr("name"),
-                    albumCount = element.longAttr("albumCount") ?: 0L,
-                    starred = element.dateAttr("starred"),
-                    coverArt = element.attr("coverArt").ifEmpty { null },
-                )
-            }.filter { it.id.isNotEmpty() }
-
-    fun albums(
-        sourceId: Long,
-        document: Document,
-    ): List<Album> = document.elements("album").map { album(sourceId, it) }.filter { it.id.isNotEmpty() }
-
-    fun album(
-        sourceId: Long,
-        element: Element,
-    ): Album =
-        Album(
+        attrs: Attributes,
+    ): Artist =
+        Artist(
             sourceId = sourceId,
-            id = element.attr("id"),
-            artistId = element.attr("artistId").ifEmpty { null },
-            name = element.attr("name"),
-            albumArtist = element.attr("artist").ifEmpty { null },
-            created = element.dateAttr("created") ?: 0L,
-            coverArt = element.attr("coverArt").ifEmpty { null },
-            genre = element.attr("genre").ifEmpty { null },
-            year = element.longAttr("year"),
-            starred = element.dateAttr("starred"),
-            songCount = element.longAttr("songCount") ?: 0L,
-            frequentRank = null,
-            recentRank = null,
-            discTitles = element.discTitles(),
+            id = attrs.attr("id"),
+            name = attrs.attr("name"),
+            albumCount = attrs.longAttr("albumCount") ?: 0L,
+            starred = attrs.dateAttr("starred"),
+            coverArt = attrs.attr("coverArt").ifEmpty { null },
         )
 
-    fun songs(
+    internal fun albumDraft(
         sourceId: Long,
-        document: Document,
-    ): List<Song> = document.elements("song").map { song(sourceId, it) }.filter { it.id.isNotEmpty() }
+        attrs: Attributes,
+    ): AlbumDraft =
+        AlbumDraft(
+            sourceId = sourceId,
+            id = attrs.attr("id"),
+            artistId = attrs.attr("artistId").ifEmpty { null },
+            name = attrs.attr("name"),
+            albumArtist = attrs.attr("artist").ifEmpty { null },
+            created = attrs.dateAttr("created") ?: 0L,
+            coverArt = attrs.attr("coverArt").ifEmpty { null },
+            genre = attrs.attr("genre").ifEmpty { null },
+            year = attrs.longAttr("year"),
+            starred = attrs.dateAttr("starred"),
+            songCount = attrs.longAttr("songCount") ?: 0L,
+        )
 
     fun song(
         sourceId: Long,
-        element: Element,
+        attrs: Attributes,
     ): Song =
         Song(
             sourceId = sourceId,
-            id = element.attr("id"),
-            albumId = element.attr("albumId").ifEmpty { null },
-            artistId = element.attr("artistId").ifEmpty { null },
-            title = element.attr("title"),
-            album = element.attr("album").ifEmpty { null },
-            artist = element.attr("artist").ifEmpty { null },
-            duration = element.longAttr("duration"),
-            track = element.longAttr("track"),
-            disc = element.longAttr("discNumber"),
-            starred = element.dateAttr("starred"),
-            genre = element.attr("genre").ifEmpty { null },
-            created = element.dateAttr("created") ?: 0L,
+            id = attrs.attr("id"),
+            albumId = attrs.attr("albumId").ifEmpty { null },
+            artistId = attrs.attr("artistId").ifEmpty { null },
+            title = attrs.attr("title"),
+            album = attrs.attr("album").ifEmpty { null },
+            artist = attrs.attr("artist").ifEmpty { null },
+            duration = attrs.longAttr("duration"),
+            track = attrs.longAttr("track"),
+            disc = attrs.longAttr("discNumber"),
+            starred = attrs.dateAttr("starred"),
+            genre = attrs.attr("genre").ifEmpty { null },
+            created = attrs.dateAttr("created") ?: 0L,
         )
 
-    fun playlists(
-        sourceId: Long,
-        document: Document,
-    ): List<Playlist> = document.elements("playlist").map { playlist(sourceId, it) }.filter { it.id.isNotEmpty() }
-
     fun playlist(
         sourceId: Long,
-        element: Element,
+        attrs: Attributes,
     ): Playlist =
         Playlist(
             sourceId = sourceId,
-            id = element.attr("id"),
-            name = element.attr("name"),
-            comment = element.attr("comment").ifEmpty { null },
-            coverArt = element.attr("coverArt").ifEmpty { null },
-            songCount = element.longAttr("songCount") ?: 0L,
-            created = element.dateAttr("created") ?: 0L,
-            changed = element.dateAttr("changed") ?: 0L,
-            duration = element.longAttr("duration") ?: 0L,
+            id = attrs.attr("id"),
+            name = attrs.attr("name"),
+            comment = attrs.attr("comment").ifEmpty { null },
+            coverArt = attrs.attr("coverArt").ifEmpty { null },
+            songCount = attrs.longAttr("songCount") ?: 0L,
+            created = attrs.dateAttr("created") ?: 0L,
+            changed = attrs.dateAttr("changed") ?: 0L,
+            duration = attrs.longAttr("duration") ?: 0L,
         )
 
-    fun playlistSongs(
+    fun playlistSong(
         sourceId: Long,
         playlistId: String,
-        document: Document,
-    ): List<PlaylistSong> =
-        document.elements("entry").mapIndexed { index, element ->
-            PlaylistSong(
-                sourceId = sourceId,
-                playlistId = playlistId,
-                songId = element.attr("id"),
-                position = index.toLong(),
-            )
-        }
-}
+        position: Long,
+        attrs: Attributes,
+    ): PlaylistSong =
+        PlaylistSong(
+            sourceId = sourceId,
+            playlistId = playlistId,
+            songId = attrs.attr("id"),
+            position = position,
+        )
 
-private fun Element.discTitles(): Map<Long, String> =
-    childElements("discTitles")
-        .mapNotNull { title ->
-            val disc = title.longAttr("disc") ?: return@mapNotNull null
-            title.attr("title").takeIf { it.isNotBlank() }?.let { disc to it }
-        }.toMap()
+    internal fun readStatus(input: InputStream) {
+        parse(input, StatusHandler())
+    }
 
-private fun Element.childElements(tag: String): List<Element> {
-    val children = childNodes
-    return (0 until children.length).mapNotNull { index ->
-        (children.item(index) as? Element)?.takeIf { it.tagName == tag }
+    internal fun <T> readEntities(
+        input: InputStream,
+        entityTag: String,
+        create: (Attributes) -> T,
+        onChild: (T, String, Attributes) -> Unit = { _, _, _ -> },
+        onEnd: (T) -> Unit,
+    ) {
+        parse(input, EntityHandler(entityTag, create, onChild, onEnd, stopAfterFirst = false))
+    }
+
+    internal fun containsEntity(
+        input: InputStream,
+        entityTag: String,
+    ): Boolean {
+        val handler = EntityHandler(entityTag, { Unit }, { _, _, _ -> }, { }, stopAfterFirst = true)
+        parse(input, handler)
+        return handler.found
     }
 }
 
-private fun Element.elements(tag: String): List<Element> {
-    val nodes = getElementsByTagName(tag)
-    return (0 until nodes.length).mapNotNull { nodes.item(it) as? Element }
+internal class AlbumDraft(
+    val sourceId: Long,
+    var id: String = "",
+    var artistId: String? = null,
+    var name: String = "",
+    var albumArtist: String? = null,
+    var created: Long = 0L,
+    var coverArt: String? = null,
+    var genre: String? = null,
+    var year: Long? = null,
+    var starred: Long? = null,
+    var songCount: Long = 0L,
+    val discTitles: MutableMap<Long, String> = mutableMapOf(),
+) {
+    fun addDiscTitle(attrs: Attributes) {
+        val disc = attrs.longAttr("disc") ?: return
+        val title = attrs.attr("title").takeIf { it.isNotBlank() } ?: return
+        discTitles[disc] = title
+    }
+
+    fun toAlbum(): Album =
+        Album(
+            sourceId = sourceId,
+            id = id,
+            artistId = artistId,
+            name = name,
+            albumArtist = albumArtist,
+            created = created,
+            coverArt = coverArt,
+            genre = genre,
+            year = year,
+            starred = starred,
+            songCount = songCount,
+            frequentRank = null,
+            recentRank = null,
+            discTitles = discTitles.toMap(),
+        )
 }
 
-private fun Document.elements(tag: String): List<Element> {
-    val nodes = getElementsByTagName(tag)
-    return (0 until nodes.length).mapNotNull { nodes.item(it) as? Element }
+internal fun Attributes.attr(name: String): String = getValue(name) ?: ""
+
+internal fun Attributes.longAttr(name: String): Long? = attr(name).takeIf { it.isNotEmpty() }?.toLongOrNull()
+
+internal fun Attributes.dateAttr(name: String): Long? = attr(name).takeIf { it.isNotEmpty() }?.let(IsoDate::parse)
+
+private const val ROOT_TAG = "subsonic-response"
+
+private class XmlFailure(
+    val code: Int,
+    message: String,
+) : RuntimeException(message)
+
+private class StopReading : RuntimeException()
+
+private abstract class SubsonicResponseHandler : DefaultHandler() {
+    private var depth = 0
+    private var sawRoot = false
+    private var statusFailed = false
+
+    override fun startElement(
+        uri: String?,
+        localName: String?,
+        qName: String,
+        attributes: Attributes,
+    ) {
+        depth++
+        if (!sawRoot) {
+            sawRoot = true
+            if (qName != ROOT_TAG) throw XmlFailure(-1, "Unexpected response from the server")
+            if (attributes.getValue("status") != "ok") statusFailed = true
+        }
+        if (qName == "error") {
+            throw XmlFailure(
+                attributes.getValue("code")?.toIntOrNull() ?: -1,
+                attributes.getValue("message") ?: "Unknown error",
+            )
+        }
+        onElementStart(qName, attributes, depth)
+    }
+
+    override fun endElement(
+        uri: String?,
+        localName: String?,
+        qName: String,
+    ) {
+        onElementEnd(qName, depth)
+        depth--
+    }
+
+    override fun endDocument() {
+        if (statusFailed) throw XmlFailure(-1, "Unknown error")
+    }
+
+    protected open fun onElementStart(
+        name: String,
+        attributes: Attributes,
+        depth: Int,
+    ) = Unit
+
+    protected open fun onElementEnd(
+        name: String,
+        depth: Int,
+    ) = Unit
 }
 
-private fun Element.attr(name: String): String = getAttribute(name)
+private class StatusHandler : SubsonicResponseHandler()
 
-private fun Element.longAttr(name: String): Long? = getAttribute(name).takeIf { it.isNotEmpty() }?.toLongOrNull()
+private class EntityHandler<T>(
+    private val entityTag: String,
+    private val create: (Attributes) -> T,
+    private val onChild: (T, String, Attributes) -> Unit,
+    private val onEnd: (T) -> Unit,
+    private val stopAfterFirst: Boolean,
+) : SubsonicResponseHandler() {
+    private var current: T? = null
+    private var entityDepth = -1
 
-private fun Element.dateAttr(name: String): Long? = getAttribute(name).takeIf { it.isNotEmpty() }?.let(IsoDate::parse)
+    var found = false
+        private set
+
+    override fun onElementStart(
+        name: String,
+        attributes: Attributes,
+        depth: Int,
+    ) {
+        val active = current
+        if (active == null) {
+            if (name == entityTag) {
+                current = create(attributes)
+                entityDepth = depth
+            }
+        } else if (depth == entityDepth + 1) {
+            onChild(active, name, attributes)
+        }
+    }
+
+    override fun onElementEnd(
+        name: String,
+        depth: Int,
+    ) {
+        val active = current ?: return
+        if (name == entityTag && depth == entityDepth) {
+            onEnd(active)
+            found = true
+            current = null
+            entityDepth = -1
+            if (stopAfterFirst) throw StopReading()
+        }
+    }
+}
+
+private fun parse(
+    input: InputStream,
+    handler: DefaultHandler,
+) {
+    try {
+        secureParserFactory().newSAXParser().parse(input, handler)
+    } catch (failure: XmlFailure) {
+        throw SubsonicException(failure.code, failure.message ?: "Unknown error")
+    } catch (_: StopReading) {
+    }
+}
+
+private fun secureParserFactory(): SAXParserFactory =
+    SAXParserFactory.newInstance().apply {
+        isNamespaceAware = false
+        isValidating = false
+        setFeatureQuietly("http://apache.org/xml/features/disallow-doctype-decl", true)
+        setFeatureQuietly("http://xml.org/sax/features/external-general-entities", false)
+        setFeatureQuietly("http://xml.org/sax/features/external-parameter-entities", false)
+        setFeatureQuietly("http://apache.org/xml/features/nonvalidating/load-external-dtd", false)
+    }
+
+private fun SAXParserFactory.setFeatureQuietly(
+    name: String,
+    enabled: Boolean,
+) {
+    try {
+        setFeature(name, enabled)
+    } catch (_: ParserConfigurationException) {
+    } catch (_: SAXException) {
+    }
+}
 
 internal object IsoDate {
     private val timezone = Regex("(Z|[+-]\\d{2}:?\\d{2})$", RegexOption.IGNORE_CASE)

app/src/main/kotlin/com/subtracks/data/sync/SyncManager.kt

@@ -36,12 +36,11 @@ class SyncManager(
 
     private suspend fun runSync() {
         _status.value = SyncStatus.Running
+        val result = sourceRepository.sync()
+        queueRepository.invalidateLibraryCache()
         _status.value =
-            sourceRepository.sync().fold(
-                onSuccess = {
-                    queueRepository.invalidateLibraryCache()
-                    SyncStatus.Success
-                },
+            result.fold(
+                onSuccess = { SyncStatus.Success },
                 onFailure = { error ->
                     Log.w(TAG, "Sync failed", error)
                     SyncStatus.Failed(error.message ?: "Sync failed")

app/src/main/kotlin/com/subtracks/data/sync/SyncService.kt

@@ -24,62 +24,132 @@ class SyncService(
 
     suspend fun sync() =
         lock.withLock {
-            val artists = source.getArtists()
-            val albums = source.getAlbums()
-            val songs = source.getSongs()
+            syncArtists()
+            syncAlbums()
+            syncSongs()
+            syncPlaylists()
+            syncPlaylistSongs()
+        }
+
+    private suspend fun syncArtists() {
+        val library = db.libraryDao()
+        val stale = library.artistIds(source.id).toHashSet()
+        source.artists().collect { batch ->
+            if (batch.isEmpty()) return@collect
+            write("artists", artistColumns, primaryKey, batch) { it.values() }
+            stale -= batch.mapTo(HashSet()) { it.id }
+        }
+        deleteStale(stale) { library.deleteArtists(source.id, it) }
+    }
+
+    private suspend fun syncAlbums() {
+        val library = db.libraryDao()
+        val staleAlbums = library.albumIds(source.id).toHashSet()
+        val staleDiscs = library.discKeys(source.id).toHashSet()
+        source.albums().collect { batch ->
+            if (batch.isEmpty()) return@collect
             val discs =
-                albums.flatMap { album ->
+                batch.flatMap { album ->
                     album.discTitles.map { (disc, title) -> Disc(album.sourceId, album.id, disc, title) }
                 }
-            val playlists = source.getPlaylists()
-            val playlistSongs = source.getPlaylistSongs(playlists)
+            writeAlbumsAndDiscs(batch, discs)
+            staleAlbums -= batch.mapTo(HashSet()) { it.id }
+            staleDiscs -= discs.mapTo(HashSet()) { DiscKey(it.albumId, it.disc) }
+        }
+        deleteStale(staleAlbums) { library.deleteAlbums(source.id, it) }
+        deleteStaleDiscs(staleDiscs)
+    }
 
-            db.useWriterConnection { transactor ->
-                transactor.immediateTransaction {
-                    upsertChanged("artists", artistColumns, primaryKey, artists) { it.values() }
-                    upsertChanged("albums", albumColumns, primaryKey, albums) { it.values() }
-                    upsertChanged("discs", discColumns, discKey, discs) { it.values() }
-                    upsertChanged("songs", songColumns, primaryKey, songs) { it.values() }
-                    upsertChanged("playlists", playlistColumns, primaryKey, playlists) { it.values() }
-                    upsertChanged("playlist_songs", playlistSongColumns, playlistSongKey, playlistSongs) { it.values() }
+    private suspend fun syncSongs() {
+        val library = db.libraryDao()
+        val stale = library.songIds(source.id).toHashSet()
+        source.songs().collect { batch ->
+            if (batch.isEmpty()) return@collect
+            write("songs", songColumns, primaryKey, batch) { it.values() }
+            stale -= batch.mapTo(HashSet()) { it.id }
+        }
+        deleteStale(stale) { library.deleteSongs(source.id, it) }
+    }
 
-                    val library = db.libraryDao()
-                    val existingArtistIds = library.artistIds(source.id)
-                    val existingAlbumIds = library.albumIds(source.id)
-                    val existingSongIds = library.songIds(source.id)
-                    val existingPlaylistIds = library.playlistIds(source.id)
-                    val existingDiscKeys = library.discKeys(source.id)
+    private suspend fun syncPlaylists() {
+        val library = db.libraryDao()
+        val stale = library.playlistIds(source.id).toHashSet()
+        source.playlists().collect { batch ->
+            if (batch.isEmpty()) return@collect
+            write("playlists", playlistColumns, primaryKey, batch) { it.values() }
+            stale -= batch.mapTo(HashSet()) { it.id }
+        }
+        deleteStale(stale) { library.deletePlaylists(source.id, it) }
+    }
 
-                    prune(existingArtistIds, artists.map { it.id }) { library.deleteArtists(source.id, it) }
-                    prune(existingAlbumIds, albums.map { it.id }) { library.deleteAlbums(source.id, it) }
-                    prune(existingSongIds, songs.map { it.id }) { library.deleteSongs(source.id, it) }
-                    prune(existingPlaylistIds, playlists.map { it.id }) { library.deletePlaylists(source.id, it) }
+    private suspend fun syncPlaylistSongs() {
+        val library = db.libraryDao()
+        val playlistIds = library.playlistIds(source.id)
+        val counts = HashMap<String, Long>()
+        source.playlistSongs(playlistIds).collect { batch ->
+            if (batch.isEmpty()) return@collect
+            write("playlist_songs", playlistSongColumns, playlistSongKey, batch) { it.values() }
+            batch.forEach { counts[it.playlistId] = (counts[it.playlistId] ?: 0L) + 1L }
+        }
+        playlistIds.forEach { library.deletePlaylistSongsFrom(source.id, it, counts[it] ?: 0L) }
+        library.deleteOrphanPlaylistSongs(source.id)
+    }
 
-                    val currentDiscKeys = discs.mapTo(HashSet()) { DiscKey(it.albumId, it.disc) }
-                    existingDiscKeys.filterNot { it in currentDiscKeys }.forEach { library.deleteDisc(source.id, it.albumId, it.disc) }
+    private suspend fun <T> write(
+        table: String,
+        columns: List<String>,
+        key: List<String>,
+        rows: List<T>,
+        values: (T) -> List<Any?>,
+    ) {
+        if (rows.isEmpty()) return
+        db.useWriterConnection { connection ->
+            connection.immediateTransaction {
+                connection.upsertChanged(table, columns, key, rows, values)
+            }
+        }
+    }
 
-                    val playlistSongCounts = playlistSongs.groupingBy { it.playlistId }.eachCount()
-                    playlists.forEach { playlist ->
-                        library.deletePlaylistSongsFrom(source.id, playlist.id, (playlistSongCounts[playlist.id] ?: 0).toLong())
+    private suspend fun writeAlbumsAndDiscs(
+        albums: List<Album>,
+        discs: List<Disc>,
+    ) {
+        db.useWriterConnection { connection ->
+            connection.immediateTransaction {
+                connection.upsertChanged("albums", albumColumns, primaryKey, albums) { it.values() }
+                connection.upsertChanged("discs", discColumns, discKey, discs) { it.values() }
+            }
+        }
+    }
+
+    private suspend fun deleteStaleDiscs(stale: Set<DiscKey>) {
+        if (stale.isEmpty()) return
+        db.useWriterConnection { connection ->
+            connection.immediateTransaction {
+                connection.usePrepared("DELETE FROM discs WHERE sourceId = ? AND albumId = ? AND disc = ?") { statement ->
+                    stale.forEach { key ->
+                        statement.bind(1, source.id)
+                        statement.bind(2, key.albumId)
+                        statement.bind(3, key.disc)
+                        statement.step()
+                        statement.reset()
+                        statement.clearBindings()
                     }
-                    (existingPlaylistIds - playlists.map { it.id }).forEach { library.deletePlaylistSongsFrom(source.id, it, 0) }
                 }
             }
         }
+    }
 
-    private suspend fun prune(
-        existing: List<String>,
-        current: List<String>,
+    private suspend fun deleteStale(
+        stale: Set<String>,
         delete: suspend (Collection<String>) -> Unit,
     ) {
-        val currentIds = current.toHashSet()
-        existing
-            .filterNot { it in currentIds }
-            .chunked(500)
-            .forEach { delete(it) }
+        stale.chunked(DELETE_CHUNK).forEach { delete(it) }
     }
 }
 
+private const val DELETE_CHUNK = 500
+
 private val primaryKey = listOf("sourceId", "id")
 
 private val artistColumns = listOf("sourceId", "id", "name", "albumCount", "starred", "coverArt")

app/src/test/kotlin/com/subtracks/data/source/subsonic/SubsonicClientTest.kt

@@ -38,7 +38,7 @@ class SubsonicClientTest {
     fun tokenAuthSendsSaltAndHash() {
         server.enqueue(MockResponse().setBody("<subsonic-response status=\"ok\" version=\"1.16.1\"/>"))
 
-        client(tokenAuth = true).get("ping")
+        client(tokenAuth = true).check("ping")
 
         val url = server.takeRequest().requestUrl!!
         assertEquals("/rest/ping.view", url.encodedPath)
@@ -53,7 +53,7 @@ class SubsonicClientTest {
     fun plaintextAuthSendsPassword() {
         server.enqueue(MockResponse().setBody("<subsonic-response status=\"ok\" version=\"1.16.1\"/>"))
 
-        client(tokenAuth = false).get("ping")
+        client(tokenAuth = false).check("ping")
 
         val url = server.takeRequest().requestUrl!!
         assertEquals("secret", url.queryParameter("p"))
@@ -64,7 +64,7 @@ class SubsonicClientTest {
     fun paramsAreForwarded() {
         server.enqueue(MockResponse().setBody("<subsonic-response status=\"ok\" version=\"1.16.1\"/>"))
 
-        client(tokenAuth = true).get("getCoverArt", mapOf("id" to "cov", "size" to "256"))
+        client(tokenAuth = true).check("getCoverArt", mapOf("id" to "cov", "size" to "256"))
 
         val url = server.takeRequest().requestUrl!!
         assertEquals("cov", url.queryParameter("id"))
@@ -79,7 +79,7 @@ class SubsonicClientTest {
             ),
         )
 
-        val error = assertThrows(SubsonicException::class.java) { client(tokenAuth = true).get("ping") }
+        val error = assertThrows(SubsonicException::class.java) { client(tokenAuth = true).check("ping") }
         assertEquals(40, error.code)
         assertEquals("Wrong username", error.message)
     }
@@ -88,6 +88,6 @@ class SubsonicClientTest {
     fun nonSubsonicResponseThrows() {
         server.enqueue(MockResponse().setBody("<html><body>Sign in</body></html>"))
 
-        assertThrows(SubsonicException::class.java) { client(tokenAuth = true).get("ping") }
+        assertThrows(SubsonicException::class.java) { client(tokenAuth = true).check("ping") }
     }
 }

app/src/test/kotlin/com/subtracks/data/source/subsonic/SubsonicSourceTest.kt

@@ -1,5 +1,6 @@
 package com.subtracks.data.source.subsonic
 
+import kotlinx.coroutines.flow.toList
 import kotlinx.coroutines.runBlocking
 import okhttp3.OkHttpClient
 import okhttp3.mockwebserver.Dispatcher
@@ -53,7 +54,33 @@ class SubsonicSourceTest {
                 }
             val source = SubsonicSource(1, client())
 
-            assertEquals(listOf("s1", "s2"), source.getSongs().map { it.id })
+            assertEquals(
+                listOf("s1", "s2"),
+                source
+                    .songs()
+                    .toList()
+                    .flatten()
+                    .map { it.id },
+            )
+        }
+
+    @Test
+    fun artistsAreEmittedInBoundedBatches() =
+        runBlocking {
+            val count = 1200
+            val body =
+                buildString {
+                    append("<subsonic-response status=\"ok\"><artists>")
+                    repeat(count) { index ->
+                        append("<artist id=\"ar$index\" name=\"Artist $index\" albumCount=\"1\"/>")
+                    }
+                    append("</artists></subsonic-response>")
+                }
+            server.enqueue(MockResponse().setBody(body))
+
+            val sizes = SubsonicSource(1, client()).artists().toList().map { it.size }
+
+            assertEquals(listOf(500, 500, 200), sizes)
         }
 
     @Test

app/src/test/kotlin/com/subtracks/data/source/subsonic/SubsonicXmlTest.kt

@@ -2,19 +2,37 @@ package com.subtracks.data.source.subsonic
 
 import org.junit.Assert.assertEquals
 import org.junit.Test
-import org.w3c.dom.Document
-import javax.xml.parsers.DocumentBuilderFactory
+import org.xml.sax.Attributes
 
 class SubsonicXmlTest {
-    private fun parse(xml: String): Document =
-        DocumentBuilderFactory
-            .newInstance()
-            .newDocumentBuilder()
-            .parse(xml.byteInputStream(Charsets.UTF_8))
+    private fun <T> parse(
+        xml: String,
+        tag: String,
+        create: (Attributes) -> T,
+        onChild: (T, String, Attributes) -> Unit = { _, _, _ -> },
+    ): List<T> {
+        val result = mutableListOf<T>()
+        SubsonicXml.readEntities(
+            input = xml.byteInputStream(Charsets.UTF_8),
+            entityTag = tag,
+            create = create,
+            onChild = onChild,
+            onEnd = { result += it },
+        )
+        return result
+    }
+
+    private fun albums(xml: String) =
+        parse(
+            xml,
+            tag = "album",
+            create = { SubsonicXml.albumDraft(1, it) },
+            onChild = { draft, name, attrs -> if (name == "discTitles") draft.addDiscTitle(attrs) },
+        ).map { it.toAlbum() }
 
     @Test
     fun mapsArtists() {
-        val document =
+        val artists =
             parse(
                 """
                 <subsonic-response status="ok">
@@ -24,10 +42,10 @@ class SubsonicXmlTest {
                   </artists>
                 </subsonic-response>
                 """.trimIndent(),
+                tag = "artist",
+                create = { SubsonicXml.artist(7, it) },
             )
 
-        val artists = SubsonicXml.artists(7, document)
-
         assertEquals(2, artists.size)
         assertEquals("Radiohead", artists[0].name)
         assertEquals(7L, artists[0].sourceId)
@@ -38,8 +56,8 @@ class SubsonicXmlTest {
 
     @Test
     fun mapsAlbumList() {
-        val document =
-            parse(
+        val albums =
+            albums(
                 """
                 <subsonic-response status="ok">
                   <albumList2>
@@ -50,8 +68,6 @@ class SubsonicXmlTest {
                 """.trimIndent(),
             )
 
-        val albums = SubsonicXml.albums(1, document)
-
         assertEquals(2, albums.size)
         assertEquals("OK Computer", albums[0].name)
         assertEquals("Kid A", albums[1].name)
@@ -62,8 +78,8 @@ class SubsonicXmlTest {
 
     @Test
     fun mapsAlbumDiscTitles() {
-        val document =
-            parse(
+        val albums =
+            albums(
                 """
                 <subsonic-response status="ok">
                   <albumList2>
@@ -77,8 +93,6 @@ class SubsonicXmlTest {
                 """.trimIndent(),
             )
 
-        val albums = SubsonicXml.albums(1, document)
-
         assertEquals(mapOf(1L to "The Calm", 2L to "The Storm"), albums.single().discTitles)
     }
 
@@ -94,28 +108,52 @@ class SubsonicXmlTest {
 
     @Test
     fun mapsSongsAndPlaylistEntries() {
-        val document =
-            parse(
-                """
-                <subsonic-response status="ok">
-                  <searchResult3>
-                    <song id="sg2" title="Idioteque" artist="Radiohead" album="Kid A" albumId="al2" artistId="ar1" duration="300" track="8" discNumber="1"/>
-                  </searchResult3>
-                  <playlist id="pl1" name="Favourites" songCount="1" created="2021-02-03T04:05:06.000Z">
-                    <entry id="sg1" title="Everything In Its Right Place" artist="Radiohead" album="Kid A" albumId="al2" artistId="ar1" duration="251" track="1" discNumber="1"/>
-                  </playlist>
-                </subsonic-response>
-                """.trimIndent(),
-            )
+        val xml =
+            """
+            <subsonic-response status="ok">
+              <searchResult3>
+                <song id="sg2" title="Idioteque" artist="Radiohead" album="Kid A" albumId="al2" artistId="ar1" duration="300" track="8" discNumber="1"/>
+              </searchResult3>
+              <playlist id="pl1" name="Favourites" songCount="1" created="2021-02-03T04:05:06.000Z">
+                <entry id="sg1" title="Everything In Its Right Place" artist="Radiohead" album="Kid A" albumId="al2" artistId="ar1" duration="251" track="1" discNumber="1"/>
+              </playlist>
+            </subsonic-response>
+            """.trimIndent()
 
-        val songs = SubsonicXml.songs(1, document)
-        val entries = SubsonicXml.playlistSongs(1, "pl1", document)
+        val songs = parse(xml, tag = "song", create = { SubsonicXml.song(1, it) })
+        var position = 0L
+        val entries = parse(xml, tag = "entry", create = { SubsonicXml.playlistSong(1, "pl1", position++, it) })
 
         assertEquals(1, songs.size)
         assertEquals("Idioteque", songs[0].title)
         assertEquals(300L, songs[0].duration)
         assertEquals(0L, entries[0].position)
         assertEquals("sg1", entries[0].songId)
-        assertEquals(251L, SubsonicXml.song(1, document.getElementsByTagName("entry").item(0) as org.w3c.dom.Element).duration)
+    }
+
+    @Test
+    fun statusOnlyReadRejectsAnHtmlLoginPage() {
+        val failure =
+            runCatching {
+                SubsonicXml.readStatus("<html><body>Sign in</body></html>".byteInputStream(Charsets.UTF_8))
+            }
+
+        assertEquals(true, failure.exceptionOrNull() is SubsonicException)
+    }
+
+    @Test
+    fun containsEntityStopsAtTheFirstMatch() {
+        val xml =
+            """
+            <subsonic-response status="ok">
+              <searchResult3>
+                <song id="s1" title="One"/>
+                <song id="s2" title="Two"/>
+              </searchResult3>
+            </subsonic-response>
+            """.trimIndent()
+
+        assertEquals(true, SubsonicXml.containsEntity(xml.byteInputStream(Charsets.UTF_8), "song"))
+        assertEquals(false, SubsonicXml.containsEntity(xml.byteInputStream(Charsets.UTF_8), "artist"))
     }
 }

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

@@ -14,7 +14,9 @@ import com.subtracks.data.model.PlaylistSong
 import com.subtracks.data.model.Song
 import com.subtracks.data.model.Source
 import com.subtracks.data.source.MusicSource
+import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.first
+import kotlinx.coroutines.flow.flow
 import kotlinx.coroutines.test.runTest
 import org.junit.After
 import org.junit.Assert.assertEquals
@@ -293,6 +295,103 @@ class SyncServiceTest {
             assertEquals(mapOf(1L to "New"), storedDiscs("al1"))
         }
 
+    @Test
+    fun syncCommitsMultipleBatchesAndPrunes() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    artists = (1..5).map { artist("a$it") },
+                    songs = (1..5).map { song("s$it") },
+                    batchSize = 2,
+                )
+
+            SyncService(db, source).sync()
+
+            assertEquals((1..5).map { "a$it" }, db.libraryDao().artistIds(1).sorted())
+            assertEquals((1..5).map { "s$it" }, db.libraryDao().songIds(1).sorted())
+
+            source.artists = (1..3).map { artist("a$it") }
+            source.songs = (1..3).map { song("s$it") }
+
+            SyncService(db, source).sync()
+
+            assertEquals((1..3).map { "a$it" }, db.libraryDao().artistIds(1).sorted())
+            assertEquals((1..3).map { "s$it" }, db.libraryDao().songIds(1).sorted())
+        }
+
+    @Test
+    fun playlistSongsSplitAcrossBatchesKeepTheirTail() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    songs = (1..5).map { song("s$it") },
+                    playlists = listOf(playlist("p1")),
+                    playlistSongs = (1..5).map { PlaylistSong(1, "p1", "s$it", (it - 1).toLong()) },
+                    batchSize = 2,
+                )
+
+            SyncService(db, source).sync()
+            assertEquals(
+                5,
+                db
+                    .libraryDao()
+                    .playlistSongs(1, "p1")
+                    .allRows()
+                    .size,
+            )
+
+            source.playlistSongs = listOf(PlaylistSong(1, "p1", "s1", 0), PlaylistSong(1, "p1", "s2", 1))
+
+            SyncService(db, source).sync()
+            assertEquals(
+                2,
+                db
+                    .libraryDao()
+                    .playlistSongs(1, "p1")
+                    .allRows()
+                    .size,
+            )
+        }
+
+    @Test
+    fun removingAPlaylistRemovesItsSongs() =
+        runTest {
+            insertSource()
+            val source =
+                FakeMusicSource(
+                    songs = listOf(song("s1")),
+                    playlists = listOf(playlist("p1")),
+                    playlistSongs = listOf(PlaylistSong(1, "p1", "s1", 0)),
+                )
+
+            SyncService(db, source).sync()
+            assertEquals(
+                1,
+                db
+                    .libraryDao()
+                    .playlistSongs(1, "p1")
+                    .allRows()
+                    .size,
+            )
+
+            source.playlists = emptyList()
+            source.playlistSongs = emptyList()
+
+            SyncService(db, source).sync()
+
+            assertEquals(0, db.libraryDao().playlistIds(1).size)
+            assertEquals(
+                0,
+                db
+                    .libraryDao()
+                    .playlistSongs(1, "p1")
+                    .allRows()
+                    .size,
+            )
+        }
+
     private suspend fun storedDiscs(albumId: String): Map<Long, String> =
         db
             .libraryDao()
@@ -359,18 +458,24 @@ private class FakeMusicSource(
     var songs: List<Song> = emptyList(),
     var playlists: List<Playlist> = emptyList(),
     var playlistSongs: List<PlaylistSong> = emptyList(),
+    private val batchSize: Int = 1000,
 ) : MusicSource {
     override val id: Long = 1
 
     override suspend fun ping() = Unit
 
-    override suspend fun getArtists(): List<Artist> = artists
+    override fun artists(): Flow<List<Artist>> = batches(artists)
 
-    override suspend fun getAlbums(): List<Album> = albums
+    override fun albums(): Flow<List<Album>> = batches(albums)
 
-    override suspend fun getSongs(): List<Song> = songs
+    override fun songs(): Flow<List<Song>> = batches(songs)
 
-    override suspend fun getPlaylists(): List<Playlist> = playlists
+    override fun playlists(): Flow<List<Playlist>> = batches(playlists)
 
-    override suspend fun getPlaylistSongs(playlists: List<Playlist>): List<PlaylistSong> = playlistSongs
+    override fun playlistSongs(playlistIds: List<String>): Flow<List<PlaylistSong>> = batches(playlistSongs)
+
+    private fun <T> batches(items: List<T>): Flow<List<T>> =
+        flow {
+            items.chunked(batchSize).forEach { emit(it) }
+        }
 }