feat(server): implement port fallback mechanism for playback proxy server

composeApp/src/commonMain/kotlin/org/spotube/spotube/core/server/LocalServer.kt

@@ -154,17 +154,26 @@ class LocalServer(
             logger.d { "Restarting playback proxy server on $host:$port (remoteControl=$allowRemoteControl)" }
             stopServerLocked()
 
-            serverState.value = embeddedServer(
-                factory = CIO,
+            val bound = runCatching {
+                startServerWithPortFallback(
                     host = host,
-                port = port,
-                module = { configureRoutes() }
-            ).also { engine ->
-                engine.start(wait = false)
+                    preferredPort = port,
+                    onBindFailure = { failedPort, cause ->
+                        logger.w(cause) { "Playback proxy could not bind to $host:$failedPort" }
                     }
-            activePort.value = port
+                ) { configureRoutes() }
+            }.getOrElse { throwable ->
+                logger.e(throwable) { "Playback proxy failed to start on $host (requested port $port)" }
+                return
+            }
+
+            if (bound.usedFallback) {
+                logger.w { "Port $port is unavailable; playback proxy is using $host:${bound.port} instead" }
+            }
+            serverState.value = bound.engine
+            activePort.value = bound.port
             activeHost.value = host
-            logger.i { "Playback proxy server started at ${baseUrl.value ?: "http://$host:$port"}" }
+            logger.i { "Playback proxy server started at ${baseUrl.value ?: "http://$host:${bound.port}"}" }
         }
     }
 

composeApp/src/commonMain/kotlin/org/spotube/spotube/core/server/ServerBinder.kt

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2026 Kingkor Roy Tirtho and Spotube Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+package org.spotube.spotube.core.server
+
+import io.ktor.server.application.Application
+import io.ktor.server.cio.CIO
+import io.ktor.server.engine.EmbeddedServer
+import io.ktor.server.engine.embeddedServer
+import kotlinx.coroutines.currentCoroutineContext
+import kotlinx.coroutines.ensureActive
+import kotlinx.coroutines.withTimeout
+import kotlin.time.Duration.Companion.seconds
+
+internal class BoundServer(val engine: EmbeddedServer<*, *>, val port: Int, val usedFallback: Boolean)
+
+/**
+ * Starts the playback proxy on [preferredPort] and confirms the socket is actually bound.
+ *
+ * CIO's `start(wait = false)` returns before binding, so a port that is already taken fails
+ * later on a worker thread and the server looks started while nothing is listening. If the
+ * preferred port can't be bound, retry on an OS-assigned free port (port 0).
+ *
+ * The caller must read [BoundServer.port] instead of assuming [preferredPort].
+ */
+internal suspend fun startServerWithPortFallback(
+    host: String,
+    preferredPort: Int,
+    onBindFailure: (port: Int, cause: Throwable) -> Unit = { _, _ -> },
+    module: Application.() -> Unit
+): BoundServer {
+    val candidates = listOf(preferredPort, 0).distinct()
+    var lastFailure: Throwable? = null
+
+    for (candidate in candidates) {
+        val engine = embeddedServer(factory = CIO, host = host, port = candidate, module = module)
+        try {
+            engine.start(wait = false)
+            val boundPort = withTimeout(5.seconds) { engine.engine.resolvedConnectors().first().port }
+            return BoundServer(engine, boundPort, usedFallback = candidate != preferredPort)
+        } catch (e: Throwable) {
+            runCatching { engine.stop(gracePeriodMillis = 0, timeoutMillis = 500) }
+            // A failed bind surfaces as the engine job's CancellationException; only rethrow
+            // when it is the caller's own coroutine that was cancelled.
+            currentCoroutineContext().ensureActive()
+            onBindFailure(candidate, e)
+            lastFailure = e
+        }
+    }
+    throw lastFailure ?: IllegalStateException("No port candidates to bind")
+}

composeApp/src/commonMain/kotlin/org/spotube/spotube/core/server/StreamProxy.kt

@@ -58,6 +58,7 @@ import okio.buffer
 import okio.use
 import org.koin.core.component.KoinComponent
 import kotlin.time.Duration.Companion.milliseconds
+import kotlin.time.TimeSource
 
 internal class StreamProxy(
     private val httpClient: HttpClient,
@@ -92,6 +93,13 @@ internal class StreamProxy(
             return
         }
 
+        logger.i {
+            "[Req] $requestMethod /stream/$trackId from=${call.request.local.remoteHost} " +
+                "version=${call.request.local.version} range=${call.request.headers[HttpHeaders.Range]} " +
+                "ua=${call.request.headers[HttpHeaders.UserAgent]} " +
+                "connection=${call.request.headers[HttpHeaders.Connection]} caching=${isCachingEnabled()}"
+        }
+
         if (isCachingEnabled()) {
             val cachedEntry = cacheManager.findCachedEntry(trackId)
             if (cachedEntry != null) {
@@ -152,7 +160,13 @@ internal class StreamProxy(
     }
 
     private suspend fun doStreamFromUpstream(call: ApplicationCall, trackId: String, requestMethod: HttpMethod) {
+        val resolveStart = TimeSource.Monotonic.markNow()
         val streamInfo = streamingUrlRepository.resolveStreamInfo(trackId)
+        logger.i {
+            "[Req] resolved stream for $trackId in ${resolveStart.elapsedNow()}: " +
+                "protocol=${streamInfo?.protocol} codec=${streamInfo?.codec} container=${streamInfo?.container} " +
+                "url=${streamInfo?.url?.take(120)}"
+        }
         if (streamInfo == null) {
             logger.w { "Unable to resolve stream for track $trackId" }
             call.respond(HttpStatusCode.NotFound, "Unable to resolve stream for track $trackId")
@@ -319,6 +333,7 @@ internal class StreamProxy(
 
         var currentUrl = streamUrl
         var attemptedRefresh = false
+        val upstreamStart = TimeSource.Monotonic.markNow()
 
         // `request()` buffers the whole body before returning, which stalls the player until
         // the full track is downloaded. `execute {}` hands over the response as soon as headers
@@ -333,6 +348,11 @@ internal class StreamProxy(
                     call.forwardRequestHeaderIfPresent(headers, HttpHeaders.Accept)
                     call.forwardRequestHeaderIfPresent(headers, HttpHeaders.UserAgent)
                 }.execute { response ->
+                    logger.i {
+                        "[Req] upstream headers for $trackId after ${upstreamStart.elapsedNow()}: " +
+                            "status=${response.status} version=${response.version} " +
+                            "headers=${response.headers.entries().joinToString { "${it.key}=${it.value}" }}"
+                    }
                     if (response.status.value < 400) {
                         respondWithUpstream(call, response, trackId, requestMethod)
                         return@execute
@@ -356,7 +376,7 @@ internal class StreamProxy(
             if (failure != null) {
                 if (failure is CancellationException) throw failure
                 if (failure.isClientDisconnect()) {
-                    logger.d { "Player closed the connection while streaming track $trackId" }
+                    logger.i(failure) { "[Req] player closed the connection while streaming track $trackId" }
                     return
                 }
                 logger.w(failure) { "Failed to fetch upstream stream for track $trackId" }
@@ -432,6 +452,18 @@ internal class StreamProxy(
                 downstreamHeaders[HttpHeaders.ContentType]?.let(ContentType.Companion::parse)
 
             override suspend fun writeTo(channel: io.ktor.utils.io.ByteWriteChannel) {
+                val writeStart = TimeSource.Monotonic.markNow()
+                logger.i { "[Req] start writing body for $trackId status=${upstream.status} cache=${cacheFilename != null} expected=$expectedLength" }
+                try {
+                    writeBody(channel)
+                    logger.i { "[Req] finished writing body for $trackId in ${writeStart.elapsedNow()}" }
+                } catch (e: Throwable) {
+                    logger.i(e) { "[Req] body write for $trackId aborted after ${writeStart.elapsedNow()}: ${e::class.simpleName}: ${e.message}" }
+                    throw e
+                }
+            }
+
+            private suspend fun writeBody(channel: io.ktor.utils.io.ByteWriteChannel) {
                 if (cacheFilename != null) {
                     val cacheDir = cacheManager.resolveCacheDir()
                     val cacheFile = cacheDir / cacheFilename.toPath()

composeApp/src/jvmTest/kotlin/org/spotube/spotube/core/server/ServerBinderTest.kt

@@ -0,0 +1,72 @@
+/*
+ * Copyright (C) 2026 Kingkor Roy Tirtho and Spotube Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+package org.spotube.spotube.core.server
+
+import io.ktor.server.response.respondText
+import io.ktor.server.routing.get
+import io.ktor.server.routing.routing
+import kotlinx.coroutines.runBlocking
+import java.net.InetAddress
+import java.net.ServerSocket
+import java.net.URI
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotEquals
+import kotlin.test.assertTrue
+
+class ServerBinderTest {
+    private val host = "127.0.0.1"
+
+    private fun health(port: Int) =
+        URI("http://$host:$port/health").toURL().readText()
+
+    @Test
+    fun usesPreferredPortWhenFree() = runBlocking {
+        val free = ServerSocket(0, 1, InetAddress.getByName(host)).use { it.localPort }
+        val bound = startServerWithPortFallback(host, free) {
+            routing { get("/health") { call.respondText("ok") } }
+        }
+        try {
+            assertEquals(free, bound.port)
+            assertTrue(!bound.usedFallback)
+            assertEquals("ok", health(bound.port))
+        } finally {
+            bound.engine.stop(0, 500)
+        }
+    }
+
+    @Test
+    fun fallsBackToAnotherPortWhenPreferredIsTaken() = runBlocking {
+        ServerSocket(0, 1, InetAddress.getByName(host)).use { blocker ->
+            val taken = blocker.localPort
+            val failures = mutableListOf<Int>()
+            val bound = startServerWithPortFallback(host, taken, { port, _ -> failures += port }) {
+                routing { get("/health") { call.respondText("ok") } }
+            }
+            try {
+                assertNotEquals(taken, bound.port)
+                assertTrue(bound.usedFallback)
+                assertEquals(listOf(taken), failures)
+                assertEquals("ok", health(bound.port))
+            } finally {
+                bound.engine.stop(0, 500)
+            }
+        }
+    }
+}

gradle/libs.versions.toml

@@ -55,7 +55,6 @@ vlcjNative = "4.12.0"
 ziplineVersion = "1.27.0"
 ktor = "3.5.1"
 dnssdkt = "1.1.0"
-kotlinStdlib = "2.4.10"
 runner = "1.7.0"
 core = "1.7.0"
 filekit = "0.14.2"
@@ -148,7 +147,7 @@ ktor-client-serialization-kotlinx-json = { module = "io.ktor:ktor-serialization-
 kotlinx-coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "kotlinx-coroutines" }
 kotlinx-coroutines-android = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-android", version.ref = "kotlinx-coroutines" }
 kotlinx-coroutines-test = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-test", version.ref = "kotlinx-coroutines" }
-kotlin-stdlib = { group = "org.jetbrains.kotlin", name = "kotlin-stdlib", version.ref = "kotlinStdlib" }
+kotlin-stdlib = { group = "org.jetbrains.kotlin", name = "kotlin-stdlib", version.ref = "kotlin" }
 androidx-runner = { group = "androidx.test", name = "runner", version.ref = "runner" }
 androidx-core = { group = "androidx.test", name = "core", version.ref = "core" }
 filekit-core = { group = "io.github.vinceglb", name = "filekit-core", version.ref = "filekit" }
@@ -186,8 +185,8 @@ jetbrainsKotlinJvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
 androidKotlinMultiplatformLibrary = { id = "com.android.kotlin.multiplatform.library", version.ref = "agp" }
 zipline-gradle-plugin = { id = "app.cash.zipline", version.ref = "ziplineVersion" }
 mavenPublish = { id = "maven-publish" }
-spotubeGradle = { id = "dev.krtirtho.spotube.gradle-plugin", version.ref = "spotube-gradle" }
-vlcjBundler = { id = "dev.krtirtho.vlcj-bundler.gradle-plugin", version.ref = "vlcjBundler" }
+spotubeGradle = { id = "org.spotube.gradle-plugin", version.ref = "spotube-gradle" }
+vlcjBundler = { id = "org.spotube.vlcj-bundler", version.ref = "vlcjBundler" }
 kotlinxAtomicFu = { id = "org.jetbrains.kotlinx.atomicfu", version.ref = "kotlinx-atomicfu" }
 uniffi = { id = "dev.gobley.uniffi", version.ref = "uniffi" }
 cargo = { id = "dev.gobley.cargo", version.ref = "uniffi" }