Refactor WebSocket ACK handling and improve message retries

Introduced `waitForAck` to centralize ACK handling logic and updated retry mechanism in `sendWithRetry` to improve reliability and readability. Cleaned up error handling in WebSocket session management and ensured proper cleanup of resources. These changes enhance maintainability and robustness of the WebSocket song list routes.

webserver/src/main/kotlin/space/mori/chzzk_bot/webserver/routes/WSSongListRoutes.kt

@@ -49,6 +49,8 @@ fun Routing.wsSongListRoutes() {
         songListScope.cancel()
     }
 
+    val ackMap = ConcurrentHashMap<String, CompletableDeferred<Boolean>>()
+
     suspend fun addSession(uid: String, session: WebSocketServerSession) {
         val oldSession = sessionMutex.withLock {
             val old = sessions[uid]
@@ -62,22 +64,33 @@ fun Routing.wsSongListRoutes() {
                         CloseReason.Codes.VIOLATED_POLICY, "Another session is already active."))
                 } catch(e: Exception) {
                     logger.warn("Error closing old session: ${e.message}")
-                    e.printStackTrace()
                 }
             }
         }
     }
+
     suspend fun removeSession(uid: String) {
         sessionMutex.withLock {
             sessions.remove(uid)
         }
     }
 
-    val ackMap = ConcurrentHashMap<String, CompletableDeferred<Boolean>>()
+    suspend fun waitForAck(ws: WebSocketServerSession, expectedUid: String): Boolean {
+        val ackDeferred = CompletableDeferred<Boolean>()
+        ackMap[expectedUid] = ackDeferred
+        return try {
+            withTimeoutOrNull(5000L) { ackDeferred.await() } ?: false
+        } catch (e: CancellationException) {
+            false
+        } finally {
+            ackMap.remove(expectedUid)
+        }
+    }
 
     suspend fun sendWithRetry(uid: String, res: SongResponse, maxRetries: Int = 5, delayMillis: Long = 3000L) {
         var attempt = 0
-        while (attempt < maxRetries) {
+        var sentSuccessfully = false
+        while (attempt < maxRetries && !sentSuccessfully) {
             val ws: WebSocketServerSession? = sessionMutex.withLock { sessions[uid] }
             if (ws == null) {
                 logger.debug("No active session for $uid. Retrying in $delayMillis ms.")
@@ -87,13 +100,10 @@ fun Routing.wsSongListRoutes() {
             }
             try {
                 ws.sendSerialized(res)
-                logger.debug("Message sent successfully to $uid on attempt $attempt")
-                val ackDeferred = CompletableDeferred<Boolean>()
-                ackMap[res.uid] = ackDeferred
-                val ackReceived = withTimeoutOrNull(delayMillis) { ackDeferred.await() } ?: false
+                val ackReceived = waitForAck(ws, res.uid)
                 if (ackReceived) {
                     logger.debug("ACK received for message to $uid on attempt $attempt.")
-                    return
+                    sentSuccessfully = true
                 } else {
                     logger.warn("ACK not received for message to $uid on attempt $attempt.")
                     attempt++
@@ -104,15 +114,16 @@ fun Routing.wsSongListRoutes() {
                 attempt++
                 logger.warn("Failed to send message to $uid on attempt $attempt: ${e.message}")
                 if (e is WebSocketException || e is IOException) {
-                    logger.warn("Connection issue detected, session may be invalid")
                     removeSession(uid)
                 }
             }
-            if (attempt < maxRetries) {
+            if (!sentSuccessfully && attempt < maxRetries) {
                 delay(delayMillis)
             }
         }
-        logger.error("Failed to send message to $uid after $maxRetries attempts.")
+        if (!sentSuccessfully) {
+            logger.error("Failed to send message to $uid after $maxRetries attempts.")
+        }
     }
 
     webSocket("/songlist") {
@@ -126,18 +137,23 @@ fun Routing.wsSongListRoutes() {
         addSession(uid, this)
         if (status[uid] == SongType.STREAM_OFF) {
             songListScope.launch {
-                sendSerialized(SongResponse(
-                    SongType.STREAM_OFF.value,
-                    uid,
-                    null,
-                    null,
-                    null,
-                ))
+                try {
+                    sendSerialized(SongResponse(
+                        SongType.STREAM_OFF.value,
+                        uid,
+                        null,
+                        null,
+                        null,
+                    ))
+                } catch (e: Exception) {
+                    logger.warn("Error sending STREAM_OFF: ${e.message}")
+                } finally {
+                    removeSession(uid)
+                }
             }
-            removeSession(uid)
+            return@webSocket
         }
-
-        songListScope.launch {
+        try {
             for (frame in incoming) {
                 when (frame) {
                     is Frame.Text -> {
@@ -148,7 +164,6 @@ fun Routing.wsSongListRoutes() {
                             val data = Json.decodeFromString<SongRequest>(text)
                             if (data.type == SongType.ACK.value) {
                                 ackMap[data.uid]?.complete(true)
-                                ackMap.remove(data.uid)
                             } else {
                                 handleSongRequest(data, user, dispatcher, logger)
                             }
@@ -158,17 +173,13 @@ fun Routing.wsSongListRoutes() {
                     else -> {}
                 }
             }
-        }
-
-        try {
-            // Keep the connection alive
-            suspendCancellableCoroutine<Unit> {}
         } catch (e: ClosedReceiveChannelException) {
             logger.error("WebSocket connection closed: ${e.message}")
-        } catch(e: Exception) {
+        } catch (e: Exception) {
             logger.error("Error in WebSocket: ${e.message}")
         } finally {
             removeSession(uid)
+            ackMap.remove(uid)
         }
     }
 
@@ -219,7 +230,7 @@ fun Routing.wsSongListRoutes() {
 }
 // 노래 처리를 위한 Mutex 추가
 private val songMutex = Mutex()
-suspend fun handleSongRequest(
+fun handleSongRequest(
     data: SongRequest,
     user: User,
     dispatcher: CoroutinesEventBus,