Feature: Gracefully handling Lavalink restarts

[jchv]

I noticed that if you restart Lavalink, the state becomes inconsistent between services. It seems likely that it would be possible to detect this scenario and handle it a bit more gracefully.

For example, trying to play again after restarting Lavalink apparently silently failed, leaving this in the Lavalink log:

2020-05-29 22:29:12.141 ERROR 1 --- [   XNIO-1 I/O-1] lavalink.server.io.SocketServer          : Exception while handling websocket message
java.lang.RuntimeException: Can't seek when not playing anything
	at lavalink.server.player.Player.seekTo(Player.java:86) ~[classes!/:na]
	at lavalink.server.io.WebSocketHandlers.seek(WebSocketHandlers.kt:92) ~[classes!/:na]
	at lavalink.server.io.SocketServer.handleTextMessageSafe(SocketServer.kt:150) ~[classes!/:na]
	at lavalink.server.io.SocketServer.handleTextMessage(SocketServer.kt:127) ~[classes!/:na]
	at org.springframework.web.socket.handler.AbstractWebSocketHandler.handleMessage(AbstractWebSocketHandler.java:43) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.handler.WebSocketHandlerDecorator.handleMessage(WebSocketHandlerDecorator.java:75) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.handler.LoggingWebSocketHandlerDecorator.handleMessage(LoggingWebSocketHandlerDecorator.java:56) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.handler.ExceptionWebSocketHandlerDecorator.handleMessage(ExceptionWebSocketHandlerDecorator.java:58) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.handleTextMessage(StandardWebSocketHandlerAdapter.java:113) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.access$000(StandardWebSocketHandlerAdapter.java:42) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$3.onMessage(StandardWebSocketHandlerAdapter.java:84) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$3.onMessage(StandardWebSocketHandlerAdapter.java:81) ~[spring-websocket-5.1.9.RELEASE.jar!/:5.1.9.RELEASE]
	at io.undertow.websockets.jsr.FrameHandler$7.run(FrameHandler.java:286) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.ServerWebSocketContainer$1.call(ServerWebSocketContainer.java:170) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.ServerWebSocketContainer$1.call(ServerWebSocketContainer.java:167) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43) ~[undertow-servlet-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.ServerWebSocketContainer.invokeEndpointMethod(ServerWebSocketContainer.java:604) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.ServerWebSocketContainer.invokeEndpointMethod(ServerWebSocketContainer.java:594) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.FrameHandler.invokeTextHandler(FrameHandler.java:266) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.FrameHandler.onFullTextMessage(FrameHandler.java:317) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.AbstractReceiveListener$2.complete(AbstractReceiveListener.java:156) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.AbstractReceiveListener$2.complete(AbstractReceiveListener.java:152) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.BufferedTextMessage.read(BufferedTextMessage.java:105) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.AbstractReceiveListener.readBufferedText(AbstractReceiveListener.java:152) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.AbstractReceiveListener.bufferFullMessage(AbstractReceiveListener.java:90) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.jsr.FrameHandler.onText(FrameHandler.java:182) ~[undertow-websockets-jsr-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.AbstractReceiveListener.handleEvent(AbstractReceiveListener.java:44) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.websockets.core.AbstractReceiveListener.handleEvent(AbstractReceiveListener.java:33) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92) ~[xnio-api-3.3.8.Final.jar!/:3.3.8.Final]
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:951) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:932) ~[undertow-core-2.0.26.Final.jar!/:2.0.26.Final]
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92) ~[xnio-api-3.3.8.Final.jar!/:3.3.8.Final]
	at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66) ~[xnio-api-3.3.8.Final.jar!/:3.3.8.Final]
	at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:88) ~[xnio-nio-3.3.8.Final.jar!/:3.3.8.Final]
	at org.xnio.nio.WorkerThread.run(WorkerThread.java:561) ~[xnio-nio-3.3.8.Final.jar!/:3.3.8.Final]

Encountered this with Fredboat version:

	Version:       4.0
	Build:         DEV
	Commit:        cede111 (dev)
	Commit time:   2020-05-27T03:12:13-0700
	JVM:           11.0.7
	Lavaplayer     1.3.38

and Lavalink version:

	Version:        606fbf3ca475d39e15183fe91ab0149fa17d0fe9-SNAPSHOT
	Build:          1061
	Build time:     29.05.2020 10:23:19 UTC
	Branch          master
	Commit:         606fbf3
	Commit time:    29.05.2020 10:19:10 UTC
	JVM:            13.0.2
	Lavaplayer      1.3.49

I'm leaving this here in case there's more to understand about this, but I am interested in possibly picking it up in the future if it doesn't get fixed and it's an easy enough fix. (Currently, I am not really sure if there's any smoother ways to update services with less disruption, but if I am missing some documentation/info on the matter please do let me know.)

[freyacodes]

I'm aware of this issue. I believe it is a Lavalink-Client thing.