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  1. Undertow
  2. UNDERTOW-2283

UT000094: Blocking await method called from IO thread. Blocking IO must be dispatched to a worker thread or deadlocks will result.

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    • Icon: Question Question
    • Resolution: Unresolved
    • Icon: Major Major
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    • Core, Web Sockets
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      Phenomenon

      Hello.

      We are using the below library for WebSocket.

      • org.springframework.boot:spring-boot-starter-undertow:2.7.4
        • io.undertow:undertow-core:2.2.19.Final
        • io.undertow:undertow-servlet:2.2.19.Final
        • io.undertow:undertow-websockets-jsr:2.2.19.Final

      Recently I received the following error message.

      java.io.IOException: UT000094: Blocking await method called from IO thread. Blocking IO must be dispatched to a worker thread or deadlocks will result.
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.awaitWritable(AbstractFramedStreamSinkChannel.java:295)
	at io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.awaitWritable(AbstractFramedStreamSinkChannel.java:289)
	at org.xnio.channels.Channels.flushBlocking(Channels.java:64)
	at io.undertow.websockets.jsr.WebSocketSessionRemoteEndpoint$BasicWebSocketSessionRemoteEndpoint.sendText(WebSocketSessionRemoteEndpoint.java:287)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketSession.sendTextMessage(StandardWebSocketSession.java:215)
	at org.springframework.web.socket.adapter.AbstractWebSocketSession.sendMessage(AbstractWebSocketSession.java:106)
	at com.kakaomobility.bone.connection.handler.WebSocketMessageHandler.handleTextMessage(WebSocketMessageHandler.java:126)
	at org.springframework.web.socket.handler.AbstractWebSocketHandler.handleMessage(AbstractWebSocketHandler.java:43)
	at org.springframework.web.socket.handler.WebSocketHandlerDecorator.handleMessage(WebSocketHandlerDecorator.java:75)
	at org.springframework.web.socket.handler.LoggingWebSocketHandlerDecorator.handleMessage(LoggingWebSocketHandlerDecorator.java:56)
	at org.springframework.web.socket.handler.ExceptionWebSocketHandlerDecorator.handleMessage(ExceptionWebSocketHandlerDecorator.java:58)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.handleTextMessage(StandardWebSocketHandlerAdapter.java:114)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter.access$000(StandardWebSocketHandlerAdapter.java:43)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$3.onMessage(StandardWebSocketHandlerAdapter.java:85)
	at org.springframework.web.socket.adapter.standard.StandardWebSocketHandlerAdapter$3.onMessage(StandardWebSocketHandlerAdapter.java:82)
	at io.undertow.websockets.jsr.FrameHandler$7.run(FrameHandler.java:288)
	at io.undertow.websockets.jsr.ServerWebSocketContainer$1.call(ServerWebSocketContainer.java:170)
	at io.undertow.websockets.jsr.ServerWebSocketContainer$1.call(ServerWebSocketContainer.java:167)
	at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43)
	at io.undertow.websockets.jsr.ServerWebSocketContainer.invokeEndpointMethod(ServerWebSocketContainer.java:610)
	at io.undertow.websockets.jsr.ServerWebSocketContainer.invokeEndpointMethod(ServerWebSocketContainer.java:600)
	at io.undertow.websockets.jsr.FrameHandler.invokeTextHandler(FrameHandler.java:268)
	at io.undertow.websockets.jsr.FrameHandler.onFullTextMessage(FrameHandler.java:319)
	at io.undertow.websockets.core.AbstractReceiveListener$2.complete(AbstractReceiveListener.java:156)
	at io.undertow.websockets.core.AbstractReceiveListener$2.complete(AbstractReceiveListener.java:152)
	at io.undertow.websockets.core.BufferedTextMessage.read(BufferedTextMessage.java:105)
	at io.undertow.websockets.core.AbstractReceiveListener.readBufferedText(AbstractReceiveListener.java:152)
	at io.undertow.websockets.core.AbstractReceiveListener.bufferFullMessage(AbstractReceiveListener.java:90)
	at io.undertow.websockets.jsr.FrameHandler.onText(FrameHandler.java:184)
	at io.undertow.websockets.core.AbstractReceiveListener.handleEvent(AbstractReceiveListener.java:44)
	at io.undertow.websockets.core.AbstractReceiveListener.handleEvent(AbstractReceiveListener.java:33)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:959)
	at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:939)
	at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92)
	at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66)
	at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:89)
	at org.xnio.nio.WorkerThread.run(WorkerThread.java:591)

       

       

      When looking at the undertow code, it is understood that it is called in the flow below.

      1. org.xnio.channels.Channels.flushBlocking()
      2. io.undertow.server.protocol.framed.AbstractFramedStreamSinkChannel.
        awaitWritable()

       

      In the last called "awaitWritable()", it is understood that if the current thread is an IO thread, it will throw an exception.


       

      Question

      Regarding this, I have a few questions.

      1. Why raise an exception when the current thread is an IO thread?
      2. Any possible causes for this error to occur?

       

      Thank you.
       

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            flaviarnn Flavia Rainone
            raccoonback KOSEUNGBIN oliver (Inactive)
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              Created:
              Updated: