November22 opened a new issue #2693:
URL: https://github.com/apache/rocketmq/issues/2693
RocketMQ日志（4.1.0-incubating版本发送异常的堆栈信息没有输出）：
2021-02-15 05:48:06 WARN MQClientFactoryScheduledThread - createChannel:
connect remote host[****:20819] timeout 3000ms,
DefaultChannelPromise@6838ce98(uncancellable)
2021-02-15 05:48:06 INFO MQClientFactoryScheduledThread - send heart beat to
broker[broker-a 0 ****:20819] failed
可以确认Broker通信没有问题，因为其他应用的心跳发送正常，在发送心跳连接超时的服务器上发现Netty异常信息如下：
2021-02-20
00:00:01.800|7596535444064e98b80e3250ff73c398|NettyClientSelector_1|WARN
|i.n.c.n.NioEventLoop-Unexpected exception in the selector loop.
io.netty.util.IllegalReferenceCountException: refCnt: 0, decrement: 1
at
io.netty.buffer.AbstractReferenceCountedByteBuf.release(AbstractReferenceCountedByteBuf.java:101)
at
io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.handleReadException(AbstractNioByteChannel.java:83)
at
io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:161)
at
io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
at
io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
at
io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
at
io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
at java.lang.Thread.run(Thread.java:748)
Netty依赖包： netty-3.10.6.Final.jar、netty-all-4.0.24.Final.jar
不确定是不是这个引起的发送异常。
解决方式：重启应用服务器，心跳发送正常。
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