symat opened a new pull request #1681:
URL: https://github.com/apache/zookeeper/pull/1681
When a ZooKeeper server realizes that an other quorum peer was shut down
(e.g. during a rolling upgrade or
rolling restart), the ServerCnxn.zkServer variable is set to null by
QuorumPear.close(). This is why in the code
we usually check the zkServer variable before using it. But this check was
missing in one place thus causing
NPE in NettyServerCnx.receiveMessage:
```
2021-02-08T12:42:08.229+0000 [myid:] - ERROR
[nioEventLoopGroup-4-1:NettyServerCnxnFactory$CnxnChannelHandler@329]-
Unexpected exception in receive
java.lang.NullPointerException: null ~[zookeeper-3.6.2.jar:3.6.2]
at
org.apache.zookeeper.server.NettyServerCnxn.receiveMessage(NettyServerCnxn.java:518)
at
org.apache.zookeeper.server.NettyServerCnxn.processMessage(NettyServerCnxn.java:368)
at org.apache.zookeeper.server.NettyServerCnxnFactory
$CnxnChannelHandler.channelRead(NettyServerCnxnFactory.java:326)
...
```
In this commit we add the necessary check and (after throwing an
IOException) we will basically ignore the
processing of the received message when the remote ZooKeeper server is
already down.
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