Potential NPE in Observer#observeLeader and Follower#followLeader

[brettKK]

@lj1043041006 found a potential NPE in ZK

Bug description:

callee Learner#findLeader will return null and callee developer check it but just log:

// code placeholder
if (leaderServer == null) {
   LOG.warn("Couldn't find the leader with id = " + current.getId());
}
return leaderServer;

caller Observer#observeLeader and Follower#followLeader will directly use return value w/o null check:

//Follower#followLeader
QuorumServer leaderServer = findLeader();
try {
    connectToLeader(leaderServer.addr, leaderServer.hostname);
    ..........
}
//Observer#observeLeader
QuorumServer leaderServer = findLeader();
LOG.info("Observing " + leaderServer.addr);
try {
    connectToLeader(leaderServer.addr, leaderServer.hostname);
}

https://issues.apache.org/jira/browse/ZOOKEEPER-3010

[lvfangmin]

The newly added exception will be caught in the QuorumPeer, which will trigger another round of look up.

This change looks good to me, it’s always safer to check, although this may not actually happen on 3.6 with dynamic config, unless there is a bug, which we should fix.

+1

[anmolnar]

@brettKK Are you still working on this patch?
Would you please rebase?