Potential race condition with Quorum Peer mutual authentication via SASL

I'm in the process of reconfiguring the ensemble to use mutual quorum peer authentication using SASL (ZOOKEEPER-1045).

In order to understand the impact on my code, I've checked how it works. Now I'm running & debugging org.apache.zookeeper.server.quorum.auth.QuorumDigestAuthTest#testValidCredentials() test case.

I have now six threads (3 peers contacting each other):

• "QuorumConnectionThread-[myid=0]-2@1483" prio=5 tid=0x2b nid=NA runnable
• "QuorumConnectionThread-[myid=0]-3@1491" prio=5 tid=0x36 nid=NA runnable
• "QuorumConnectionThread-[myid=1]-1@1481" prio=5 tid=0x2d nid=NA runnable
• "QuorumConnectionThread-[myid=1]-4@1505" prio=5 tid=0x3c nid=NA runnable
• "QuorumConnectionThread-[myid=2]-2@1495" prio=5 tid=0x37 nid=NA runnable
• "QuorumConnectionThread-[myid=2]-4@1506" prio=5 tid=0x3d nid=NA runnable

at this point of invocation:

  java.lang.Thread.State: RUNNABLE
	  at org.apache.zookeeper.server.quorum.auth.SaslQuorumServerCallbackHandler.handleNameCallback(SaslQuorumServerCallbackHandler.java:101)
	  at org.apache.zookeeper.server.quorum.auth.SaslQuorumServerCallbackHandler.handle(SaslQuorumServerCallbackHandler.java:82)
	  at com.sun.security.sasl.digest.DigestMD5Server.validateClientResponse(DigestMD5Server.java:589)
	  at com.sun.security.sasl.digest.DigestMD5Server.evaluateResponse(DigestMD5Server.java:244)
	  at org.apache.zookeeper.server.quorum.auth.SaslQuorumAuthServer.authenticate(SaslQuorumAuthServer.java:100)
	  at org.apache.zookeeper.server.quorum.QuorumCnxManager.handleConnection(QuorumCnxManager.java:467)
	  at org.apache.zookeeper.server.quorum.QuorumCnxManager.receiveConnection(QuorumCnxManager.java:386)
	  at org.apache.zookeeper.server.quorum.QuorumCnxManager$QuorumConnectionReceiverThread.run(QuorumCnxManager.java:422)
	  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	  at java.lang.Thread.run(Thread.java:748)

which is this line:

private void handleNameCallback(NameCallback nc) {
    // check to see if this user is in the user password database.
    if (credentials.get(nc.getDefaultName()) == null) {
        LOG.warn("User '{}' not found in list of DIGEST-MD5 authenticateable users.",
                nc.getDefaultName());
        return;
    }
    nc.setName(nc.getDefaultName());
    /* >>> */ userName = nc.getDefaultName(); /* <<< */
}

each pair of threads is operating on single instance of org.apache.zookeeper.server.quorum.auth.SaslQuorumServerCallbackHandler#userName. In the stack trace we have both shared and local variables/fields:

• o.a.z.server.quorum.QuorumCnxManager.QuorumConnectionReceiverThread#sock is thread-specific (ok)
• o.a.z.server.quorum.QuorumCnxManager#authServer is peer-specific (instance of o.a.z.server.quorum.auth.SaslQuorumAuthServer) but without a state that changes
• javax.security.sasl.SaslServer is thread-specific (ok) - this instance is created to handle sasl authentication, but is created using peer-specific JAAS subject (which is ok) and peer-specific o.a.z.server.quorum.auth.SaslQuorumAuthServer#serverLogin.callbackHadler which is potentially a problem

Each (out of six) thread handles different connection, but each pair (for given QuorumPeer) calls o.a.z.server.quorum.auth.SaslQuorumServerCallbackHandler#handleNameCallback() which modifies shared (peer-specific) field - userName.

I understand that according to the example from Wiki all peers may use the same credentials (in simplest case).

But the "userName" comes from data sent by each peer, like this:

charset=utf-8,\
username="test",\
realm="zk-quorum-sasl-md5",\
nonce="iBqYWtaCrEE013S6Dv6xiOsR9uX2l/qKZcEZ1pm2",\
nc=00000001,\
cnonce="LVaL9XYFjNxVBPCjPewXjEBsj9GuwIfBN/RXsKt5",\
digest-uri="zookeeper-quorum/zk-quorum-sasl-md5",\
maxbuf=65536,\
response=dd4e9e2115ec2e304484d5191f3fc771,\
qop=auth,\
authzid="test"

And I can imagine such JAAS configuration for DIGEST-MD5 SASL algorithm, that each peer uses own credentials and is able to validate other peers' specific credentials.:

QuorumServer {
       org.apache.zookeeper.server.auth.DigestLoginModule required
       user_peer1="peer1";
       user_peer2="peer2";
       user_peer3="peer3";
};
QuorumLearner1 {
       org.apache.zookeeper.server.auth.DigestLoginModule required
       username="peer1"
       password="peer1";
};
QuorumLearner2 {
       org.apache.zookeeper.server.auth.DigestLoginModule required
       username="peer2"
       password="peer2";
};
QuorumLearner2 {
       org.apache.zookeeper.server.auth.DigestLoginModule required
       username="peer3"
       password="peer3";
};

Isn't it a race condition? Like this (having 3 peers):

Please verify.