sodonnel opened a new pull request #1551: HDDS-2199 In SCMNodeManager 
dnsToUuidMap cannot track multiple DNs on the same host
URL: https://github.com/apache/hadoop/pull/1551
 
 
   Often in test clusters and tests, we start multiple datanodes on the same 
host.
   
   In SCMNodeManager.register() there is a map of hostname -> datanode UUID 
called dnsToUuidMap.
   
   If several DNs register from the same host, the entry in the map will be 
overwritten and the last DN to register will 'win'.
   
   This means that the method getNodeByAddress() does not return the correct 
DatanodeDetails object when many hosts are registered from the same address.
   
   This method is only used in SCMBlockProtocolServer.sortDatanodes() to allow 
it to see if one of the nodes matches the client, but it need to be used by the 
Decommission code.
   
   Perhaps we could change the getNodeByAddress() method to returns a list of 
DNs? In normal production clusters, there should only be one returned, but in 
test clusters, there may be many. Any code looking for a specific DN entry 
would need to iterate the list and match on the port number too, as host:port 
would be the unique definition of a datanode.

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