stack commented on HBASE-19997:

Ok. Asking 0.98 to reopen hbase:meta throws an NPE whether the Master is a 0.98 
Master or a 2.0.0 Master. I killed the znode for hbase:meta so the 098 Master 
would reassign the region and it ends up looping on this...

2018-03-06 14:45:33,037 WARN  [master:] 
master.AssignmentManager: Failed assignment of hbase:meta,,1.1588230740 to,60020,1520375936764, trying to assign elsewhere instead; try=9 of 
java.io.IOException: java.io.IOException
        at org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:2247)
        at org.apache.hadoop.hbase.ipc.CallRunner.run(CallRunner.java:104)
        at org.apache.hadoop.hbase.ipc.RpcExecutor$1.run(RpcExecutor.java:108)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException

... which is same place as 200 Master gets stuck at. 0.98 takes the region name 
and tries to scan hbase:meta to find a record. It won't find one if the region 
is hbase:meta . itself. In an hbase2 cluster we'll just report success if we 
already have hbase:meta up and open. Let me add handling to the hbase2 master.

> [rolling upgrade] 1.x => 2.x
> ----------------------------
>                 Key: HBASE-19997
>                 URL: https://issues.apache.org/jira/browse/HBASE-19997
>             Project: HBase
>          Issue Type: Umbrella
>            Reporter: stack
>            Priority: Blocker
>             Fix For: 2.0.0
> An umbrella issue of issues needed so folks can do a rolling upgrade from 
> hbase-1.x to hbase-2.x.
> (Recent) Notables:
>  * hbase-1.x can't read hbase-2.x WALs -- hbase-1.x doesn't know the 
> AsyncProtobufLogWriter class used writing the WAL -- see 
> https://issues.apache.org/jira/browse/HBASE-19166?focusedCommentId=16362897&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16362897
>  for exception.
>  ** Might be ok... means WAL split fails on an hbase1 RS... must wait till an 
> hbase-2.x RS picks up the WAL for it to be split.
>  * hbase-1 can't open regions from tables created by hbase-2; it can't find 
> the Table descriptor. See 
> https://issues.apache.org/jira/browse/HBASE-19116?focusedCommentId=16363276&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16363276
>  ** This might be ok if the tables we are doing rolling upgrade over were 
> written with hbase-1.

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