Jian He commented on YARN-3021:

thanks Yongjun, some comments on the patch !

- DelegationTokenRenewer: the skipTokenRenewal check should be done under the 
existing code {{if (token.getKind().equals(new 
Text("HDFS_DELEGATION_TOKEN")))}} as below. And I think only doing this check 
is enough, we don't need checks in other places.
      if (token.isManaged()) {
        if (token.getKind().equals(new Text("HDFS_DELEGATION_TOKEN"))) {
          LOG.info(applicationId + " found existing hdfs token " + token);
          hasHdfsToken = true;
          Text renewer = ((Token<AbstractDelegationTokenIdentifier>) token).
          if ((renewer != null && renewer.toString()
              .equals(Token.NON_RENEWER))) {

- does conf.getStrings strip off the leading or ending empty strings? if not, 
we may strip those off.
String [] nns =
- given that this is a work-around fix, maybe not adding the NON_RENEWER 
publicly in common ? just check for null ?
- Did you test the patch on real cluster ?

> YARN's delegation-token handling disallows certain trust setups to operate 
> properly over DistCp
> -----------------------------------------------------------------------------------------------
>                 Key: YARN-3021
>                 URL: https://issues.apache.org/jira/browse/YARN-3021
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: security
>    Affects Versions: 2.3.0
>            Reporter: Harsh J
>            Assignee: Yongjun Zhang
>         Attachments: YARN-3021.001.patch, YARN-3021.002.patch, 
> YARN-3021.003.patch, YARN-3021.004.patch, YARN-3021.patch
> Consider this scenario of 3 realms: A, B and COMMON, where A trusts COMMON, 
> and B trusts COMMON (one way trusts both), and both A and B run HDFS + YARN 
> clusters.
> Now if one logs in with a COMMON credential, and runs a job on A's YARN that 
> needs to access B's HDFS (such as a DistCp), the operation fails in the RM, 
> as it attempts a renewDelegationToken(…) synchronously during application 
> submission (to validate the managed token before it adds it to a scheduler 
> for automatic renewal). The call obviously fails cause B realm will not trust 
> A's credentials (here, the RM's principal is the renewer).
> In the 1.x JobTracker the same call is present, but it is done asynchronously 
> and once the renewal attempt failed we simply ceased to schedule any further 
> attempts of renewals, rather than fail the job immediately.
> We should change the logic such that we attempt the renewal but go easy on 
> the failure and skip the scheduling alone, rather than bubble back an error 
> to the client, failing the app submission. This way the old behaviour is 
> retained.

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