HADOOP-18691. Add a CallerContext getter on the Schedulable interface

[xBis7]

Description of PR

This patch will help us solve a FairCallQueue impersonation issue we have on the Apache Ozone project.

The issue: On Ozone the FairCallQueue doesn't work when used with the Ozone S3G. The OzoneManager and the S3G are using a single permanent connection for communication, which is hiding all S3G client users under a special S3G user. For every request that comes through the S3G, getUserGroupInformation() from the Schedulable returns the special s3g user. Therefore, there is no impersonation and the FairCallQueue is ineffective.

We would like to expose the CallerContext field from the Call class and use that to carry the information needed by the IdentityProvider. On Ozone, we will set the correct information on the CallerContext before issuing the request and then provide an IdentityProvider implementation which will access the new CallerContext getter and use it to return the username to makeIdentity().

Here is a draft patch on Ozone side that utilizes these changes: apache/ozone#4116

hadoop jira: https://issues.apache.org/jira/browse/HADOOP-18691

How was this patch tested?

No new tests were added for this patch as it doesn't make any functional changes but only exposing existing data. It was tested on Ozone side.

Here is a gist with steps and information on how the FairCallQueue was tested using this patch and an OzoneIdentityProvider impl: https://gist.github.com/xBis7/b8247986e718417a4b48320eab6efeda

[smengcl]

Thanks @xBis7 . lgtm apart from one comment above.

I'd like to invite @steveloughran to vet the interface method addition here as well.

[xBis7]

@smengcl Thanks for looking into this PR. I've addressed your comment.

[smengcl]

lgtm. Pending CI

[xBis7]

@smengcl Thanks for the review!

[xBis7]

@steveloughran Thanks for reviewing this PR. I have made the changes you requested. Let me know how the patch looks now.

[smengcl]

Thanks @xBis7 . Latest changes lgtm. Looks like all comments from @steveloughran are addressed.

I will merge after a few days or when Steve approves this.

[smengcl]

Thanks @xBis7 for the PR.

[smengcl]

Backported to branch-3.3 (3.3.9): 57ff8bd