Github user HyukjinKwon commented on the issue:

    https://github.com/apache/spark/pull/21060
  
    > withCallback was added in Spark 1.6 release 
https://issues.apache.org/jira/browse/SPARK-11068 Since then, my understanding 
is we never clearly define which should be part of withCallback. Thus, it is 
hard to say this is a bug fix.
    
    The callback works for `collect` in R and Scala but Python doesn't. I think 
we should at least match the behaviour. I wonder why it's hard to say a bug 
when `collect` is detected in some APIs but not in some APIs.
    
    > We hit the similar issue in #18064. At that time, we did not backport the 
PR to the previous releases too.
    
    That's because the change was big and invasive. I wouldn't backport it too; 
however, this fix is relatively small.
    
    > Thus, I do not think we should make an exception for this PR just because 
the customers of @HyukjinKwon hit this issue
    
    It's not because my customers but I am saying it fixes an actual usecase 
and it affects actual users.
    
    > If we make an exception, it becomes harder to decide which PRs are 
qualified for a backport.
    
    I think we usually use committer's judgement when we make an exception. I 
already have been seeing many backports that actually causes behaviour changes 
and I did this because it looks being backported in general. This is the reason 
why we should formally document it if this is actually the rule.
    
    What I am less sure is, why this one specifically prompted you.


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