Github user ilooner commented on the issue:

    https://github.com/apache/drill/pull/1105
  
    @arina-ielchiieva @vrozov I believe I have a solution. There were several 
issues with the original code.
    
    1. It made incorrect assumptions about how cache invalidation works with 
java **synchronized**.
    2. It assumed **innerNext** and **close** would be called sequentially.
    
    I believe this PR fixes these issues now and I have gone into more detail 
about the problems below.
    
    # 1. Incorrect Cache Invalidation Assumptions
    
    The original code was trying to be smart by trying to reduce 
synchronization overhead on **innerNext**. So the code in **innerNext** did not 
synchronize before changing the partitioner object since this would be called 
often. The code in **close()** and ** receivingFragmentFinished()** 
synchronized before accessing the partitioner with the intention that this 
would trigger an update of the partitioner variable state across all threads. 
Unfortunately, this assumption is invalid (see 
https://stackoverflow.com/questions/22706739/does-synchronized-guarantee-a-thread-will-see-the-latest-value-of-a-non-volatile).
 Every thread that accesses a variable must synchronize before accessing a 
variable in order to properly invalidate cached data on a core. 
    
    For example if **Thread A** modifies **Variable 1** then **Thread B** 
synchronizes before accessing **Variable 1** then there is no guarantee 
**Thread B** will see the most updated value for **Variable 1** since it might .
    
    ## Solution
    
    In summary the right thing to do is the simple thing. Make the methods 
synchronized. Unfortunately there is no way to outsmart the system and reduce 
synchronization overhead without causing race conditions.
    
    # 2. Concurrent InnerNext and Close Calls
    
    The original code did not consider the case that innerNext was in the 
middle of execution when close was called. It did try to handle the case where 
**innerNext** could be called after **close** by setting the **ok** variable. 
But it didn't even do that right because there was no synchronization around 
the **ok** variable.
    
    ## Solution
    
    The right thing to do is the simple thing. Make sure the methods are 
synchronized so close has to wait until innerNext is done before executing. 
Also when a query is cancelled the executing thread should be interrupted the 
thread running innerNext incase it is on a blocking call.


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