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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