Randall Hauch commented on KAFKA-5896:

[~ewencp], do you have any thoughts on this? I know in the past you've talked 
about not wanting to do this since some developers won't properly implement the 
interruption. I agree that not everyone implements it correctly, but we could 
at least _try_ to cancel the tasks. 

> Kafka Connect task threads never interrupted
> --------------------------------------------
>                 Key: KAFKA-5896
>                 URL: https://issues.apache.org/jira/browse/KAFKA-5896
>             Project: Kafka
>          Issue Type: Bug
>          Components: KafkaConnect
>            Reporter: Nick Pillitteri
>            Assignee: Nick Pillitteri
>            Priority: Minor
> h2. Problem
> Kafka Connect tasks associated with connectors are run in their own threads. 
> When tasks are stopped or restarted, a flag is set - {{stopping}} - to 
> indicate the task should stop processing records. However, if the thread the 
> task is running in is blocked (waiting for a lock or performing I/O) it's 
> possible the task will never stop.
> I've created a connector specifically to demonstrate this issue (along with 
> some more detailed instructions for reproducing the issue): 
> https://github.com/smarter-travel-media/hang-connector
> I believe this is an issue because it means that a single badly behaved 
> connector (any connector that does I/O without timeouts) can cause the Kafka 
> Connect worker to get into a state where the only solution is to restart the 
> JVM.
> I think, but couldn't reproduce, that this is the cause of this problem on 
> Stack Overflow: 
> https://stackoverflow.com/questions/43802156/inconsistent-connector-state-connectexception-task-already-exists-in-this-work
> h2. Expected Result
> I would expect the Worker to eventually interrupt the thread that the task is 
> running in. In the past across various other libraries, this is what I've 
> seen done when a thread needs to be forcibly stopped.
> h2. Actual Result
> In actuality, the Worker sets a {{stopping}} flag and lets the thread run 
> indefinitely. It uses a timeout while waiting for the task to stop but after 
> this timeout has expired it simply sets a {{cancelled}} flag. This means that 
> every time a task is restarted, a new thread running the task will be 
> created. Thus a task may end up with multiple instances all running in their 
> own threads when there's only supposed to be a single thread.
> h2. Steps to Reproduce
> The problem can be replicated by using the connector available here: 
> https://github.com/smarter-travel-media/hang-connector
> Apologies for how involved the steps are.
> I've created a patch that forcibly interrupts threads after they fail to 
> gracefully shutdown here: 
> https://github.com/smarter-travel-media/kafka/commit/295c747a9fd82ee8b30556c89c31e0bfcce5a2c5
> I've confirmed that this fixes the issue. I can add some unit tests and 
> submit a PR if people agree that this is a bug and interrupting threads is 
> the right fix.
> Thanks!

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