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### _Why are the changes needed?_
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We shall avoid calling cancelJobGroup if the sc is already stopped.
We shall stop the engine to stop serving before we stop the sc during the shutdown hook
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [ ] [Run test](https://kyuubi.readthedocs.io/en/latest/tools/testing.html#running-tests) locally before make a pull request
Closes#868 from yaooqinn/shutdownhook.
Closes#868
14810875 [Kent Yao] comment
972e2741 [Kent Yao] Avoid calling a stopped SparkContext
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>
d5104ed277