[GitHub] [arrow] lidavidm commented on pull request #10259: ARROW-12668: [C++][Dataset] Fix segfault in CountRows

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lidavidm commented on pull request #10259:
URL: https://github.com/apache/arrow/pull/10259#issuecomment-833699168


   > LGTM, is the test case related though? It think it's a good case to have but just want to check my understanding.
   
   Yes - the test case, with the old code, would cause a use-after-free (since the callback in CountRows would run and try to append to a vector on the stack which doesn't exist anymore). I decided I couldn't capture the original failure effectively in a unit test (two callbacks run concurrently and interleave their calls to push_back) but both are symptoms of the same thing (don't take shared state by reference and then try to use it concurrently!).


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