Commits on Feb 8, 2023

1. Swallow potential exceptions from showtraceback() …

   The nbgrader project is aware of a form of cheating where students
   disrupt `InteractiveShell.showtraceback` in hopes of hiding exceptions
   to avoid losing points. They have implemented a solution to prevent this
   cheating from working on the client side, and have some tests to
   demonstrate this technique:
   
   https://github.com/jupyter/nbgrader/blob/main/nbgrader/tests/apps/files/submitted-cheat-attempt.ipynb
   https://github.com/jupyter/nbgrader/blob/main/nbgrader/tests/apps/files/submitted-cheat-attempt-alternative.ipynb
   
   In essence, these attacks import the interactive shell and erase the
   traceback handler so that their failing tests won't report failures.
   
       import IPython.core.interactiveshell
       IPython.core.interactiveshell.InteractiveShell.showtraceback = None
   
   The problem is that this causes an exception inside the kernel, leading
   to a stalled execution. The kernel has stopped working, but the client
   continues to wait for messages. So far, nbgrader's solution to this is
   to require a timeout value so the client can eventually decide it is
   done. This prevents allowing a value of `None` for `Execute.timeout`
   because this would cause a test case to infinitely hang.
   
   This commit addresses the problem by making `InteractiveShell._run_cell`
   a little more protective around it's call to `showtraceback()`. There is
   already a try/except block around running the cell. This commit adds a
   finally clause so that the method will _always_ return an
   `ExecutionResult`, even if a new exception is thrown within the except
   clause. For the record, the exception thrown is:
   
       TypeError: 'NoneType' object is not callable
   
   Accepting this change will allow nbgrader to update
   `nbgrader.preprocessors.Execute` to support a type of
   `Integer(allow_none=True)` as the parent `NotebookClient` intended.
   
   Discussion about this is ongoing in jupyter/nbgrader#1690.

   

   shreve committed Feb 8, 2023
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2. Add some regression tests for this change

   

   shreve committed Feb 8, 2023
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   c7a9470 View commit details

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3. Isolate the attack tests with setUp and tearDown methods

   

   shreve committed Feb 8, 2023
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   a011765 View commit details

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