Swallow potential exceptions from showtraceback()

[shreve]

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]

Of course! The tests all pass on my machine, so I have no idea why they're failing in CI. Converting to draft until I figure this out.

[Carreau]

That looks good to me, as test are now passing I'm marking as ready to merge, I'll leave the be for a few hours/a day and will merge, unless someone else stop by and merge in the meantime.
Thanks !