gh-144475: Fix a heap buffer overflow in partial_repr

[bkap123]

• Issue: heap-buffer-overflow in functools.partial.__repr__() #144475

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[picnixz]

Please:

• add regression tests and check that the test failed before your change
• remove blank lines in NEWS

[picnixz]

By the way, @dr-carlos already suggested to open a PR. It is polite to ask them if they want to contribute themselves. As such, I'm going to close this one unless they are fine with you making the PR (we don't really want people "sniping" work of others)

[bkap123]

Thanks for the feedback. I missed that @dr-carlos suggested to fix it.

I’m happy to close this PR if @dr-carlos is already working on it.

[dr-carlos]

Thanks for the feedback. I missed that @dr-carlos suggested to fix it.

I’m happy to close this PR if @dr-carlos is already working on it.

Hi, thanks for asking!
I'm happy for you to continue with the PR :)

[bkap123]

Here are the changes I made:

• I added a kw local pointer, as a similar segfault happens for keywords
• I added an fn local pointer so that repr uses its original state when generating its final representation.
• I got rid of the error goto and merged it with the done goto as I needed to decrement the reference count of fn, args, and kw, and I found that decrementing them in the done goto was the easiest.
  Update: I changed the goto logic to reduce repetitive calls to Py_DECREF
• I added a test based on @Qanux's original code in issue heap-buffer-overflow in functools.partial.__repr__() #144475. I extended it to also check for changes in the fn and kw arguments.