gh-123657: Fix crash and refleak in decimal.getcontext()

[neonene]

• Issue: decimal.getcontext crashes with --with-decimal-contextvar=no and -X showrefcount #123657

[neonene]

@ZeroIntensity Could you verify this fix?

cc @kumaraditya303@erlend-aasland

[ZeroIntensity]

I can take a look in an hour or so.

[ZeroIntensity]

Confirmed that this fixes the assertion failure, as well as the memory leak:

./python -Xshowrefcount -c "import decimal; decimal.getcontext()"
[0 refs, 0 blocks]

[ZeroIntensity]

@encukou, this needs the 3.13 backport label.

[encukou]

This makes setcontext leak in the --with-decimal-contextvar=no build:

$ ./python -Xshowrefcount -c "import decimal; decimal.getcontext(); decimal.setcontext(decimal.Context())"
[7122 refs, 4233 blocks]

Since cached_context is now an owned reference, PyDec_SetCurrentContext needs a Py_CLEAR instead of just setting to NULL.
Please also adjust cached_context's comment in the decimal_state struct: explicity mention it's a strong reference.

[miss-islington-app]

Thanks @neonene for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-123774 is a backport of this pull request to the 3.13 branch.

[encukou]

Thanks!
(FWIW, A big reason why I wanted “owned reference” mentioned is that it's not immediately clear that this is a PyObject subclass and so it even can have strong reference. It didn't need to be emphasized this much. But, that's a minor point.)

[vstinner]

Thanks for this nice fix. It does fix this leak.

Before:

$ ./python -X showrefcount -c 'import decimal; decimal.getcontext()'
[207 refs, 138 blocks]

After (with this fix):

$ ./python -X showrefcount -c 'import decimal; decimal.getcontext()'
[0 refs, 0 blocks]

cc @picnixz