Creation of an internal decimal context for `_pylong` has side effects on `decimal`

[picnixz]

Bug report

Bug description:

The _decimal module was recently fixed in #123243. I also found that _pylong leaks globals due to the creation of a global context to use as a local context:

_unbounded_dec_context = decimal.getcontext().copy()
_unbounded_dec_context.prec = decimal.MAX_PREC
_unbounded_dec_context.Emax = decimal.MAX_EMAX
_unbounded_dec_context.Emin = decimal.MIN_EMIN
_unbounded_dec_context.traps[decimal.Inexact] = 1 # sanity check

This context is used as with decimal.localcontext(_unbounded_dec_context). On the other hand decimal.getcontext().copy() returns a copy of the decimal context for the current thread (or set it to decimal.DefaultContext if none exists). Instead of setting it like that, I suggest that we construct a decimal.Context() object directly in _pylong instead of possibly polluting the decimal module (whether it's the C or the python implementation).

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-123284: fix _pylong decimal context creation #123285

[vstinner]

_pylong leaks globals due to the creation of a global context to use as a local context

How do you see such leak?

[picnixz]

I just used ./python -X showrefcounts -c 'import _pylong'. I don't really know if it's the _pylong module itself that leaks or if it's the decimal module that leaks because decimal.getcontext() was called.

What I however know is that there is a side-effect on decimal because _pylong calls decimal.getcontext() and thus sets the decimal default's context. I'm not really sure whether it's an issue or not, but I was just looking at all ./python -X showrefcounts -c 'import MODULE' for every module in Lib to see whether a module leaked or not. Feel free to close if you think this is a non-issue though.

[vstinner]

The problem is unrelated to _pylong, but related to decimal:

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

[picnixz]

Ok, one interrogation confirmed. I'll try to see what I can do tomorrow (not sure if I'll be able to though).

[picnixz]

So the reference leaks will be fixed by another PR. I won't bother about setting the decimal's default thread context