gh-155526: correct errno handling in complex_abs()

[skirpichev]

• Issue: complex_abs() should call _Py_c_abs() with errno=0 set #155526

[skirpichev]

CC @vstinner

[skirpichev]

I decided to use alternative approach, ignoring errno, as proposed in #156145. This is in a separate commit and can be easily reverted.

Edit: this will benefit from new private API in #156694. So, I would appreciate if that pr will be reviewed/merged first.

[serhiy-storchaka]

This breaks cmath.polar() for an underflowing atan2():

>>> import cmath
>>> cmath.polar(complex(1e300, 1e-320))
Traceback (most recent call last):
  ...
OverflowError: math range error

atan2(1e-320, 1e300) underflows and sets errno to ERANGE. Currently _Py_c_abs() resets it to 0, so cmath_polar_impl() does not see it. cmath.phase() fails this way already -- gh-153144.

[skirpichev]

@serhiy-storchaka, are you suggesting (1) reversion of errno = 0 stuff or (2) you are ok with moving errno = 0 down in cmath_polar_impl()?

[vstinner]

I applied the 2 @hpkfft's suggestions (fixing typos).

[skirpichev]

I understand that this change fix a bug in math.isnan() if errno is non-zero.

It fixes abs(complex(nan)), when errno was set.

Would it make sense to backport the fix to 3.13-3.15 branches, without the C API change?

Looks to be a very minor issue. Though, I can revert all C-API changes for a separate PR.

[vstinner]

Looks to be a very minor issue. Though, I can revert all C-API changes for a separate PR.

Oh no, there is no need to revert C API changes in this PR.

[vstinner]

LGTM. Thanks for the updated PR. Errno is now tested by all C _Py_c_abs() tests. There is also a test on Python abs() setting/checking errno. The changes are now well documented in Changelog entries and in the documentation. Good!

[vstinner]

Merged, thanks for the fix, and sorry for being picky on documenting the change :-D

@skirpichev: So what do you think of backporting the fix to Python 3.15 and older without changing _Py_c_abs() API?

[vstinner]

For the backport, I suppose that we can do simply what the issue title says:

"complex_abs() should call _Py_c_abs() with errno=0 set": just set errno to 0 before calling _Py_c_abs().

[bedevere-app]

GH-157341 is a backport of this pull request to the 3.15 branch.

[skirpichev]

So what do you think of backporting the fix to Python 3.15 and older

Should I create backports for <3.15 by hand?