gh-81793: Always call linkat() from os.link(), if available

[serhiy-storchaka]

This fixes os.link() on platforms (like Linux and OpenIndiana) where the system link() function does not follow symlinks.

• On Linux and OpenIndiana, it now follows symlinks by default or if follow_symlinks=True is specified.
• On Windows, it now raises error if follow_symlinks=True is passed.
• On macOS, it now raises error if follow_symlinks=False is passed and the system linkat() function is not available at runtime.
• On other platforms, it now raises error if follow_symlinks is passed with a value that does not match the system link() function behavior if if the behavior is not known.

• Issue: os.link(..., follow_symlinks=True) broken on Linux #81793

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📚 Documentation preview 📚: https://cpython-previews--132517.org.readthedocs.build/

[serhiy-storchaka]

Thank you for grammar fixes @StanFromIreland.

[StanFromIreland]

No problem:-)

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 Android 3.x (tier-3) has failed when building commit 5a57248.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1594/builds/2228) and take a look at the build logs.
4. Check if the failure is related to this commit (5a57248) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1594/builds/2228

Failed tests:

• test_posix

Failed subtests:

• test_link_follow_symlinks - test.test_posix.PosixTester.test_link_follow_symlinks

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/data/user/0/org.python.testbed/files/python/lib/python3.14/test/test_posix.py", line 1548, in test_link_follow_symlinks
    posix.link(symlink, link, follow_symlinks=False)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: '@test_1785_tmpæsymlink' -> '@test_1785_tmpælink_nofollow'


Traceback (most recent call last):
  File "/data/user/0/org.python.testbed/files/python/lib/python3.14/test/test_posix.py", line 1560, in test_link_follow_symlinks
    posix.link(symlink, link, follow_symlinks=True)
    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: '@test_1785_tmpæsymlink' -> '@test_1785_tmpælink_following'


Traceback (most recent call last):
  File "/data/user/0/org.python.testbed/files/python/lib/python3.14/test/test_posix.py", line 1537, in test_link_follow_symlinks
    posix.link(symlink, link)
    ~~~~~~~~~~^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: '@test_1785_tmpæsymlink' -> '@test_1785_tmpælink'

[freakboy3742]

Not sure of the source of the Android failure here, but it looks related to the subject ticket. Investigation ongoing.

[serhiy-storchaka]

It was fixed on #133388.

[freakboy3742]

It was fixed on #133388.

Thanks for the fast response!