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GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows#148036

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itamaro merged 5 commits intopython:mainfrom
chris-eibl:CheckTailCalling
Apr 12, 2026
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GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows#148036
itamaro merged 5 commits intopython:mainfrom
chris-eibl:CheckTailCalling

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@chris-eibl chris-eibl commented Apr 3, 2026 •
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Rather than failing late when compiling e.g. a debug configuration

build.bat -c debug --tail-call-interp

with hundreds of

error C4737: Unable to perform required tail call. Performance may be degraded.

lets fail early with an explicit error message for configurations that are not supported by MSVC.

This is a follow-up on #140513 / #140548.

@chris-eibl chris-eibl requested a review from a team as a code owner April 3, 2026 13:54
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chris-eibl commented Apr 3, 2026 •
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TSan https://github.com/python/cpython/actions/runs/23948631142/job/69850491565?pr=148036 failure for sure unreleated: this is a pure Windows change and would result in a build error on onsupported (Windows) hosts when using --tail-call-interp. Note that there are no such hosts in CI nor in buildbots, and if there were, they would have already failed (but with tons of Unable to perform required tail call messages).

I've checked manually that I get all the expected errors for unsupported configurations.

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Fidget-Spinner commented Apr 3, 2026

Please open a separate issue for this, rather than reusing the same closed issue. Thanks!

@chris-eibl chris-eibl changed the title GH-139922: Check early whether tail calling is possible GH-148047: Check early whether tail calling is possible Apr 3, 2026
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@chris-eibl chris-eibl added OS-windows build The build process and cross-build and removed skip news labels Apr 3, 2026
@chris-eibl chris-eibl changed the title GH-148047: Check early whether tail calling is possible GH-148047: Check early whether tail-calling is possible for MSVC builds on Windows Apr 3, 2026
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Overall looks good, thank you for this!

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bedevere-app bot commented Apr 4, 2026

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

@itamaro itamaro merged commit cbd81d5 into python:main Apr 12, 2026
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bedevere-bot commented Apr 12, 2026

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot x86 Debian Installed with X 3.x (no tier) has failed when building commit cbd81d5.

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/1244/builds/7983) and take a look at the build logs.
  4. Check if the failure is related to this commit (cbd81d5) 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/1244/builds/7983

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

==

Click to see traceback logs 
Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
    return func(*args, **kwds)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m649�[0m, in �[35mtest_interrupt�[0m
    exitcode = self._kill_process(multiprocessing.Process.interrupt)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
    return func(*args, **kwds)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m630�[0m, in �[35m_kill_process�[0m
    self.assertEqual(�[31mjoin�[0m�[1;31m()�[0m, None)
                     �[31m~~~~�[0m�[1;31m^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m303�[0m, in �[35m__call__�[0m
    return �[31mself.func�[0m�[1;31m(*args, **kwds)�[0m
           �[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/process.py"�[0m, line �[35m156�[0m, in �[35mjoin�[0m
    res = self._popen.wait(timeout)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m44�[0m, in �[35mwait�[0m
    return �[31mself.poll�[0m�[1;31m(os.WNOHANG if timeout == 0.0 else 0)�[0m
           �[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m28�[0m, in �[35mpoll�[0m
    pid, sts = �[31mos.waitpid�[0m�[1;31m(self.pid, flag)�[0m
               �[31m~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m626�[0m, in �[35mhandler�[0m
    raise RuntimeError('join took too long: %s' % p)
�[1;35mRuntimeError�[0m: �[35mjoin took too long: <Process name='Process-158' pid=28453 parent=26696 started daemon>�[0m


Traceback (most recent call last):
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
    return func(*args, **kwds)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m649�[0m, in �[35mtest_interrupt�[0m
    exitcode = self._kill_process(multiprocessing.Process.interrupt)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/contextlib.py"�[0m, line �[35m85�[0m, in �[35minner�[0m
    return func(*args, **kwds)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m630�[0m, in �[35m_kill_process�[0m
    self.assertEqual(�[31mjoin�[0m�[1;31m()�[0m, None)
                     �[31m~~~~�[0m�[1;31m^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m303�[0m, in �[35m__call__�[0m
    return �[31mself.func�[0m�[1;31m(*args, **kwds)�[0m
           �[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/process.py"�[0m, line �[35m156�[0m, in �[35mjoin�[0m
    res = self._popen.wait(timeout)
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m44�[0m, in �[35mwait�[0m
    return �[31mself.poll�[0m�[1;31m(os.WNOHANG if timeout == 0.0 else 0)�[0m
           �[31m~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/multiprocessing/popen_fork.py"�[0m, line �[35m28�[0m, in �[35mpoll�[0m
    pid, sts = �[31mos.waitpid�[0m�[1;31m(self.pid, flag)�[0m
               �[31m~~~~~~~~~~�[0m�[1;31m^^^^^^^^^^^^^^^^�[0m
  File �[35m"/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.15/test/_test_multiprocessing.py"�[0m, line �[35m626�[0m, in �[35mhandler�[0m
    raise RuntimeError('join took too long: %s' % p)
�[1;35mRuntimeError�[0m: �[35mjoin took too long: <Process name='Process-1' pid=2089 parent=2087 started daemon>�[0m

@chris-eibl chris-eibl deleted the CheckTailCalling branch April 12, 2026 05:58
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