Detect QEMU linux-user emulation in subprocess._use_posix_spawn()

[andreas-schwab]

The qemu linux-user emulation does not support CLONE_VFORK and emulates it
with a true fork. That breaks posix_spawn because it cannot report errors
back to the parent process. The subprocess modules should detect that is
it being executed in qemu linux-user emulation and return False in
_use_posix_spawn. One way to do that is to look for "uarch *: qemu" in
/proc/cpuinfo.

[vstinner]

How is it an issue to use vfork() if it's emulated with fork()? Is it a performance issue? Do you have examples of benchmark showing the performance problem?

[andreas-schwab]

Without a true vfork, the posix_spawn implementation in glibc cannot
report a proper errno value for errors that happen in the child before
execve.

See https://lists.gnu.org/archive/html/qemu-devel/2025-03/msg04706.html
for more information.

[vstinner]

cc @gpshead

[vstinner]

Without a true vfork, the posix_spawn implementation in glibc cannot report a proper errno value for errors that happen in the child before execve.

Do you have an example to reproduce the issue?

[andreas-schwab]

It causes a failure while building the MozillaFirefox package:

4:55.15 Traceback (most recent call last):
4:55.16 File "/home/abuild/rpmbuild/BUILD/MozillaFirefox-136.0.1-build/firefox-136.0.1/security/nss/./coreconf/werror.py", line 80, in
4:55.16 main()
4:55.16 ~~~~^^
4:55.16 File "/home/abuild/rpmbuild/BUILD/MozillaFirefox-136.0.1-build/firefox-136.0.1/security/nss/./coreconf/werror.py", line 10, in main
4:55.16 cc_is_clang = 'clang' in subprocess.check_output(
4:55.16 ~~~~~~~~~~~~~~~~~~~~~~~^
4:55.16 [cc, '--version'], universal_newlines=True, stderr=sink)
4:55.16 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
4:55.16 File "/usr/lib64/python3.13/subprocess.py", line 474, in check_output
4:55.16 return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
4:55.16 ~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
4:55.17 **kwargs).stdout
4:55.17 ^^^^^^^^^
4:55.17 File "/usr/lib64/python3.13/subprocess.py", line 579, in run
4:55.17 raise CalledProcessError(retcode, process.args,
4:55.17 output=stdout, stderr=stderr)
4:55.17 subprocess.CalledProcessError: Command '['/usr/bin/ccache /usr/bin/gcc', '--version']' returned non-zero exit status 127.

The issue here is that posix_spawn should have set errno to ENOENT.

Also, the python testsuite tests this in test_subprocess.py.

[andreas-schwab]

For exmaple:

FAIL: test_exception_bad_args_0 (test.test_subprocess.POSIXProcessTestCase.test_exception_bad_args_0)
Test error in the child raised in the parent for a bad args[0].

Traceback (most recent call last):
File "/home/abuild/rpmbuild/BUILD/python313-3.13.2-build/Python-3.13.2/Lib/test/test_subprocess.py", line 1903, in test_exception_bad_args_0
self.fail("Expected OSError: %s" % desired_exception)
~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Expected OSError: [Errno 2] No such file or directory: '/_this/pa.th/does/not/exist'

[gpshead]

FWIW I agreee with the approach of detecting this unusual/incomplete environment and having _use_posix_spawn() return False there.