gh-129204: Add _PYTHON_SUBPROCESS_USE_POSIX_SPAWN environment knob (GH-132184)

* Add _PYTHON_SUBPROCESS_USE_POSIX_SPAWN environment knob

Add support for disabling the use of `posix_spawn` via a variable in
the process environment.

While it was previously possible to toggle this by modifying the value
of `subprocess._USE_POSIX_SPAWN`, this required either patching CPython
or modifying it within the interpreter instance which is not always
possible, such as when running applications or scripts not under a
user's control.

Signed-off-by: Vincent Fazio <[email protected]>

* fixup NEWS entry

---------

Signed-off-by: Vincent Fazio <[email protected]>

Author: vfazio

Lib/subprocess.py

@@ -715,6 +715,9 @@ def _use_posix_spawn():
         # os.posix_spawn() is not available
         return False
 
+    if ((_env := os.environ.get('_PYTHON_SUBPROCESS_USE_POSIX_SPAWN')) in ('0', '1')):
+        return bool(int(_env))
+
     if sys.platform in ('darwin', 'sunos5'):
         # posix_spawn() is a syscall on both macOS and Solaris,
         # and properly reports errors

Misc/NEWS.d/next/Library/2025-04-06-19-25-12.gh-issue-129204.sAVFO6.rst

@@ -0,0 +1,2 @@
+Introduce new ``_PYTHON_SUBPROCESS_USE_POSIX_SPAWN`` environment variable knob in
+:mod:`subprocess` to control the use of :func:`os.posix_spawn`.