[3.9] pythongh-112736: Refactor del-safe symbol handling in subprocess (python#112738)

Refactor delete-safe symbol handling in subprocess.

Only module globals are force-cleared during interpreter finalization, using a class reference instead of individually listing the constants everywhere is simpler.

Author: freakboy3742

Lib/subprocess.py

@@ -69,16 +69,19 @@
            # NOTE: We intentionally exclude list2cmdline as it is
            # considered an internal implementation detail.  issue10838.
 
+# use presence of msvcrt to detect Windows-like platforms (see bpo-8110)
 try:
     import msvcrt
-    import _winapi
-    _mswindows = True
 except ModuleNotFoundError:
     _mswindows = False
-    import _posixsubprocess
-    import select
-    import selectors
 else:
+    _mswindows = True
+
+# some platforms do not support subprocesses
+_can_fork_exec = sys.platform not in {"ios", "tvos", "watchos"}
+
+if _mswindows:
+    import _winapi
     from _winapi import (CREATE_NEW_CONSOLE, CREATE_NEW_PROCESS_GROUP,
                          STD_INPUT_HANDLE, STD_OUTPUT_HANDLE,
                          STD_ERROR_HANDLE, SW_HIDE,
@@ -99,6 +102,12 @@
                     "NORMAL_PRIORITY_CLASS", "REALTIME_PRIORITY_CLASS",
                     "CREATE_NO_WINDOW", "DETACHED_PROCESS",
                     "CREATE_DEFAULT_ERROR_MODE", "CREATE_BREAKAWAY_FROM_JOB"])
+else:
+    if _can_fork_exec:
+        import _posixsubprocess
+
+    import select
+    import selectors
 
 
 # Exception classes used by this module.
@@ -762,6 +771,11 @@ def __init__(self, args, bufsize=-1, executable=None,
                  pass_fds=(), *, user=None, group=None, extra_groups=None,
                  encoding=None, errors=None, text=None, umask=-1):
         """Create new Popen instance."""
+        if not _can_fork_exec:
+            raise OSError(
+                errno.ENOTSUP, f"{sys.platform} does not support processes."
+            )
+
         _cleanup()
         # Held while anything is calling waitpid before returncode has been
         # updated to prevent clobbering returncode if wait() or poll() are

Misc/NEWS.d/next/Library/2023-12-05-01-19-28.gh-issue-112736.rdHDrU.rst

@@ -0,0 +1 @@
+The use of del-safe symbols in ``subprocess`` was refactored to allow for use in cross-platform build environments.