gh-104536: Improve multiprocessing.process._cleanup logic

Lib/multiprocessing/process.py

@@ -61,7 +61,7 @@ def parent_process():
 def _cleanup():
     # check for processes which have finished
     for p in list(_children):
-        if p._popen.poll() is not None:
+        if (child_popen := p._popen) and child_popen.poll() is not None:
             _children.discard(p)
 
 #

Misc/NEWS.d/next/Library/2023-05-16-10-07-16.gh-issue-104536.hFWD8f.rst

@@ -0,0 +1,3 @@
+Fix a race condition in the internal :mod:`multiprocessing.process` cleanup
+logic that could manifest as an unintended ``AttributeError`` when calling
+``process.close()``.