bpo-42406: Fix whichmodule() with multiprocessing (GH-23403)

* bpo-42406: Fix whichmodule() with multiprocessing

Signed-off-by: Renato L. de F. Cunha <[email protected]>
Co-authored-by: Gregory P. Smith <[email protected]>

Author: renatolfc

Lib/pickle.py

@@ -340,7 +340,9 @@ def whichmodule(obj, name):
     # Protect the iteration by using a list copy of sys.modules against dynamic
     # modules that trigger imports of other modules upon calls to getattr.
     for module_name, module in sys.modules.copy().items():
-        if module_name == '__main__' or module is None:
+        if (module_name == '__main__'
+            or module_name == '__mp_main__'  # bpo-42406
+            or module is None):
             continue
         try:
             if _getattribute(module, name)[0] is obj:

Misc/NEWS.d/next/Library/2020-11-19-10-44-41.bpo-42406.r9rNCj.rst

@@ -0,0 +1,3 @@
+We fixed an issue in `pickle.whichmodule` in which importing
+`multiprocessing` could change the how pickle identifies which module an
+object belongs to, potentially breaking the unpickling of those objects.