Fix _whichmodule with multiprocessing

CHANGES.md

@@ -5,6 +5,9 @@
   dynamic functions and classes.
   ([PR #524](https://github.com/cloudpipe/cloudpickle/pull/524))
 
+- Fix a problem with the joint usage of cloudpickle's `_whichmodule` and
+  `multiprocessing`.
+  ([PR #529](https://github.com/cloudpipe/cloudpickle/pull/529))
 
 3.0.0
 =====

cloudpickle/cloudpickle.py

@@ -213,6 +213,7 @@ def _whichmodule(obj, name):
         # sys.modules
         if (
             module_name == "__main__"
+            or module_name == "__mp_main__"
             or module is None
             or not isinstance(module, types.ModuleType)
         ):

tests/cloudpickle_test.py

@@ -29,6 +29,7 @@
 import pickle
 
 import pytest
+from pathlib import Path
 
 try:
     # try importing numpy and scipy. These are not hard dependencies and
@@ -1479,6 +1480,29 @@ def __getattr__(self, name):
         finally:
             sys.modules.pop("NonModuleObject")
 
+    def test_importing_multiprocessing_does_not_impact_whichmodule(self):
+        # non-regression test for #528
+        pytest.importorskip("numpy")
+        script = textwrap.dedent("""
+        import multiprocessing
+        import cloudpickle
+        from numpy import exp
+
+        print(cloudpickle.cloudpickle._whichmodule(exp, exp.__name__))
+        """)
+        script_path = Path(self.tmpdir) / "whichmodule_and_multiprocessing.py"
+        with open(script_path, mode="w") as f:
+            f.write(script)
+
+        proc = subprocess.Popen(
+            [sys.executable, str(script_path)],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+        )
+        out, _ = proc.communicate()
+        self.assertEqual(proc.wait(), 0)
+        self.assertEqual(out, b"numpy.core._multiarray_umath\n")
+
     def test_unrelated_faulty_module(self):
         # Check that pickling a dynamically defined function or class does not
         # fail when introspecting the currently loaded modules in sys.modules