Add a failing test if we %run something with multiprocessing

IPython/core/tests/test_run.py

@@ -539,6 +539,35 b' def test_run_tb():'
         del ip.user_ns['bar']
         del ip.user_ns['foo']
 
+
+def test_multiprocessing_run():
+    """Set we can run mutiprocesgin without messing up up main namespace
+
+    Note that import `nose.tools as nt` mdify the value s
+    sys.module['__mp_main__'] so wee need to temporarily set it to None to test
+    the issue.
+    """
+    with TemporaryDirectory() as td:
+        mpm = sys.modules.get('__mp_main__')
+        assert mpm is not None
+        sys.modules['__mp_main__'] = None
+        try:
+            path = pjoin(td, 'test.py')
+            with open(path, 'w') as f:
+                f.write("import multiprocessing\nprint('hoy')")
+            with capture_output() as io:
+                _ip.run_line_magic('run', path)
+                _ip.run_cell("i_m_undefined")
+            out = io.stdout
+            nt.assert_in("hoy", out)
+            nt.assert_not_in("AttributeError", out)
+            nt.assert_in("NameError", out)
+            nt.assert_equal(out.count("---->"), 1)
+        except:
+            raise
+        finally:
+            sys.modules['__mp_main__'] = mpm
+
 @dec.knownfailureif(sys.platform == 'win32', "writes to io.stdout aren't captured on Windows")
 def test_script_tb():
     """Test traceback offset in `ipython script.py`"""