bpo-38762: Extend logging.test_multiprocessing to cover missing cases. (GH-22142)

Lib/test/test_logging.py

@@ -4354,15 +4354,65 @@ class LogRecordTest(BaseTest):
         r.removeHandler(h)
         h.close()
 
-    def test_multiprocessing(self):
-        r = logging.makeLogRecord({})
-        self.assertEqual(r.processName, 'MainProcess')
+    @staticmethod # pickled as target of child process in the following test
+    def _extract_logrecord_process_name(key, logMultiprocessing, conn=None):
+        prev_logMultiprocessing = logging.logMultiprocessing
+        logging.logMultiprocessing = logMultiprocessing
         try:
             import multiprocessing as mp
+            name = mp.current_process().name
+
+            r1 = logging.makeLogRecord({'msg': f'msg1_{key}'})
+            del sys.modules['multiprocessing']
+            r2 = logging.makeLogRecord({'msg': f'msg2_{key}'})
+
+            results = {'processName'  : name,
+                       'r1.processName': r1.processName,
+                       'r2.processName': r2.processName,
+                      }
+        finally:
+            logging.logMultiprocessing = prev_logMultiprocessing
+        if conn:
+            conn.send(results)
+        else:
+            return results
+
+    def test_multiprocessing(self):
+        multiprocessing_imported = 'multiprocessing' in sys.modules
+        try:
+            # logMultiprocessing is True by default
+            self.assertEqual(logging.logMultiprocessing, True)
+
+            LOG_MULTI_PROCESSING = True
+            # When logMultiprocessing == True:
+            # In the main process processName = 'MainProcess'
             r = logging.makeLogRecord({})
-            self.assertEqual(r.processName, mp.current_process().name)
-        except ImportError:
-            pass
+            self.assertEqual(r.processName, 'MainProcess')
+
+            results = self._extract_logrecord_process_name(1, LOG_MULTI_PROCESSING)
+            self.assertEqual('MainProcess', results['processName'])
+            self.assertEqual('MainProcess', results['r1.processName'])
+            self.assertEqual('MainProcess', results['r2.processName'])
+
+            # In other processes, processName is correct when multiprocessing in imported,
+            # but it is (incorrectly) defaulted to 'MainProcess' otherwise (bpo-38762).
+            import multiprocessing
+            parent_conn, child_conn = multiprocessing.Pipe()
+            p = multiprocessing.Process(
+                target=self._extract_logrecord_process_name,
+                args=(2, LOG_MULTI_PROCESSING, child_conn,)
+            )
+            p.start()
+            results = parent_conn.recv()
+            self.assertNotEqual('MainProcess', results['processName'])
+            self.assertEqual(results['processName'], results['r1.processName'])
+            self.assertEqual('MainProcess', results['r2.processName'])
+            p.join()
+
+        finally:
+            if multiprocessing_imported:
+                import multiprocessing
+
 
     def test_optional(self):
         r = logging.makeLogRecord({})