bpo-31804: multiprocessing calls flush on sys.stdout at exit even if …

…it is None (pythonw) If you start Python by pythonw.exe on Windows platform then sys.stdout and sys.stderr are set to None. If you also use multiprocessing then when the child process finishes BaseProcess._bootstrap calls sys.stdout.flush() and sys.stderr.flush() finally. This causes the process return code to be not zero (it is 1).
SlaushVunter · Feb 2, 2018 · Feb 12, 2018 · Feb 22, 2018 · Feb 22, 2018 · Feb 2, 2018
commit cab49f209ebede629e2d431cb65297cdee18570b
diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
@@ -314,8 +314,10 @@ def _bootstrap(self):
         finally:
             threading._shutdown()
             util.info('process exiting with exitcode %d' % exitcode)
-            sys.stdout.flush()
-            sys.stderr.flush()
+            if sys.stdout is not None and not sys.stdout.closed:
+                sys.stdout.flush()
+            if sys.stderr is not None and not sys.stderr.closed:
+                sys.stderr.flush()
 
         return exitcode
 

diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
@@ -653,6 +653,47 @@ def test_forkserver_sigkill(self):
             self.check_forkserver_death(signal.SIGKILL)
 
 
+class TestStdOutAndErr(unittest.TestCase):
+    @staticmethod
+    def closeIO(stream_name):
+        getattr(sys, stream_name).close()
+
+    @staticmethod
+    def removeIO(stream_name):
+        setattr(sys, stream_name, None)
+
+    def test_closed_stdio(self):
+        """
+        bpo-28326: multiprocessing.Process depends on sys.stdout being open
+        """
+        self.run_process(self.closeIO)
+
+    def test_no_stdio(self):
+        """
+        bpo-31804: If you start Python by pythonw then sys.stdout and
+        sys.stderr are set to None. If you also use multiprocessing
+        then when the child process finishes BaseProcess._bootstrap
+        calls sys.stdout.flush() and sys.stderr.flush() finally.
+        This causes the process return code to be not zero (it is 1).
+
+        This unit test sets sys.stdio and sys.stderr to None, instead of
+        changing the Python interpreter to use when starting a child process
+        to pythonw.exe because that is Windows specific. This method
+        cannot test if there is an error (because of stdout or stderr is None)
+        before the target function is called. However the errors have occurred
+        in the multiprocessing outro so this can test the previously mentioned
+        bug. Also this way the feature can be tested on other operating
+        systems too.
+        """
+        self.run_process(self.removeIO)
+
+    def run_process(self, target):
+        for stream_name in ('stdout', 'stderr'):
+            proc = multiprocessing.Process(target=target, args=(stream_name,))
+            proc.start()
+            proc.join()
+            self.assertEqual(proc.exitcode, 0)
+
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