From 653ed40222ab95426bf750249af214dbea1c5beb Mon Sep 17 00:00:00 2001
From: Antoine Pitrou <antoine@python.org>
Date: Sun, 11 Mar 2018 19:04:23 +0100
Subject: [PATCH] bpo-31804: Fix multiprocessing.Process with broken standard
 streams

In some conditions the standard streams will be None or closed in the child process
(for example if using "pythonw" instead of "python" on Windows).  Avoid failing
with a non-0 exit code in those conditions.

Report and initial patch by poxthegreat.
---
 Lib/multiprocessing/popen_fork.py             |  9 +-----
 Lib/multiprocessing/process.py                |  3 +-
 Lib/multiprocessing/util.py                   | 14 +++++++++
 Lib/test/_test_multiprocessing.py             | 31 +++++++++++++++++--
 .../2018-03-11-19-03-52.bpo-31804.i8KUMp.rst  |  2 ++
 5 files changed, 47 insertions(+), 12 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2018-03-11-19-03-52.bpo-31804.i8KUMp.rst

diff --git a/Lib/multiprocessing/popen_fork.py b/Lib/multiprocessing/popen_fork.py
index b0fc01396eb24c..008b97b366280f 100644
--- a/Lib/multiprocessing/popen_fork.py
+++ b/Lib/multiprocessing/popen_fork.py
@@ -14,14 +14,7 @@ class Popen(object):
     method = 'fork'
 
     def __init__(self, process_obj):
-        try:
-            sys.stdout.flush()
-        except (AttributeError, ValueError):
-            pass
-        try:
-            sys.stderr.flush()
-        except (AttributeError, ValueError):
-            pass
+        util._flush_std_streams()
         self.returncode = None
         self.finalizer = None
         self._launch(process_obj)
diff --git a/Lib/multiprocessing/process.py b/Lib/multiprocessing/process.py
index 8fff3e105ead4c..cd592d0bdf0910 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -314,8 +314,7 @@ def _bootstrap(self):
         finally:
             threading._shutdown()
             util.info('process exiting with exitcode %d' % exitcode)
-            sys.stdout.flush()
-            sys.stderr.flush()
+            util._flush_std_streams()
 
         return exitcode
 
diff --git a/Lib/multiprocessing/util.py b/Lib/multiprocessing/util.py
index f0827f0a9693ae..0c4eb2473273b4 100644
--- a/Lib/multiprocessing/util.py
+++ b/Lib/multiprocessing/util.py
@@ -391,6 +391,20 @@ def _close_stdin():
     except (OSError, ValueError):
         pass
 
+#
+# Flush standard streams, if any
+#
+
+def _flush_std_streams():
+    try:
+        sys.stdout.flush()
+    except (AttributeError, ValueError):
+        pass
+    try:
+        sys.stderr.flush()
+    except (AttributeError, ValueError):
+        pass
+
 #
 # Start a program with only specified fds kept open
 #
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 1e497a572a760f..940fe584e7522d 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -584,10 +584,19 @@ def test_wait_for_threads(self):
         self.assertTrue(evt.is_set())
 
     @classmethod
-    def _test_error_on_stdio_flush(self, evt):
+    def _test_error_on_stdio_flush(self, evt, break_std_streams={}):
+        for stream_name, action in break_std_streams.items():
+            if action == 'close':
+                stream = io.StringIO()
+                stream.close()
+            else:
+                assert action == 'remove'
+                stream = None
+            setattr(sys, stream_name, None)
         evt.set()
 
-    def test_error_on_stdio_flush(self):
+    def test_error_on_stdio_flush_1(self):
+        # Check that Process works with broken standard streams
         streams = [io.StringIO(), None]
         streams[0].close()
         for stream_name in ('stdout', 'stderr'):
@@ -601,6 +610,24 @@ def test_error_on_stdio_flush(self):
                     proc.start()
                     proc.join()
                     self.assertTrue(evt.is_set())
+                    self.assertEqual(proc.exitcode, 0)
+                finally:
+                    setattr(sys, stream_name, old_stream)
+
+    def test_error_on_stdio_flush_2(self):
+        # Same as test_error_on_stdio_flush_1(), but standard streams are
+        # broken by the child process
+        for stream_name in ('stdout', 'stderr'):
+            for action in ('close', 'remove'):
+                old_stream = getattr(sys, stream_name)
+                try:
+                    evt = self.Event()
+                    proc = self.Process(target=self._test_error_on_stdio_flush,
+                                        args=(evt, {stream_name: action}))
+                    proc.start()
+                    proc.join()
+                    self.assertTrue(evt.is_set())
+                    self.assertEqual(proc.exitcode, 0)
                 finally:
                     setattr(sys, stream_name, old_stream)
 
diff --git a/Misc/NEWS.d/next/Library/2018-03-11-19-03-52.bpo-31804.i8KUMp.rst b/Misc/NEWS.d/next/Library/2018-03-11-19-03-52.bpo-31804.i8KUMp.rst
new file mode 100644
index 00000000000000..7fcede297aca31
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-03-11-19-03-52.bpo-31804.i8KUMp.rst
@@ -0,0 +1,2 @@
+Avoid failing in multiprocessing.Process if the standard streams are closed
+or None at exit.