bpo-31804: Fix multiprocessing.Process with broken standard streams (#6079)

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.

3 files changed, 16 insertions, 10 deletions

diff --git a/Lib/multiprocessing/popen_fork.py b/Lib/multiprocessing/popen_fork.py
index b0fc01396e..008b97b366 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 8fff3e105e..cd592d0bdf 100644
--- a/Lib/multiprocessing/process.py
+++ b/Lib/multiprocessing/process.py
@@ -314,8 +314,7 @@ class BaseProcess(object):
         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 f0827f0a96..0c4eb24732 100644
--- a/Lib/multiprocessing/util.py
+++ b/Lib/multiprocessing/util.py
@@ -392,6 +392,20 @@ def _close_stdin():
         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
 #