[3.8] bpo-35182: fix communicate() crash after child closes its pipes (GH-18117) (GH-18148)

When communicate() is called in a loop, it crashes when the child process
has already closed any piped standard stream, but still continues to be running

Co-authored-by: Andriy Maletsky <andriy.maletsky@gmail.com>.
(cherry picked from commit d3ae95e1e9)

Co-authored-by: Alex Rebert <alex@forallsecure.com>





https://bugs.python.org/issue35182

Lib/subprocess.py

@@ -1848,9 +1848,9 @@ class Popen(object):
             with _PopenSelector() as selector:
                 if self.stdin and input:
                     selector.register(self.stdin, selectors.EVENT_WRITE)
-                if self.stdout:
+                if self.stdout and not self.stdout.closed:
                     selector.register(self.stdout, selectors.EVENT_READ)
-                if self.stderr:
+                if self.stderr and not self.stderr.closed:
                     selector.register(self.stderr, selectors.EVENT_READ)
 
                 while selector.get_map():

Lib/test/test_subprocess.py

@@ -2910,6 +2910,17 @@ class POSIXProcessTestCase(BaseTestCase):
 
         self.assertEqual(returncode, -3)
 
+    def test_communicate_repeated_call_after_stdout_close(self):
+        proc = subprocess.Popen([sys.executable, '-c',
+                                 'import os, time; os.close(1), time.sleep(2)'],
+                                stdout=subprocess.PIPE)
+        while True:
+            try:
+                proc.communicate(timeout=0.1)
+                return
+            except subprocess.TimeoutExpired:
+                pass
+
 
 @unittest.skipUnless(mswindows, "Windows specific tests")
 class Win32ProcessTestCase(BaseTestCase):

Misc/NEWS.d/next/Library/2019-10-31-19-23-25.bpo-35182.hzeNl9.rst

@@ -0,0 +1,3 @@
+Fixed :func:`Popen.communicate` subsequent call crash when the child process
+has already closed any piped standard stream, but still continues to be
+running. Patch by Andriy Maletsky.