bpo-30121: Fix debug assert in subprocess on Windows (#1224)

* bpo-30121: Fix debug assert in subprocess on Windows

This is caused by closing HANDLEs using os.close which is for CRT file
descriptors and not for HANDLEs.

* bpo-30121: Suppress debug assertion in test_subprocess when ran directly

Lib/subprocess.py

@@ -727,6 +727,9 @@ class Popen(object):
                     to_close.append(self._devnull)
                 for fd in to_close:
                     try:
+                        if _mswindows and isinstance(fd, Handle):
+                            fd.Close()
+                        else:
                             os.close(fd)
                     except OSError:
                         pass
@@ -1007,6 +1010,9 @@ class Popen(object):
                     errwrite.Close()
                 if hasattr(self, '_devnull'):
                     os.close(self._devnull)
+                # Prevent a double close of these handles/fds from __init__
+                # on error.
+                self._closed_child_pipe_fds = True
 
             # Retain the process handle, but close the thread handle
             self._child_created = True

Lib/test/test_subprocess.py

@@ -1116,6 +1116,7 @@ class ProcessTestCase(BaseTestCase):
             p.stdin.write(line) # expect that it flushes the line in text mode
             os.close(p.stdin.fileno()) # close it without flushing the buffer
             read_line = p.stdout.readline()
+            with support.SuppressCrashReport():
                 try:
                     p.stdin.close()
                 except OSError: