bpo-37267: Do not check for FILE_TYPE_CHAR in os.dup() on Windows (GH-14051)

On Windows, os.dup() no longer creates an inheritable fd when handling a
character file.

Lib/test/test_os.py

@@ -3382,6 +3382,15 @@ class FDInheritanceTests(unittest.TestCase):
         self.addCleanup(os.close, fd2)
         self.assertEqual(os.get_inheritable(fd2), False)
 
+    @unittest.skipUnless(sys.platform == 'win32', 'win32-specific test')
+    def test_dup_nul(self):
+        # os.dup() was creating inheritable fds for character files.
+        fd1 = os.open('NUL', os.O_RDONLY)
+        self.addCleanup(os.close, fd1)
+        fd2 = os.dup(fd1)
+        self.addCleanup(os.close, fd2)
+        self.assertFalse(os.get_inheritable(fd2))
+
     @unittest.skipUnless(hasattr(os, 'dup2'), "need os.dup2()")
     def test_dup2(self):
         fd = os.open(__file__, os.O_RDONLY)

Misc/NEWS.d/next/Windows/2019-06-13-04-15-51.bpo-37267.Ygo5ef.rst

+On Windows, :func:`os.dup` no longer creates an inheritable fd when handling
+a character file.

Python/fileutils.c

@@ -1776,7 +1776,6 @@ _Py_dup(int fd)
 {
 #ifdef MS_WINDOWS
     HANDLE handle;
-    DWORD ftype;
 #endif
 
     assert(PyGILState_Check());
@@ -1790,9 +1789,6 @@ _Py_dup(int fd)
         return -1;
     }
 
-    /* get the file type, ignore the error if it failed */
-    ftype = GetFileType(handle);
-
     Py_BEGIN_ALLOW_THREADS
     _Py_BEGIN_SUPPRESS_IPH
     fd = dup(fd);
@@ -1803,14 +1799,11 @@ _Py_dup(int fd)
         return -1;
     }
 
-    /* Character files like console cannot be make non-inheritable */
-    if (ftype != FILE_TYPE_CHAR) {
-        if (_Py_set_inheritable(fd, 0, NULL) < 0) {
-            _Py_BEGIN_SUPPRESS_IPH
-            close(fd);
-            _Py_END_SUPPRESS_IPH
-            return -1;
-        }
+    if (_Py_set_inheritable(fd, 0, NULL) < 0) {
+        _Py_BEGIN_SUPPRESS_IPH
+        close(fd);
+        _Py_END_SUPPRESS_IPH
+        return -1;
     }
 #elif defined(HAVE_FCNTL_H) && defined(F_DUPFD_CLOEXEC)
     Py_BEGIN_ALLOW_THREADS