return the new file descriptor from os.dup2 (closes bpo-32441)

Doc/library/os.rst

@@ -735,13 +735,17 @@ as internal buffering of data.
 
 .. function:: dup2(fd, fd2, inheritable=True)
 
-   Duplicate file descriptor *fd* to *fd2*, closing the latter first if necessary.
-   The file descriptor *fd2* is :ref:`inheritable <fd_inheritance>` by default,
-   or non-inheritable if *inheritable* is ``False``.
+   Duplicate file descriptor *fd* to *fd2*, closing the latter first if
+   necessary. Return *fd2*. The new file descriptor is :ref:`inheritable
+   <fd_inheritance>` by default or non-inheritable if *inheritable*
+   is ``False``.
 
    .. versionchanged:: 3.4
       Add the optional *inheritable* parameter.
 
+   .. versionchanged:: 3.7
+      Return *fd2* on success. Previously, ``None`` was always returned.
+
 
 .. function:: fchmod(fd, mode)
 

Lib/test/test_os.py

@@ -3079,19 +3079,15 @@ def test_dup2(self):
 
         # inheritable by default
         fd2 = os.open(__file__, os.O_RDONLY)
-        try:
-            os.dup2(fd, fd2)
-            self.assertEqual(os.get_inheritable(fd2), True)
-        finally:
-            os.close(fd2)
+        self.addCleanup(os.close, fd2)
+        self.assertEqual(os.dup2(fd, fd2), fd2)
+        self.assertTrue(os.get_inheritable(fd2))
 
         # force non-inheritable
         fd3 = os.open(__file__, os.O_RDONLY)
-        try:
-            os.dup2(fd, fd3, inheritable=False)
-            self.assertEqual(os.get_inheritable(fd3), False)
-        finally:
-            os.close(fd3)
+        self.addCleanup(os.close, fd3)
+        self.assertEqual(os.dup2(fd, fd3, inheritable=False), fd3)
+        self.assertFalse(os.get_inheritable(fd3))
 
     @unittest.skipUnless(hasattr(os, 'openpty'), "need os.openpty()")
     def test_openpty(self):

Misc/NEWS.d/next/Library/2017-12-28-21-30-40.bpo-32441.LqlboJ.rst

@@ -0,0 +1,2 @@
+Return the new file descriptor (i.e., the second argument) from ``os.dup2``.
+Previously, ``None`` was always returned.

Modules/clinic/posixmodule.c.h

@@ -3486,7 +3486,7 @@ PyDoc_STRVAR(os_dup2__doc__,
 #define OS_DUP2_METHODDEF    \
     {"dup2", (PyCFunction)os_dup2, METH_FASTCALL|METH_KEYWORDS, os_dup2__doc__},
 
-static PyObject *
+static int
 os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable);
 
 static PyObject *
@@ -3498,12 +3498,17 @@ os_dup2(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwn
     int fd;
     int fd2;
     int inheritable = 1;
+    int _return_value;
 
     if (!_PyArg_ParseStackAndKeywords(args, nargs, kwnames, &_parser,
         &fd, &fd2, &inheritable)) {
         goto exit;
     }
-    return_value = os_dup2_impl(module, fd, fd2, inheritable);
+    _return_value = os_dup2_impl(module, fd, fd2, inheritable);
+    if ((_return_value == -1) && PyErr_Occurred()) {
+        goto exit;
+    }
+    return_value = PyLong_FromLong((long)_return_value);
 
 exit:
     return return_value;
@@ -6405,4 +6410,4 @@ os_getrandom(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject
 #ifndef OS_GETRANDOM_METHODDEF
     #define OS_GETRANDOM_METHODDEF
 #endif /* !defined(OS_GETRANDOM_METHODDEF) */
-/*[clinic end generated code: output=b6ade5f170d5a431 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=6345053cd5992caf input=a9049054013a1b77]*/

Modules/posixmodule.c

@@ -7770,17 +7770,17 @@ os_dup_impl(PyObject *module, int fd)
 
 
 /*[clinic input]
-os.dup2
+os.dup2 -> int
     fd: int
     fd2: int
     inheritable: bool=True
 
 Duplicate file descriptor.
 [clinic start generated code]*/
 
-static PyObject *
+static int
 os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
-/*[clinic end generated code: output=db832a2d872ccc5f input=76e96f511be0352f]*/
+/*[clinic end generated code: output=bc059d34a73404d1 input=c3cddda8922b038d]*/
 {
     int res;
 #if defined(HAVE_DUP3) && \
@@ -7789,8 +7789,10 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
     int dup3_works = -1;
 #endif
 
-    if (fd < 0 || fd2 < 0)
-        return posix_error();
+    if (fd < 0 || fd2 < 0) {
+        posix_error();
+        return -1;
+    }
 
     /* dup2() can fail with EINTR if the target FD is already open, because it
      * then has to be closed. See os_close_impl() for why we don't handle EINTR
@@ -7802,13 +7804,16 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
     res = dup2(fd, fd2);
     _Py_END_SUPPRESS_IPH
     Py_END_ALLOW_THREADS
-    if (res < 0)
-        return posix_error();
+    if (res < 0) {
+        posix_error();
+        return -1;
+    }
+    res = fd2; // msvcrt dup2 returns 0 on success.
 
     /* Character files like console cannot be make non-inheritable */
     if (!inheritable && _Py_set_inheritable(fd2, 0, NULL) < 0) {
         close(fd2);
-        return NULL;
+        return -1;
     }
 
 #elif defined(HAVE_FCNTL_H) && defined(F_DUP2FD_CLOEXEC)
@@ -7818,8 +7823,10 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
     else
         res = dup2(fd, fd2);
     Py_END_ALLOW_THREADS
-    if (res < 0)
-        return posix_error();
+    if (res < 0) {
+        posix_error();
+        return -1;
+    }
 
 #else
 
@@ -7831,8 +7838,10 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
         if (res < 0) {
             if (dup3_works == -1)
                 dup3_works = (errno != ENOSYS);
-            if (dup3_works)
-                return posix_error();
+            if (dup3_works) {
+                posix_error();
+                return -1;
+            }
         }
     }
 
@@ -7842,20 +7851,22 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
         Py_BEGIN_ALLOW_THREADS
         res = dup2(fd, fd2);
         Py_END_ALLOW_THREADS
-        if (res < 0)
-            return posix_error();
+        if (res < 0) {
+            posix_error();
+            return -1;
+        }
 
         if (!inheritable && _Py_set_inheritable(fd2, 0, NULL) < 0) {
             close(fd2);
-            return NULL;
+            return -1;
         }
 #ifdef HAVE_DUP3
     }
 #endif
 
 #endif
 
-    Py_RETURN_NONE;
+    return res;
 }