Issue 4957

Let os.ftruncate raise OSError like documented.

Lib/test/test_os.py

@@ -570,10 +570,9 @@ class TestInvalidFD(unittest.TestCase):
         if hasattr(os, "fpathconf"):
             self.assertRaises(OSError, os.fpathconf, 10, "PC_NAME_MAX")
 
-    #this is a weird one, it raises IOError unlike the others
     def test_ftruncate(self):
         if hasattr(os, "ftruncate"):
-            self.assertRaises(IOError, os.ftruncate, 10, 0)
+            self.assertRaises(OSError, os.ftruncate, 10, 0)
 
     def test_lseek(self):
         if hasattr(os, "lseek"):

Modules/posixmodule.c

@@ -6633,7 +6633,7 @@ posix_ftruncate(PyObject *self, PyObject *args)
 	res = ftruncate(fd, length);
 	Py_END_ALLOW_THREADS
 	if (res < 0) {
-		PyErr_SetFromErrno(PyExc_IOError);
+		posix_error();
 		return NULL;
 	}
 	Py_INCREF(Py_None);