remove a optimization that resulted in unexpected behavior #8929

Lib/test/test_select.py

@@ -21,6 +21,13 @@ class SelectTestCase(unittest.TestCase):
         self.assertRaises(TypeError, select.select, [self.Almost()], [], [])
         self.assertRaises(TypeError, select.select, [], [], [], "not a number")
 
+    def test_returned_list_identity(self):
+        # See issue #8329
+        r, w, x = select.select([], [], [], 1)
+        self.assertIsNot(r, w)
+        self.assertIsNot(r, x)
+        self.assertIsNot(w, x)
+
     def test_select(self):
         cmd = 'for i in 0 1 2 3 4 5 6 7 8 9; do echo testing...; sleep 1; done'
         p = os.popen(cmd, 'r')

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.7 beta 1?
 Core and Builtins
 -----------------
 
+- Issue #8329: Don't return the same lists from select.select when no fds are
+  changed.
+
 - Issue #8259: 1L << (2**31) no longer produces an 'outrageous shift error'
   on 64-bit machines.  The shift count for either left or right shift is
   permitted to be up to sys.maxsize.

Modules/selectmodule.c

@@ -287,14 +287,6 @@ select_select(PyObject *self, PyObject *args)
 		PyErr_SetFromErrno(SelectError);
 	}
 #endif
-	else if (n == 0) {
-                /* optimization */
-		ifdlist = PyList_New(0);
-		if (ifdlist) {
-			ret = PyTuple_Pack(3, ifdlist, ifdlist, ifdlist);
-			Py_DECREF(ifdlist);
-		}
-	}
 	else {
 		/* any of these three calls can raise an exception.  it's more
 		   convenient to test for this after all three calls... but