Fix SF bug #667147, Segmentation fault printing str subclass

Fix infinite recursion which occurred when printing an object
whose __str__() returned self.

Will backport

Lib/test/test_descr.py

@@ -1,6 +1,6 @@
 # Test enhancements related to descriptors and new-style classes
 
-from test.test_support import verify, vereq, verbose, TestFailed, TESTFN
+from test.test_support import verify, vereq, verbose, TestFailed, TESTFN, get_original_stdout
 from copy import deepcopy
 import warnings
 
@@ -1821,6 +1821,29 @@ def specials():
     unsafecmp(1, 1L)
     unsafecmp(1L, 1)
 
+    class Letter(str):
+        def __new__(cls, letter):
+            if letter == 'EPS':
+                return str.__new__(cls)
+            return str.__new__(cls, letter)
+        def __str__(self):
+            if not self:
+                return 'EPS'
+            return self 
+
+    # sys.stdout needs to be the original to trigger the recursion bug
+    import sys
+    test_stdout = sys.stdout
+    sys.stdout = get_original_stdout()
+    try:
+        # nothing should actually be printed, this should raise an exception
+        print Letter('w')
+    except RuntimeError:
+        pass
+    else:
+        raise TestFailed, "expected a RuntimeError for print recursion"
+    sys.stdout = test_stdout
+
 def weakrefs():
     if verbose: print "Testing weak references..."
     import weakref

Misc/NEWS

@@ -18,6 +18,9 @@ Core and builtins
   Passing None is semantically identical to calling sort() with no
   arguments.
 
+- Fixed crash when printing a subclass of str and __str__ returned self.
+  See SF bug #667147.
+
 Extension modules
 -----------------
 

Objects/object.c

@@ -158,10 +158,15 @@ _PyObject_Del(PyObject *op)
 	PyObject_FREE(op);
 }
 
-int
-PyObject_Print(PyObject *op, FILE *fp, int flags)
+/* Implementation of PyObject_Print with recursion checking */
+static int
+internal_print(PyObject *op, FILE *fp, int flags, int nesting)
 {
 	int ret = 0;
+	if (nesting > 10) {
+		PyErr_SetString(PyExc_RuntimeError, "print recursion");
+		return -1;
+	}
 	if (PyErr_CheckSignals())
 		return -1;
 #ifdef USE_STACKCHECK
@@ -187,7 +192,8 @@ PyObject_Print(PyObject *op, FILE *fp, int flags)
 			if (s == NULL)
 				ret = -1;
 			else {
-				ret = PyObject_Print(s, fp, Py_PRINT_RAW);
+				ret = internal_print(s, fp, Py_PRINT_RAW,
+						     nesting+1);
 			}
 			Py_XDECREF(s);
 		}
@@ -204,6 +210,13 @@ PyObject_Print(PyObject *op, FILE *fp, int flags)
 	return ret;
 }
 
+int
+PyObject_Print(PyObject *op, FILE *fp, int flags)
+{
+	return internal_print(op, fp, flags, 0);
+}
+
+
 /* For debugging convenience.  See Misc/gdbinit for some useful gdb hooks */
 void _PyObject_Dump(PyObject* op)
 {