bpo-18533: Avoid RecursionError from repr() of recursive dictview (#4823)

dictview_repr(): Use a Py_ReprEnter() / Py_ReprLeave() pair to check
for recursion, and produce "..." if so.

test_recursive_repr(): Check for the string rather than a
RecursionError.  (Test cannot be any tighter as contents are
implementation-dependent.)

test_deeply_nested_repr(): Add new test, replacing the original
test_recursive_repr().  It checks that a RecursionError is raised in
the case of a non-recursive but deeply nested structure.  (Very
similar to what test_repr_deep() in test/test_dict.py does for a
normal dict.)

OrderedDictTests: Add new test case, to test behavior on OrderedDict
instances containing their own values() or items().

Lib/test/test_dictviews.py

 import collections.abc
 import copy
 import pickle
+import sys
 import unittest
 
 class DictSetTest(unittest.TestCase):
@@ -202,6 +203,20 @@ class DictSetTest(unittest.TestCase):
     def test_recursive_repr(self):
         d = {}
         d[42] = d.values()
+        r = repr(d)
+        # Cannot perform a stronger test, as the contents of the repr
+        # are implementation-dependent.  All we can say is that we
+        # want a str result, not an exception of any sort.
+        self.assertIsInstance(r, str)
+        d[42] = d.items()
+        r = repr(d)
+        # Again.
+        self.assertIsInstance(r, str)
+
+    def test_deeply_nested_repr(self):
+        d = {}
+        for i in range(sys.getrecursionlimit() + 100):
+            d = {42: d.values()}
         self.assertRaises(RecursionError, repr, d)
 
     def test_copy(self):

Lib/test/test_ordered_dict.py

@@ -355,6 +355,20 @@ class OrderedDictTests:
         self.assertEqual(repr(od),
             "OrderedDict([('a', None), ('b', None), ('c', None), ('x', ...)])")
 
+    def test_repr_recursive_values(self):
+        OrderedDict = self.OrderedDict
+        od = OrderedDict()
+        od[42] = od.values()
+        r = repr(od)
+        # Cannot perform a stronger test, as the contents of the repr
+        # are implementation-dependent.  All we can say is that we
+        # want a str result, not an exception of any sort.
+        self.assertIsInstance(r, str)
+        od[42] = od.items()
+        r = repr(od)
+        # Again.
+        self.assertIsInstance(r, str)
+
     def test_setdefault(self):
         OrderedDict = self.OrderedDict
         pairs = [('c', 1), ('b', 2), ('a', 3), ('d', 4), ('e', 5), ('f', 6)]

Misc/NEWS.d/next/Core and Builtins/2017-12-13-16-46-23.bpo-18533.Dlk8d7.rst

+``repr()`` on a dict containing its own ``values()`` or ``items()`` no
+longer raises ``RecursionError``; OrderedDict similarly.  Instead, use
+``...``, as for other recursive structures.  Patch by Ben North.

Objects/dictobject.c

@@ -3853,14 +3853,22 @@ static PyObject *
 dictview_repr(_PyDictViewObject *dv)
 {
     PyObject *seq;
-    PyObject *result;
+    PyObject *result = NULL;
+    Py_ssize_t rc;
 
+    rc = Py_ReprEnter((PyObject *)dv);
+    if (rc != 0) {
+        return rc > 0 ? PyUnicode_FromString("...") : NULL;
+    }
     seq = PySequence_List((PyObject *)dv);
-    if (seq == NULL)
-        return NULL;
-
+    if (seq == NULL) {
+        goto Done;
+    }
     result = PyUnicode_FromFormat("%s(%R)", Py_TYPE(dv)->tp_name, seq);
     Py_DECREF(seq);
+
+Done:
+    Py_ReprLeave((PyObject *)dv);
     return result;
 }