bpo-42536: GC track recycled tuples (GH-23623)

Several built-in and standard library types now ensure that their internal result tuples are always tracked by the garbage collector:

- collections.OrderedDict.items
- dict.items
- enumerate
- functools.reduce
- itertools.combinations
- itertools.combinations_with_replacement
- itertools.permutations
- itertools.product
- itertools.zip_longest
- zip

Previously, they could have become untracked by a prior garbage collection.

Lib/test/test_itertools.py

@@ -12,6 +12,8 @@ from functools import reduce
 import sys
 import struct
 import threading
+import gc
+
 maxsize = support.MAX_Py_ssize_t
 minsize = -maxsize-1
 
@@ -1573,6 +1575,51 @@ class TestBasicOps(unittest.TestCase):
             self.assertRaises(StopIteration, next, f(lambda x:x, []))
             self.assertRaises(StopIteration, next, f(lambda x:x, StopNow()))
 
+    @support.cpython_only
+    def test_combinations_result_gc(self):
+        # bpo-42536: combinations's tuple-reuse speed trick breaks the GC's
+        # assumptions about what can be untracked. Make sure we re-track result
+        # tuples whenever we reuse them.
+        it = combinations([None, []], 1)
+        next(it)
+        gc.collect()
+        # That GC collection probably untracked the recycled internal result
+        # tuple, which has the value (None,). Make sure it's re-tracked when
+        # it's mutated and returned from __next__:
+        self.assertTrue(gc.is_tracked(next(it)))
+
+    @support.cpython_only
+    def test_combinations_with_replacement_result_gc(self):
+        # Ditto for combinations_with_replacement.
+        it = combinations_with_replacement([None, []], 1)
+        next(it)
+        gc.collect()
+        self.assertTrue(gc.is_tracked(next(it)))
+
+    @support.cpython_only
+    def test_permutations_result_gc(self):
+        # Ditto for permutations.
+        it = permutations([None, []], 1)
+        next(it)
+        gc.collect()
+        self.assertTrue(gc.is_tracked(next(it)))
+
+    @support.cpython_only
+    def test_product_result_gc(self):
+        # Ditto for product.
+        it = product([None, []])
+        next(it)
+        gc.collect()
+        self.assertTrue(gc.is_tracked(next(it)))
+
+    @support.cpython_only
+    def test_zip_longest_result_gc(self):
+        # Ditto for zip_longest.
+        it = zip_longest([[]])
+        gc.collect()
+        self.assertTrue(gc.is_tracked(next(it)))
+
+
 class TestExamples(unittest.TestCase):
 
     def test_accumulate(self):