bpo-28416: Break reference cycles in Pickler and Unpickler subclasses (#4080)

with the persistent_id() and persistent_load() methods.

Lib/test/test_pickle.py

@@ -6,6 +6,7 @@ import io
 import collections
 import struct
 import sys
+import weakref
 
 import unittest
 from test import support
@@ -117,6 +118,66 @@ class PyIdPersPicklerTests(AbstractIdentityPersistentPicklerTests,
     pickler = pickle._Pickler
     unpickler = pickle._Unpickler
 
+    @support.cpython_only
+    def test_pickler_reference_cycle(self):
+        def check(Pickler):
+            for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+                f = io.BytesIO()
+                pickler = Pickler(f, proto)
+                pickler.dump('abc')
+                self.assertEqual(self.loads(f.getvalue()), 'abc')
+            pickler = Pickler(io.BytesIO())
+            self.assertEqual(pickler.persistent_id('def'), 'def')
+            r = weakref.ref(pickler)
+            del pickler
+            self.assertIsNone(r())
+
+        class PersPickler(self.pickler):
+            def persistent_id(subself, obj):
+                return obj
+        check(PersPickler)
+
+        class PersPickler(self.pickler):
+            @classmethod
+            def persistent_id(cls, obj):
+                return obj
+        check(PersPickler)
+
+        class PersPickler(self.pickler):
+            @staticmethod
+            def persistent_id(obj):
+                return obj
+        check(PersPickler)
+
+    @support.cpython_only
+    def test_unpickler_reference_cycle(self):
+        def check(Unpickler):
+            for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+                unpickler = Unpickler(io.BytesIO(self.dumps('abc', proto)))
+                self.assertEqual(unpickler.load(), 'abc')
+            unpickler = Unpickler(io.BytesIO())
+            self.assertEqual(unpickler.persistent_load('def'), 'def')
+            r = weakref.ref(unpickler)
+            del unpickler
+            self.assertIsNone(r())
+
+        class PersUnpickler(self.unpickler):
+            def persistent_load(subself, pid):
+                return pid
+        check(PersUnpickler)
+
+        class PersUnpickler(self.unpickler):
+            @classmethod
+            def persistent_load(cls, pid):
+                return pid
+        check(PersUnpickler)
+
+        class PersUnpickler(self.unpickler):
+            @staticmethod
+            def persistent_load(pid):
+                return pid
+        check(PersUnpickler)
+
 
 class PyPicklerUnpicklerObjectTests(AbstractPicklerUnpicklerObjectTests):
 
@@ -197,7 +258,7 @@ if has_c_implementation:
         check_sizeof = support.check_sizeof
 
         def test_pickler(self):
-            basesize = support.calcobjsize('5P2n3i2n3iP')
+            basesize = support.calcobjsize('6P2n3i2n3iP')
             p = _pickle.Pickler(io.BytesIO())
             self.assertEqual(object.__sizeof__(p), basesize)
             MT_size = struct.calcsize('3nP0n')
@@ -214,7 +275,7 @@ if has_c_implementation:
                 0)  # Write buffer is cleared after every dump().
 
         def test_unpickler(self):
-            basesize = support.calcobjsize('2Pn2P 2P2n2i5P 2P3n6P2n2i')
+            basesize = support.calcobjsize('2P2n2P 2P2n2i5P 2P3n6P2n2i')
             unpickler = _pickle.Unpickler
             P = struct.calcsize('P')  # Size of memo table entry.
             n = struct.calcsize('n')  # Size of mark table entry.