bpo-41422: Visit the Pickler's and Unpickler's memo in tp_traverse (GH-21664)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: kale-smoothie

Misc/NEWS.d/next/Library/2020-07-28-20-48-05.bpo-41422.iMwnMu.rst

@@ -0,0 +1,2 @@
+Fixed memory leaks of :class:`pickle.Pickler` and :class:`pickle.Unpickler` involving cyclic references via the
+internal memo mapping.

Modules/_pickle.c

@@ -4707,6 +4707,14 @@ Pickler_traverse(PicklerObject *self, visitproc visit, void *arg)
     Py_VISIT(self->fast_memo);
     Py_VISIT(self->reducer_override);
     Py_VISIT(self->buffer_callback);
+    PyMemoTable *memo = self->memo;
+    if (memo && memo->mt_table) {
+        Py_ssize_t i = memo->mt_allocated;
+        while (--i >= 0) {
+            Py_VISIT(memo->mt_table[i].me_key);
+        }
+    }
+
     return 0;
 }
 
@@ -7175,6 +7183,13 @@ Unpickler_traverse(UnpicklerObject *self, visitproc visit, void *arg)
     Py_VISIT(self->stack);
     Py_VISIT(self->pers_func);
     Py_VISIT(self->buffers);
+    PyObject **memo = self->memo;
+    if (memo) {
+        Py_ssize_t i = self->memo_size;
+        while (--i >= 0) {
+            Py_VISIT(memo[i]);
+        }
+    }
     return 0;
 }