GH-89988: Fix memory leak in pickle.Pickler dispatch_table lookup (GH-94298)

(cherry picked from commit 01ef1f9)

Co-authored-by: Kumar Aditya <[email protected]>

Author: miss-islington

Lib/test/test_pickle.py

@@ -154,6 +154,29 @@ def persistent_id(obj):
                 return obj
         check(PersPickler)
 
+    @support.cpython_only
+    def test_custom_pickler_dispatch_table_memleak(self):
+        # See https://github.com/python/cpython/issues/89988
+
+        class Pickler(self.pickler):
+            def __init__(self, *args, **kwargs):
+                self.dispatch_table = table
+                super().__init__(*args, **kwargs)
+
+        class DispatchTable:
+            pass
+
+        table = DispatchTable()
+        pickler = Pickler(io.BytesIO())
+        self.assertIs(pickler.dispatch_table, table)
+        table_ref = weakref.ref(table)
+        self.assertIsNotNone(table_ref())
+        del pickler
+        del table
+        support.gc_collect()
+        self.assertIsNone(table_ref())
+
+
     @support.cpython_only
     def test_unpickler_reference_cycle(self):
         def check(Unpickler):

Misc/NEWS.d/next/Library/2022-06-26-10-59-15.gh-issue-89988.K8rnmt.rst

@@ -0,0 +1 @@
+Fix memory leak in :class:`pickle.Pickler` when looking up :attr:`dispatch_table`. Patch by Kumar Aditya.

Modules/_pickle.c

@@ -4773,7 +4773,9 @@ _pickle_Pickler___init___impl(PicklerObject *self, PyObject *file,
     {
         return -1;
     }
-
+    if (self->dispatch_table != NULL) {
+        return 0;
+    }
     if (_PyObject_LookupAttr((PyObject *)self, &_Py_ID(dispatch_table),
                              &self->dispatch_table) < 0) {
         return -1;