Apply cpython patch bpo-39492 for the reference counting issue in pickle5 (#7177)

* apply cpython patch bpo-39492 for the reference count issue

build.sh

@@ -130,7 +130,7 @@ WORK_DIR=`mktemp -d`
 pushd $WORK_DIR
 git clone https://github.com/suquark/pickle5-backport
 pushd pickle5-backport
-  git checkout 43551fbb9add8ac2e8551b96fdaf2fe5a3b5997d
+  git checkout 8ffe41ceba9d5e2ce8a98190f6b3d2f3325e5a72
   "$PYTHON_EXECUTABLE" setup.py bdist_wheel
   unzip -o dist/*.whl -d "$ROOT_DIR/python/ray/pickle5_files"
 popd

python/ray/tests/test_basic.py

@@ -10,6 +10,7 @@ import sys
 import threading
 import time
 import pickle
+import weakref
 
 import numpy as np
 import pytest
@@ -453,6 +454,27 @@ def test_ray_recursive_objects(ray_start_regular):
                 ray.put(obj)
 
 
+def test_reducer_override_no_reference_cycle(ray_start_regular):
+    # bpo-39492: reducer_override used to induce a spurious reference cycle
+    # inside the Pickler object, that could prevent all serialized objects
+    # from being garbage-collected without explicity invoking gc.collect.
+    f = lambda: 4669201609102990671853203821578
+
+    wr = weakref.ref(f)
+
+    bio = io.BytesIO()
+    from ray.cloudpickle import CloudPickler, loads
+    p = CloudPickler(bio, protocol=5)
+    p.dump(f)
+    new_f = loads(bio.getvalue())
+    assert new_f() == 4669201609102990671853203821578
+
+    del p
+    del f
+
+    assert wr() is None
+
+
 def test_passing_arguments_by_value_out_of_the_box(ray_start_regular):
     @ray.remote
     def f(x):