bpo-35053: Enhance tracemalloc to trace free lists (GH-10063)

tracemalloc now tries to update the traceback when an object is
reused from a "free list" (optimization for faster object creation,
used by the builtin list type for example).

Changes:

* Add _PyTraceMalloc_NewReference() function which tries to update
  the Python traceback of a Python object.
* _Py_NewReference() now calls _PyTraceMalloc_NewReference().
* Add an unit test.

1 files changed, 20 insertions, 0 deletions

diff --git a/Lib/test/test_tracemalloc.py b/Lib/test/test_tracemalloc.py
index 76a6159..c386648 100644
--- a/Lib/test/test_tracemalloc.py
+++ b/Lib/test/test_tracemalloc.py
@@ -111,6 +111,26 @@ class TestTracemallocEnabled(unittest.TestCase):
         traceback = tracemalloc.get_object_traceback(obj)
         self.assertEqual(traceback, obj_traceback)
 
+    def test_new_reference(self):
+        tracemalloc.clear_traces()
+        # gc.collect() indirectly calls PyList_ClearFreeList()
+        support.gc_collect()
+
+        # Create a list and "destroy it": put it in the PyListObject free list
+        obj = []
+        obj = None
+
+        # Create a list which should reuse the previously created empty list
+        obj = []
+
+        nframe = tracemalloc.get_traceback_limit()
+        frames = get_frames(nframe, -3)
+        obj_traceback = tracemalloc.Traceback(frames)
+
+        traceback = tracemalloc.get_object_traceback(obj)
+        self.assertIsNotNone(traceback)
+        self.assertEqual(traceback, obj_traceback)
+
     def test_set_traceback_limit(self):
         obj_size = 10