deduperreload: patch NULL for empty closure rather than None (ipython#14978)

When patching an empty closure into some function, we should patch a
value of NULL (corresponding to address 0) rather than than the address
of None, as per
python/cpython#138267 (comment).
This has actually always been a problem with deduperreload, but it
wasn't until
ipython@f6fd3f5
that there was a test case that actually manifested this bug.

Anyway, this PR should fix
python/cpython#138267

Author: krassowski

IPython/extensions/deduperreload/deduperreload_patching.py

@@ -4,6 +4,7 @@
 from typing import Any
 
 NOT_FOUND: object = object()
+NULL: object = object()
 _MAX_FIELD_SEARCH_OFFSET = 50
 
 if sys.maxsize > 2**32:
@@ -55,12 +56,17 @@ def try_write_readonly_attr(
         if offset == -1:
             return
         obj_addr = ctypes.c_void_p.from_buffer(ctypes.py_object(obj)).value
-        new_value_addr = ctypes.c_void_p.from_buffer(ctypes.py_object(new_value)).value
+        if new_value is NULL:
+            new_value_addr: int | None = 0
+        else:
+            new_value_addr = ctypes.c_void_p.from_buffer(
+                ctypes.py_object(new_value)
+            ).value
         if obj_addr is None or new_value_addr is None:
             return
         if prev_value is not None:
             ctypes.pythonapi.Py_DecRef(ctypes.py_object(prev_value))
-        if new_value is not None:
+        if new_value not in (None, NULL):
             ctypes.pythonapi.Py_IncRef(ctypes.py_object(new_value))
         ctypes.cast(
             obj_addr + WORD_N_BYTES * offset, ctypes.POINTER(WORD_TYPE)
@@ -108,12 +114,10 @@ def try_patch_attr(
     def patch_function(
         cls, to_patch_to: Any, to_patch_from: Any, is_method: bool
     ) -> None:
-        new_freevars = []
         new_closure = []
         for freevar, closure_val in zip(
             to_patch_from.__code__.co_freevars or [], to_patch_from.__closure__ or []
         ):
-            new_freevars.append(freevar)
             if (
                 callable(closure_val.cell_contents)
                 and freevar in to_patch_to.__code__.co_freevars
@@ -125,23 +129,19 @@ def patch_function(
                 )
             else:
                 new_closure.append(closure_val)
-        code_with_new_freevars = to_patch_from.__code__.replace(
-            co_freevars=tuple(new_freevars)
-        )
         # lambdas may complain if there is more than one freevar
-        cls.try_patch_attr(
-            to_patch_to, code_with_new_freevars, "__code__", new_is_value=True
-        )
+        cls.try_patch_attr(to_patch_to, to_patch_from, "__code__")
         offset = -1
         if to_patch_to.__closure__ is None and to_patch_from.__closure__ is not None:
             offset = cls.infer_field_offset(to_patch_from, "__closure__")
-        cls.try_patch_readonly_attr(
-            to_patch_to,
-            tuple(new_closure) or None,
-            "__closure__",
-            new_is_value=True,
-            offset=offset,
-        )
+        if to_patch_to.__closure__ is not None or to_patch_from.__closure__ is not None:
+            cls.try_patch_readonly_attr(
+                to_patch_to,
+                tuple(new_closure) or NULL,
+                "__closure__",
+                new_is_value=True,
+                offset=offset,
+            )
         for attr in ("__defaults__", "__kwdefaults__", "__doc__", "__dict__"):
             cls.try_patch_attr(to_patch_to, to_patch_from, attr)
         if is_method: