Fix PostAttributeChangeCallback value typing (#36928)

As documented in the TODO, the callback requires the caller to pass a
NULL-terminated C-string, but it’s actually a length-buffer pair.
The python lighting example receives values with incorrect length due to this
issue.

This PR fixes the FFI interface by wrapping the callback function, and converts
the length-buffer pair to a proper python bytes value.

Author: sasdf

src/controller/python/chip/native/__init__.py

@@ -16,6 +16,7 @@
 
 import ctypes
 import enum
+import functools
 import glob
 import os
 import platform
@@ -146,21 +147,31 @@ def __ne__(self, other):
         return not self == other
 
 
-PostAttributeChangeCallback = ctypes.CFUNCTYPE(
+c_PostAttributeChangeCallback = ctypes.CFUNCTYPE(
     None,
     ctypes.c_uint16,
     ctypes.c_uint16,
     ctypes.c_uint16,
     ctypes.c_uint8,
     ctypes.c_uint16,
-    # TODO: This should be a pointer to uint8_t, but ctypes does not provide
-    #       such a type. The best approximation is c_char_p, however, this
-    #       requires the caller to pass NULL-terminate C-string which might
-    #       not be the case here.
-    ctypes.c_char_p,
+    ctypes.POINTER(ctypes.c_char),
 )
 
 
+def PostAttributeChangeCallback(func):
+    @functools.wraps(func)
+    def wrapper(
+        endpoint: int,
+        clusterId: int,
+        attributeId: int,
+        xx_type: int,
+        size: int,
+        value: ctypes.POINTER(ctypes.c_char),
+    ):
+        return func(endpoint, clusterId, attributeId, xx_type, size, value[:size])
+    return c_PostAttributeChangeCallback(wrapper)
+
+
 def FindNativeLibraryPath(library: Library) -> str:
     """Find the native CHIP dll/so path."""
 
@@ -234,7 +245,7 @@ def _GetLibraryHandle(lib: Library, expectAlreadyInitialized: bool) -> _Handle:
                        [ctypes.POINTER(PyChipError), ctypes.c_char_p, ctypes.c_uint32])
         elif lib == Library.SERVER:
             setter.Set("pychip_server_native_init", PyChipError, [])
-            setter.Set("pychip_server_set_callbacks", None, [PostAttributeChangeCallback])
+            setter.Set("pychip_server_set_callbacks", None, [c_PostAttributeChangeCallback])
 
     handle = _nativeLibraryHandles[lib]
     if expectAlreadyInitialized and not handle.initialized:

src/controller/python/chip/server/__init__.py

@@ -17,11 +17,20 @@
 import ctypes
 
 from chip import native
-from chip.native import PostAttributeChangeCallback
+from chip.native import PostAttributeChangeCallback, c_PostAttributeChangeCallback
 
+__all__ = [
+    "GetLibraryHandle",
+    "PostAttributeChangeCallback",
+]
 
-def GetLibraryHandle(cb: PostAttributeChangeCallback) -> ctypes.CDLL:
-    """Get a memoized handle to the chip native code dll."""
+
+def GetLibraryHandle(cb: c_PostAttributeChangeCallback) -> ctypes.CDLL:
+    """Get a memoized handle to the chip native code dll.
+
+    Args:
+      cb: A callback decorated by PostAttributeChangeCallback decorator.
+    """
 
     handle = native._GetLibraryHandle(native.Library.SERVER, False)
     if not handle.initialized: