Fix stack overflow when partially-__init__ Node raises exception. (ap…

…ache#7481)

* Fix stack overflow when partially-__init__ Node raises exception.

 * If a Node subclass raises an exception and ctypes is in use before
   __init_handle_by_constructor__ is called (or self.handle is
   otherwise set), a Python stack overflow could result. This is
   because the unset handle slot causes self.handle accesses to
   fallback on the getattr(self, 'handle') method, invoking
   NodeGetAttr.
 * Then I believe this causes an infinite loop.
 * The fix is to make Node.__getattr__ raise AttributeError for all
   attributes in __slots__, then make __del__ tolerant to missing
   self.handle.
 * I don't believe cython is affected because it implements a
   descriptor to access its underlying chandle and that shouldn't be unset.

* black format

* actually use handle instead of self.handle

python/tvm/_ffi/_ctypes/object.py

@@ -106,7 +106,12 @@ class ObjectBase(object):
 
     def __del__(self):
         if _LIB is not None:
-            check_call(_LIB.TVMObjectFree(self.handle))
+            try:
+                handle = self.handle
+            except AttributeError:
+                return
+
+            check_call(_LIB.TVMObjectFree(handle))
 
     def __init_handle_by_constructor__(self, fconstructor, *args):
         """Initialize the handle by calling constructor function.

python/tvm/runtime/object.py

@@ -56,6 +56,9 @@ def __dir__(self):
         return sorted([fnames(i) for i in range(size)] + class_names)
 
     def __getattr__(self, name):
+        if name in self.__slots__:
+            raise AttributeError(f"{name} is not set")
+
         try:
             return _ffi_node_api.NodeGetAttr(self, name)
         except AttributeError: