context/data: memleak due to API (mis)use

Memory management is a foreign concept to python in general, howevrer it
appears DNode and Context require .free() and .destroy() to be called,
respectively, to avoid a memory leak.

This seems easily avoidable by simply attaching a destructor to the
class so the python garbage collector can clean up automatically when all
references go out of scope.

This change should also be compatible with existing integrations which may be
aware of this as it sets the internal `cdata` reference to None and checks
it to ensure it doesn't run the cleanup again.

Signed-off-by: Brad House <[email protected]>

Author: bradh352

libyang/context.py

@@ -259,6 +259,10 @@ def __init__(
             raise self.error("cannot create context")
         self.external_module_loader = ContextExternalModuleLoader(self.cdata)
 
+
+    def __del__(self):
+        self.destroy()
+
     def compile_schema(self):
         ret = lib.ly_ctx_compile(self.cdata)
         if ret != lib.LY_SUCCESS:

libyang/data.py

@@ -291,6 +291,9 @@ def __init__(self, context: "libyang.Context", cdata):
         self.attributes = None
         self.free_func = None  # type: Callable[DNode]
 
+    def __del__(self):
+        self.free()
+
     def meta(self):
         ret = {}
         item = self.cdata.meta
@@ -996,13 +999,14 @@ def free_internal(self, with_siblings: bool = True) -> None:
             lib.lyd_free_tree(self.cdata)
 
     def free(self, with_siblings: bool = True) -> None:
-        try:
-            if self.free_func:
-                self.free_func(self)  # pylint: disable=not-callable
-            else:
-                self.free_internal(with_siblings)
-        finally:
-            self.cdata = ffi.NULL
+        if self.cdata is not None:
+            try:
+                if self.free_func:
+                    self.free_func(self)  # pylint: disable=not-callable
+                else:
+                    self.free_internal(with_siblings)
+            finally:
+                self.cdata = None
 
     def leafref_link_node_tree(self) -> None:
         """