[nft PATCH v5 4/6] py: fix exception during cleanup of half-initialized Nftables

[Thomas Haller <thaller@redhat.com>]

When we create a Nftables instance against an older library version,
we might not find a symbol and fail with an exception when initializing
the context object.

Then, __del__() is still called, but resulting in a second exception
because self.__ctx is not set. Avoid that second exception.

    $ python -c 'import nftables; nftables.Nftables()'
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/data/src/nftables/py/nftables.py", line 90, in __init__
        self.nft_ctx_input_get_flags = lib.nft_ctx_input_get_flags
                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/ctypes/__init__.py", line 389, in __getattr__
        func = self.__getitem__(name)
               ^^^^^^^^^^^^^^^^^^^^^^
      File "/usr/lib64/python3.11/ctypes/__init__.py", line 394, in __getitem__
        func = self._FuncPtr((name_or_ordinal, self))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    AttributeError: /lib64/libnftables.so.1: undefined symbol: nft_ctx_input_get_flags
    Exception ignored in: <function Nftables.__del__ at 0x7f6315a2c540>
    Traceback (most recent call last):
      File "/data/src/nftables/py/nftables.py", line 166, in __del__
        self.nft_ctx_free(self.__ctx)
        ^^^^^^^^^^^^^^^^^
    AttributeError: 'Nftables' object has no attribute 'nft_ctx_free'

Signed-off-by: Thomas Haller <thaller@redhat.com>
Reviewed-by: Phil Sutter <phil@nwl.cc>
---
 py/src/nftables.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/py/src/nftables.py b/py/src/nftables.py
index 68fcd7dd103c..b1186781ab5c 100644
--- a/py/src/nftables.py
+++ b/py/src/nftables.py
@@ -74,6 +74,8 @@ class Nftables:
         is requested from the library and buffering of output and error streams
         is turned on.
         """
+        self.__ctx = None
+
         lib = cdll.LoadLibrary(sofile)
 
         ### API function definitions
@@ -150,7 +152,9 @@ class Nftables:
         self.nft_ctx_buffer_error(self.__ctx)
 
     def __del__(self):
-        self.nft_ctx_free(self.__ctx)
+        if self.__ctx is not None:
+            self.nft_ctx_free(self.__ctx)
+            self.__ctx = None
 
     def __get_output_flag(self, name):
         flag = self.output_flags[name]
-- 
2.41.0