vyos.utils: T9269: no zero-component key path for an empty dict

dict_to_key_paths() yields the path leading to an empty dict, which is a
valueless leaf node. An empty dict at the top level means "no
configuration" and yielded a single zero-component path, making
dict_to_paths_values() report an option named ''.

A bare "interfaces ethernet eth1" - as found in a container, where no
hw-id is deployed - could therefore not be added to a bond:

  Cannot add interface "eth1" to bond, it has a "" assigned!

interfaces_ethernet.py recorded the same phantom path in
bond_blocked_changes. configdiff.py was immune, owner([]) is falsy.

1 files changed, 6 insertions, 1 deletions

diff --git a/python/vyos/utils/dict.py b/python/vyos/utils/dict.py
index 3fcda902f..058717265 100644
--- a/python/vyos/utils/dict.py
+++ b/python/vyos/utils/dict.py
@@ -315,7 +315,12 @@ def dict_to_key_paths(d: dict) -> list:
     """
     def func(d, path):
         if isinstance(d, dict):
-            if not d:
+            # An empty dict is a valueless leaf node and yields the path that
+            # leads to it - but only if there is one. An empty dict at the top
+            # level means "no configuration at all", which must yield nothing
+            # instead of a single empty path: consumers join the components
+            # into a dotted option name, and '' is not an option
+            if not d and path:
                 yield path
             for k, v in d.items():
                 for r in func(v, path + [k]):