Fix a memory leak, an assert and an error message

[daniloegea]

Description

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

Thanks for spotting and fixing more papercuts!

I put one tiny suggestion inline (up to you if you want it that way!), and one comment about your fix in nm.c which I'd rather like to see fixed at its origin in parse-nm.c.

[slyon]

Your approach wfm! It's good to warn the user when an entry is deleted/ignored.

[slyon]

Two more inline comments:

• We cannot be sure the renderer already got set in the parse.c stage, we need to check that in validation.c stage, maybe using a has_backend_settings_nm helper variable.
• We might want to drop the NetplanBackendSettings union all together. backend_settings.networkd.union doesn't seem to be used anywhere. Replacing the union by just struct NetplanNMSettings should be ABI-safe IMO.

[daniloegea]

I moved the wireguard part to another PR (#352).

I converted the NetplanBackendSettings union to a struct as suggested, I think it makes sense.

[slyon]

Thank you very much, this is looking good now!

Just two little nitpicks, maybe you can squash them into their relevant commits, so we can merge this PR with a clean history:

• nitpick: nm.c:write_fallback_key_value() "backend_settings.nm.passthrough" in comment should be adopted, too.
• See inline comment

E.g.:

diff --git a/src/nm.c b/src/nm.c
index 112e1ff6..1d90e54b 100644
--- a/src/nm.c
+++ b/src/nm.c
@@ -517,7 +517,7 @@ write_vxlan_parameters(const NetplanNetDefinition* def, GKeyFile* kf)
 
 /**
  * Special handling for passthrough mode: read key-value pairs from
- * "backend_settings.nm.passthrough" and inject them into the keyfile as-is.
+ * "backend_settings.passthrough" and inject them into the keyfile as-is.
  */
 static void
 write_fallback_key_value(GQuark key_id, gpointer value, gpointer user_data)
diff --git a/src/validation.c b/src/validation.c
index e159a7c2..c87d7f67 100644
--- a/src/validation.c
+++ b/src/validation.c
@@ -413,7 +413,7 @@ validate_netdef_grammar(const NetplanParser* npp, NetplanNetDefinition* nd, GErr
         backend = npp->global_backend;
 
     if (nd->has_backend_settings_nm && backend != NETPLAN_BACKEND_NM) {
-            return yaml_error(npp, NULL, error, "%s: networkmanager.passthrough settings found but renderer is not NetworkManager.", nd->id);
+            return yaml_error(npp, NULL, error, "%s: networkmanager backend settings found but renderer is not NetworkManager.", nd->id);
     }
 
     valid = TRUE;