parse: handle redefinition of a place holder netdef

If we find an interface with the same name of a place holder created
earlier, we just change the place holder type to the type of the
new interface and use it to store the configuration.

Without this logic the parser would fail if it finds an interface with
the same name of the place holder but of a different type.

src/parse.c

@@ -3071,8 +3071,13 @@ handle_network_type(NetplanParser* npp, yaml_node_t* node, const char* key_prefi
         npp->current.netdef = npp->parsed_defs ? g_hash_table_lookup(npp->parsed_defs, scalar(key)) : NULL;
         if (npp->current.netdef) {
             /* already exists, overriding/amending previous definition */
-            if (npp->current.netdef->type != GPOINTER_TO_UINT(data))
-                return yaml_error(npp, key, error, "Updated definition '%s' changes device type", scalar(key));
+            if (npp->current.netdef->type != GPOINTER_TO_UINT(data)) {
+                /* If the existing netdef is a place holder, we just repurpose it */
+                if (npp->current.netdef->type == NETPLAN_DEF_TYPE_NM_PLACEHOLDER_)
+                    npp->current.netdef->type = GPOINTER_TO_UINT(data);
+                else
+                    return yaml_error(npp, key, error, "Updated definition '%s' changes device type", scalar(key));
+            }
         } else {
             npp->current.netdef = netplan_netdef_new(npp, scalar(key), GPOINTER_TO_UINT(data), npp->current.backend);
         }

tests/generator/test_vlans.py

@@ -331,3 +331,21 @@ def test_vlan_parent_is_allowed_to_be_missing_for_nm(self):
 [ipv6]
 method=ignore
 '''})
+
+    def test_vlan_with_missing_netdef_found_in_the_next_file_wont_fail(self):
+        ''' If eth0 was registered as missing (and created as a placeholder netdef)
+        if shouldn't fail if it's defined in a file that was parsed *after* the file where
+        the VLAN is defined
+        '''
+        out = self.generate('''network:
+  renderer: NetworkManager
+  version: 2
+  vlans:
+    vlan100:
+      id: 100
+      link: eth0''', confs={'b': '''network:
+  renderer: NetworkManager
+  version: 2
+  ethernets: {eth0: {}}'''})
+
+        self.assertNotIn('Updated definition \'eth0\' changes device type', out)