feat: write controller config atomically

It's possible to corrupt the controller config file if there is an
exception during writing of the file. Instead, we want to only update
the controller config atomically. This involves writing the yaml to
a temp file and then do a atomic file replace (POSIX). This ensures
that it will always be valid if the data is valid.

Author: SimonRichardson

src/charm.py

@@ -9,6 +9,8 @@
 import secrets
 import urllib.parse
 import yaml
+import tempfile
+import os
 
 from charms.prometheus_k8s.v0.prometheus_scrape import MetricsEndpointProvider
 from ops.charm import CharmBase, CollectStatusEvent
@@ -238,8 +240,13 @@ def _update_config_file(self, bind_addresses):
             conf = dict()
         conf[self.ALL_BIND_ADDRS_KEY] = bind_addresses
 
-        with open(file_path, 'w') as conf_file:
-            yaml.dump(conf, conf_file)
+        dir_name = os.path.dirname(file_path)
+        with tempfile.NamedTemporaryFile('w', dir=dir_name, delete=False) as tmp_file:
+            yaml.dump(conf, tmp_file)
+            temp_name = tmp_file.name
+
+        os.replace(temp_name, file_path)
+        os.remove(temp_name)
 
         self._request_config_reload()
         self._stored.all_bind_addresses = bind_addresses