[DPE-3314] remove secrets on broken (#336)

## FYI
most of the work here is done on the `mongos` side, but some changes are
needed in the libs that are owned by this charm

## Issue
Secretes associated with username, password, uri are still present on
the `host-application` and `mongos sub-charm` relation after relation
with `config-server` and `mongos sub-charm` is broken

## Solution
Call appropriate functions from mongos charm

## Future PRs
1. Implementation + Integration tests added on mongos side

lib/charms/mongodb/v0/config_server_interface.py

@@ -14,7 +14,7 @@
 )
 from charms.mongodb.v1.helpers import add_args_to_env, get_mongos_args
 from charms.mongodb.v1.mongos import MongosConnection
-from ops.charm import CharmBase, EventBase
+from ops.charm import CharmBase, EventBase, RelationBrokenEvent
 from ops.framework import Object
 from ops.model import ActiveStatus, MaintenanceStatus, WaitingStatus
 
@@ -35,7 +35,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 3
+LIBPATCH = 4
 
 
 class ClusterProvider(Object):
@@ -182,8 +182,13 @@ def __init__(
         self.framework.observe(
             charm.on[self.relation_name].relation_changed, self._on_relation_changed
         )
-
-        # TODO Future PRs handle scale down
+        self.framework.observe(
+            charm.on[self.relation_name].relation_departed,
+            self.charm.check_relation_broken_or_scale_down,
+        )
+        self.framework.observe(
+            charm.on[self.relation_name].relation_broken, self._on_relation_broken
+        )
 
     def _on_database_created(self, event) -> None:
         if not self.charm.unit.is_leader():
@@ -228,6 +233,30 @@ def _on_relation_changed(self, event) -> None:
 
         self.charm.unit.status = ActiveStatus()
 
+    def _on_relation_broken(self, event: RelationBrokenEvent) -> None:
+        # Only relation_deparated events can check if scaling down
+        if not self.charm.has_departed_run(event.relation.id):
+            logger.info(
+                "Deferring, must wait for relation departed hook to decide if relation should be removed."
+            )
+            event.defer()
+            return
+
+        if not self.charm.proceed_on_broken_event(event):
+            logger.info("Skipping relation broken event, broken event due to scale down")
+            return
+
+        self.charm.stop_mongos_service()
+        logger.info("Stopped mongos daemon")
+
+        if not self.charm.unit.is_leader():
+            return
+
+        logger.info("Database and user removed for mongos application")
+        self.charm.remove_secret(Config.Relations.APP_SCOPE, Config.Secrets.USERNAME)
+        self.charm.remove_secret(Config.Relations.APP_SCOPE, Config.Secrets.PASSWORD)
+        self.charm.remove_connection_info()
+
     # BEGIN: helper functions
 
     def is_mongos_running(self) -> bool:

lib/charms/mongodb/v1/shards_interface.py

@@ -51,7 +51,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 3
+LIBPATCH = 4
 KEYFILE_KEY = "key-file"
 HOSTS_KEY = "host"
 OPERATOR_PASSWORD_KEY = MongoDBUser.get_password_key_name_for_user(OperatorUser.get_username())
@@ -146,6 +146,17 @@ def pass_hook_checks(self, event: EventBase) -> bool:
             event.defer()
             return False
 
+        if isinstance(event, RelationBrokenEvent):
+            if not self.charm.has_departed_run(event.relation.id):
+                logger.info(
+                    "Deferring, must wait for relation departed hook to decide if relation should be removed."
+                )
+                event.defer()
+                return False
+
+            if not self.charm.proceed_on_broken_event(event):
+                return False
+
         return True
 
     def _on_relation_event(self, event):
@@ -159,9 +170,7 @@ def _on_relation_event(self, event):
 
         departed_relation_id = None
         if isinstance(event, RelationBrokenEvent):
-            departed_relation_id = self.charm.proceed_on_broken_event(event)
-            if not departed_relation_id:
-                return
+            departed_relation_id = event.relation.id
 
         try:
             logger.info("Adding/Removing shards not present in cluster.")

src/charm.py

@@ -1323,27 +1323,25 @@ def set_scaling_down(self, event: RelationDepartedEvent) -> bool:
         self.unit_peer_data[rel_departed_key] = json.dumps(scaling_down)
         return scaling_down
 
-    def proceed_on_broken_event(self, event) -> int:
+    def proceed_on_broken_event(self, event) -> bool:
         """Returns relation_id if relation broken event occurred due to a removed relation."""
-        departed_relation_id = None
-
         # Only relation_deparated events can check if scaling down
         departed_relation_id = event.relation.id
         if not self.has_departed_run(departed_relation_id):
             logger.info(
                 "Deferring, must wait for relation departed hook to decide if relation should be removed."
             )
             event.defer()
-            return
+            return False
 
         # check if were scaling down and add a log message
         if self.is_scaling_down(departed_relation_id):
             logger.info(
                 "Relation broken event occurring due to scale down, do not proceed to remove users."
             )
-            return
+            return False
 
-        return departed_relation_id
+        return True
 
     @staticmethod
     def _generate_relation_departed_key(rel_id: int) -> str: