[DPE-3182] mongodb remove user (#334)

## Issue
When relation is broken between `mongos` charm and `config-server` charm
the user created for the `mongos` application is still accessible

## Solution
When relation is broken between mongos charm and config-sever process
users

## Tested by hand
```
 # deploy both charms
cd ~/mongos-operator
# juju add-model test-0
tox -e build
juju deploy ./*charm

cd ~/mongos-operator/tests/integration/application
charmcraft pack 
juju deploy ./*charm -n2

# get shared cluster up and running
cd ~/mongodb-operator
charmcraft pack
juju deploy ./*charm --config role="config-server" config-server -n2
juju deploy ./*charm --config role="shard" shard

# integrate
juju integrate mongos application
juju integrate config-server:config-server shard:sharding
juju integrate config-server:cluster mongos:cluster

# get URI
juju show-unit application/0
juju show-secret --reveal <secret URI>

# access user
juju ssh application/0
sudo charmed-mongodb.mongosh <URI>

# scale application and repeat access user step
juju remove-unit config-server/1
juju remove-unit application/0

# remove relation and  repeat access user step (except this time verify we cannot access the user)
juju remove-relation  config-server:cluster mongos:cluster
```

## Future PRs
1. [user int tests](https://warthogs.atlassian.net/browse/DPE-3292)
2. [have mongos sub-charm remove connection info for host application]()

lib/charms/mongodb/v0/config_server_interface.py

@@ -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 = 2
+LIBPATCH = 3
 
 
 class ClusterProvider(Object):
@@ -54,8 +54,13 @@ def __init__(
             charm.on[self.relation_name].relation_changed, self._on_relation_changed
         )
 
-        # TODO Future PRs handle scale down
-        # TODO Future PRs handle changing of units/passwords to be propagated to mongos
+        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 pass_hook_checks(self, event: EventBase) -> bool:
         """Runs the pre-hooks checks for ClusterProvider, returns True if all pass."""
@@ -97,6 +102,26 @@ def _on_relation_changed(self, event) -> None:
                 },
             )
 
+    def _on_relation_broken(self, event) -> None:
+        # Only relation_deparated events can check if scaling down
+        departed_relation_id = event.relation.id
+        if not self.charm.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
+
+        if not self.pass_hook_checks(event):
+            logger.info("Skipping relation broken event: hook checks did not pass")
+            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.client_relations.oversee_users(departed_relation_id, event)
+
     def update_config_server_db(self, event):
         """Provides related mongos applications with new config server db."""
         if not self.pass_hook_checks(event):
@@ -157,6 +182,7 @@ def __init__(
         self.framework.observe(
             charm.on[self.relation_name].relation_changed, self._on_relation_changed
         )
+
         # TODO Future PRs handle scale down
 
     def _on_database_created(self, event) -> None:

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 = 2
+LIBPATCH = 3
 KEYFILE_KEY = "key-file"
 HOSTS_KEY = "host"
 OPERATOR_PASSWORD_KEY = MongoDBUser.get_password_key_name_for_user(OperatorUser.get_username())
@@ -148,28 +148,6 @@ def pass_hook_checks(self, event: EventBase) -> bool:
 
         return True
 
-    def _proceed_on_broken_event(self, event) -> int:
-        """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.charm.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
-
-        # check if were scaling down and add a log message
-        if self.charm.is_scaling_down(event.relation.id):
-            logger.info(
-                "Relation broken event occurring due to scale down, do not proceed to remove users."
-            )
-            return
-
-        return departed_relation_id
-
     def _on_relation_event(self, event):
         """Handles adding and removing of shards.
 
@@ -181,7 +159,7 @@ def _on_relation_event(self, event):
 
         departed_relation_id = None
         if isinstance(event, RelationBrokenEvent):
-            departed_relation_id = self._proceed_on_broken_event(event)
+            departed_relation_id = self.charm.proceed_on_broken_event(event)
             if not departed_relation_id:
                 return
 

src/charm.py

@@ -1319,10 +1319,32 @@ def set_scaling_down(self, event: RelationDepartedEvent) -> bool:
         # check if relation departed is due to current unit being removed. (i.e. scaling down the
         # application.)
         rel_departed_key = self._generate_relation_departed_key(event.relation.id)
-        scaling_down = json.dumps(event.departing_unit == self.unit)
-        self.unit_peer_data[rel_departed_key] = scaling_down
+        scaling_down = event.departing_unit == self.unit
+        self.unit_peer_data[rel_departed_key] = json.dumps(scaling_down)
         return scaling_down
 
+    def proceed_on_broken_event(self, event) -> int:
+        """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
+
+        # 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 departed_relation_id
+
     @staticmethod
     def _generate_relation_departed_key(rel_id: int) -> str:
         """Generates the relation departed key for a specified relation id."""