[DPE-2613] fix rel tests (6/edge)

lib/charms/mongodb/v0/mongodb_provider.py

@@ -29,7 +29,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 4
+LIBPATCH = 5
 
 logger = logging.getLogger(__name__)
 REL_NAME = "database"
@@ -64,6 +64,9 @@ def __init__(self, charm: CharmBase, substrate="k8s", relation_name: str = "data
         self.substrate = substrate
 
         super().__init__(charm, self.relation_name)
+        self.framework.observe(
+            charm.on[self.relation_name].relation_departed, self._on_relation_departed
+        )
         self.framework.observe(
             charm.on[self.relation_name].relation_broken, self._on_relation_event
         )
@@ -79,6 +82,16 @@ def __init__(self, charm: CharmBase, substrate="k8s", relation_name: str = "data
             self.database_provides.on.database_requested, self._on_relation_event
         )
 
+    def _on_relation_departed(self, event):
+        """Checks if users should be removed on the following event (relation-broken)."""
+        # relation departed and relation broken events occur during scaling down or during relation
+        # removal, only relation departed events have access to metadata to determine which case.
+        self.charm.set_scaling_down(event)
+
+        # check if were scaling down and add a log message
+        if self.charm.is_scaling_down(event.relation.id):
+            logger.info("Scaling down the application, no need to remove external users.")
+
     def _on_relation_event(self, event):
         """Handle relation joined events.
 
@@ -110,7 +123,25 @@ def _on_relation_event(self, event):
 
         departed_relation_id = None
         if type(event) is RelationBrokenEvent:
+            # 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(departed_relation_id):
+                logger.info(
+                    "Relation broken event occurring due to scale down, do not proceed to remove users."
+                )
+                return
+
+            logger.info(
+                "Relation broken event occurring due to relation removal, proceed to remove user."
+            )
 
         try:
             self.oversee_users(departed_relation_id, event)
@@ -186,6 +217,9 @@ def _diff(self, event: RelationChangedEvent) -> Diff:
             a Diff instance containing the added, deleted and changed
                 keys from the event relation databag.
         """
+        if not isinstance(event, RelationChangedEvent):
+            logger.info("Cannot compute diff of event type: %s", type(event))
+            return
         # TODO import marvelous unit tests in a future PR
         # Retrieve the old data from the data key in the application relation databag.
         old_data = json.loads(event.relation.data[self.charm.model.app].get("data", "{}"))

src/charm.py

@@ -1407,6 +1407,28 @@ def _juju_secret_remove(self, scope: Scopes, key: str) -> None:
         secret.set_content(secret_cache)
         logging.debug(f"Secret {scope}:{key}")
 
+    def is_scaling_down(self, rel_id: int) -> bool:
+        """Returns True if the application is scaling down."""
+        rel_departed_key = self._generate_relation_departed_key(rel_id)
+        return json.loads(self.unit_peer_data[rel_departed_key])
+
+    def has_departed_run(self, rel_id: int) -> bool:
+        """Returns True if the relation departed event has run."""
+        rel_departed_key = self._generate_relation_departed_key(rel_id)
+        return rel_departed_key in self.unit_peer_data
+
+    def set_scaling_down(self, event: RelationDepartedEvent) -> None:
+        """Sets whether or not the current unit is scaling down."""
+        # 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)
+        self.unit_peer_data[rel_departed_key] = json.dumps(event.departing_unit == self.unit)
+
+    @staticmethod
+    def _generate_relation_departed_key(rel_id: int) -> str:
+        """Generates the relation departed key for a specified relation id."""
+        return f"relation_{rel_id}_departed"
+
     # END: helper functions