[DPE-4224] Remove relation_{joined,changed}.emit() calls (#266)

We should replace calls to `relation_{joined,changed}.emit()` by their
end-method's handler. Otherwise we will face situations like #265, where
a deferred event constantly gets called up, generates new events which
also get deferred and end in a snow ball of deferrals.

Closes #265

---------

Co-authored-by: Mehdi Bendriss <bendrissmehdi@gmail.com>
Co-authored-by: Carl Csaposs <carl.csaposs@canonical.com>

lib/charms/opensearch/v0/helper_charm.py

@@ -4,10 +4,7 @@
 """Utility functions for charms related operations."""
 import re
 import typing
-from datetime import datetime
 
-from charms.data_platform_libs.v0.data_interfaces import Scope
-from charms.opensearch.v0.constants_charm import PeerRelationName
 from charms.opensearch.v0.helper_enums import BaseStrEnum
 from ops import CharmBase
 from ops.model import ActiveStatus, StatusBase
@@ -118,12 +115,3 @@ def relation_departure_reason(charm: CharmBase, relation_name: str) -> RelDepart
         return RelDepartureReason.SCALE_DOWN
 
     return RelDepartureReason.REL_BROKEN
-
-
-def trigger_leader_peer_rel_changed(charm: CharmBase) -> None:
-    """Force trigger a peer rel changed event by leader."""
-    if not charm.unit.is_leader():
-        return
-
-    charm.peers_data.put(Scope.APP, "triggered", datetime.now().timestamp())
-    charm.on[PeerRelationName].relation_changed.emit(charm.model.get_relation(PeerRelationName))

lib/charms/opensearch/v0/opensearch_base_charm.py

@@ -238,6 +238,10 @@ def __init__(self, *args, distro: Type[OpenSearchDistribution] = None):
         self.framework.observe(self.on.set_password_action, self._on_set_password_action)
         self.framework.observe(self.on.get_password_action, self._on_get_password_action)
 
+        # Ensure that only one instance of the `_on_peer_relation_changed` handler exists
+        # in the deferred event queue
+        self._is_peer_rel_changed_deferred = False
+
     @property
     @abc.abstractmethod
     def _upgrade(self) -> typing.Optional[upgrade.Upgrade]:
@@ -429,8 +433,14 @@ def _on_peer_relation_changed(self, event: RelationChangedEvent):
             and self.opensearch.is_node_up()
             and self.health.apply() in [HealthColors.UNKNOWN, HealthColors.YELLOW_TEMP]
         ):
+            if self._is_peer_rel_changed_deferred:
+                # We already deferred this event during this Juju event. Retry on the next
+                # Juju event.
+                return
             # we defer because we want the temporary status to be updated
             event.defer()
+            # If the handler is called again within this Juju hook, we will abandon the event
+            self._is_peer_rel_changed_deferred = True
 
         for relation in self.model.relations.get(ClientRelationName, []):
             self.opensearch_provider.update_endpoints(relation)
@@ -593,9 +603,7 @@ def _on_config_changed(self, event: ConfigChangedEvent):  # noqa C901
             # since when an IP change happens, "_on_peer_relation_joined" won't be called,
             # we need to alert the leader that it must recompute the node roles for any unit whose
             # roles were changed while the current unit was cut-off from the rest of the network
-            self.on[PeerRelationName].relation_joined.emit(
-                self.model.get_relation(PeerRelationName)
-            )
+            self._on_peer_relation_joined(event)
 
         previous_deployment_desc = self.opensearch_peer_cm.deployment_desc()
         if self.unit.is_leader():

lib/charms/opensearch/v0/opensearch_peer_clusters.py

@@ -88,12 +88,6 @@ def run(self) -> None:
                 Scope.APP, "deployment-description", deployment_desc.to_dict()
             )
 
-            if deployment_desc.start == StartMode.WITH_GENERATED_ROLES:
-                # role generation logic
-                self._charm.on[PeerRelationName].relation_changed.emit(
-                    self._charm.model.get_relation(PeerRelationName)
-                )
-
         self.apply_status_if_needed(deployment_desc)
 
         # TODO: once peer clusters relation implemented, we should apply all directives

lib/charms/opensearch/v0/opensearch_relation_peer_cluster.py

@@ -137,9 +137,6 @@ def _on_peer_cluster_relation_joined(self, event: RelationJoinedEvent):
 
         self.refresh_relation_data(event)
 
-        # TODO: is the below still needed
-        # self.charm.trigger_leader_peer_rel_changed()
-
     def _on_peer_cluster_relation_changed(self, event: RelationChangedEvent):
         """Event received by all units in sub-cluster when a new sub-cluster joins the relation."""
         if not self.charm.unit.is_leader():
@@ -415,7 +412,6 @@ def __init__(self, charm: "OpenSearchBaseCharm"):
 
     def _on_peer_cluster_relation_joined(self, event: RelationJoinedEvent):
         """Event received when a new main-failover cluster unit joins the fleet."""
-        # self.charm.trigger_leader_peer_rel_changed()
         pass
 
     def _on_peer_cluster_relation_changed(self, event: RelationChangedEvent):