[DPE-3456] add messages for incorrect rels to mongos (#346)

## Issue
Right now Charmed MongoDB applications that are running as shard or
replication can be integrated with mongos , this goes against the
proposed architecture from the [sharding spec
doc](https://docs.google.com/document/d/1kx5HUnMR2tiQQpCoWIyxJRUO2fdX4hhzVlo86noh-ts/edit?usp=sharingConnect)

 
## Solution
for cases in which either a Charmed MongoDB instance is in the role of
`shard` or `replication` and a relation has been made to `mongos` then
go into BlockedState

lib/charms/mongodb/v0/config_server_interface.py

@@ -16,7 +16,7 @@
 from charms.mongodb.v1.mongos import MongosConnection
 from ops.charm import CharmBase, EventBase, RelationBrokenEvent
 from ops.framework import Object
-from ops.model import ActiveStatus, MaintenanceStatus, WaitingStatus
+from ops.model import ActiveStatus, BlockedStatus, MaintenanceStatus, WaitingStatus
 
 from config import Config
 
@@ -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 = 5
+LIBPATCH = 6
 
 
 class ClusterProvider(Object):
@@ -69,9 +69,9 @@ def pass_hook_checks(self, event: EventBase) -> bool:
             event.defer()
             return False
 
-        if not self.charm.is_role(Config.Role.CONFIG_SERVER):
+        if not self.is_valid_mongos_integration():
             logger.info(
-                "Skipping %s. ShardingProvider is only be executed by config-server", type(event)
+                "Skipping %s. ClusterProvider is only be executed by config-server", type(event)
             )
             return False
 
@@ -80,9 +80,24 @@ def pass_hook_checks(self, event: EventBase) -> bool:
 
         return True
 
+    def is_valid_mongos_integration(self) -> bool:
+        """Returns true if the integration to mongos is valid."""
+        is_integrated_to_mongos = len(
+            self.charm.model.relations[Config.Relations.CLUSTER_RELATIONS_NAME]
+        )
+
+        if not self.charm.is_role(Config.Role.CONFIG_SERVER) and is_integrated_to_mongos:
+            return False
+
+        return True
+
     def _on_relation_changed(self, event) -> None:
         """Handles providing mongos with KeyFile and hosts."""
         if not self.pass_hook_checks(event):
+            if not self.is_valid_mongos_integration():
+                self.charm.unit.status = BlockedStatus(
+                    "Relation to mongos not supported, config role must be config-server"
+                )
             logger.info("Skipping relation joined event: hook checks did not pass")
             return
 
@@ -208,7 +223,6 @@ def _on_relation_changed(self, event) -> None:
             event.relation.id, CONFIG_SERVER_DB_KEY
         )
         if not key_file_contents or not config_server_db:
-            event.defer()
             self.charm.unit.status = WaitingStatus("Waiting for secrets from config-server")
             return
 

lib/charms/mongodb/v1/shards_interface.py

@@ -55,7 +55,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 5
+LIBPATCH = 6
 KEYFILE_KEY = "key-file"
 HOSTS_KEY = "host"
 OPERATOR_PASSWORD_KEY = MongoDBUser.get_password_key_name_for_user(OperatorUser.get_username())
@@ -539,6 +539,11 @@ def pass_hook_checks(self, event):
             logger.info("skipping %s is only be executed by shards", type(event))
             return False
 
+        # occasionally, broken events have no application, in these scenarios nothing should be
+        # processed.
+        if not event.relation.app:
+            return False
+
         mongos_hosts = event.relation.data[event.relation.app].get(HOSTS_KEY, None)
         if isinstance(event, RelationBrokenEvent) and not mongos_hosts:
             logger.info("Config-server relation never set up, no need to process broken event.")

src/charm.py

@@ -529,11 +529,11 @@ def _on_storage_detaching(self, event: StorageDetachingEvent) -> None:
             logger.error("Failed to remove %s from replica set, error=%r", self.unit.name, e)
 
     def _on_update_status(self, event: UpdateStatusEvent):
-        # cannot have both legacy and new relations since they have different auth requirements
-        if self.client_relations._get_users_from_relations(
-            None, rel="obsolete"
-        ) and self.client_relations._get_users_from_relations(None):
-            self.unit.status = BlockedStatus("cannot have both legacy and new relations")
+        # user-made mistakes might result in other incorrect statues. Prioritise informing users of
+        # their mistake.
+        invalid_integration_status = self.get_invalid_integration_status()
+        if invalid_integration_status:
+            self.unit.status = invalid_integration_status
             return
 
         # no need to report on replica set status until initialised
@@ -1353,6 +1353,18 @@ def _is_removing_last_replica(self) -> bool:
         """Returns True if the last replica (juju unit) is getting removed."""
         return self.app.planned_units() == 0 and len(self._peers.units) == 0
 
+    def get_invalid_integration_status(self) -> Optional[StatusBase]:
+        """Returns a status if an invalid integration is present."""
+        if self.client_relations._get_users_from_relations(
+            None, rel="obsolete"
+        ) and self.client_relations._get_users_from_relations(None):
+            return BlockedStatus("cannot have both legacy and new relations")
+
+        if not self.cluster.is_valid_mongos_integration():
+            return BlockedStatus(
+                "Relation to mongos not supported, config role must be config-server"
+            )
+
     def get_status(self) -> StatusBase:
         """Returns the status with the highest priority from backups, sharding, and mongod.
 

tests/integration/sharding_tests/test_sharding_relations.py

@@ -1,6 +1,8 @@
 #!/usr/bin/env python3
 # Copyright 2023 Canonical Ltd.
 # See LICENSE file for licensing details.
+import time
+
 import pytest
 from juju.errors import JujuAPIError
 from pytest_operator.plugin import OpsTest
@@ -11,6 +13,8 @@
 REPLICATION_APP_NAME = "replication"
 APP_CHARM_NAME = "application"
 LEGACY_APP_CHARM_NAME = "legacy-application"
+MONGOS_APP_NAME = "mongos"
+MONGOS_HOST_APP_NAME = "application-host"
 
 SHARDING_COMPONENTS = [SHARD_ONE_APP_NAME, CONFIG_SERVER_ONE_APP_NAME]
 
@@ -28,7 +32,11 @@
 
 @pytest.mark.abort_on_fail
 async def test_build_and_deploy(
-    ops_test: OpsTest, application_charm, legacy_charm, database_charm
+    ops_test: OpsTest,
+    application_charm,
+    legacy_charm,
+    database_charm,
+    mongos_host_application_charm,
 ) -> None:
     """Build and deploy a sharded cluster."""
     await ops_test.model.deploy(
@@ -44,6 +52,18 @@ async def test_build_and_deploy(
     await ops_test.model.deploy(
         database_charm, config={"role": "shard"}, application_name=SHARD_ONE_APP_NAME
     )
+    await ops_test.model.deploy(
+        MONGOS_APP_NAME,
+        channel="6/edge",
+        revision=3,
+    )
+
+    # TODO: Future PR, once data integrator works with mongos charm deploy that charm instead of
+    # packing and deploying the charm in the application dir.
+    await ops_test.model.deploy(
+        mongos_host_application_charm, application_name=MONGOS_HOST_APP_NAME
+    )
+
     await ops_test.model.deploy(database_charm, application_name=REPLICATION_APP_NAME)
     await ops_test.model.deploy(application_charm, application_name=APP_CHARM_NAME)
     await ops_test.model.deploy(legacy_charm, application_name=LEGACY_APP_CHARM_NAME)
@@ -59,6 +79,19 @@ async def test_build_and_deploy(
         timeout=TIMEOUT,
     )
 
+    await ops_test.model.integrate(
+        f"{MONGOS_APP_NAME}",
+        f"{MONGOS_HOST_APP_NAME}",
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[MONGOS_HOST_APP_NAME, MONGOS_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+        raise_on_error=False,
+    )
+
 
 async def test_only_one_config_server_relation(ops_test: OpsTest) -> None:
     """Verify that a shard can only be related to one config server."""
@@ -218,3 +251,65 @@ async def test_replication_shard_relation(ops_test: OpsTest):
         raise_on_blocked=False,
         timeout=TIMEOUT,
     )
+
+
+async def test_replication_mongos_relation(ops_test: OpsTest) -> None:
+    """Verifies connecting a replica to a mongos router fails."""
+    # attempt to add a replication deployment as a shard to the config server.
+    await ops_test.model.integrate(
+        f"{REPLICATION_APP_NAME}",
+        f"{MONGOS_APP_NAME}",
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[REPLICATION_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )
+
+    replication_unit = ops_test.model.applications[REPLICATION_APP_NAME].units[0]
+    assert (
+        replication_unit.workload_status_message
+        == "Relation to mongos not supported, config role must be config-server"
+    ), "replica cannot be related to mongos."
+
+    # clean up relations
+    await ops_test.model.applications[REPLICATION_APP_NAME].remove_relation(
+        f"{REPLICATION_APP_NAME}:cluster",
+        f"{MONGOS_APP_NAME}:cluster",
+    )
+
+    # TODO remove this and wait for mongos to be active
+    # right now we cannot wait for `mongos` to be active after removing the relation due to a bug
+    # in the mongos charm. To fix the bug it is first necessary to publish the updated library
+    # lib/charms/mongodb/v0/config_server.py
+    time.sleep(60)
+
+
+async def test_shard_mongos_relation(ops_test: OpsTest) -> None:
+    """Verifies connecting a shard to a mongos router fails."""
+    # attempt to add a replication deployment as a shard to the config server.
+    await ops_test.model.integrate(
+        f"{SHARD_ONE_APP_NAME}",
+        f"{MONGOS_APP_NAME}",
+    )
+
+    await ops_test.model.wait_for_idle(
+        apps=[SHARD_ONE_APP_NAME],
+        idle_period=20,
+        raise_on_blocked=False,
+        timeout=TIMEOUT,
+    )
+
+    shard_unit = ops_test.model.applications[SHARD_ONE_APP_NAME].units[0]
+    assert (
+        shard_unit.workload_status_message
+        == "Relation to mongos not supported, config role must be config-server"
+    ), "replica cannot be related to mongos."
+
+    # clean up relations
+    await ops_test.model.applications[SHARD_ONE_APP_NAME].remove_relation(
+        f"{MONGOS_APP_NAME}:cluster",
+        f"{SHARD_ONE_APP_NAME}:cluster",
+    )

tests/unit/test_config_server_lib.py

@@ -62,6 +62,7 @@ def is_not_config_mock_call(*args):
             return False
 
         self.harness.charm.app_peer_data["db_initialised"] = "True"
+        self.harness.add_relation("cluster", "mongos")
 
         # fails due to being run on non-config-server
         self.harness.charm.is_role = is_not_config_mock_call