CLOUDP-324655 - fix MongoDBMultiCluster not ready if clusterSpecList contains 0 members (#484)

# Summary

Fixes issue in `MongoDBMultiCluster` not ready if `clusterSpecList`
contains 0 members.

## Proof of Work

Passing CI with new test case where we scale down to 0 members in one
cluster.

## Checklist

- [x] Have you linked a jira ticket and/or is the ticket in the title?
- [x] Have you checked whether your jira ticket required DOCSP changes?
- [x] Have you added changelog file?
    - use `skip-changelog` label if not needed
- refer to [Changelog files and Release
Notes](https://github.com/mongodb/mongodb-kubernetes/blob/master/CONTRIBUTING.md#changelog-files-and-release-notes)
section in CONTRIBUTING.md for more details

Author: MaciejKaras

changelog/20251001_fix_mongodbmulticluster_not_ready_if_0_members.md

@@ -0,0 +1,6 @@
+---
+kind: fix
+date: 2025-10-01
+---
+
+* **MongoDBMultiCluster**: fix resource stuck in Pending state if any `clusterSpecList` item has 0 members. After the fix, a value of 0 members is handled correctly, similarly to how it's done in the **MongoDB** resource.

controllers/operator/mongodbmultireplicaset_controller.go

@@ -235,23 +235,13 @@ func (r *ReconcileMongoDbMultiReplicaSet) Reconcile(ctx context.Context, request
 		return r.updateStatus(ctx, &mrs, workflow.Failed(xerrors.Errorf("Failed to set annotation: %w", err)), log)
 	}
 
-	// for purposes of comparison, we don't want to compare entries with 0 members since they will not be present
-	// as a desired entry.
 	desiredSpecList := mrs.GetDesiredSpecList()
 	actualSpecList, err := mrs.GetClusterSpecItems()
 	if err != nil {
 		return r.updateStatus(ctx, &mrs, workflow.Failed(err), log)
 	}
 
-	effectiveSpecList := filterClusterSpecItem(actualSpecList, func(item mdb.ClusterSpecItem) bool {
-		return item.Members > 0
-	})
-
-	// sort both actual and desired to match the effective and desired list version before comparing
-	sortClusterSpecList(desiredSpecList)
-	sortClusterSpecList(effectiveSpecList)
-
-	needToRequeue := !clusterSpecListsEqual(effectiveSpecList, desiredSpecList)
+	needToRequeue := !clusterSpecListsEqual(actualSpecList, desiredSpecList)
 	if needToRequeue {
 		return r.updateStatus(ctx, &mrs, workflow.Pending("MongoDBMultiCluster deployment is not yet ready, requeuing reconciliation."), log)
 	}
@@ -1276,26 +1266,39 @@ func (r *ReconcileMongoDbMultiReplicaSet) deleteManagedResources(ctx context.Con
 	return errs
 }
 
-// filterClusterSpecItem filters items out of a list based on provided predicate.
-func filterClusterSpecItem(items mdb.ClusterSpecList, fn func(item mdb.ClusterSpecItem) bool) mdb.ClusterSpecList {
-	var result mdb.ClusterSpecList
-	for _, item := range items {
-		if fn(item) {
-			result = append(result, item)
-		}
-	}
-	return result
-}
-
 func sortClusterSpecList(clusterSpecList mdb.ClusterSpecList) {
 	sort.SliceStable(clusterSpecList, func(i, j int) bool {
 		return clusterSpecList[i].ClusterName < clusterSpecList[j].ClusterName
 	})
 }
 
 func clusterSpecListsEqual(effective, desired mdb.ClusterSpecList) bool {
+	// for purposes of comparison, we don't want to compare entries with 0 members
+	effective = filterClusterSpecItem(effective, func(item mdb.ClusterSpecItem) bool {
+		return item.Members > 0
+	})
+	desired = filterClusterSpecItem(desired, func(item mdb.ClusterSpecItem) bool {
+		return item.Members > 0
+	})
+
+	// sort both actual and desired to match the effective and desired list version before comparing
+	sortClusterSpecList(effective)
+	sortClusterSpecList(desired)
+
 	comparer := cmp.Comparer(func(x, y automationconfig.MemberOptions) bool {
 		return true
 	})
+
 	return cmp.Equal(effective, desired, comparer)
 }
+
+// filterClusterSpecItem filters items out of a list based on provided predicate.
+func filterClusterSpecItem(items mdb.ClusterSpecList, fn func(item mdb.ClusterSpecItem) bool) mdb.ClusterSpecList {
+	var result mdb.ClusterSpecList
+	for _, item := range items {
+		if fn(item) {
+			result = append(result, item)
+		}
+	}
+	return result
+}

docker/mongodb-kubernetes-tests/tests/multicluster/multi_cluster_replica_set_scale_down.py

@@ -2,6 +2,7 @@
 
 import kubernetes
 import pytest
+from kubetester import try_load
 from kubetester.automation_config_tester import AutomationConfigTester
 from kubetester.certs_mongodb_multi import create_multi_cluster_mongodb_tls_certs
 from kubetester.kubetester import fixture as yaml_fixture
@@ -59,7 +60,10 @@ def server_certs(
 
 @pytest.fixture(scope="module")
 def mongodb_multi(mongodb_multi_unmarshalled: MongoDBMulti, server_certs: str) -> MongoDBMulti:
-    return mongodb_multi_unmarshalled.create()
+    if try_load(mongodb_multi_unmarshalled):
+        return mongodb_multi_unmarshalled
+
+    return mongodb_multi_unmarshalled.update()
 
 
 @pytest.mark.e2e_multi_cluster_replica_set_scale_down
@@ -101,8 +105,9 @@ def test_ops_manager_has_been_updated_correctly_before_scaling():
 def test_scale_mongodb_multi(mongodb_multi: MongoDBMulti):
     mongodb_multi.load()
     mongodb_multi["spec"]["clusterSpecList"][0]["members"] = 1
-    mongodb_multi["spec"]["clusterSpecList"][1]["members"] = 1
-    mongodb_multi["spec"]["clusterSpecList"][2]["members"] = 1
+    # Testing scaling down to zero is required to test fix for https://jira.mongodb.org/browse/CLOUDP-324655
+    mongodb_multi["spec"]["clusterSpecList"][1]["members"] = 0
+    mongodb_multi["spec"]["clusterSpecList"][2]["members"] = 2
     mongodb_multi.update()
 
     mongodb_multi.assert_reaches_phase(Phase.Running, timeout=1800)
@@ -120,11 +125,11 @@ def test_statefulsets_have_been_scaled_down_correctly(
 
     cluster_two_client = member_cluster_clients[1]
     cluster_two_sts = statefulsets[cluster_two_client.cluster_name]
-    assert cluster_two_sts.status.ready_replicas == 1
+    assert cluster_two_sts.status.ready_replicas is None
 
     cluster_three_client = member_cluster_clients[2]
     cluster_three_sts = statefulsets[cluster_three_client.cluster_name]
-    assert cluster_three_sts.status.ready_replicas == 1
+    assert cluster_three_sts.status.ready_replicas == 2
 
 
 @pytest.mark.e2e_multi_cluster_replica_set_scale_down