From 6e9960895e9e3f522e4e52695bb5899920ce7a84 Mon Sep 17 00:00:00 2001
From: Brian Pandola <bpandola@gmail.com>
Date: Tue, 7 Nov 2023 02:36:36 -0800
Subject: [PATCH] RDS: Can't delete DBCluster with active DBInstances (#6998)

---
 moto/rds/exceptions.py                        |  8 ++++
 moto/rds/models.py                            |  4 +-
 .../test_rds_clusters_with_instances.py       | 39 +++++++++++++++++++
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/moto/rds/exceptions.py b/moto/rds/exceptions.py
index f6af07161..d6c015311 100644
--- a/moto/rds/exceptions.py
+++ b/moto/rds/exceptions.py
@@ -82,6 +82,14 @@ class InvalidDBClusterStateFaultError(RDSClientError):
         )
 
 
+class DBClusterToBeDeletedHasActiveMembers(RDSClientError):
+    def __init__(self) -> None:
+        super().__init__(
+            "InvalidDBClusterStateFault",
+            "Cluster cannot be deleted, it still contains DB instances in non-deleting state.",
+        )
+
+
 class InvalidDBInstanceStateError(RDSClientError):
     def __init__(self, database_identifier: str, istate: str):
         estate = (
diff --git a/moto/rds/models.py b/moto/rds/models.py
index 5e39aaef4..db0042cac 100644
--- a/moto/rds/models.py
+++ b/moto/rds/models.py
@@ -19,6 +19,7 @@ from .exceptions import (
     DBClusterNotFoundError,
     DBClusterSnapshotAlreadyExistsError,
     DBClusterSnapshotNotFoundError,
+    DBClusterToBeDeletedHasActiveMembers,
     DBInstanceNotFoundError,
     DBSnapshotNotFoundError,
     DBSecurityGroupNotFoundError,
@@ -2344,7 +2345,8 @@ class RDSBackend(BaseBackend):
                 raise InvalidParameterValue(
                     "Can't delete Cluster with protection enabled"
                 )
-
+            if cluster.cluster_members:
+                raise DBClusterToBeDeletedHasActiveMembers()
             global_id = cluster.global_cluster_identifier or ""
             if global_id in self.global_clusters:
                 self.remove_from_global_cluster(global_id, cluster_identifier)
diff --git a/tests/test_rds/test_rds_clusters_with_instances.py b/tests/test_rds/test_rds_clusters_with_instances.py
index 09dec8c56..66caeed7f 100644
--- a/tests/test_rds/test_rds_clusters_with_instances.py
+++ b/tests/test_rds/test_rds_clusters_with_instances.py
@@ -89,3 +89,42 @@ def test_add_instance_to_serverless_cluster():
     err = exc.value.response["Error"]
     assert err["Code"] == "InvalidParameterValue"
     assert err["Message"] == "Instances cannot be added to Aurora Serverless clusters."
+
+
+@mock_rds
+def test_delete_db_cluster_fails_if_cluster_contains_db_instances():
+    cluster_identifier = "test-cluster"
+    instance_identifier = "test-instance"
+    client = boto3.client("rds", "us-east-1")
+    client.create_db_cluster(
+        DBClusterIdentifier=cluster_identifier,
+        Engine="aurora-postgresql",
+        MasterUsername="test-user",
+        MasterUserPassword="password",
+    )
+    client.create_db_instance(
+        DBClusterIdentifier=cluster_identifier,
+        Engine="aurora-postgresql",
+        DBInstanceIdentifier=instance_identifier,
+        DBInstanceClass="db.t4g.medium",
+    )
+    with pytest.raises(ClientError) as exc:
+        client.delete_db_cluster(
+            DBClusterIdentifier=cluster_identifier,
+            SkipFinalSnapshot=True,
+        )
+    err = exc.value.response["Error"]
+    assert err["Code"] == "InvalidDBClusterStateFault"
+    assert (
+        err["Message"]
+        == "Cluster cannot be deleted, it still contains DB instances in non-deleting state."
+    )
+    client.delete_db_instance(
+        DBInstanceIdentifier=instance_identifier,
+        SkipFinalSnapshot=True,
+    )
+    cluster = client.delete_db_cluster(
+        DBClusterIdentifier=cluster_identifier,
+        SkipFinalSnapshot=True,
+    ).get("DBCluster")
+    assert cluster["DBClusterIdentifier"] == cluster_identifier