localstack · simonrw · Jan 31, 2024 · Jan 31, 2024 · Jan 31, 2024 · Jan 31, 2024
diff --git a/localstack/services/cloudformation/engine/template_deployer.py b/localstack/services/cloudformation/engine/template_deployer.py
@@ -40,7 +40,7 @@
 from localstack.utils.strings import to_bytes, to_str
 from localstack.utils.threads import start_worker_thread
 
-from localstack.services.cloudformation.models import *  # noqa: F401, isort:skip
+from localstack.services.cloudformation.models import *  # noqa: F401, F403, isort:skip
 from localstack.utils.urls import localstack_host
 
 ACTION_CREATE = "create"
@@ -169,7 +169,9 @@ def resolve_ref(
     # resource
     resource = resources.get(ref)
     if not resource:
-        raise Exception("Should be detected earlier.")
+        raise Exception(
+            f"Resource target for `Ref {ref}` could not be found. Is there a resource with name {ref} in your stack?"
+        )
 
     return resources[ref].get("PhysicalResourceId")
 
@@ -325,7 +327,10 @@ def _resolve_refs_recursively(
             )
             # TODO: we should check the deployment state and not try to GetAtt from a resource that is still IN_PROGRESS or hasn't started yet.
             if resolved_getatt is None:
-                raise DependencyNotYetSatisfied(resource_ids=resource_logical_id, message="")
+                raise DependencyNotYetSatisfied(
+                    resource_ids=resource_logical_id,
+                    message=f"Could not resolve attribute '{attribute_name}' on resource '{resource_logical_id}'",
+                )
             return resolved_getatt
 
         if stripped_fn_lower == "join":
@@ -361,7 +366,7 @@ def _resolve_refs_recursively(
             none_values = [v for v in join_values if v is None]
             if none_values:
                 raise Exception(
-                    "Cannot resolve CF fn::Join %s due to null values: %s" % (value, join_values)
+                    f"Cannot resolve CF Fn::Join {value} due to null values: {join_values}"
                 )
             return value[keys_list[0]][0].join([str(v) for v in join_values])
 
@@ -375,7 +380,7 @@ def _resolve_refs_recursively(
             item_to_sub[1].update(attr_refs)
 
             for key, val in item_to_sub[1].items():
-                val = resolve_refs_recursively(
+                resolved_val = resolve_refs_recursively(
                     account_id,
                     region_name,
                     stack_name,
@@ -385,11 +390,13 @@ def _resolve_refs_recursively(
                     parameters,
                     val,
                 )
-                if not isinstance(val, str):
+                if not isinstance(resolved_val, str):
                     # We don't have access to the resource that's a dependency in this case,
                     # so do the best we can with the resource ids
-                    raise DependencyNotYetSatisfied(resource_ids=key, message="")
-                result = result.replace("${%s}" % key, val)
+                    raise DependencyNotYetSatisfied(
+                        resource_ids=key, message=f"Could not resolve {val} to terminal value type"
+                    )
+                result = result.replace("${%s}" % key, resolved_val)
 
             # resolve placeholders
             result = resolve_placeholders_in_string(
@@ -1284,7 +1291,7 @@ def do_apply_changes_in_loop(self, changes, stack):
                 break
             if not updated:
                 raise Exception(
-                    "Resource deployment loop completed, pending resource changes: %s" % changes
+                    f"Resource deployment loop completed, pending resource changes: {changes}"
                 )
 
         # clean up references to deleted resources in stack

diff --git a/tests/aws/services/cloudformation/engine/test_references.py b/tests/aws/services/cloudformation/engine/test_references.py
@@ -1,8 +1,10 @@
+import json
 import os
 
 import pytest
 
 from localstack.testing.pytest import markers
+from localstack.testing.pytest.fixtures import StackDeployError
 from localstack.utils.files import load_file
 from localstack.utils.strings import short_uid
 
@@ -53,3 +55,42 @@ def test_fn_sub_cases(self, deploy_cfn_template, aws_client, snapshot):
         )
 
         snapshot.match("outputs", deployment.outputs)
+
+
+@markers.aws.only_localstack
+def test_useful_error_when_invalid_ref(deploy_cfn_template):
+    """
+    When trying to resolve a non-existent !Ref, make sure the error message includes the name of the !Ref
+    to clarify which !Ref cannot be resolved.
+    """
+    logical_resource_id = "Topic"
+    ref_name = "InvalidRef"
+
+    template = json.dumps(
+        {
+            "Resources": {
+                logical_resource_id: {
+                    "Type": "AWS::SNS::Topic",
+                    "Properties": {
+                        "Name": {
+                            "Ref": ref_name,
+                        },
+                    },
+                }
+            }
+        }
+    )
+
+    with pytest.raises(StackDeployError) as exc_info:
+        deploy_cfn_template(template=template)
+
+    # get the exception error message from the events list
+    message = None
+    for event in exc_info.value.events:
+        if (
+            event["LogicalResourceId"] == logical_resource_id
+            and event["ResourceStatus"] == "CREATE_FAILED"
+        ):
+            message = event["ResourceStatusReason"]
+
+    assert ref_name in message