bug: CloudFormation fails to process nested Fn::Select with Fn::FindInMap in conditions #12775
Description
Is there an existing issue for this?
- I have searched the existing issues
Current Behavior
When deploying a CloudFormation template with nested intrinsic functions in conditions (specifically Fn::Select with Fn::FindInMap), Localstack fails with a KeyError: '0'. This prevents testing of common CloudFormation patterns locally.
Error: Failed to create changeset for the stack: test, An error occurred (InternalError) when calling the CreateChangeSet operation: exception while calling cloudformation.CreateChangeSet: ... KeyError: '0'
Expected Behavior
The template should process successfully as it does in real AWS.
How are you starting LocalStack?
With a docker-compose file
Steps To Reproduce
How are you starting localstack (e.g., bin/localstack command, arguments, or docker-compose.yml)
docker-compose.yml:
services:
localstack:
container_name: "${LOCALSTACK_DOCKER_NAME:-localstack-main}"
image: localstack/localstack
ports:
- "127.0.0.1:4566:4566" # LocalStack Gateway
- "127.0.0.1:4510-4559:4510-4559" # external services port range
environment:
# LocalStack configuration: https://docs.localstack.cloud/references/configuration/
- DEBUG=${DEBUG:-1}
volumes:
- "${LOCALSTACK_VOLUME_DIR:-./volume}:/var/lib/localstack"
- "/var/run/docker.sock:/var/run/docker.sock"Example Template
template.yml
AWSTemplateFormatVersion: '2010-09-09'
Description: 'Minimal template to demonstrate Localstack condition issue'
Mappings:
RegionMap:
us-east-1:
AZs:
- "empty"
Conditions:
UseFnGetAZs:
Fn::Equals:
- "empty"
- Fn::Select:
- "0"
- Fn::FindInMap:
- "RegionMap"
- Ref: "AWS::Region"
- "AZs"
Resources:
TestBucket:
Type: 'AWS::S3::Bucket'
Condition: UseFnGetAZs
Properties:
BucketName: test-bucket-3492340Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)
Deploying to real AWS account is fine
sam deploy --stack-name test --region us-east-1results in normal sam deployment:
...
Successfully created/updated stack - test in us-east-1
Deploying to LocalStack fails
samlocal deploy --stack-name test --region us-east-1fails with
Error: Failed to create changeset for the stack: test, An error occurred (InternalError) when calling the CreateChangeSet operation (reached max retries: 4): exception while calling cloudformation.CreateChangeSet: Traceback (most recent call last):
File "/opt/code/localstack/.venv/lib/python3.11/site-packages/rolo/gateway/chain.py", line 166, in handle
handler(self, self.context, response)
File "/opt/code/localstack/localstack-core/localstack/aws/handlers/service.py", line 113, in __call__
handler(chain, context, response)
File "/opt/code/localstack/localstack-core/localstack/aws/handlers/service.py", line 83, in __call__
skeleton_response = self.skeleton.invoke(context)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/aws/skeleton.py", line 154, in invoke
return self.dispatch_request(serializer, context, instance)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/aws/skeleton.py", line 168, in dispatch_request
result = handler(context, instance) or {}
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/aws/skeleton.py", line 118, in __call__
return self.fn(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/aws/api/core.py", line 178, in operation_marker
return fn(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/services/cloudformation/provider.py", line 731, in create_change_set
resolved_stack_conditions = resolve_stack_conditions(
^^^^^^^^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/services/cloudformation/engine/template_utils.py", line 112, in resolve_stack_conditions
result[condition_name] = resolve_condition(
^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/services/cloudformation/engine/template_utils.py", line 291, in resolve_condition
right = resolve_condition(
^^^^^^^^^^^^^^^^^^
File "/opt/code/localstack/localstack-core/localstack/services/cloudformation/engine/template_utils.py", line 336, in resolve_condition
return options[index]
~~~~~~~^^^^^^^
KeyError: '0'Environment
- OS: macOS 15.5 (24F74)
- LocalStack:
LocalStack version: 4.4.1.dev66
LocalStack Docker image sha: 28a7fe23332dd8180089cad651548e996bdccc67ece26d66de4e06fcf6d7d7f8
LocalStack build date: 2025-06-02
LocalStack build git hash: 374183c42Anything else?
No response