(core): CDK creates unsupported list outputs from cross-stack references

[gshpychka]

Describe the bug

When doing cross-stack references that point to a list value, CDK creates a CfnOutput with the list as its value. Cloudformation does not support outputting list values - it only supports strings. So this fails either during synth (if you explicitly pass a list), or during template validation (if you do a cdk.Fn.select and pass that to another stack).

Expected Behavior

Cross-stack references with list values work properly

Current Behavior

Cross-stack references with list values generate a synth-time error when passed explicitly:
jsii.errors.JSIIError: Resolution error: Resolution error: Found an encoded list token string in a scalar string context. Use 'Fn.select(0, list)' (not 'list[0]') to extract elements from token lists...
or a deploy-time validation error when doing a cdk.Fn.select:
Template format error: The Value field of every Outputs member must evaluate to a String.

Reproduction Steps

Here's a repro that produces both the synth-time and deploy-time errors. Uncomment ConsumerSynthTimeError to get a synth-time error.

from aws_cdk import aws_ec2 as ec2, aws_ssm as ssm
import aws_cdk as cdk
from constructs import Construct

app = cdk.App()

class Producer(cdk.Stack):
    def __init__(self, scope: Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)

        vpc = ec2.Vpc(self, "vpc")
        self.endpoint = ec2.InterfaceVpcEndpoint(
            self,
            "endpoint",
            vpc=vpc,
            service=ec2.InterfaceVpcEndpointAwsService.SECRETS_MANAGER,
        )
        self.single_dns_entry = cdk.Fn.select(0, self.endpoint.vpc_endpoint_dns_entries)

class ConsumerSynthTimeError(cdk.Stack):
    def __init__(
        self, scope: Construct, id: str, vpc_endpoint_dns_entries: list[str], **kwargs
    ) -> None:
        super().__init__(scope, id, **kwargs)

        ssm.StringListParameter(
            self, "parameter", string_list_value=vpc_endpoint_dns_entries
        )

class ConsumerDeployTimeError(cdk.Stack):
    def __init__(
        self, scope: Construct, id: str, vpc_endpoint_dns_entry: str, **kwargs
    ) -> None:
        super().__init__(scope, id, **kwargs)

        ssm.StringParameter(self, "parameter", string_value=vpc_endpoint_dns_entry)

producer = Producer(app, "producer")

# ConsumerSynthTimeError(
#     app,
#     "consumerSynthTime",
#     vpc_endpoint_dns_entries=producer.endpoint.vpc_endpoint_dns_entries,
# )

ConsumerDeployTimeError(
    app,
    "consumer",
    vpc_endpoint_dns_entry=producer.single_dns_entry,
)

app.synth()

Possible Solution

cdk.Fn.select should be placed in the producer stack instead of the consumer, and the result should be passed the output.

In cases where a whole list has to be passed, CDK could do a cdk.Fn.join in the producer, output that, and a cdk.Fn.split in the consumer.

Additional Information/Context

No response

CDK CLI Version

2.38.1

Framework Version

2.38.1

Node.js Version

18.7.0

OS

Linux

Language

Python

Language Version

3.10

Other information

No response

[comcalvi]

We've fixed this in one region with #22873, but it doesn't work for cross-account / region.

[gshpychka]

My issue was about CloudFormation outputs, so single-environment by definition. Not sure whether the new cross-env references are relevant here as well.