Add a rule to avoid non-strings in template outputs

[francipvb]

Is this feature request related to a new rule or cfn-lint capabilities?

No response

Describe the feature you'd like to request

Recently I had an issue deploying to cloudformation and I found that the issue was an exported number value.

Therefore I suggest you to add a lint rule to check output values for references to numbered values (just for references to resource attributes).

Describe the solution you'd like

I assume the linter have access to the JSON schema definition and the solution should be a matter of reference solving and checking.

I think a check for the GetAtt function should be enough.

Additional context

I was trying to deploy a MemoryDB cluster and a sibling stack was referencing the cluster port. I'm using the CDK.

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request

Would this feature include a breaking change?

• ⚠️ This feature might incur a breaking change

[kddejong]

Agreed. Looking into this. I thought we had covered this but I need to clean some of the logic up. Turns out if you do Value: 1 that it works but if you do Value: !Gett Resource.NonStringProperty it will fail.

Its bringing up that this issue extends beyond just Output Value and also exists in Sub, Joins, etc.

[Dantemss]

@kddejong after this change, it's not clear to me how we're supposed to Output for example an RDS DBCluster's Port, which is a number.

Value: !GetAtt DBCluster.Port fails with E6101
Attempting to cast it with something like Value: !Sub ${DBCluster.Port} also fails with E6101

Unless that is something that changed very recently, CloudFormation used to happily accept Value: !GetAtt DBCluster.Port

[kddejong]

@Dantemss and this is to a AWS::RDS::DBCluster? And are you using Endpoint.Port? or .Port? Resource docs

[Dantemss]

@kddejong sorry, my mistake. Our case is actually an AWS::DocDB::DBCluster: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-docdb-dbcluster.html