[cfn-include] Ref'd Parameters in Conditions are not replaced when parameters are set to ""

[ds17f]

I have a CloudFormation template which has both Parameters and Conditions. The Conditions Ref the Parameters. When I set the parameters field on the props while instantiating a CfnInclude construct, and then run cdk synth (or deploy or diff), I see an exception if I set the value of the parameter to the empty string.

If the parameter is named SomeParameter, The error message reads:

Element used in Ref expression with logical ID: 'SomeParameter' not found

Reproduction Steps

Bug.json - A cfn template with a parameter and condition

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Description": "Example of bug with conditions ref'ing parameters",
    "Parameters": {
        "SomeParameter": {
            "Type": "String",
            "Description": "Some Value"
        }
    },
    "Conditions": {
        "SomeParameterIsNotEmpty": {
            "Fn::Not": [
                {
                    "Fn::Equals": [
                        {
                            "Ref": "SomeParameter"
                        },
                        ""
                    ]
                }
            ]
        }
    },
    "Resources": {
        "SSMValue": {
            "Type": "AWS::SSM:Parameter",
            "Properties": {
                "Type": "String",
                "Name": "SomeValue",
                "Value": {
                    "Ref": "SomeParameter"
                }
            }
        }
    }
}

A CfnInclude construct to add to a stack

        // An example of the bug
        const template = new cfn_inc.CfnInclude(this, 'BugConstruct', {
            templateFile: `${__dirname}/Bug.json`,
            parameters: {
                "SomeParameter": ""
            }
        });

What did you expect to happen?

I'd expect to see the value "" replaced wherever a Ref points to SomeParameter. This works as expected with a non-empty string value.

What actually happened?

The cdk synth command failed producing the following error:

Element used in Ref expression with logical ID: 'SomeParameter' not found

Environment

• CLI Version : 1.61.1
• Framework Version: 1.61.1
• Node.js Version: v10.16.3
• OS : macOS 10.12.6
• Language (Version): TypeScript (3.8.3)

Other

I did some digging and I'm fairly sure that the issue lies here:
https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/core/lib/cfn-parse.ts#L435

if (specialRef) {

and is a result of the value returned from this.specialCaseRefs here:
https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/core/lib/cfn-parse.ts#L651-L666

this.specialCaseRefs appears to have 3 classes of return value:

1. the value of the parameter
2. an enum related to the AWS::* flag
3. undefined if it's none of these.

if the value of the parameter is falsey (such as an empty string) then the returned value will be seen as NOT a specialRef and will not be replaced.

I think the fix is simple, but I haven't tested it. Will try to open a PR if possible. The fix should just be:

if (specialRef !== undefined) {

---

This is 🐛 Bug Report

[skinny85]

Hey @ds17f ,

thanks a lot for opening the issue. Yes, I'm fairly certain the fix is exactly to change

if (specialRef) {

to

if (specialRef !== undefined) {

Would appreciate a PR :) Check out our Contributing docs for some tips on how to do that!

Thanks,
Adam

[skinny85]

@ds17f how's it going? Do you need something for the PR? I'm here for you 🙂.

[ds17f]

@skinny85 Work wasn't in scope for the story. :) As simple as the fix is I don't have an env setup for cdk build work. I made a gitpod but it started to eat up time that I didn't have allotted.

I will get to this ASAP, but honestly I'm guessing that you could probably wrap this before I actually got it done. It's really just the test that's holding me up here too. I know the fix is stupid easy.

[skinny85]

OK, I'll take this one then, no worries 🙂