fix: Add intrinsic support for AWS::Serverless::Api BasePath property

[reedham-aws]

Issue #, if available

N/a

Description of changes

The BasePath attribute of Aws::Serverless::Api does not currently accept !Sub as a parameter, instead leaving it as blank or returning None. As an example,

Resources:
  ApiGatewayApi:
    Type: AWS::Serverless::Api
    Properties:
      StageName: prod
      CacheClusterEnabled: true
      CacheClusterSize: '0.5'
      MethodSettings:
        - ResourcePath: /
          HttpMethod: GET
          CachingEnabled: true
          CacheTtlInSeconds: 300
      Domain:
        BasePath: !Sub ${ Service }-api
        CertificateArn: !Ref CertificateArn
        DomainName: example.myinstance.com
        Route53:
          HostedZoneId: !Ref HostedZoneId

Transforms to the below, without the actual BasePath being added.

"ApiGatewayApiBasePathMapping": {
    "Type": "AWS::ApiGateway::BasePathMapping",
    "Properties": {
        "DomainName": {
            "Ref": "ApiGatewayDomainName619a1fb284"
        },
        "RestApiId": {
            "Ref": "ApiGatewayApi"
        },
        "Stage": {
            "Ref": "ApiGatewayApiprodStage"
        }
    }
}

After updates

With the updates to allow intrinsics for this property, the output now looks like:

"Parameters": {
    "Service": {
        "Type": "String",
        "Default": "NewServ"
    }
},
...
"TestApiBasePathMapping": {
    "Type": "AWS::ApiGateway::BasePathMapping",
    "Properties": {
        "BasePath": {
            "Fn::Sub": "NewServ-api"
         },
        "DomainName": {
            "Ref": "ApiGatewayDomainName54e3d11755"
        },
        "RestApiId": {
            "Ref": "TestApi"
        },
        "Stage": {
            "Ref": "TestApiprodStage"
        }
    }
}

Description of how you validated changes

Manually ran the transform with and without the updated changes and compared the templates.

Updated the transform tests to include this new case. I didn't add any new tests because there are other locations that have BasePath without the intrinsics, so I figured it was fine to add this in the api_with_basic_custom_domain_intrinsics tests.

Also confirmed that the added tests failed with the older transform version.

Checklist

• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values
• Add/update integration tests

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[valerena]

So the parameter NormalizeBasePath is not used anymore if the BasePath is passed as an intrinsic?

[valerena]

what happens if there are different base paths all defined with intrinsics? Will they just have the same logical id? Should we add some sort of hash for the intrinsic values here, so we actually create different resources?

[valerena]

Can you still keep the list, but do with intrinsics instead?

Something like

Suggested change

	BasePath: !Sub '${AWS::Region}-api'
	BasePath:
	- !Sub '${AWS::Region}-get'
	- !Sub '${AWS::Region}-fetch'

[valerena]

Thoughts on adding this same support on http_api_generator.py file?