Cloudformation to covert string into list on apigateway policy document with condition

Question

Below is the my cloudformation template. I want to convert aws:SourceVpc into list in the resource policy document. I tried with spilt but cft below error.

Invalid policy document. Please check the policy syntax and ensure that Principals are valid. (Service: AmazonApiGateway; Status Code: 400; Error Code: BadRequestException; Request ID: xxxxx; Proxy: null)

Also devl and sdbx have 1 value where as others acnt and acpt have two values in mappings how can do it. Please suggest.

Output should like below. aws:SourceVpc: ["vpc-7830jkd", "vpc-a1236"]

Mappings:
 vpcid:
us-east-1:
  sdbx: "vpc-1234"
  devl: "vpc-2345"
  acpt: "vpc-7830jkd,vpc-a1236"
us-east-2:
  acnt: "vpc-a1236,vpc-7830jkd"

Parameters:
  Env:
   Type: String

Resources:
apigateway:
  Type: "AWS::ApiGateway::RestApi"
    Properties:
   Name: mygateway
  EndpointConfiguration:
    Types:
      - "PRIVATE"
  Policy: !Sub
    - |-
      {
        "Version": "2012-10-17",
        "Statement": [
          {
            "Effect": "Allow",
            "Principal": "*",
            "Action": "execute-api:Invoke",
            "Resource": [
              "execute-api:/*"
            ]
          },
          {
            "Effect": "Deny",
            "Principal": "*",
            "Action": "execute-api:Invoke",
            "Resource": [
              "execute-api:/*"
            ],
            "Condition": {
              "StringNotEquals": {
                "aws:SourceVpc": !Split ["," , "${myappid}"]
              }
            }
          }
        ]
      }
    - { myappid: !FindInMap [ vpcid, !Ref "AWS::Region",  !Ref "Env" ] }

Answer 1

Output should like below. aws:SourceVpc: ["vpc-7830jkd", "vpc-a1236"]

Sadly, you can't do this. CFN is very limited in what it can do, and what you want is simply not achievable with plain CFN. You have to modify your Maps and provide all the vpc ids individually, not mix lists with individual values.

The other way would be through development of CFN macro or custom resource.