Computed variables for CloudFormation using an identity function, as by @claude on StackOverflow (https://serverfault.com/a/812040/6438)

cloud-formation-computed-variables.yaml

##
#
# Custom resource setup to provide an identity function that can be used to
# implement "custom variables".
# See: https://serverfault.com/a/812040/6438
#
# Usage:
# 
# After deploying the below code to your stack, you can define computed
# variables thus:
# ```
#  GlobalStackName:
#    Type: Custom::Variable
#    Properties:
#      ServiceToken: !GetAtt [ IdentityFunc, Arn ]
#      Value: !Join [ "-", [ { Ref: "AWS::StackName" }, { Ref: "AWS::Region" } ]]
# ```
# And use it like this:
# 
# ```
# !GetAtt [ GlobalStackName, Value ]
# ```
# Any attribute name can be used instead of `Value` in the definition and fetched
# with the `GetAtt`. It may be useful to define all computed values in a single
# variable, each with its own attribute name.
#
# Its also possible to use such custom variables to resolve conditions to values, in which case
# pay attention to put the `Fn::If` as the top entry under `Properties`:
# ```
#  ResolveConditionToValue:
#    Type: Custom::Variable
#    Properties:
#      "Fn::If":
#        - ConditionName
#        - ServiceToken: { "Fn::GetAtt": [ IdentityFunc, Arn ] }
#          Value: !Ref SomeParam
#        - ServiceToken: { "Fn::GetAtt": [ IdentityFunc, Arn ] }
#          Value: !Ref SomeOtherParam
# ```
# It may seem redundant, but if you do anything else, the `Fn::If` and its content will be sent as the value
# to the function instead of being resolved.
#
# Articles used in the construction of this snippet:
# * https://serverfault.com/a/812040/6438
# * https://github.com/arjenderijke/aws-cloud-examples/blob/master/cloudformation/python-custom-resource.template
# * https://blog.jayway.com/2015/07/04/extending-cloudformation-with-lambda-backed-custom-resources/
# * http://2ndwatch.com/blog/a-step-by-step-guide-on-using-aws-lambda-backed-custom-resources-with-amazon-cfts/
Resources:
  IdentityFunc:
    Type: AWS::Lambda::Function
    Properties:
      Code:
        ZipFile: >
          var r = require('cfn-response');
          exports.handler = function(ev, ctx) {
            r.send(ev, ctx, r.SUCCESS, ev.ResourceProperties);
          }; 
      Handler: index.handler
      Runtime: nodejs6.10
      Timeout: 30
      Role: !GetAtt [ IdentityFunctionExecutionRole, Arn ]
  
  IdentityFunctionExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal: { Service: [ lambda.amazonaws.com ] }
            Action: sts:AssumeRole
      Policies:
        - PolicyName: 
            Fn::Join: [ '-', [ identity-custom-res, { Ref: 'AWS::StackName' }, { Ref: 'AWS::Region' } ] ]
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action:
                  - logs:CreateLogGroup
                  - logs:CreateLogStream
                  - logs:PutLogEvents
                Resource: "arn:aws:logs:*:*:*"
              - Effect: Allow
                Action: cloudformation:DescribeStacks
                Resource: "arn:aws:cloudformation:*:*:*"