A basic !InlineFile YAML tag/function resolver for inlining Lambda into CloudFormation; assumes two-space indentation

inline_file.rb

PATTERN = %r{
  ^
  (?<indent>\s*)
  (?<key>\S+):
  \s*
  !InlineFile
  \s*
  (?<file>.*)
  $
}x

def inline_file(match)
  puts "#{match[:indent]}#{match[:key]}: |"
  File.open(match[:file]) do |f|
    f.each_line do |line|
      puts(match[:indent] + "  " + line)
    end
  end
end

File.open(ARGV[0]) do |template|
  template.each_line do |line|
    if match = PATTERN.match(line)
      inline_file(match)
    else
      puts line
    end
  end
end

sample_in.yaml

AWSTemplateFormatVersion: '2010-09-09'
Description: Lambda function with cfn-response.
Resources:
  primer:
    Type: AWS::Lambda::Function
    Properties:
      Runtime: nodejs12.x
      Role: arn:aws:iam::123456789012:role/lambda-role
      Handler: index.handler
      Code:
        ZipFile: !InlineFile lambda.js
      Description: Invoke a function during stack creation.
      TracingConfig:
        Mode: Active

sample_out.yaml

AWSTemplateFormatVersion: '2010-09-09'
Description: Lambda function with cfn-response.
Resources:
  primer:
    Type: AWS::Lambda::Function
    Properties:
      Runtime: nodejs12.x
      Role: arn:aws:iam::123456789012:role/lambda-role
      Handler: index.handler
      Code:
        ZipFile: |
          var aws = require('aws-sdk')
          var response = require('cfn-response')
          exports.handler = function(event, context) {
              console.log("REQUEST RECEIVED:\n" + JSON.stringify(event))
              // For Delete requests, immediately send a SUCCESS response.
              if (event.RequestType == "Delete") {
                  response.send(event, context, "SUCCESS")
                  return
              }
              var responseStatus = "FAILED"
              var responseData = {}
              var functionName = event.ResourceProperties.FunctionName
              var lambda = new aws.Lambda()
              lambda.invoke({ FunctionName: functionName }, function(err, invokeResult) {
                  if (err) {
                      responseData = {Error: "Invoke call failed"}
                      console.log(responseData.Error + ":\n", err)
                  }
                  else responseStatus = "SUCCESS"
                  response.send(event, context, responseStatus, responseData)
              })
          }
      Description: Invoke a function during stack creation.
      TracingConfig:
        Mode: Active