Is it possible to populate an S3 bucket through a CloudFormation template?

I'm afraid it's not possible to upload to a bucket created from the same template. However if the bucket is pre-existing, say an account-wide deployment bucket, or one of those cf-templates-... buckets, you've got some options.

• Upload the auxiliary files using Ansible playbook and in the same playbook create/update the CloudFormation stack. That's a very popular method for CI/CD pipelines. In your Ansible playbook you'll have something like this:

  tasks:
  - name: "Upload files to s3://{{deployment_bucket}}/lambda/"
    s3_sync:
      bucket: "{{deployment_bucket}}"
      file_root: lambda/
      prefix: lambda/
      permission: private
  
  - name: "Create CloudFormation Stack"
    cloudformation:
      stack_name: "some-stack-name"
      state: present
      template: template.yml
      template_parameters:
        DeploymentS3Path: "s3://{{deployment_bucket}}/lambda/"
  

• Let aws cloudformation package zip up and upload the files to S3 and then aws cloudformation deploy create and execute a CloudFormation change set. Again quite a popular method that works well with CI/CD.

  In your CloudFormation template you can then refer to local files like this:

  MyLambda:
    Type: AWS::Lambda::Function
    Properties:
      Code: lambda/
  

  And when aws cloudformation package is run it will output a modified template with the code path expanded:

  MyLambda:
    Properties:
      Code:
        S3Bucket: cf-templates-1a2b3c4d5e6f-ap-southeast-2
        S3Key: e24e45d4f5f2ab3c5d437659fa2246a7
  

  You then aws cloudformation deploy this expanded template. Or use some other method for deployment - the template is ready to use.

  The beauty of this package / deploy method is that it handles the upload of local files to S3 for you.

In different projects we use both methods in various CI/CD pipelines and both work great.

Hope that helps :)