Preview diff on deploy/package

[dannyrandall]

This issue is a design proposal for adding a flag to copilot [noun] deploy and copilot [noun] package that previews changes that will be made to the existing cloudformation stack. This will show what changes Copilot will make to existing and new resources, which will become especially useful when testing/deploying extensions via #4208 or #4209.

Proposal

A new flag, --diff, will be added to deploy and package commands that calculates and prints the diff between the deployed stack (including the addons stack!) and the stack that will be deployed or generated.

After displaying the diff, copilot [noun] deploy --diff will prompt with a confirmation like:

Continue with deployment? [y/N]:

before deploying the proposed changes.

copilot [noun] package --diff will print the diff and exit. The exit code will be as follows:
0 = no diffs found
1 = diffs found
2 = error producing diff

This should allow for a script to skip deploying a stack if no changes will occur.

UX Examples

Some examples of how a diff will look in different scenarios:

Adding a lambda function

~ Resources:
  + EnvControllerFunction:
    + Type: AWS::Lambda::Function
    + Properties:
      + Code:
        + S3Bucket: "s3://location"
        + S3Key: "s3://location"
      + Timeout: 900
      + MemorySize: 512

Adding a statement to an IAM Role AssumeRolePolicyDocument:

~ Resources:
  ~ TaskRole:
    ~ Properties:
      ~ AssumeRolePolicyDocument:
        ~ Statement:
          # (1 unchanged list entry)
          + - Action: "*"
              Effect: "Allow"
              Principal:
                Service: "ecs-tasks.amazonaws.com"
          # (2 unchanged list entries)

Updating an attribute of an IAM Role AssumeRolePolicyDocument:

~ Resources:
  ~ TaskRole:
    ~ Properties:
      ~ AssumeRolePolicyDocument:
        ~ Statement:
          # (1 unchanged list entry)
          ~ - Effect: "Allow" -> "Deny"
          # (2 unchanged list entries)

Changing the ContainerPort parameter

~ Parameters:
  ~ ContainerPort:
    ~ Value: 80 -> 443

[efekarakus]

Hi folks!!

I'm really excited to say that AWS Copilot v1.27 is out and preview with --diff is now supported! 🚀

• Blog post: https://aws.github.io/copilot-cli/blogs/release-v127/#preview-aws-cloudformation-template-changes
• Release notes: https://github.com/aws/copilot-cli/releases/tag/v1.27.0

I'll close the issue now! If you have any questions, feedback, or suggestions please feel free to open new GitHub issues 🥳