Support variables in rules:changes

Problem to solve

I have some CI jobs which run the same steps on different files and try to DRY my CI by using extends. However, GitLab does not currently evaluate variables in rules:changes, which makes this difficult. I can work around it by replacing extends with the explicit properties, but it makes my CI file unnecessarily verbose.

When setting an environment variable as a path and using that variable in rules:changes, the variable is ignored.

Example

1. Create a env variable named HELM_DIR and set it to helm in the UI.
2. Create a project that contains helm/test.txt
3. Add the below gitlab-ci.yml script to the project:

test0:
  script:
    - echo "This is test0 and will complete"
    - echo $CI_COMMIT_BRANCH
    - echo $CI_PROJECT_DIR

test1:
  script:
    - echo $CI_COMMIT_BRANCH
    - echo $CI_PROJECT_DIR
    - echo $HELM_DIR
    - echo "This is working"
    - ls
  rules:
    - if: $CI_COMMIT_BRANCH
      changes:
      - $HELM_DIR/**/*
      when: on_success

4. Make a change to the helm/test.txt file or any file in that folder.

The below works when explicitly setting the file path:

test0:
  script:
    - echo "This is test0 and will complete"
    - echo $CI_COMMIT_BRANCH
    - echo $CI_PROJECT_DIR

test1:
  script:
    - echo $CI_COMMIT_BRANCH
    - echo $CI_PROJECT_DIR
    - echo "This is working"
    - ls
  rules:
    - if: $CI_COMMIT_BRANCH
      changes:
      - helm/**/*
      when: on_success

Intended users

Further details

Benefit would be cleaner-looking CI that is easier to maintain.

Proposal

Support variable use in the rules:changes clause

Note that $ is a valid character in a path, so we need to enforce a rule that if $SOMETHING variable exists then we will replace it, otherwise we should leave $SOMETHING unchanged.

Permissions and Security

No impact

Documentation

Testing

What does success look like, and how can we measure that?

Support variable use in the rules:changes clause

What is the type of buyer?

Any tier where rules:changes is available (i.e. free/core and above)

Links / references