GitLab: add merge request comments via CI/CD pipeline

Why?

The snippet below is useful if you want to notify user via Merge Request comments.

This is especially useful when rolling out git policies - naming conventions for git branches or commit messages, mandatory checks, etc. To ease the deployment of these policies you could add these checks as non-blocking and let users know after certain date these pipelines will fail. This will give users time to adjust their tools.

This snippet can be further changed to include a check of the merge request description to make sure it includes all necessary information.

This snippet:

• Only runs on merge requests
• Only runs once per merge request (via caching, so if caching is cleaned - message will be sent again)
• Requires Project Access Token - add it as CI/CD environment variable with name PAT_TOKEN
• Name of the generated PAT_TOKEN will be used by GitLab as the bot user, so give it a user friendly name
• Tested with GitLab 16.5

Code

stages:
  - mr-checks

mr-checks:
  stage: mr-checks
  rules:
    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
  cache:
    key: $CI_MERGE_REQUEST_IID
    paths:
      - .gitlab-check
  script:
    - |
      if [ -f .gitlab-check ]; then
        echo "Notification was already sent";
        exit 0;
      fi
    - |
      # add your checks here.
      # It can be - checking branch names, commit messages for required info,
      # files, anything else.
      if [[ <CONDITION GOES HERE> ]]; then
        echo "Check passed";
        exit 0;
      fi
    - export GITLAB_TOKEN=$PAT_TOKEN
    - |
      curl --location --request POST \
        "https://gitlab.com/api/v4/projects/$CI_MERGE_REQUEST_PROJECT_ID/merge_requests/$CI_MERGE_REQUEST_IID/notes" \
        --header "PRIVATE-TOKEN: $GITLAB_TOKEN" \
        --header "Content-Type: application/json" \
        --data-raw "{ \"body\": \"Comment messages goes here.\" }"
    - touch .gitlab-check