How can I set a minimum unit test coverage with GitLab CI?

Question

I have a project hosted on GitLab that already has working CI configuration. I would like to add the notion of "minimal code coverage".

What I would love is to force a positive delta (the code coverage of the Merge Request must be greater than the one of the target branch, except if it's already 100%).

I would settle to a "Minimum 80% coverage" kind of rule, but I'm sure I can do better.

I can't find anything in the doc, except the keyword coverage that grabs the coverage to display on the main page.

I would like something like this in .gitlab-ci.yml:

check_coverage:
  stage: test
  rules:
    - if: '$CI_OPEN_MERGE_REQUESTS && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME == $CI_DEFAULT_BRANCH'
  script:
    - compare-coverage.sh $CI_CURRENT_COVERAGE $CI_TARGET_COVERAGE # something that fails if delta < 0

How can I achieve that?

Answer 1

I did it that way:

# .gitlab-ci.yml
image: python:3.9

stages:
  - tests
  - min-coverage

variables:
  MIN_COVERAGE: 80  # set the threshold at 80%

unit-test:
  stage: tests
  before_script:
    - pip install pytest pytest-cov
  script:
    - coverage run -m pytest
    - coverage json
    - coverage report -m
  artifacts:
    paths:
      - coverage.json
  coverage: '/TOTAL.+ ([0-9]{1,3}%)/'

min-coverage-test:
  stage: min-coverage
  before_script:
    - apt-get update && apt-get install jq -y
  script:
    - jsonStr="$(cat $CI_PROJECT_DIR/coverage.json)"
    - coverage="$(echo $jsonStr | jq .totals.percent_covered)"
    - if (( $coverage < $MIN_COVERAGE )) ; then echo "$coverage% of code coverage below threshold of $MIN_COVERAGE%" && exit 1 ; else exit 0 ; fi