GitLab CI: Execute a Job only if one of the two previous jobs succeeded

Question

We are trying to develop a GitLab ci pipeline. We have three Jobs: A,B and C. We want to execute the C job only in the case when either A or B is executed.

How to define such job C?

I have tried using needs but it give error as the needed job is not executed:

Found errors in your .gitlab-ci.yml:
'C' job needs 'A' job, but 'A' is not in any previous stage.
'C' job needs 'B' job, but 'B' is not in any previous stage.

My attempt:

C:
  stage: push
  image: alpine
  tags:
    - DOCKER
  needs:
    - job: B
      artifacts: true

Answer 1

There is no concise, built-in way to do this that I am aware of, but the needs: optional keyword may be helpful in conjunction with rules.

I could provide a more complete example if you were to post the rules of jobs A and B. Let's pretend that these are the rules defined for jobs A and B.

# Job runs only on the main branch
A:
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
    - when: never

B:
  rules:
    - if: $CI_COMMIT_BRANCH =~ /feature\/*/i
    - when: never

C:
  stage: push
  image: alpine
  tags:
    - DOCKER
  needs:
    - job: A
      optional: true
      artifacts: true
    - job: B
      optional: true
      artifacts: true
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
    - if: $CI_COMMIT_BRANCH =~ /feature\/*/i
    - when: never

Beware that if the rules defined allow for a situation where job C is triggered without triggering either of the dependent jobs(A and B) it will not fail automatically since the dependent jobs are considered optional.