Run a job when build is cancelled

Description

Often Gitlab CI pipelines will leave various garbage when cancelled. Things like unstopped docker containers, orphaned build files etc. With Gitlabs new auto-cancelling of redundant pipelines, this issue became more pronounced. Also, project users often manually cancel jobs without manually running cleanup jobs.

Proposal

Add when: on-cancel or similar feature to .gitlab-ci.yml. Could be implemented as a standard Job.

[...]
cleanup:
  stage: cleanup
  script:
    - docker-compose down -v
    - rm -rf tmp/
  when: on_cancel

This would be more in line with how other when: clauses work, but would be limited to a single, catch-all cleanup job

Alternatively, this could be implemented on a per-job basis, similar to Environments on_stop feature

compile:
  stage: build
  script:
    - compile stuff
  on_cancel: clean-compile-job

clean-compile-job:
  stage: cleanup
  script:
    - rm -rf tmp/*

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/8998

https://docs.gitlab.com/ee/ci/yaml/README.html#when

Documentation blurb

Overview

The feature would make auto-cancelling more complete, allowing for less maintenance around gitlab-runner environments.

Use cases

Developer pushes a quick follow-up commit to the repository, triggering cancellation of the previous, still running build. The cancelled build runs a job cleaning up any leftovers from the unfinished build.