Gitlab CI pipeline - continue to next stage only on a certain condition

Based on Fzgregors suggestion, this is how I solved my problem: If there was a difference and I wanted my second stage to actually do some work I created a file called "continue" and made it available as an artifact.

The second stage will look for that file and use an IF statement to decide if it should do something or just exit nicely

Get_inventory_dump:
  stage: compare  
  only:
    - schedules
  script: 
    - 'curl -k --output "previous-inventory.json" --header "PRIVATE-TOKEN: $user_token" "https://url/to/get/artifact/from/last/successful/run"'
    - python3 auto_config_scripts/dump_device_inventory_api_to_json.py -p $pass -o /inventory.json -u https://url/for/inventory/dump -y
    - /usr/bin/cmp previous-inventory.json inventory.json && echo "No Change in inventory since last successful run" || echo "Inventory has changed since last run, continue" && touch continue
  artifacts:
    when: on_success
    expire_in: 4 weeks
    paths:
     - inventory.json
     - continue

Generate_icinga_config:
  stage: build
  only:
    - schedules
  when: on_success
  script: 
    - if [[ -f continue ]]; then
        do some stuff;
      else
        echo "No Change in inventory, nothing to do";
      fi

This allowed me to keep my inventory artifact but at the same time let the next stage know if it needed to do some work or just do nothing and exit with code 0

There are two solutions I can think of. Unfortunately, they either come slightly confusing UI behavior or you have to adapt all jobs.

Job attributes like only or changes are only concerned with the state of or the files of the git repository (see https://docs.gitlab.com/ee/ci/yaml/) and therefore not of use here as the file is only created during CI and not part of the repository.

Solution 1: You can allow_failure: true to the first job. This will mark the pipeline as successful despite the job failing and subsequent jobs will not be executed as the first job did not succeed. The drawback is that when you investigate the pipeline there will be an exclamation mark instead of a green check for this job.

Solution 2: Instead of failing the first job when there are no changes the inventory.json file is removed. And all subsequent jobs directly terminate with exit code 0 when the file doesn't exist. Note that this only works because inventory.json is marked as an artifact.