[BUG] danger ci not running in a gitlab.com MR

Describe the bug Danger doesn’t think it’s running in a CI environment or a PR context in a gitlab.com MR (I can’t quite tell which, DEBUG logging is a little sparse on startup).

Skipping Danger due to this run not executing on a PR.

To Reproduce Steps to reproduce the behavior:

1. I added a simple Dangerfile

import { message, danger } from "danger"

const modifiedMD = danger.git.modified_files.join("- ")
message("Changed Files in this PR: \n - " + modifiedMD)

1. And a basic .gitlab-ci.yml

danger:
    image: node:lts
    before_script:
        - printenv
    script:
        - DEBUG="*" npx danger ci

1. To a branch and opened an MR
2. Check the pipeline logs (snipped some of the env output)

$ printenv
...
GITLAB_CI=true
CI=true
DANGER_GITLAB_API_TOKEN=[MASKED]
...
 $ DEBUG="*" npx danger ci
 2020-04-02T15:34:30.576Z danger:process_runner Debug mode on for Danger v10.0.0
 2020-04-02T15:34:30.578Z danger:process_runner Starting sub-process run
 Skipping Danger due to this run not executing on a PR.

Expected behavior

I hoped that the presence of the CI and GITLAB_CI and other env vars would inform danger that it was running in a GitLab CI environment!

Screenshots If applicable, add screenshots to help explain your problem.

Your Environment

software	version
danger.js	10.0.0
node	12.6.1
npm	6.13.4
Operating System

Additional context Add any other context about the problem here.

[AnthonyMastrean]

Oh, the gitlab documentation for the CI_MERGE_REQUEST_IID variable says

The IID of the merge request if the pipelines are for merge requests. Available only if only: [merge_requests] or rules syntax is used and the merge request is created.

https://docs.gitlab.com/ee/ci/variables/predefined_variables.html

That means I need to add this to my .gitlab-ci.yml

only:
  - merge_requests

I could send a PR with updated GitLab docs if you like?

[mat813]

I usually don’t have my ci run on merge requests but on branch commits, attached to merge requests, so I never have CI_MERGE_REQUEST_IID. On the other hand, there is a CI_OPEN_MERGE_REQUESTS that contains project/path!id.

I ended up having:

  script:
    - export CI_MERGE_REQUEST_IID=${CI_OPEN_MERGE_REQUESTS#*!}
    - yarn danger ci --failOnErrors