Pipelines for Merge Requests

Introduced in GitLab 11.6.

In a basic configuration, GitLab runs a pipeline each time changes are pushed to a branch.

If you want the pipeline to run jobs only when merge requests are created or updated, you can use pipelines for merge requests.

In the UI, these pipelines are labeled as detached.

Merge request page

A few notes:

Configuring pipelines for merge requests

To configure pipelines for merge requests, configure your CI/CD configuration file. There are a few different ways to do this.

Enable pipelines for merge requests for all jobs

The recommended method for enabling pipelines for merge requests for all jobs in a pipeline is to use workflow:rules.

In this example, the pipeline always runs for all merge requests, as well as for all changes to the master branch:

workflow:
  rules:
    - if: $CI_MERGE_REQUEST_ID               # Execute jobs in merge request context
    - if: $CI_COMMIT_BRANCH == 'master'      # Execute jobs when a new commit is pushed to master branch

build:
  stage: build
  script: ./build

test:
  stage: test
  script: ./test

deploy:
  stage: deploy
  script: ./deploy

Enable pipelines for merge requests for specific jobs

To enable pipelines for merge requests for specific jobs, you can use rules.

In the following example:

  • The build job runs for all changes to the master branch, as well as for all merge requests.
  • The test job runs for all merge requests.
  • The deploy job runs for all changes to the master branch, but does not run for merge requests.
build:
  stage: build
  script: ./build
  rules:
    - if: $CI_COMMIT_BRANCH == 'master'   # Execute jobs when a new commit is pushed to master branch
    - if: $CI_MERGE_REQUEST_ID            # Execute jobs in merge request context

test:
  stage: test
  script: ./test
  rules:
    - if: $CI_MERGE_REQUEST_ID             # Execute jobs in merge request context

deploy:
  stage: deploy
  script: ./deploy
  rules:
    - if: $CI_COMMIT_BRANCH == 'master'    # Execute jobs when a new commit is pushed to master branch

Use only or except to run pipelines for merge requests

Note: The only / except keywords are going to be deprecated and you should not use them.

To enable pipelines for merge requests, you can use only / except. When you use this method, you have to specify only: - merge_requests for each job.

In this example, the pipeline contains a test job that is configured to run on merge requests.

The build and deploy jobs don’t have the only: - merge_requests parameter, so they will not run on merge requests.

build:
  stage: build
  script: ./build
  only:
  - master

test:
  stage: test
  script: ./test
  only:
  - merge_requests

deploy:
  stage: deploy
  script: ./deploy
  only:
  - master

Excluding certain jobs

The behavior of the only: [merge_requests] parameter is such that only jobs with that parameter are run in the context of a merge request; no other jobs will be run.

However, you can invert this behavior and have all of your jobs run except for one or two.

Consider the following pipeline, with jobs A, B, and C. Imagine you want:

  • All pipelines to always run A and B.
  • C to run only for merge requests.

To achieve this, you can configure your .gitlab-ci.yml file as follows:

.only-default: &only-default
  only:
    - master
    - merge_requests
    - tags

A:
  <<: *only-default
  script:
    - ...

B:
  <<: *only-default
  script:
    - ...

C:
  script:
    - ...
  only:
    - merge_requests

Therefore:

  • Since A and B are getting the only: rule to execute in all cases, they will always run.
  • Since C specifies that it should only run for merge requests, it will not run for any pipeline except a merge request pipeline.

This helps you avoid having to add the only: rule to all of your jobs in order to make them always run. You can use this format to set up a Review App, helping to save resources.

Excluding certain branches

Pipelines for merge requests require special treatment when using only/except. Unlike ordinary branch refs (for example refs/heads/my-feature-branch), merge request refs use a special Git reference that looks like refs/merge-requests/:iid/head. Because of this, the following configuration will not work as expected:

# Does not exclude a branch named "docs-my-fix"!
test:
  only: [merge_requests]
  except: [/^docs-/]

Instead, you can use the $CI_COMMIT_REF_NAME predefined environment variable in combination with only:variables to accomplish this behavior:

test:
  only: [merge_requests]
  except:
    variables:
      - $CI_COMMIT_REF_NAME =~ /^docs-/

Pipelines for Merged Results

Read the documentation on Pipelines for Merged Results.

Merge Trains

Read the documentation on Merge Trains.

Important notes about merge requests from forked projects

Note that the current behavior is subject to change. In the usual contribution flow, external contributors follow the following steps:

  1. Fork a parent project.
  2. Create a merge request from the forked project that targets the master branch in the parent project.
  3. A pipeline runs on the merge request.
  4. A maintainer from the parent project checks the pipeline result, and merge into a target branch if the latest pipeline has passed.

Currently, those pipelines are created in a forked project, not in the parent project. This means you cannot completely trust the pipeline result, because, technically, external contributors can disguise their pipeline results by tweaking their GitLab Runner in the forked project.

There are multiple reasons why GitLab doesn’t allow those pipelines to be created in the parent project, but one of the biggest reasons is security concern. External users could steal secret variables from the parent project by modifying .gitlab-ci.yml, which could be some sort of credentials. This should not happen.

We’re discussing a secure solution of running pipelines for merge requests that are submitted from forked projects, see the issue about the permission extension.

Additional predefined variables

By using pipelines for merge requests, GitLab exposes additional predefined variables to the pipeline jobs. Those variables contain information of the associated merge request, so that it’s useful to integrate your job with GitLab Merge Request API.

You can find the list of available variables in the reference sheet. The variable names begin with the CI_MERGE_REQUEST_ prefix.