Pipelines for merge requests

Note: As of GitLab 11.10, pipelines for merge requests require GitLab Runner 11.9 or higher due to the recent refspecs changes. Anything lower will cause the pipeline to fail.

Introduced in GitLab 11.6.

Usually, when you create a new merge request, a pipeline runs with the new change and checks if it’s qualified to be merged into a target branch. This pipeline should contain only necessary jobs for validating the new changes. For example, unit tests, lint checks, and Review Apps are often used in this cycle.

With pipelines for merge requests, you can design a specific pipeline structure for when you are running a pipeline in a merge request. This could be either adding or removing steps in the pipeline, to make sure that your pipelines are as efficient as possible.

Configuring pipelines for merge requests

To configure pipelines for merge requests, add the only: merge_requests parameter to the jobs that you want to run only for merge requests.

Then, when developers create or update merge requests, a pipeline runs every time a commit is pushed to GitLab.

Note: If you use this feature with merge when pipeline succeeds, pipelines for merge requests take precedence over the other regular pipelines.

For example, consider the following .gitlab-ci.yml:

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

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

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

After the merge request is updated with new commits:

  • GitLab detects that changes have occurred and creates a new pipeline for the merge request.
  • The pipeline fetches the latest code from the source branch and run tests against it.

In the above example, the pipeline contains only a test job. Since the build and deploy jobs don’t have the only: merge_requests parameter, they will not run in the merge request.

Pipelines tagged with the detached badge indicate that they were triggered when a merge request was created or updated. For example:

Merge request page

Pipelines for Merged Results

Introduced in GitLab Premium 11.10. This feature is disabled by default until we resolve issues with contention handling, but can be enabled manually.

It’s possible for your source and target branches to diverge, which can result in the scenario that source branch’s pipeline was green, the target’s pipeline was green, but the combined output fails.

By having your merge request pipeline automatically create a new ref that contains the merge result of the source and target branch (then running a pipeline on that ref), we can better test that the combined result is also valid.

GitLab can run pipelines for merge requests on this merged result. That is, where the source and target branches are combined into a new ref and a pipeline for this ref validates the result prior to merging.

Merge request pipeline as the head pipeline

There are some cases where creating a combined ref is not possible or not wanted. For example, a source branch that has conflicts with the target branch or a merge request that is still in WIP status. In this case, the merge request pipeline falls back to a “detached” state and runs on the source branch ref as if it was a regular pipeline.

The detached state serves to warn you that you are working in a situation subjected to merge problems, and helps to highlight that you should get out of WIP status or resolve merge conflicts as soon as possible.

Enabling Pipelines for Merged Results

To enable pipelines on merged results at the project level:

  1. Visit your project’s Settings > General and expand Merge requests.
  2. Check Merge pipelines will try to validate the post-merge result prior to merging.
  3. Click Save changes button.

Merge request pipeline config

Warning: Make sure your gitlab-ci.yml file is configured properly for pipelines for merge requests, otherwise pipelines for merged results won’t run and your merge requests will be stuck in an unresolved state.

Pipelines for Merged Result’s limitations

  • This feature requires GitLab Runner 11.9 or newer.
  • This feature requires Gitaly 1.21.0 or newer.
  • Forking/cross-repo workflows are not currently supported. To follow progress, see #9713.
  • This feature is not available for fast forward merges yet. To follow progress, see #58226.

Merge Trains

Introduced in GitLab Premium 12.0. This feature is disabled by default, but can be enabled manually.

Pipelines for merged results introduces running a build on the result of the merged code prior to merging, as a way to keep master green. There’s a scenario, however, for teams with a high number of changes in the target branch (typically master) where in many or even all cases, by the time the merged code is validated another commit has made it to master, invalidating the merged result. You’d need some kind of queuing, cancellation or retry mechanism for these scenarios in order to ensure an orderly flow of changes into the target branch.

Each MR that joins a merge train joins as the last item in the train, just as it works in the current state. However, instead of queuing and waiting, each item takes the completed state of the previous (pending) merge ref, adds its own changes, and starts the pipeline immediately in parallel under the assumption that everything is going to pass. In this way, if all the pipelines in the train merge successfully, no pipeline time is wasted either queuing or retrying. If the button is subsequently pressed in a different MR, instead of creating a new pipeline for the target branch, it creates a new pipeline targeting the merge result of the previous MR plus the target branch. Pipelines invalidated through failures are immediately canceled and requeued.

Caution: At the moment, each merge train can generate a merge ref and run a pipeline one at a time. We plan to make the pipelines for merged results run in parallel in a future release.

Enabling Merge Trains

To enable merge trains at the project level:

  1. Visit your project’s Settings > General and expand Merge requests.
  2. Check Allow merge trains.
  3. Click Save changes button.

Merge request pipeline config

Warning: This feature requires Pipelines for merged results to be configured properly.

How to add a merge request to a merge train

To add a merge request to a merge train:

  1. Click “Start/Add merge train” button in a merge request

Start merge train

How to remove a merge request from a merge train

  1. Click “Remove from merge train” button in the merge request widget.

Cancel merge train

Tips: Start/Add to merge train when pipeline succeeds

You can add a merge request to a merge train only when the latest pipeline in the merge request finished. While the pipeline is running or pending, you cannot add the merge request to a train because the current change of the merge request may be broken thus it could affect the following merge requests.

In this case, you can schedule to add the merge request to a merge train when the latest pipeline succeeds. You can see the following button instead of the regular “Start/Add merge train” button while the latest pipeline is running.

Add to merge train when pipeline succeeds

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 may want to reverse this behavior, having all of your jobs to 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.

As you can see, this will help you avoid a lot of boilerplate where you’d need to add that only: rule to all of your jobs in order to make them always run. You can use this for scenarios like having only pipelines with merge requests get a Review App set up, helping to save resources.

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 about 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 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.