Pipelines for Merged Results

When you submit a merge request, you are requesting to merge changes from a source branch into a target branch. By default, the CI pipeline runs jobs against the source branch.

With pipelines for merged results, the pipeline runs as if the changes from the source branch have already been merged into the target branch.

If the pipeline fails due to a problem in the target branch, you can wait until the target is fixed and re-run the pipeline. This new pipeline will run as if the source is merged with the updated target, and you will not need to rebase.

The pipeline does not automatically run when the target branch changes. Only changes to the source branch trigger a new pipeline. If a long time has passed since the last successful pipeline, you may want to re-run it before merge, to ensure that the source changes can still be successfully merged into the target.

When the merge request can’t be merged, the pipeline runs against the source branch only. For example, when:

  • The target branch has changes that conflict with the changes in the source branch.
  • The merge request is a work in progress.

In these cases, the pipeline runs as a pipeline for merge requests and is labeled as detached. If these cases no longer exist, new pipelines will again run against the merged results.

Requirements and limitations

Pipelines for merged results require a GitLab Runner 11.9 or newer.

In addition, pipelines for merged results have the following limitations:

  • Forking/cross-repo workflows are not currently supported. To follow progress, see #11934.
  • This feature is not available for fast forward merges yet. To follow progress, see #58226.

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.

Automatic pipeline cancelation

GitLab CI/CD can detect the presence of redundant pipelines, and will cancel them automatically in order to conserve CI resources.

When a user merges a merge request immediately within an ongoing merge train, the train will be reconstructed, as it will recreate the expected post-merge commit and pipeline. In this case, the merge train may already have pipelines running against the previous expected post-merge commit. These pipelines are considered redundant and will be automatically canceled.

Troubleshooting

Pipelines for merged results not created even with new change pushed to merge request

Can be caused by some disabled feature flags. Please make sure that the following feature flags are enabled on your GitLab instance:

  • :ci_use_merge_request_ref
  • :merge_ref_auto_sync

To check and set these feature flag values, please ask an administrator to:

  1. Log into the Rails console of the GitLab instance:

    sudo gitlab-rails console
    
  2. Check if the flags are enabled or not:

    Feature.enabled?(:ci_use_merge_request_ref)
    Feature.enabled?(:merge_ref_auto_sync)
    
  3. If needed, enable the feature flags:

    Feature.enable(:ci_use_merge_request_ref)
    Feature.enable(:merge_ref_auto_sync)
    

Intermittently pipelines fail by fatal: reference is not a tree: error

Since pipelines for merged results are a run on a merge ref of a merge request (refs/merge-requests/<iid>/merge), the Git reference could be overwritten at an unexpected timing. For example, when a source or target branch is advanced. In this case, the pipeline fails because of fatal: reference is not a tree: error, which indicates that the checkout-SHA is not found in the merge ref.

This behavior was improved at GitLab 12.4 by introducing Persistent pipeline refs. You should be able to create pipelines at any timings without concerning the error.

Using Merge Trains

By enabling Pipelines for merged results, GitLab will automatically display a Start/Add Merge Train button as the most recommended merge strategy.

Generally, this is a safer option than merging merge requests immediately as your merge request will be evaluated with an expected post-merge result before the actual merge happens.

For more information, read the documentation on Merge Trains.