Pipelines for Merged Results

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, GitLab doesn’t create a merge commit and the pipeline runs on source branch, instead, which is a default behavior of Pipelines for merge requests i.e. detached label is shown to the pipelines.

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.

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 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 these feature flag values, please ask administrator to execute the following commands:

> sudo gitlab-rails console                         # Login to Rails console of GitLab instance.
> Feature.enabled?(:ci_use_merge_request_ref)       # Check if it's enabled or not.
> Feature.enable(:ci_use_merge_request_ref)         # Enable the feature flag.

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.