Pipelines for merged results (PREMIUM)

Introduced in GitLab Premium 11.10.

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. The commit shown for the pipeline does not exist on the source or target branches but represents the combined target and source branches.

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 runs as if the source is merged with the updated target, and you don't 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 Draft merge request.

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 again run against the merged results.

Any user who has developer permissions can run a pipeline for merged results.

Prerequisites

To enable pipelines for merge results:

You must have the Maintainer role.
You must be using GitLab Runner 11.9 or later.
You must not be using fast forward merges yet. To follow progress, see #58226.
Your repository must be a GitLab repository, not an external repository.

Enable pipelines for merged results

To enable pipelines for merged results for your project:

Configure your CI/CD configuration file so that the pipeline or individual jobs run for merge requests.
Visit your project's Settings > General and expand Merge requests.
Check Enable merged results pipelines.
Click Save changes.

WARNING: If you select the checkbox but don't configure your CI/CD to use pipelines for merge requests, your merge requests may become stuck in an unresolved state or your pipelines may be dropped.

Using Merge Trains

When you enable Pipelines for merged results, GitLab automatically displays a Start/Add Merge Train button.

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

For more information, read the documentation on Merge Trains.

Automatic pipeline cancellation

Introduced in GitLab Premium 12.3.

GitLab CI/CD can detect the presence of redundant pipelines, and cancels them to conserve CI resources.

When a user merges a merge request immediately in an ongoing merge train, the train is reconstructed, because it recreates 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 are 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:

:merge_ref_auto_sync

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

Log into the Rails console of the GitLab instance:
```
sudo gitlab-rails console
```
Check if the flags are enabled or not:
```
Feature.enabled?(:merge_ref_auto_sync)
```
If needed, enable the feature flags:
```
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.