---
type: reference
last_update: 2019-07-03
---

# Pipelines for Merged Results **(PREMIUM)**

> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/7380) in [GitLab Premium](https://about.gitlab.com/pricing/) 11.10.

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](img/merged_result_pipeline_v12_3.png)

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](../index.md)
 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][runner] 11.9 or newer.

[runner]: https://gitlab.com/gitlab-org/gitlab-runner

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

- Forking/cross-repo workflows are not currently supported. To follow progress,
  see [#11934](https://gitlab.com/gitlab-org/gitlab/issues/11934).
- This feature is not available for
  [fast forward merges](../../../user/project/merge_requests/fast_forward_merge.md) yet.
  To follow progress, see [#58226](https://gitlab.com/gitlab-org/gitlab-foss/issues/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**.
1. Check **Merge pipelines will try to validate the post-merge result prior to merging**.
1. Click **Save changes** button.

![Merge request pipeline config](img/merge_request_pipeline_config.png)

CAUTION: **Warning:**
Make sure your `gitlab-ci.yml` file is [configured properly for pipelines for merge requests](../index.md#configuring-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

> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/12996) in [GitLab Premium](https://about.gitlab.com/pricing/) 12.3.

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

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

   ```shell
   sudo gitlab-rails console
   ```

1. Check if the flags are enabled or not:

   ```ruby
   Feature.enabled?(:ci_use_merge_request_ref)
   Feature.enabled?(:merge_ref_auto_sync)
   ```

1. If needed, enable the feature flags:

   ```ruby
   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](../../pipelines.md#persistent-pipeline-refs).
You should be able to create pipelines at any timings without concerning the error.

## Using Merge Trains **(PREMIUM)**

By enabling [Pipelines for merged results](#pipelines-for-merged-results-premium),
GitLab will [automatically display](merge_trains/index.md#how-to-add-a-merge-request-to-a-merge-train)
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](merge_trains/index.md).