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# Pipelines for merge requests
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NOTE: **Note** :
As of GitLab 11.10, pipelines for merge requests require GitLab Runner 11.9
or higher due to the [recent refspecs
changes](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25504).
Anything lower will cause the pipeline to fail.
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> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/15310) in GitLab 11.6.
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Usually, when you create a new merge request, a pipeline runs with the
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new change and checks if it's qualified to be merged into a target branch. This
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pipeline should contain only necessary jobs for validating the new changes.
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For example, unit tests, lint checks, and [Review Apps ](../review_apps/index.md )
are often used in this cycle.
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With pipelines for merge requests, you can design a specific pipeline structure
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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.
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## Configuring pipelines for merge requests
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To configure pipelines for merge requests, add the `only: merge_requests` parameter to
the jobs that you want to run only for merge requests.
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Then, when developers create or update merge requests, a pipeline runs
every time a commit is pushed to GitLab.
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NOTE: **Note** :
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If you use this feature with [merge when pipeline succeeds ](../../user/project/merge_requests/merge_when_pipeline_succeeds.md ),
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pipelines for merge requests take precedence over the other regular pipelines.
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For example, consider the following [`.gitlab-ci.yml` ](../yaml/README.md ):
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```yaml
build:
stage: build
script: ./build
only:
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- master
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test:
stage: test
script: ./test
only:
- merge_requests
deploy:
stage: deploy
script: ./deploy
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only:
- master
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```
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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.
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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.
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Pipelines tagged with the **detached** badge indicate that they were triggered
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when a merge request was created or updated. For example:
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![Merge request page ](img/merge_request.png )
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## Combined ref pipelines **[PREMIUM]**
> [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
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create a new ref that contains the merge result of the source and target branch
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(then running a pipeline on that ref), we can better test that the combined result
is also valid.
From GitLab 11.10, pipelines for merge requests run by default
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/merge_request_pipeline.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, 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.
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### Enabling combined ref pipelines
This feature disabled by default until we resolve issues with [contention handling ](https://gitlab.com/gitlab-org/gitlab-ee/issues/9186 ). It can be enabled 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 )
### Combined ref pipeline's limitations
- This feature requires [GitLab Runner ](https://gitlab.com/gitlab-org/gitlab-runner ) 11.9 or newer.
- This feature requires [Gitaly ](https://gitlab.com/gitlab-org/gitaly ) 1.21.0 or newer.
- After the merge request pipeline succeeds, if the target branch has moved forward, the result of the pipeline is stale and must be retried. In busy repos, this can become a problem as it is highly probable that the target branch will have moved ahead. Improvements are [planned ](https://gitlab.com/gitlab-org/gitlab-ee/issues/9186 ) for future versions of GitLab.
- Forking/cross-repo workflows are not currently supported. To follow progress, see [#9713 ](https://gitlab.com/gitlab-org/gitlab-ee/issues/9713 ).
- 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-ce/issues/58226 ).
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## Excluding certain jobs
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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.
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However, you may want to reverse this behavior, having all of your jobs to run _except_
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for one or two.
Consider the following pipeline, with jobs `A` , `B` , and `C` . Imagine you want:
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- All pipelines to always run `A` and `B` .
- `C` to run only for merge requests.
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To achieve this, you can configure your `.gitlab-ci.yml` file as follows:
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``` yaml
.only-default: & only-default
only:
- master
- merge_requests
- tags
A:
< < : * only-default
script:
- ...
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B:
< < : * only-default
script:
- ...
C:
script:
- ...
only:
- merge_requests
```
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Therefore:
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- 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
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except a merge request pipeline.
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As you can see, this will help you avoid a lot of boilerplate where you'd need
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to add that `only:` rule to all of your jobs in order to make them always run. You
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can use this for scenarios like having only pipelines with merge requests get a
Review App set up, helping to save resources.
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## 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.
1. Create a merge request from the forked project that targets the `master` branch
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in the parent project.
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1. A pipeline runs on the merge request.
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1. A maintainer from the parent project checks the pipeline result, and merge
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into a target branch if the latest pipeline has passed.
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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
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created in the parent project, but one of the biggest reasons is security concern.
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External users could steal secret variables from the parent project by modifying
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`.gitlab-ci.yml` , which could be some sort of credentials. This should not happen.
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We're discussing a secure solution of running pipelines for merge requests
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that submitted from forked projects,
see [the issue about the permission extension ](https://gitlab.com/gitlab-org/gitlab-ce/issues/23902 ).
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## 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 ](../../api/merge_requests.md ).
You can find the list of avilable variables in [the reference sheet ](../variables/predefined_variables.md ).
The variable names begin with the `CI_MERGE_REQUEST_` prefix.