Introduce direct relation between pipeline and merge request

Currently, relation between pipeline and merge request is indirect.

We use commits stored in merge requests diffs to find SHAs, which is a little tricky. It may also sometimes require access to the file system what is inefficient from performance perspective. We will benefit from improving this relation. We need to think how to solve that and ensure backwards compatibility.

/cc @ayufan @markpundsack

Mentioned in issue #23492 (closed)

Yes please!

It's a part of bigger problem, not only relationship.

It's more about creating a merge requests in context of upstream project. Given that:

• we have to create a pipeline for merge request,
• pipeline has to be created in parent project,
• we have to adapt runners to limit which runners can run merge requests build (untrusted code),
• we have to decide what secure variables are available for a build when running for merge request,
• we have to expose CI_MERGE_REQUEST_ID for build,
• we have to introduce only: and except: in .gitlab-ci.yml,
• we have to extend deployments to mark them as created on merge request,
• we have to support current way of merge requests (branch on source project) and a new way (merge request on target project),

I wonder if this can be solved by adding one missing concept to our domain model. We currently do not have Git::Push entity class, which may be missing piece to create relations between pipelines and merge requests. We do have push event class, but although this seems to be related concept, it is also a little different.

@rymai @rspeicher What do you think about that?

Mentioned in issue #23779 (moved)

I did create a separate issue to discuss the proposal: https://gitlab.com/gitlab-org/gitlab-ce/issues/23902

Milestone changed to %8.15

Status changed to closed

I'm closing this issue as the #23902 (moved) covers much more of actual flow.

mentioned in issue #27687 (closed)

mentioned in issue #31040 (closed)