Add `pipelines` endpoint to merge requests API

changelogs/unreleased/39214__pipeline_api.yml

+---
+title: Add `pipelines` endpoint to merge requests API
+merge_request: 15454
+author: Tony Rom <thetonyrom@gmail.com>
+type: added

doc/api/merge_requests.md

@@ -431,6 +431,32 @@ Parameters:
 }
 ```
 
+## List MR pipelines
+
+Get a list of merge request pipelines.
+
+```
+GET /projects/:id/merge_requests/:merge_request_iid/pipelines
+```
+
+Parameters:
+
+- `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
+- `merge_request_iid` (required) - The internal ID of the merge request
+
+Example of response
+
+```json
+[
+  {
+    "id": 77,
+    "sha": "959e04d7c7a30600c894bd3c0cd0e1ce7f42c11d",
+    "ref": "master",
+    "status": "success"
+  }
+]
+```
+
 ## Create MR
 
 Creates a new merge request.

lib/api/merge_requests.rb

@@ -24,6 +24,12 @@ module API
           .preload(:notes, :author, :assignee, :milestone, :latest_merge_request_diff, :labels, :timelogs)
       end
 
+      def merge_request_pipelines_with_access(access_level = :read_pipeline)
+        authorize! access_level, user_project
+        mr = find_merge_request_with_access(params[:merge_request_iid])
+        mr.all_pipelines
+      end
+
       params :merge_requests_params do
         optional :state, type: String, values: %w[opened closed merged all], default: 'all',
                          desc: 'Return opened, closed, merged, or all merge requests'
@@ -203,6 +209,15 @@ module API
         present merge_request, with: Entities::MergeRequestChanges, current_user: current_user
       end
 
+      desc 'Get the merge request pipelines' do
+        success Entities::PipelineBasic
+      end
+      get ':id/merge_requests/:merge_request_iid/pipelines' do
+        pipelines = merge_request_pipelines_with_access
+
+        present paginate(pipelines), with: Entities::PipelineBasic
+      end
+
       desc 'Update a merge request' do
         success Entities::MergeRequest
       end

spec/fixtures/api/schemas/public_api/v4/pipelines.json

+{
+  "type": "array",
+  "items": { "$ref": "pipeline/basic.json" }
+}

spec/requests/api/merge_requests_spec.rb

@@ -525,6 +525,43 @@ describe API::MergeRequests do
     end
   end
 
+  describe 'GET /projects/:id/merge_requests/:merge_request_iid/pipelines' do
+    context 'when authorized' do
+      let!(:pipeline) { create(:ci_empty_pipeline, project: project, user: user, ref: merge_request.source_branch, sha: merge_request.diff_head_sha) }
+      let!(:pipeline2) { create(:ci_empty_pipeline, project: project) }
+
+      it 'returns a paginated array of corresponding pipelines' do
+        get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/pipelines")
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(response).to include_pagination_headers
+        expect(json_response).to be_an Array
+        expect(json_response.count).to eq(1)
+        expect(json_response.first['id']).to eq(pipeline.id)
+      end
+
+      it 'exposes basic attributes' do
+        get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/pipelines")
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(response).to match_response_schema('public_api/v4/pipelines')
+      end
+    end
+
+    context 'when unauthorized' do
+      it 'returns 403' do
+        project = create(:project, public_builds: false)
+        merge_request = create(:merge_request, :simple, source_project: project)
+        guest = create(:user)
+        project.team << [guest, :guest]
+
+        get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/pipelines", guest)
+
+        expect(response).to have_gitlab_http_status(403)
+      end
+    end
+  end
+
   describe "POST /projects/:id/merge_requests" do
     context 'between branches projects' do
       it "returns merge_request" do