Add a rebase API endpoint for merge requests

changelogs/unreleased/33705-merge-request-rebase-api.yml

+---
+title: Add a rebase API endpoint for merge requests
+merge_request: 23296
+author:
+type: added

doc/api/merge_requests.md

@@ -1206,6 +1206,31 @@ Parameters:
 }
 ```
 
+## Rebase a merge request
+
+Automatically rebase the `source_branch` of the merge request against its
+`target_branch`.
+
+If you don't have permissions to push to the merge request's source branch -
+you'll get a `403 Forbidden` response.
+
+```
+PUT /projects/:id/merge_requests/:merge_request_iid/rebase
+```
+
+| Attribute           | Type    | Required | Description                          |
+| ---------           | ----    | -------- | -----------                          |
+| `id`                | integer/string | yes      | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user                  |
+| `merge_request_iid` | integer | yes      | The internal ID of the merge request |
+
+```bash
+curl --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" https://gitlab.example.com/api/v4/projects/76/merge_requests/1/rebase
+```
+
+This is an asynchronous request. The API will return an empty `202 Accepted`
+response if the request is enqueued successfully. You should poll the
+[Get single MR](#get-single-mr) endpoint to determine success or failure.
+
 ## Comments on merge requests
 
 Comments are done via the [notes](notes.md) resource.

lib/api/merge_requests.rb

@@ -74,6 +74,19 @@ module API
         options
       end
 
+      def authorize_push_to_merge_request!(merge_request)
+        forbidden!('Source branch does not exist') unless
+          merge_request.source_branch_exists?
+
+        user_access = Gitlab::UserAccess.new(
+          current_user,
+          project: merge_request.source_project
+        )
+
+        forbidden!('Cannot push to source branch') unless
+          user_access.can_push_to_branch?(merge_request.source_branch)
+      end
+
       params :merge_requests_params do
         optional :state, type: String, values: %w[opened closed locked merged all], default: 'all',
                          desc: 'Return opened, closed, locked, merged, or all merge requests'
@@ -378,6 +391,19 @@ module API
           .cancel(merge_request)
       end
 
+      desc 'Rebase the merge request against its target branch' do
+        detail 'This feature was added in GitLab 11.6'
+      end
+      put ':id/merge_requests/:merge_request_iid/rebase' do
+        merge_request = find_project_merge_request(params[:merge_request_iid])
+
+        authorize_push_to_merge_request!(merge_request)
+
+        RebaseWorker.perform_async(merge_request.id, current_user.id)
+
+        status :accepted
+      end
+
       desc 'List issues that will be closed on merge' do
         success Entities::MRNote
       end

spec/requests/api/merge_requests_spec.rb

@@ -1181,6 +1181,26 @@ describe API::MergeRequests do
     end
   end
 
+  describe 'PUT :id/merge_requests/:merge_request_iid/rebase' do
+    it 'enques a rebase of the merge request against the target branch' do
+      Sidekiq::Testing.fake! do
+        put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/rebase", user)
+      end
+
+      expect(response).to have_gitlab_http_status(202)
+      expect(RebaseWorker.jobs.size).to eq(1)
+    end
+
+    it 'returns 403 if the user cannot push to the branch' do
+      guest = create(:user)
+      project.add_guest(guest)
+
+      put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/rebase", guest)
+
+      expect(response).to have_gitlab_http_status(403)
+    end
+  end
+
   describe 'Time tracking' do
     let(:issuable) { merge_request }