Automatically update MR merge-ref along merge status

This couples the code that transitions the `MergeRequest#merge_status`
and refs/merge-requests/:iid/merge ref update.

In general, instead of directly telling `MergeToRefService` to update
the merge ref, we should rely on `MergeabilityCheckService` to keep
both the merge status and merge ref synced. Now, if the merge_status is
`can_be_merged` it means the merge-ref is also updated to the latest.

We've also updated the logic to be more systematic and less user-based.

2 files changed, 7 insertions, 19 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 25e9fdd5fce..ead01dc53f7 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -703,7 +703,7 @@ module API
       # See https://gitlab.com/gitlab-org/gitlab-ce/issues/42344 for more
       # information.
       expose :merge_status do |merge_request|
-        merge_request.check_if_can_be_merged
+        merge_request.check_mergeability
         merge_request.merge_status
       end
       expose :diff_head_sha, as: :sha
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 955624404f1..bf87e9ec2ff 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -397,28 +397,16 @@ module API
         present merge_request, with: Entities::MergeRequest, current_user: current_user, project: user_project
       end
 
-      desc 'Merge a merge request to its default temporary merge ref path'
-      params do
-        optional :merge_commit_message, type: String, desc: 'Custom merge commit message'
-      end
-      put ':id/merge_requests/:merge_request_iid/merge_to_ref' do
+      desc 'Returns the up to date merge-ref HEAD commit'
+      get ':id/merge_requests/:merge_request_iid/merge_ref' do
         merge_request = find_project_merge_request(params[:merge_request_iid])
 
-        authorize! :admin_merge_request, user_project
-
-        merge_params = {
-          commit_message: params[:merge_commit_message]
-        }
-
-        result = ::MergeRequests::MergeToRefService
-          .new(merge_request.target_project, current_user, merge_params)
-          .execute(merge_request)
+        result = ::MergeRequests::MergeabilityCheckService.new(merge_request).execute(recheck: true)
 
-        if result[:status] == :success
-          present result.slice(:commit_id), 200
+        if result.success?
+          present :commit_id, result.payload.dig(:merge_ref_head, :commit_id)
         else
-          http_status = result[:http_status] || 400
-          render_api_error!(result[:message], http_status)
+          render_api_error!(result.message, 400)
         end
       end