REMOVE THIS COMMIT: allow undoing last resolution

app/controllers/projects/merge_requests_controller.rb

@@ -10,7 +10,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   before_action :module_enabled
   before_action :merge_request, only: [
     :edit, :update, :show, :diffs, :commits, :conflicts, :builds, :merge, :merge_check,
-    :ci_status, :toggle_subscription, :cancel_merge_when_build_succeeds, :remove_wip, :resolve_conflicts
+    :ci_status, :toggle_subscription, :cancel_merge_when_build_succeeds, :remove_wip, :resolve_conflicts,
+    :undo_last_resolution
   ]
   before_action :validates_merge_request, only: [:show, :diffs, :commits, :builds]
   before_action :define_show_vars, only: [:show, :diffs, :commits, :conflicts, :builds]
@@ -160,6 +161,16 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     end
   end
 
+  def undo_last_resolution
+    rugged = @merge_request.project.repository.rugged
+    ref = Gitlab::Git::BRANCH_REF_PREFIX + @merge_request.source_branch
+    prev = rugged.ref(ref).target
+
+    rugged.references.update(ref, prev.parents.first.oid) if prev.parents.length == 2
+
+    head :ok
+  end
+
   def builds
     respond_to do |format|
       format.html do

config/routes.rb

@@ -729,6 +729,7 @@ Rails.application.routes.draw do
             post :remove_wip
             get :diff_for_path
             post :resolve_conflicts
+            post :undo_last_resolution
           end
 
           collection do

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -638,6 +638,19 @@ describe Projects::MergeRequestsController do
       it 'redirects to the MR show page' do
         expect(response).to redirect_to([merge_request_with_conflicts.target_project.namespace.becomes(Namespace), merge_request_with_conflicts.target_project, merge_request_with_conflicts])
       end
+
+      context 'undoing the resolution' do
+        before do
+          post :undo_last_resolution,
+               namespace_id: merge_request_with_conflicts.project.namespace.to_param,
+               project_id: merge_request_with_conflicts.project.to_param,
+               id: merge_request_with_conflicts.iid
+        end
+
+        it 'undoes the commit' do
+          expect(original_head_sha).to eq(merge_request_with_conflicts.source_branch_head.sha)
+        end
+      end
     end
 
     context 'when sections are missing' do