Don't allow merges with new commits

Set a `sha` parameter on the MR form. If this doesn't match the HEAD of
the source branch when the form is submitted, show a warning (like with
a merge conflict) and don't merge the branch.

CHANGELOG

@@ -14,6 +14,7 @@ v 8.9.0 (unreleased)
   - Redesign account and email confirmation emails
   - Use gitlab-shell v3.0.0
   - Add `sha` parameter to MR merge API, to ensure only reviewed changes are merged
+  - Don't allow MRs to be merged when commits were added since the last review / page load
   - Add DB index on users.state
   - Add rake task 'gitlab:db:configure' for conditionally seeding or migrating the database
   - Changed the Slack build message to use the singular duration if necessary (Aran Koning)

app/controllers/projects/merge_requests_controller.rb

@@ -190,6 +190,11 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       return
     end
 
+    if params[:sha] != @merge_request.source_sha
+      @status = :sha_mismatch
+      return
+    end
+
     TodoService.new.merge_merge_request(merge_request, current_user)
 
     @merge_request.update(merge_error: nil)

app/views/projects/merge_requests/merge.js.haml

@@ -5,6 +5,9 @@
 - when :merge_when_build_succeeds
   :plain
     $('.mr-widget-body').html("#{escape_javascript(render('projects/merge_requests/widget/open/merge_when_build_succeeds'))}");
+- when :sha_mismatch
+  :plain
+    $('.mr-widget-body').html("#{escape_javascript(render('projects/merge_requests/widget/open/sha_mismatch'))}");
 - else
   :plain
     $('.mr-widget-body').html("#{escape_javascript(render('projects/merge_requests/widget/open/reload'))}");

app/views/projects/merge_requests/widget/open/_accept.html.haml

@@ -2,6 +2,7 @@
 
 = form_for [:merge, @project.namespace.becomes(Namespace), @project, @merge_request], remote: true, method: :post, html: { class: 'accept-mr-form js-quick-submit js-requires-input' } do |f|
   = hidden_field_tag :authenticity_token, form_authenticity_token
+  = hidden_field_tag :sha, @merge_request.source_sha
   .accept-merge-holder.clearfix.js-toggle-container
     .clearfix
       .accept-action

app/views/projects/merge_requests/widget/open/_merge_when_build_succeeds.html.haml

@@ -16,7 +16,7 @@
   - if remove_source_branch_button || user_can_cancel_automatic_merge
     .clearfix.prepend-top-10
       - if remove_source_branch_button
-        = link_to merge_namespace_project_merge_request_path(@merge_request.target_project.namespace, @merge_request.target_project, @merge_request, merge_when_build_succeeds: true, should_remove_source_branch: true), remote: true, method: :post, class: "btn btn-grouped btn-primary btn-sm remove_source_branch" do
+        = link_to merge_namespace_project_merge_request_path(@merge_request.target_project.namespace, @merge_request.target_project, @merge_request, merge_when_build_succeeds: true, should_remove_source_branch: true, sha: @merge_request.source_sha), remote: true, method: :post, class: "btn btn-grouped btn-primary btn-sm remove_source_branch" do
           = icon('times')
           Remove Source Branch When Merged
 

app/views/projects/merge_requests/widget/open/_sha_mismatch.html.haml

+%h4
+  = icon("exclamation-triangle")
+  This merge request has new commits since the page was loaded.
+
+%p
+  Please review the new commits before merging.

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -185,6 +185,92 @@ describe Projects::MergeRequestsController do
     end
   end
 
+  describe 'POST #merge' do
+    let(:base_params) do
+      {
+        namespace_id: project.namespace.path,
+        project_id: project.path,
+        id: merge_request.iid,
+        format: 'raw'
+      }
+    end
+
+    context 'when the user does not have access' do
+      before do
+        project.team.truncate
+        project.team << [user, :reporter]
+        post :merge, base_params
+      end
+
+      it 'returns not found' do
+        expect(response).to be_not_found
+      end
+    end
+
+    context 'when the merge request is not mergeable' do
+      before do
+        merge_request.update_attributes(title: "WIP: #{merge_request.title}")
+
+        post :merge, base_params
+      end
+
+      it 'returns :failed' do
+        expect(assigns(:status)).to eq(:failed)
+      end
+    end
+
+    context 'when the sha parameter does not match the source SHA' do
+      before { post :merge, base_params.merge(sha: 'foo') }
+
+      it 'returns :sha_mismatch' do
+        expect(assigns(:status)).to eq(:sha_mismatch)
+      end
+    end
+
+    context 'when the sha parameter matches the source SHA' do
+      def merge_with_sha
+        post :merge, base_params.merge(sha: merge_request.source_sha)
+      end
+
+      it 'returns :success' do
+        merge_with_sha
+
+        expect(assigns(:status)).to eq(:success)
+      end
+
+      it 'starts the merge immediately' do
+        expect(MergeWorker).to receive(:perform_async).with(merge_request.id, anything, anything)
+
+        merge_with_sha
+      end
+
+      context 'when merge_when_build_succeeds is passed' do
+        def merge_when_build_succeeds
+          post :merge, base_params.merge(sha: merge_request.source_sha, merge_when_build_succeeds: '1')
+        end
+
+        before do
+          create(:ci_empty_commit, project: project, sha: merge_request.source_sha, ref: merge_request.source_branch)
+        end
+
+        it 'returns :merge_when_build_succeeds' do
+          merge_when_build_succeeds
+
+          expect(assigns(:status)).to eq(:merge_when_build_succeeds)
+        end
+
+        it 'sets the MR to merge when the build succeeds' do
+          service = double(:merge_when_build_succeeds_service)
+
+          expect(MergeRequests::MergeWhenBuildSucceedsService).to receive(:new).with(project, anything, anything).and_return(service)
+          expect(service).to receive(:execute).with(merge_request)
+
+          merge_when_build_succeeds
+        end
+      end
+    end
+  end
+
   describe "DELETE #destroy" do
     it "denies access to users unless they're admin or project owner" do
       delete :destroy, namespace_id: project.namespace.path, project_id: project.path, id: merge_request.iid