Check the HEAD of branch when doing Merge When Succeeded

app/services/merge_requests/merge_when_build_succeeds_service.rb

@@ -24,10 +24,14 @@ module MergeRequests
 
       merge_requests.each do |merge_request|
         next unless merge_request.merge_when_build_succeeds?
+        next unless merge_request.mergeable?
 
-        if merge_request.ci_commit && merge_request.ci_commit.success? && merge_request.mergeable?
-          MergeWorker.perform_async(merge_request.id, merge_request.merge_user_id, merge_request.merge_params)
-        end
+        ci_commit = merge_request.ci_commit
+        next unless ci_commit
+        next unless ci_commit.sha == commit_status.sha
+        next unless ci_commit.success?
+
+        MergeWorker.perform_async(merge_request.id, merge_request.merge_user_id, merge_request.merge_params)
       end
     end
 
@@ -51,6 +55,8 @@ module MergeRequests
       # This is for ref-less builds
       branches ||= @project.repository.branch_names_contains(commit_status.sha)
 
+      return [] if branches.blank?
+
       merge_requests = @project.origin_merge_requests.opened.where(source_branch: branches).to_a
       merge_requests += @project.fork_merge_requests.opened.where(source_branch: branches).to_a
 

spec/services/merge_requests/merge_when_build_succeeds_service_spec.rb

@@ -66,9 +66,25 @@ describe MergeRequests::MergeWhenBuildSucceedsService do
       end
     end
 
+    context 'triggered by an old build' do
+      let(:old_build) { create(:ci_build, ref: mr_merge_if_green_enabled.source_branch, status: "success") }
+      let(:build)     { create(:ci_build, ref: mr_merge_if_green_enabled.source_branch, status: "success") }
+
+      it "merges all merge requests with merge when build succeeds enabled" do
+        allow_any_instance_of(MergeRequest).to receive(:ci_commit).and_return(ci_commit)
+        allow(ci_commit).to receive(:success?).and_return(true)
+        allow(old_build).to receive(:sha).and_return('1234abcdef')
+
+        expect(MergeWorker).to_not receive(:perform_async)
+        service.trigger(old_build)
+      end
+    end
+
     context 'commit status without ref' do
       let(:commit_status) { create(:generic_commit_status, status: 'success') }
 
+      before { mr_merge_if_green_enabled }
+
       it "doesn't merge a requests for status on other branch" do
         allow(project.repository).to receive(:branch_names_contains).with(commit_status.sha).and_return([])