Allow fetching owners for MR

ee/app/models/ee/merge_request.rb

@@ -50,5 +50,11 @@ module EE
     def participant_approvers
       approval_needed? ? approvers_left : []
     end
+
+    def code_owners
+      strong_memoize(:code_owners) do
+        ::Gitlab::CodeOwners.for_merge_request(self).freeze
+      end
+    end
   end
 end

ee/lib/gitlab/code_owners.rb

@@ -12,5 +12,20 @@ module Gitlab
         User.none
       end
     end
+
+    # @param merge_request [MergeRequest]
+    # @param merge_request_diff [MergeRequestDiff]
+    #   Find code owners at a particular MergeRequestDiff.
+    #   Assumed to be the most recent one if not provided.
+    def self.for_merge_request(merge_request, merge_request_diff: nil)
+      return [] if merge_request.source_project.nil? || merge_request.source_branch.nil?
+      return [] unless merge_request.target_project.feature_available?(:code_owners)
+
+      Loader.new(
+        merge_request.target_project,
+        merge_request.target_branch,
+        merge_request.modified_paths(past_merge_request_diff: merge_request_diff)
+      ).members.where_not_in(merge_request.author).to_a
+    end
   end
 end

ee/spec/lib/gitlab/code_owners_spec.rb

@@ -7,18 +7,22 @@ describe Gitlab::CodeOwners do
 
   let!(:code_owner) { create(:user, username: 'owner-1') }
   let(:project) { create(:project, :repository) }
-  let(:blob) do
-    project.repository.blob_at(TestEnv::BRANCH_SHA['with-codeowners'], 'docs/CODEOWNERS')
-  end
   let(:codeowner_content) { "docs/CODEOWNERS @owner-1" }
   let(:codeowner_blob) { fake_blob(path: 'CODEOWNERS', data: codeowner_content) }
+  let(:codeowner_blob_ref) { fake_blob(path: 'CODEOWNERS', data: codeowner_content) }
 
   before do
     project.add_developer(code_owner)
-    allow(project.repository).to receive(:code_owners_blob).and_return(codeowner_blob)
+    allow(project.repository).to receive(:code_owners_blob)
+      .with(ref: codeowner_lookup_ref)
+      .and_return(codeowner_blob)
   end
 
   describe '.for_blob' do
+    let(:branch) { TestEnv::BRANCH_SHA['with-codeowners'] }
+    let(:blob) { project.repository.blob_at(branch, 'docs/CODEOWNERS') }
+    let(:codeowner_lookup_ref) { branch }
+
     context 'when the feature is available' do
       before do
         stub_licensed_features(code_owners: true)
@@ -39,4 +43,59 @@ describe Gitlab::CodeOwners do
       end
     end
   end
+
+  describe '.for_merge_request' do
+    let(:codeowner_lookup_ref) { merge_request.target_branch }
+    let(:merge_request) do
+      build(
+        :merge_request,
+        source_project: project,
+        source_branch: 'feature',
+        target_project: project,
+        target_branch: 'with-codeowners'
+      )
+    end
+
+    context 'when the feature is available' do
+      before do
+        stub_licensed_features(code_owners: true)
+      end
+
+      it 'returns owners for merge request' do
+        expect(merge_request).to receive(:modified_paths).with(past_merge_request_diff: nil).and_return(['docs/CODEOWNERS'])
+
+        expect(described_class.for_merge_request(merge_request)).to eq([code_owner])
+      end
+
+      context 'when owner is merge request author' do
+        let(:merge_request) { build(:merge_request, target_project: project, author: code_owner) }
+
+        it 'excludes author' do
+          expect(merge_request).to receive(:modified_paths).with(past_merge_request_diff: nil).and_return(['docs/CODEOWNERS'])
+
+          expect(described_class.for_merge_request(merge_request)).to eq([])
+        end
+      end
+
+      context 'when merge_request_diff is specified' do
+        let(:merge_request_diff) { double(:merge_request_diff) }
+
+        it 'returns owners at the specified ref' do
+          expect(merge_request).to receive(:modified_paths).with(past_merge_request_diff: merge_request_diff).and_return(['docs/CODEOWNERS'])
+
+          expect(described_class.for_merge_request(merge_request, merge_request_diff: merge_request_diff)).to eq([code_owner])
+        end
+      end
+    end
+
+    context 'when the feature is not available' do
+      before do
+        stub_licensed_features(code_owners: false)
+      end
+
+      it 'returns no users' do
+        expect(described_class.for_merge_request(merge_request)).to eq([])
+      end
+    end
+  end
 end

ee/spec/models/merge_request_spec.rb

@@ -14,6 +14,18 @@ describe MergeRequest do
     it { is_expected.to have_many(:approved_by_users) }
   end
 
+  describe '#code_owners' do
+    subject(:merge_request) { build(:merge_request) }
+    let(:owners) { [double(:owner)] }
+
+    it 'returns code owners, frozen' do
+      allow(::Gitlab::CodeOwners).to receive(:for_merge_request).with(subject).and_return(owners)
+
+      expect(subject.code_owners).to eq(owners)
+      expect(subject.code_owners).to be_frozen
+    end
+  end
+
   describe '#approvals_before_merge' do
     where(:license_value, :db_value, :expected) do
       true  | 5   | 5