Extract the code owner validation logic from DiffCheck

ee/lib/ee/gitlab/checks/diff_check.rb

@@ -36,27 +36,12 @@ module EE
 
         def validate_code_owners
           lambda do |paths|
-            loader = ::Gitlab::CodeOwners::Loader.new(project, branch_name, paths)
+            validator = ::Gitlab::CodeOwners::Validator.new(project, branch_name, paths)
 
-            return if loader.entries.blank?
-
-            assemble_error_msg_for_codeowner_matches(loader)
+            validator.execute
           end
         end
 
-        def assemble_error_msg_for_codeowner_matches(loader)
-          matched_rules = loader.entries.collect { |e| "- #{e.pattern}" }
-          code_owner_path = project.repository.code_owners_blob(ref: branch_name).path || "CODEOWNERS"
-
-          msg = "Pushes to protected branches that contain changes to files that\n" \
-            "match patterns defined in `#{code_owner_path}` are disabled for\n" \
-            "this project. Please submit these changes via a merge request.\n\n" \
-            "The following pattern(s) from `#{code_owner_path}` were matched:\n" \
-            "#{matched_rules.join('\n')}\n"
-
-          updated_from_web? ? msg.tr("\n", " ") : msg
-        end
-
         def validate_path_locks?
           strong_memoize(:validate_path_locks) do
             project.feature_available?(:file_locks) &&

ee/lib/gitlab/code_owners/validator.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module CodeOwners
+    class Validator
+      def initialize(project, branch_name, paths)
+        @project     = project
+        @branch_name = branch_name
+        @paths       = Array(paths)
+      end
+
+      def execute
+        return if loader.entries.blank?
+
+        assemble_error_msg_for_codeowner_matches
+      end
+
+      private
+
+      def assemble_error_msg_for_codeowner_matches
+        "Pushes to protected branches that contain changes to files that\n" \
+          "match patterns defined in `#{code_owner_path}` are disabled for\n" \
+          "this project. Please submit these changes via a merge request.\n\n" \
+          "The following pattern(s) from `#{code_owner_path}` were matched:\n" \
+          "#{matched_rules.join('\n')}\n"
+      end
+
+      def matched_rules
+        loader.entries.collect { |e| "- #{e.pattern}" }
+      end
+
+      def code_owner_path
+        @project.repository.code_owners_blob(ref: @branch_name).path || "CODEOWNERS"
+      end
+
+      def loader
+        @loader ||= ::Gitlab::CodeOwners::Loader.new(@project, @branch_name, @paths)
+      end
+    end
+  end
+end

ee/spec/lib/gitlab/code_owners/validator_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::CodeOwners::Validator do
+  include FakeBlobHelpers
+
+  let_it_be(:group) { create(:group) }
+  let_it_be(:project) { create(:project, namespace: group) }
+
+  let(:codeowner_content) do
+    <<~CODEOWNERS
+    docs/* @documentation-owner
+    docs/CODEOWNERS @owner-1 owner2@gitlab.org @owner-3 @documentation-owner
+    spec/* @test-owner @test-group @test-group/nested-group
+    CODEOWNERS
+  end
+
+  let!(:owner_1) { create(:user, username: 'owner-1') }
+  let!(:email_owner) { create(:user, username: 'owner-2') }
+  let!(:owner_3) { create(:user, username: 'owner-3') }
+  let!(:documentation_owner) { create(:user, username: 'documentation-owner') }
+  let!(:test_owner) { create(:user, username: 'test-owner') }
+  let(:codeowner_blob) { fake_blob(path: 'CODEOWNERS', data: codeowner_content) }
+  let(:paths) { 'docs/CODEOWNERS' }
+
+  subject(:validator) { described_class.new(project, 'with-codeowners', paths) }
+
+  before do
+    project.add_developer(owner_1)
+    project.add_developer(email_owner)
+    project.add_developer(documentation_owner)
+    project.add_developer(test_owner)
+
+    create(:email, user: email_owner, email: 'owner2@gitlab.org')
+
+    allow(project.repository).to receive(:code_owners_blob).and_return(codeowner_blob)
+  end
+
+  describe "#execute" do
+    context "when paths match entries in the codeowners file" do
+      it "returns an error message" do
+        expect(subject.execute).to include("Pushes to protected branches")
+      end
+    end
+
+    context "when paths do not match entries in the codeowners file" do
+      let(:paths) { "not/a/matching/path" }
+
+      it "returns nil" do
+        expect(subject.execute).to be_nil
+      end
+    end
+  end
+end