Add feature flag to skip codeowners in the web UI

ee/changelogs/unreleased/security-apply-codeowners-checks-to-web_ui.yml

+---
+title: Apply CODEOWNERS validations to web requests
+merge_request: 31283
+author:
+type: security

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

@@ -12,13 +12,28 @@ module EE
         def path_validations
           validations = [super].flatten
 
-          if !updated_from_web? && project.branch_requires_code_owner_approval?(branch_name)
+          if validate_code_owners?
             validations << validate_code_owners
           end
 
           validations
         end
 
+        def validate_code_owners?
+          return false if updated_from_web? && skip_web_ui_code_owner_validations?
+
+          project.branch_requires_code_owner_approval?(branch_name)
+        end
+
+        # To allow self-hosted installations to ignore CODEOWNERS rules when
+        # clicking Merge in the UI. By default, these rules are not skipped.
+        #
+        # Issue to remove this feature flag:
+        # https://gitlab.com/gitlab-org/gitlab/-/issues/217427
+        def skip_web_ui_code_owner_validations?
+          ::Feature.enabled?(:skip_web_ui_code_owner_validations, project)
+        end
+
         def validate_code_owners
           lambda do |paths|
             loader = ::Gitlab::CodeOwners::Loader.new(project, branch_name, paths)
@@ -34,11 +49,13 @@ module EE
           matched_rules = loader.entries.collect { |e| "- #{e.pattern}" }
           code_owner_path = project.repository.code_owners_blob(ref: branch_name).path || "CODEOWNERS"
 
-          "Pushes to protected branches that contain changes to files that\n" \
+          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?

ee/spec/lib/gitlab/checks/diff_check_spec.rb

@@ -89,8 +89,22 @@ describe Gitlab::Checks::DiffCheck do
             stub_feature_flags(sectional_codeowners: false)
           end
 
+          context "for a non-web-based request" do
             it "returns an error message" do
               expect(validation_result).to include("Pushes to protected branches")
+              expect(validation_result).to include("\n")
+            end
+          end
+
+          context "for a web-based request" do
+            before do
+              allow(subject).to receive(:updated_from_web?).and_return(true)
+            end
+
+            it "returns an error message with newline chars removed" do
+              expect(validation_result).to include("Pushes to protected branches")
+              expect(validation_result).not_to include("\n")
+            end
           end
         end
 
@@ -211,6 +225,26 @@ describe Gitlab::Checks::DiffCheck do
         context "updated_from_web? == true" do
           before do
             expect(subject).to receive(:updated_from_web?).and_return(true)
+          end
+
+          context "when skip_web_ui_code_owner_validations is disabled" do
+            before do
+              stub_feature_flags(skip_web_ui_code_owner_validations: false)
+              allow(project).to receive(:branch_requires_code_owner_approval?)
+                .once.and_return(true)
+            end
+
+            it "returns an array of Proc(s)" do
+              validations = subject.send(:path_validations)
+
+              expect(validations.any?).to be_truthy
+              expect(validations.any? { |v| !v.is_a? Proc }).to be_falsy
+            end
+          end
+
+          context "when skip_web_ui_code_owner_validations is enabled" do
+            before do
+              stub_feature_flags(skip_web_ui_code_owner_validations: true)
               expect(project).not_to receive(:branch_requires_code_owner_approval?)
             end
 
@@ -220,6 +254,7 @@ describe Gitlab::Checks::DiffCheck do
           end
         end
       end
+    end
 
     context 'file name rules' do
       # Notice that the commit used creates a file named 'README'