Add :default_branch_name as permitted attribute

app/controllers/groups_controller.rb

@@ -241,7 +241,8 @@ class GroupsController < Groups::ApplicationController
       :two_factor_grace_period,
       :project_creation_level,
       :subgroup_creation_level,
-      :default_branch_protection
+      :default_branch_protection,
+      :default_branch_name
     ]
   end
 

app/models/namespace_setting.rb

@@ -3,7 +3,19 @@
 class NamespaceSetting < ApplicationRecord
   belongs_to :namespace, inverse_of: :namespace_settings
 
+  validate :default_branch_name_content
+
+  NAMESPACE_SETTINGS_PARAMS = [:default_branch_name].freeze
+
   self.primary_key = :namespace_id
+
+  def default_branch_name_content
+    return if default_branch_name.nil?
+
+    if default_branch_name.blank?
+      errors.add(:default_branch_name, "can not be an empty string")
+    end
+  end
 end
 
 NamespaceSetting.prepend_if_ee('EE::NamespaceSetting')

app/services/groups/update_service.rb

@@ -23,6 +23,8 @@ module Groups
 
       before_assignment_hook(group, params)
 
+      handle_namespace_settings
+
       group.assign_attributes(params)
 
       begin
@@ -40,6 +42,18 @@ module Groups
 
     private
 
+    def handle_namespace_settings
+      settings_params = params.slice(*::NamespaceSetting::NAMESPACE_SETTINGS_PARAMS)
+
+      return if settings_params.empty?
+
+      ::NamespaceSetting::NAMESPACE_SETTINGS_PARAMS.each do |nsp|
+        params.delete(nsp)
+      end
+
+      ::NamespaceSettings::UpdateService.new(current_user, group, settings_params).execute
+    end
+
     def valid_path_change_with_npm_packages?
       return true unless group.packages_feature_enabled?
       return true if params[:path].blank?

ee/app/services/ee/namespace_settings/update_service.rb

@@ -9,16 +9,14 @@ module EE
 
       override :execute
       def execute
+        super
+
         unless can_update_prevent_forking?
           group.errors.add(
             :prevent_forking_outside_group,
             s_('GroupSettings|Prevent forking setting was not saved')
           )
-
-          return
         end
-
-        super
       end
 
       private

spec/controllers/groups_controller_spec.rb

@@ -551,6 +551,37 @@ RSpec.describe GroupsController, factory_default: :keep do
       end
     end
 
+    context "updating default_branch_name" do
+      let(:example_branch_name) { "example_branch_name" }
+
+      subject(:update_action) do
+        put :update,
+          params: {
+            id: group.to_param,
+            group: { default_branch_name: example_branch_name }
+          }
+      end
+
+      it "updates the attribute" do
+        expect { subject }
+          .to change { group.namespace_settings.reload.default_branch_name }
+          .from(nil)
+          .to(example_branch_name)
+
+        expect(response).to have_gitlab_http_status(:found)
+      end
+
+      context "to empty string" do
+        let(:example_branch_name) { '' }
+
+        it "does not update the attribute" do
+          subject
+
+          expect(group.namespace_settings.reload.default_branch_name).not_to eq('')
+        end
+      end
+    end
+
     context 'when there is a conflicting group path' do
       let!(:conflict_group) { create(:group, path: SecureRandom.hex(12) ) }
       let!(:old_name) { group.name }

spec/models/namespace_setting_spec.rb

@@ -3,5 +3,45 @@
 require 'spec_helper'
 
 RSpec.describe NamespaceSetting, type: :model do
+  # Relationships
+  #
   it { is_expected.to belong_to(:namespace) }
+
+  describe "validations" do
+    describe "#default_branch_name_content" do
+      let_it_be(:group) { create(:group) }
+
+      let(:namespace_settings) { group.namespace_settings }
+
+      shared_examples "doesn't return an error" do
+        it "doesn't return an error" do
+          expect(namespace_settings.valid?).to be_truthy
+          expect(namespace_settings.errors.full_messages).to be_empty
+        end
+      end
+
+      context "when not set" do
+        it_behaves_like "doesn't return an error"
+      end
+
+      context "when set" do
+        before do
+          namespace_settings.default_branch_name = "example_branch_name"
+        end
+
+        it_behaves_like "doesn't return an error"
+      end
+
+      context "when an empty string" do
+        before do
+          namespace_settings.default_branch_name = ''
+        end
+
+        it "returns an error" do
+          expect(namespace_settings.valid?).to be_falsey
+          expect(namespace_settings.errors.full_messages).not_to be_empty
+        end
+      end
+    end
+  end
 end

spec/services/namespace_settings/update_service_spec.rb

@@ -33,5 +33,16 @@ RSpec.describe NamespaceSettings::UpdateService do
         end.not_to change { NamespaceSetting.count }
       end
     end
+
+    context "updating :default_branch_name" do
+      let(:example_branch_name) { "example_branch_name" }
+      let(:settings) { { default_branch_name: example_branch_name } }
+
+      it "changes settings" do
+        expect { service.execute }
+          .to change { group.namespace_settings.default_branch_name }
+          .from(nil).to(example_branch_name)
+      end
+    end
   end
 end