Add validation to ensure Primary is not disabled

ee/app/models/geo_node.rb

@@ -21,6 +21,7 @@ class GeoNode < ActiveRecord::Base
   validate :check_url_is_valid
 
   validates :primary, uniqueness: { message: 'node already exists' }, if: :primary
+  validates :enabled, if: :primary, acceptance: { message: 'Geo primary node cannot be disabled' }
 
   validates :access_key, presence: true
   validates :encrypted_secret_access_key, presence: true

ee/spec/lib/gitlab/geo/jwt_request_decoder_spec.rb

@@ -13,8 +13,7 @@ describe Gitlab::Geo::JwtRequestDecoder do
     end
 
     it 'fails to decode when node is disabled' do
-      primary_node.enabled = false
-      primary_node.save
+      primary_node.update_attribute(:enabled, false)
 
       expect(subject.decode).to be_nil
     end

ee/spec/models/geo_node_spec.rb

@@ -27,6 +27,15 @@ describe GeoNode, type: :model do
     it { is_expected.to validate_numericality_of(:files_max_capacity).is_greater_than_or_equal_to(0) }
     it { is_expected.to validate_numericality_of(:verification_max_capacity).is_greater_than_or_equal_to(0) }
     it { is_expected.to validate_numericality_of(:minimum_reverification_interval).is_greater_than_or_equal_to(1) }
+
+    context 'primary node' do
+      it 'cannot be disabled' do
+        primary_node.enabled = false
+
+        expect(primary_node).not_to be_valid
+        expect(primary_node.errors).to include(:enabled)
+      end
+    end
   end
 
   context 'default values' do