Drop OpenStruct use from FakeApplicationSettings

OpenStruct has been pointed out as problematic to use,
and the way it is used here actually breaks under Ruby 3.

We cannot override OpenStruct props through module
methods anymore, due to OpenStruct defining methods
eagerly in Ruby 3.

Therefore, this patch drops the use of OpenStruct entirely
in favor of some hand-rolled meta-programming that mimics
OStruct behavior from 2.7 where relevant.

lib/gitlab/fake_application_settings.rb

 # frozen_string_literal: true
 
-# This class extends an OpenStruct object by adding predicate methods to mimic
+# Fakes ActiveRecord attribute storage by adding predicate methods to mimic
 # ActiveRecord access. We rely on the initial values being true or false to
 # determine whether to define a predicate method because for a newly-added
 # column that has not been migrated yet, there is no way to determine the
 # column type without parsing db/structure.sql.
 module Gitlab
-  class FakeApplicationSettings < OpenStruct
-    include ApplicationSettingImplementation
-
-    # Mimic ActiveRecord predicate methods for boolean values
-    def self.define_predicate_methods(options)
-      options.each do |key, value|
-        next if key.to_s.end_with?('?')
-        next unless [true, false].include?(value)
-
-        define_method "#{key}?" do
-          actual_key = key.to_s.chomp('?')
-          self[actual_key]
+  class FakeApplicationSettings
+    prepend ApplicationSettingImplementation
+
+    def self.define_properties(settings)
+      settings.each do |key, value|
+        define_method key do
+          read_attribute(key)
+        end
+
+        if [true, false].include?(value)
+          define_method "#{key}?" do
+            read_attribute(key)
+          end
+        end
+
+        define_method "#{key}=" do |v|
+          @table[key.to_sym] = v
         end
       end
     end
 
-    def initialize(options = {})
-      super
+    def initialize(settings = {})
+      @table = settings.dup
 
-      FakeApplicationSettings.define_predicate_methods(options)
+      FakeApplicationSettings.define_properties(settings)
     end
 
-    alias_method :read_attribute, :[]
-    alias_method :has_attribute?, :[]
+    def read_attribute(key)
+      @table[key.to_sym]
+    end
+
+    def has_attribute?(key)
+      @table.key?(key.to_sym)
+    end
+
+    # Mimic behavior of OpenStruct, which absorbs any calls into undefined
+    # properties to return `nil`.
+    def method_missing(*)
+      nil
+    end
   end
 end
 

spec/lib/gitlab/fake_application_settings_spec.rb

@@ -6,27 +6,35 @@ RSpec.describe Gitlab::FakeApplicationSettings do
   let(:defaults) do
     described_class.defaults.merge(
       foobar: 'asdf',
-      'test?' => 123
+      'test?'.to_sym => 123,
+      # these two settings have no default in ApplicationSettingImplementation,
+      # so we need to set one here
+      domain_denylist: [],
+      archive_builds_in_seconds: nil
     )
   end
 
   let(:setting) { described_class.new(defaults) }
 
-  it 'wraps OpenStruct variables properly' do
+  it 'defines methods for default attributes' do
     expect(setting.password_authentication_enabled_for_web).to be_truthy
     expect(setting.signup_enabled).to be_truthy
     expect(setting.foobar).to eq('asdf')
   end
 
-  it 'defines predicate methods' do
+  it 'defines predicate methods for boolean properties' do
     expect(setting.password_authentication_enabled_for_web?).to be_truthy
     expect(setting.signup_enabled?).to be_truthy
   end
 
-  it 'does not define a predicate method' do
+  it 'does not define a predicate method for non-boolean properties' do
     expect(setting.foobar?).to be_nil
   end
 
+  it 'returns nil for undefined attributes' do
+    expect(setting.does_not_exist).to be_nil
+  end
+
   it 'does not override an existing predicate method' do
     expect(setting.test?).to eq(123)
   end