Add simple feature flag system (#34038)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

config/application.rb

@@ -35,6 +35,7 @@ require_relative '../lib/paperclip/type_corrector'
 require_relative '../lib/paperclip/response_with_limit_adapter'
 require_relative '../lib/terrapin/multi_pipe_extensions'
 require_relative '../lib/mastodon/snowflake'
+require_relative '../lib/mastodon/feature'
 require_relative '../lib/mastodon/version'
 require_relative '../lib/mastodon/rack_middleware'
 require_relative '../lib/public_file_server_middleware'

config/mastodon.yml

@@ -1,5 +1,6 @@
 ---
 shared:
+  experimental_features: <%= ENV.fetch('EXPERIMENTAL_FEATURES', nil) %>
   self_destruct_value: <%= ENV.fetch('SELF_DESTRUCT', nil) %>
   software_update_url: <%= ENV.fetch('UPDATE_CHECK_URL', 'https://api.joinmastodon.org/update-check') %>
   source:

lib/mastodon/feature.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module Mastodon::Feature
+  class << self
+    def enabled_features
+      @enabled_features ||=
+        (Rails.configuration.x.mastodon.experimental_features || '').split(',').map(&:strip)
+    end
+
+    def method_missing(name)
+      if respond_to_missing?(name)
+        feature = name.to_s.delete_suffix('_enabled?')
+        enabled = enabled_features.include?(feature)
+        define_singleton_method(name) { enabled }
+
+        return enabled
+      end
+
+      super
+    end
+
+    def respond_to_missing?(name)
+      name.to_s.end_with?('_enabled?')
+    end
+  end
+end

spec/lib/mastodon/feature_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Mastodon::Feature do
+  around do |example|
+    original_value = Rails.configuration.x.mastodon.experimental_features
+    Rails.configuration.x.mastodon.experimental_features = 'fasp,fetch_all_replies'
+    example.run
+    Rails.configuration.x.mastodon.experimental_features = original_value
+  end
+
+  describe '::fasp_enabled?' do
+    subject { described_class.fasp_enabled? }
+
+    it { is_expected.to be true }
+  end
+
+  describe '::fetch_all_replies_enabled?' do
+    subject { described_class.fetch_all_replies_enabled? }
+
+    it { is_expected.to be true }
+  end
+
+  describe '::unspecified_feature_enabled?' do
+    subject { described_class.unspecified_feature_enabled? }
+
+    it { is_expected.to be false }
+  end
+end