Add simple feature flag system

Allows specifying enabled features as a comma separated list
in `ENV['EXPERIMENTAL_FEATURES']`.

Can be queries as `Mastodon::Feature.<name>_enabled?`.

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'

lib/mastodon/feature.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module Mastodon::Feature
+  class << self
+    def enabled_features
+      @enabled_features ||=
+        (ENV['EXPERIMENTAL_FEATURES'] || '').split(',').map(&:strip)
+    end
+
+    def method_missing(name)
+      if respond_to_missing?(name)
+        feature = name.to_s.sub(/_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 +1,29 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Mastodon::Feature do
+  around do |example|
+    ClimateControl.modify EXPERIMENTAL_FEATURES: 'fasp,fetch_all_replies' do
+      example.run
+    end
+  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