DEV: Detect circular site setting dependencies (#38165)

### Background

In #36061, dependent site settings were introduced through the
`depends_on` key. This is used internally to construct a dependency
graph and sort settings in order-of-dependency before bulk updates.

However, no safeguards were introduced to avoid circular dependencies.
This means a `TSort::Cyclic` could come along and ruin the day at
arbitrary points during runtime.

### What is this change?

This change does two things:

1. Rescue, format, and re-raise `TSort::Cyclic` exceptions.
2. Eagerly order settings on app reload to catch cyclic dependencies
up-front.

config/initializers/005-site_settings.rb

@@ -23,6 +23,9 @@ Rails.application.config.to_prepare do
                SiteSetting.push_api_secret_key.length == 32
         SiteSetting.push_api_secret_key = SecureRandom.hex
       end
+
+      # Check for circular dependencies in site settings.
+      SiteSetting.type_supervisor.dependencies.order
     rescue ActiveRecord::StatementInvalid
       # This will happen when migrating a new database
     end

lib/site_settings/dependency_graph.rb

@@ -6,6 +6,8 @@ end
 class SiteSettings::DependencyGraph
   include TSort
 
+  CircularDependency = Class.new(StandardError)
+
   attr_reader :dependencies, :behaviors
 
   def initialize(dependencies = {})
@@ -44,6 +46,15 @@ class SiteSettings::DependencyGraph
 
   def order
     @order ||= tsort
+  rescue TSort::Cyclic
+    cycles =
+      strongly_connected_components.reject(&:one?).map { |cycle| cycle.join(" <-> ") }.join("\n")
+
+    raise CircularDependency.new(<<~MESSAGE)
+      Circular dependencies in site settings:
+
+        #{cycles}
+    MESSAGE
   end
 
   private

lib/site_settings/type_supervisor.rb

@@ -159,7 +159,7 @@ class SiteSettings::TypeSupervisor
       @validators[name] = { class: validator_type, opts: validator_opts }
     end
 
-    @dependencies[name] = opts[:depends_on] || []
+    @dependencies[name] = (opts[:depends_on] || []).map(&:to_sym)
     @dependencies.change_behavior(name, opts[:depends_behavior]) if opts[:depends_behavior]
   end
 

spec/lib/site_settings/dependency_graph_spec.rb

@@ -16,6 +16,21 @@ RSpec.describe SiteSettings::DependencyGraph do
     it "topologically sorts the dependencies" do
       expect(depencency_graph.order).to match_array(%i[foo qux bar baz quux])
     end
+
+    context "with circular dependencies" do
+      let(:dependencies) { { foo: [:bar], bar: [:foo] } }
+
+      it "raises an exception" do
+        expect { depencency_graph.order }.to raise_error(
+          described_class::CircularDependency,
+          <<~MESSAGE,
+          Circular dependencies in site settings:
+
+            foo <-> bar
+        MESSAGE
+        )
+      end
+    end
   end
 
   describe "#dependents" do

spec/services/site_setting/update_spec.rb

@@ -128,6 +128,8 @@ RSpec.describe SiteSetting::Update do
     end
 
     context "when updating dependent settings" do
+      before { SiteSetting.allow_user_locale = false }
+
       let(:settings) do
         [
           # The first setting depends on the second one, which will fail