PERF: remove superflous queries from initial page loads

stop doing expensive work to figure out discourse style sheet

app/models/category.rb

@@ -38,6 +38,8 @@ class Category < ActiveRecord::Base
   after_destroy :publish_categories_list
   after_update :rename_category_definition, if: :name_changed?
 
+  after_save :publish_discourse_stylesheet
+
   has_one :category_search_data
   belongs_to :parent_category, class_name: 'Category'
   has_many :subcategories, class_name: 'Category', foreign_key: 'parent_category_id'
@@ -362,6 +364,10 @@ SQL
       topic.update_column(:title, I18n.t("category.topic_prefix", category: name))
     end
   end
+
+  def publish_discourse_stylesheet
+    MessageBus.publish("/discourse_stylesheet", self.name)
+  end
 end
 
 # == Schema Information

app/models/color_scheme.rb

@@ -9,6 +9,7 @@ class ColorScheme < ActiveRecord::Base
   scope :current_version, ->{ where(versioned_id: nil) }
 
   after_destroy :destroy_versions
+  after_save :publish_discourse_stylesheet
 
   validates_associated :color_scheme_colors
 
@@ -93,6 +94,10 @@ class ColorScheme < ActiveRecord::Base
     ColorScheme.where(versioned_id: self.id).destroy_all
   end
 
+  def publish_discourse_stylesheet
+    MessageBus.publish("/discourse_stylesheet", self.name)
+  end
+
 end
 
 # == Schema Information

lib/sass/discourse_stylesheets.rb

@@ -7,13 +7,37 @@ class DiscourseStylesheets
   MANIFEST_FULL_PATH = "#{MANIFEST_DIR}/stylesheet-manifest"
 
   @lock = Mutex.new
+  @links = {}
+
+  def self.ensure_subscribed!
+    unless @subscribed
+      @lock.synchronize do
+        MessageBus.subscribe("/discourse_stylesheet") do |message|
+          @lock.synchronize do
+            @links[message.site_id] = nil
+          end
+        end
+        @subscribed = true
+      end
+    end
+  end
 
   def self.stylesheet_link_tag(target = :desktop)
-    builder = self.new(target)
+    ensure_subscribed!
     @lock.synchronize do
-      builder.compile unless File.exists?(builder.stylesheet_fullpath)
+      links = (@links[RailsMultisite::ConnectionManagement.current_db] ||= {})
-      builder.ensure_digestless_file
+      tag = links[target]
-      %[<link href="#{Rails.env.production? ? builder.stylesheet_relpath : builder.stylesheet_relpath_no_digest + '?body=1'}" media="all" rel="stylesheet" />].html_safe
+
+      if !tag
+        builder = self.new(target)
+        builder.compile unless File.exists?(builder.stylesheet_fullpath)
+        builder.ensure_digestless_file
+        tag = %[<link href="#{Rails.env.production? ? builder.stylesheet_relpath : builder.stylesheet_relpath_no_digest + '?body=1'}" media="all" rel="stylesheet" />].html_safe
+
+        links[target] = tag
+      end
+
+      tag
     end
   end