PERF: Memoize core svgs in memory to avoid expensive XML parsing.

The XML parsing of SVGs is done whenever the cache expires or on the
first load after a reboot. In one of our production instance, parsing
ranges from 30ms to 70ms which is not ideal. Instead, we've decided to
make a small memory trade off here by memoizing the core SVGs once on
boot to avoid parsing of the SVG files during the duration of a request.
The memozied hash will take up 57440 bytes or 0.05744 megabytes in size.

lib/discourse.rb

@@ -914,9 +914,9 @@ module Discourse
 
     schema_cache = ActiveRecord::Base.connection.schema_cache
 
+    RailsMultisite::ConnectionManagement.safe_each_connection do
       # load up schema cache for all multisite assuming all dbs have
       # an identical schema
-    RailsMultisite::ConnectionManagement.safe_each_connection do
       dup_cache = schema_cache.dup
       # this line is not really needed, but just in case the
       # underlying implementation changes lets give it a shot
@@ -948,6 +948,9 @@ module Discourse
       },
       Thread.new {
         LetterAvatar.image_magick_version
+      },
+      Thread.new {
+        SvgSprite.core_svgs
       }
     ].each(&:join)
   ensure

lib/svg_sprite/svg_sprite.rb

@@ -225,6 +225,7 @@ module SvgSprite
     get_set_cache("custom_svg_sprites_#{Theme.transform_ids(theme_ids).join(',')}") do
       custom_sprite_paths = Dir.glob("#{Rails.root}/plugins/*/svg-icons/*.svg")
 
+      if theme_ids.present?
         ThemeField.where(type_id: ThemeField.types[:theme_upload_var], name: THEME_SPRITE_VAR_NAME, theme_id: Theme.transform_ids(theme_ids))
           .pluck(:upload_id).each do |upload_id|
 
@@ -239,6 +240,7 @@ module SvgSprite
 
           custom_sprite_paths << original_path if original_path.present?
         end
+      end
 
       custom_sprite_paths
     end
@@ -275,7 +277,33 @@ module SvgSprite
   end
 
   def self.sprite_sources(theme_ids)
-    CORE_SVG_SPRITES | custom_svg_sprites(theme_ids)
+    sources = CORE_SVG_SPRITES
+
+    if theme_ids.present?
+      sources = sources + custom_svg_sprites(theme_ids)
+    end
+
+    sources
+  end
+
+  def self.core_svgs
+    @core_svgs ||= begin
+      symbols = {}
+
+      CORE_SVG_SPRITES.each do |filename|
+        svg_filename = "#{File.basename(filename, ".svg")}"
+
+        Nokogiri::XML(File.open(filename)) do |config|
+          config.options = Nokogiri::XML::ParseOptions::NOBLANKS
+        end.css('symbol').each do |sym|
+          icon_id = prepare_symbol(sym, svg_filename)
+          sym.attributes['id'].value = icon_id
+          symbols[icon_id] = sym.to_xml
+        end
+      end
+
+      symbols
+    end
   end
 
   def self.bundle(theme_ids = [])
@@ -288,15 +316,23 @@ License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL
 <svg xmlns='http://www.w3.org/2000/svg' style='display: none;'>
 """.dup
 
-    sprite_sources(theme_ids).each do |fname|
+    core_svgs.each do |icon_id, sym|
+      if icons.include?(icon_id)
+        svg_subset << sym
+      end
+    end
+
+    if theme_ids.present?
+      custom_svg_sprites(theme_ids).each do |fname|
         svg_file = Nokogiri::XML(File.open(fname)) do |config|
           config.options = Nokogiri::XML::ParseOptions::NOBLANKS
         end
 
         svg_filename = "#{File.basename(fname, ".svg")}"
 
-      svg_file.css('symbol').each do |sym|
+        svg_file.css("symbol").each do |sym|
           icon_id = prepare_symbol(sym, svg_filename)
+
           if icons.include? icon_id
             sym.attributes['id'].value = icon_id
             sym.css('title').each(&:remove)
@@ -304,6 +340,7 @@ License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL
           end
         end
       end
+    end
 
     svg_subset << '</svg>'
   end
@@ -405,6 +442,8 @@ License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL
   end
 
   def self.theme_icons(theme_ids)
+    return [] if theme_ids.blank?
+
     theme_icon_settings = []
 
     # Need to load full records for default values
@@ -422,6 +461,8 @@ License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL
   end
 
   def self.custom_icons(theme_ids)
+    return [] if theme_ids.blank?
+
     # Automatically register icons in sprites added via themes or plugins
     icons = []
     custom_svg_sprites(theme_ids).each do |fname|