PERF: Defer setting of distributed cache in performance critical paths.

Setting a key/value pair in DistributedCache involves waiting on the
write to Redis to finish. In most cases, we don't need to wait on the
setting of the cache to finish. We just need to take our return value
and move on.

app/models/theme.rb

@@ -155,10 +155,10 @@ class Theme < ActiveRecord::Base
   end
 
   def self.get_set_cache(key, &blk)
-    if @cache.hash.key? key.to_s
-      return @cache[key]
-    end
-    @cache[key] = blk.call
+    return @cache[key] if @cache[key]
+    value = blk.call
+    @cache.defer_set(key, value)
+    value
   end
 
   def self.theme_ids

lib/distributed_cache.rb

@@ -11,4 +11,12 @@ class DistributedCache < MessageBus::DistributedCache
       app_version: Discourse.git_version
     )
   end
+
+  # Defer setting of the key in the cache for performance critical path to avoid
+  # waiting on MessageBus to publish the message which involves writing to Redis.
+  def defer_set(k, v)
+    Scheduler::Defer.later("#{@key}_set") do
+      self[k] = v
+    end
+  end
 end

lib/site_icon_manager.rb

@@ -62,7 +62,10 @@ module SiteIconManager
   private
 
   def self.get_set_cache(key)
-    @cache[key] ||= yield
+    return @cache[key] if @cache[key]
+    value = yield
+    @cache.defer_set(key, value)
+    value
   end
 
   def self.resolve_original(info)

lib/stylesheet/manager.rb

@@ -88,14 +88,16 @@ class Stylesheet::Manager
             builder.compile unless File.exists?(builder.stylesheet_fullpath)
             href = builder.stylesheet_path(current_hostname)
           end
-          cache[cache_key] = href
+
+          cache.defer_set(cache_key, href)
         end
 
         data[:theme_id] = theme_id if theme_id.present? && data[:theme_id].blank?
         data[:new_href] = href
         stylesheets << data
       end
-      cache[array_cache_key] = stylesheets.freeze
+
+      cache.defer_set(array_cache_key, stylesheets.freeze)
       stylesheets
     end
   end
@@ -128,7 +130,7 @@ class Stylesheet::Manager
 
     href = builder.stylesheet_path(current_hostname)
     stylesheet[:new_href] = href
-    cache[cache_key] = stylesheet.freeze
+    cache.defer_set(cache_key, stylesheet.freeze)
     stylesheet
   end
 

lib/svg_sprite/svg_sprite.rb

@@ -478,7 +478,10 @@ License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL
   end
 
   def self.get_set_cache(key)
-    cache[key] ||= yield
+    return cache[key] if cache[key]
+    value = yield
+    cache.defer_set(key, value)
+    value
   end
 
   def self.cache