PERF: guard so we only create one optimized image per process

This protectd sidekiq and other cases where optimized images are created on
demand so they do not dominate the machine.

app/models/optimized_image.rb

@@ -9,11 +9,22 @@ class OptimizedImage < ActiveRecord::Base
   # BUMP UP if optimized image algorithm changes
   VERSION = 1
 
+  def self.lock(upload_id, width, height)
+    # note, the extra lock here ensures we only optimize one image per process
+    # this can very easily lead to runaway CPU so slowing it down is beneficial
+    @mutex ||= Mutex.new
+    @mutex.synchronize do
+      DistributedMutex.synchronize("optimized_image_#{upload_id}_#{width}_#{height}") do
+        yield
+      end
+    end
+  end
+
   def self.create_for(upload, width, height, opts = {})
     return unless width > 0 && height > 0
     return if upload.try(:sha1).blank?
 
-    DistributedMutex.synchronize("optimized_image_#{upload.id}_#{width}_#{height}") do
+    lock(upload.id, width, height) do
       # do we already have that thumbnail?
       thumbnail = find_by(upload_id: upload.id, width: width, height: height)