Fixes issue ##2504 - Login into Zammad not possible, application server puma will raise 100% or more cpu.

app/models/store.rb

@@ -281,6 +281,10 @@ returns
     file.provider
   end
 
+  def self.semaphore
+    @semaphore ||= Mutex.new
+  end
+
   private
 
   def image_resize(content, width)
@@ -295,6 +299,9 @@ returns
     temp_file.binmode
     temp_file.write(content)
     temp_file.close
+
+    temp_file_resize = nil
+    self.class.semaphore.synchronize do
       image = Rszr::Image.load(temp_file.path)
 
       # do not resize image if image is smaller or already same size
@@ -308,6 +315,9 @@ returns
       image.resize!(width, :auto)
       temp_file_resize = ::Tempfile.new.path
       image.save(temp_file_resize)
+      image.send(:handle).finalize!
+    end
+
     image_resized = ::File.binread(temp_file_resize)
 
     Cache.write(cache_key, image_resized, { expires_in: 6.months })