PERF: stop running background work between requests

Use a dedicated thread to run Scheduler::Defer

This avoids blocking of a worker during operations that require waiting.

In particular uploads risked blocking a unicorn.

This also add a queue "length" that discourse prometheus consumes.

config/initializers/099-defer.rb

@@ -0,0 +1 @@
+require_dependency 'scheduler/defer'

config/initializers/099-unicorn.rb

@@ -1,7 +0,0 @@
-require_dependency 'scheduler/defer'
-
-if defined? Unicorn::HttpServer
-  ObjectSpace.each_object(Unicorn::HttpServer) do |s|
-    s.extend(Scheduler::Defer::Unicorn)
-  end
-end

lib/scheduler/defer.rb

@@ -8,6 +8,10 @@ module Scheduler
       @thread = nil
     end
 
+    def length
+      @queue.length
+    end
+
     def pause
       stop!
       @paused = true
@@ -80,16 +84,6 @@ module Scheduler
   end
 
   class Defer
-
-    module Unicorn
-      def process_client(client)
-        Defer.pause
-        super(client)
-        Defer.do_all_work
-        Defer.resume
-      end
-    end
-
     extend Deferrable
     initialize
   end

spec/components/scheduler/defer_spec.rb

@@ -31,6 +31,8 @@ describe Scheduler::Defer do
       x = 2
     end
 
+    expect(@defer.length).to eq(1)
+
     @defer.do_all_work
 
     expect(x).to eq(2)