Prevent ThreadedWorker from leaking enqueue lifecycle callbacks

lib/delayed_job/threaded_worker.rb

@@ -1,5 +1,18 @@
 module Delayed
   class ThreadedWorker < Delayed::Worker
+    # Delayed::Plugin callbacks register against the base Delayed::Worker.lifecycle,
+    # but setup_lifecycle rebuilds the lifecycle on the class it is called on. On this
+    # subclass it would reset a different lifecycle than the one plugins append to,
+    # so every ThreadedWorker.new would leak another callback into the base lifecycle.
+    # Delegate to the base class so each instantiation rebuilds the same lifecycle.
+    def self.setup_lifecycle
+      Delayed::Worker.setup_lifecycle
+    end
+
+    def self.lifecycle
+      Delayed::Worker.lifecycle
+    end
+
     def initialize(options={})
       super
       @num_threads = options[:num_threads]

spec/unit/lib/delayed_job/threaded_worker_spec.rb

@@ -22,6 +22,16 @@
       worker = Delayed::ThreadedWorker.new({ num_threads: 2 })
       expect(worker.instance_variable_get(:@grace_period_seconds)).to eq(30)
     end
+
+    it 'does not accumulate enqueue lifecycle callbacks across instantiations' do
+      before_enqueue_callbacks = lambda do
+        Delayed::Worker.lifecycle.instance_variable_get(:@callbacks)[:enqueue].instance_variable_get(:@before).size
+      end
+
+      baseline = before_enqueue_callbacks.call
+      5.times { Delayed::ThreadedWorker.new(options) }
+      expect(before_enqueue_callbacks.call).to eq(baseline)
+    end
   end
 
   describe '#start' do