Allow TimerTask to be safely restarted after shutdown and avoid duplicate tasks

lib/concurrent-ruby/concurrent/timer_task.rb

@@ -2,6 +2,7 @@
 require 'concurrent/concern/dereferenceable'
 require 'concurrent/concern/observable'
 require 'concurrent/atomic/atomic_boolean'
+require 'concurrent/atomic/atomic_fixnum'
 require 'concurrent/executor/executor_service'
 require 'concurrent/executor/ruby_executor_service'
 require 'concurrent/executor/safe_task_executor'
@@ -236,6 +237,7 @@ def execute
       synchronize do
         if @running.false?
           @running.make_true
+          @age.increment
           schedule_next_task(@run_now ? 0 : @execution_interval)
         end
       end
@@ -309,6 +311,7 @@ def ns_initialize(opts, &task)
       @task = Concurrent::SafeTaskExecutor.new(task)
       @executor = opts[:executor] || Concurrent.global_io_executor
       @running = Concurrent::AtomicBoolean.new(false)
+      @age = Concurrent::AtomicFixnum.new(0)
       @value = nil
 
       self.observers = Collection::CopyOnNotifyObserverSet.new
@@ -328,13 +331,15 @@ def ns_kill_execution
 
     # @!visibility private
     def schedule_next_task(interval = execution_interval)
-      ScheduledTask.execute(interval, executor: @executor, args: [Concurrent::Event.new], &method(:execute_task))
+      ScheduledTask.execute(interval, executor: @executor, args: [Concurrent::Event.new, @age.value], &method(:execute_task))
       nil
     end
 
     # @!visibility private
-    def execute_task(completion)
+    def execute_task(completion, age_when_scheduled)
       return nil unless @running.true?
+      return nil unless @age.value == age_when_scheduled
+
       start_time = Concurrent.monotonic_time
       _success, value, reason = @task.execute(self)
       if completion.try?

spec/concurrent/timer_task_spec.rb

@@ -1,5 +1,6 @@
 require_relative 'concern/dereferenceable_shared'
 require_relative 'concern/observable_shared'
+require 'concurrent/atomic/atomic_fixnum'
 require 'concurrent/timer_task'
 
 module Concurrent
@@ -116,6 +117,19 @@ def trigger_observable(observable)
           sleep(0.1)
           expect(task.shutdown).to be_truthy
         end
+
+        it 'will cancel pre-shutdown task even if restarted to avoid double-runs' do
+          counter = Concurrent::AtomicFixnum.new(0)
+          task = TimerTask.execute(execution_interval: 0.2, run_now: true) { counter.increment }
+          sleep 0.05
+          expect(counter.value).to eq 1
+
+          task.shutdown
+          task.execute
+
+          sleep 0.25
+          expect(counter.value).to eq 3
+        end
       end
     end