Guard against the case of maintenance tasks not having started before shutdown

And add an observer for the Supervisor's maintenance task and the
heartbeat task as well.

Author: rosa

lib/solid_queue/dispatcher/concurrency_maintenance.rb

@@ -25,7 +25,7 @@ def start
     end
 
     def stop
-      @concurrency_maintenance_task.shutdown
+      @concurrency_maintenance_task&.shutdown
     end
 
     private

lib/solid_queue/log_subscriber.rb

@@ -76,7 +76,7 @@ def shutdown_process(event)
       hostname: process.hostname
     }.merge(process.metadata)
 
-    info formatted_event(event, action: "Shut down #{process.kind}", **attributes)
+    info formatted_event(event, action: "Shutdown #{process.kind}", **attributes)
   end
 
   def register_process(event)

lib/solid_queue/processes/registrable.rb

@@ -36,6 +36,10 @@ def launch_heartbeat
           wrap_in_app_executor { heartbeat }
         end
 
+        @heartbeat_task.add_observer do |_, _, error|
+          handle_thread_error(error) if error
+        end
+
         @heartbeat_task.execute
       end
 

lib/solid_queue/supervisor/maintenance.rb

@@ -10,7 +10,13 @@ module Supervisor::Maintenance
       def launch_maintenance_task
         @maintenance_task = Concurrent::TimerTask.new(run_now: true, execution_interval: SolidQueue.process_alive_threshold) do
           prune_dead_processes
-        end.tap(&:execute)
+        end
+
+        @maintenance_task.add_observer do |_, _, error|
+          handle_thread_error(error) if error
+        end
+
+        @maintenance_task.execute
       end
 
       def stop_maintenance_task