alerts-server: Add ShutdownableTaskRunner

Using this class we can create task runners that work with
the ShutdownHandler to perform graceful shutdowns.

alerts-server/app/com/alexitc/coinalerts/tasks/ShutdownableTaskRunner.scala

@@ -0,0 +1,67 @@
+package com.alexitc.coinalerts.tasks
+
+import akka.actor.ActorSystem
+import com.alexitc.coinalerts.core.ShutdownHandler
+import org.slf4j.LoggerFactory
+
+import scala.concurrent.ExecutionContext.Implicits.global
+import scala.concurrent.Future
+import scala.concurrent.duration.FiniteDuration
+import scala.util.control.NonFatal
+
+trait ShutdownableTaskRunner {
+
+  private val logger = LoggerFactory.getLogger(this.getClass)
+
+  protected def shutdownHandler: ShutdownHandler
+
+  protected def actorSystem: ActorSystem
+
+  protected def initialDelay: FiniteDuration
+
+  protected def interval: FiniteDuration
+
+  protected def run(): Future[Unit]
+
+  @volatile
+  private var running = false
+
+  protected def register() = {
+    logger.info("Registering task")
+
+    shutdownHandler.addShutdownHook(() => Future {
+
+      if (running) {
+        logger.info("Waiting for task to complete")
+      }
+
+      // if we are shutting down, do we care no making a mess with threads? I hope we don't
+      scala.concurrent.blocking {
+        while (running) {
+          Thread.sleep(1000)
+        }
+      }
+    })
+
+    val _ = actorSystem.scheduler.schedule(
+      initialDelay = initialDelay,
+      interval = interval) { execute() }
+  }
+
+  private def execute() = {
+    if (!shutdownHandler.isShuttingDown) {
+      running = true
+
+      logger.info("Running task")
+
+      run()
+          .recover {
+            case NonFatal(ex) =>
+              logger.error("Failed to run task", ex)
+          }
+          .foreach { _ =>
+            running = false
+          }
+    }
+  }
+}