Add ability to register a jvm shutdown hook to close app

Add ability to register a jvm shutdown hook to close app

Differential Revision: https://phabricator.twitter.biz/D1229286

Author: Benjamin Erichsen

util-app/src/main/scala/com/twitter/app/App.scala

@@ -10,7 +10,9 @@ import java.util.concurrent.ConcurrentLinkedQueue
 import java.util.concurrent.atomic.AtomicReference
 import scala.collection.mutable
 import scala.jdk.CollectionConverters._
+import scala.sys.ShutdownHookThread
 import scala.util.control.NonFatal
+import com.twitter.finagle.util.enableJvmShutdownHook
 
 /**
  * A composable application trait that includes flag parsing as well
@@ -444,6 +446,11 @@ trait App extends ClosableOnce with CloseOnceAwaitably with Lifecycle {
         catch {
           case _: NoSuchMethodException => None
         }
+      if (enableJvmShutdownHook()) {
+        ShutdownHookThread {
+          Await.result(close(defaultCloseGracePeriod))
+        }
+      }
 
       // Invoke main() if it exists.
       mainMethod.foreach { method =>

util-app/src/main/scala/com/twitter/finagle/util/flags.scala

@@ -1,6 +1,7 @@
 package com.twitter.finagle.util
 
 import com.twitter.app
+import com.twitter.app.GlobalFlag
 
 /**
  * A deny list of implementations to ignore. Keys are the fully qualified class names.
@@ -44,3 +45,12 @@ object loadServiceIgnoredPaths
       Seq.empty[String],
       "Additional packages to be excluded from recursive directory scan"
     )
+
+/**
+ * This flag enables another way for graceful shutdown.
+ */
+object enableJvmShutdownHook
+    extends GlobalFlag[Boolean](
+      false,
+      "Registers a JVM shutdown hook which invokes close()"
+    )