ForceExpunge for missing task is a no-op, rather than a failure

Summary: Address #5142

Test Plan: add tests

Reviewers: jasongilanfarr, meichstedt

Subscribers: marathon-team

Differential Revision: https://phabricator.mesosphere.com/D498

src/main/scala/mesosphere/marathon/core/instance/update/InstanceUpdateOpResolver.scala

@@ -46,28 +46,36 @@ private[marathon] class InstanceUpdateOpResolver(
         createInstance(op.instanceId)(updater.reserve(op, clock.now()))
 
       case op: ForceExpunge =>
-        updateExistingInstance(op.instanceId)(updater.forceExpunge(_, clock.now()))
+        updateExistingInstance(op.instanceId)(
+          applyOperation = updater.forceExpunge(_, clock.now() ),
+          ifNotExist = InstanceUpdateEffect.Noop(_))
 
       case op: Revert =>
         Future.successful(updater.revert(op.instance))
     }
   }
 
+  private[this] def failIfNotExist(id: Instance.Id): InstanceUpdateEffect = {
+    InstanceUpdateEffect.Failure(
+      new IllegalStateException(s"$id of app [${id.runSpecId}] does not exist"))
+  }
+
   /**
     * Helper method that verifies that an instance already exists. If it does, it will apply the given function and
     * return the resulting effect; otherwise this will result in a. [[InstanceUpdateEffect.Failure]].
     * @param id ID of the instance that is expected to exist.
     * @param applyOperation the operation that shall be applied to the instance
     * @return The [[InstanceUpdateEffect]] that results from applying the given operation.
     */
-  private[this] def updateExistingInstance(id: Instance.Id)(applyOperation: Instance => InstanceUpdateEffect)(implicit ec: ExecutionContext): Future[InstanceUpdateEffect] = {
+  private[this] def updateExistingInstance(id: Instance.Id)(
+    applyOperation: Instance => InstanceUpdateEffect,
+    ifNotExist: Instance.Id => InstanceUpdateEffect = failIfNotExist(_))(implicit ec: ExecutionContext): Future[InstanceUpdateEffect] = {
     directInstanceTracker.instance(id).map {
       case Some(existingInstance) =>
         applyOperation(existingInstance)
 
       case None =>
-        InstanceUpdateEffect.Failure(
-          new IllegalStateException(s"$id of app [${id.runSpecId}] does not exist"))
+        ifNotExist(id)
     }
   }
 

src/test/scala/mesosphere/marathon/core/instance/update/InstanceUpdateOpResolverTest.scala

@@ -32,8 +32,8 @@ class InstanceUpdateOpResolverTest extends UnitTest {
       When("call taskTracker.task")
       verify(instanceTracker).instance(notExistingInstanceId)
 
-      Then("result in a Failure")
-      stateChange shouldBe a[InstanceUpdateEffect.Failure]
+      Then("result in a Noop")
+      stateChange shouldBe a[InstanceUpdateEffect.Noop]
 
       verifyNoMoreInteractions()
     }