client/allocrunner: depend on internal task state

Alloc runner already tracks tasks associated with alloc.  Here, we
become defensive by relying on the alloc runner tracked tasks, rather
than depend on server never updating the job unexpectedly.

client/allocrunner/alloc_runner.go

@@ -571,11 +571,11 @@ func (ar *allocRunner) clientAlloc(taskStates map[string]*structs.TaskState) *st
 		// Make sure we have marked the finished at for every task. This is used
 		// to calculate the reschedule time for failed allocations.
 		now := time.Now()
-		for _, task := range alloc.Job.LookupTaskGroup(alloc.TaskGroup).Tasks {
-			ts, ok := a.TaskStates[task.Name]
+		for taskName, _ := range ar.tasks {
+			ts, ok := a.TaskStates[taskName]
 			if !ok {
 				ts = &structs.TaskState{}
-				a.TaskStates[task.Name] = ts
+				a.TaskStates[taskName] = ts
 			}
 			if ts.FinishedAt.IsZero() {
 				ts.FinishedAt = now