fix(server): fix some issues of the distributed scheduler

Author: Tsukilc

hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/task/DistributedTaskScheduler.java

@@ -436,17 +436,15 @@ private <V> HugeTask<V> waitUntilTaskCompleted(Id id, long seconds,
         long passes = seconds * 1000 / intervalMs;
         HugeTask<V> task = null;
         for (long pass = 0; ; pass++) {
-            HugeTask<V> previousTask = task;
-            task = this.taskWithoutResult(id);
-            if (task == null) {
-                // Task not found in DB
-                if (previousTask != null && previousTask.completed()) {
-                    // Task was completed and then deleted (ephemeral task case)
-                    assert previousTask.id().asLong() < 0L : previousTask.id();
+            try {
+                task = this.taskWithoutResult(id);
+            } catch (NotFoundException e) {
+                if (task != null && task.completed()) {
+                    assert task.id().asLong() < 0L : task.id();
                     sleep(intervalMs);
-                    return previousTask;
+                    return task;
                 }
-                throw new NotFoundException("Can't find task with id '%s'", id);
+                throw e;
             }
             if (task.completed()) {
                 // Wait for task result being set after status is completed

hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/task/TaskAndResultScheduler.java

@@ -219,6 +219,10 @@ protected <V> HugeTask<V> taskWithoutResult(Id id) {
             return HugeTask.fromVertex(vertex);
         });
 
+        if (result == null) {
+            throw new NotFoundException("Can't find task with id '%s'", id);
+        }
+
         return result;
     }