Improve coverage

indexing-service/src/test/java/org/apache/druid/indexing/seekablestream/supervisor/SeekableStreamSupervisorStateTest.java

@@ -1313,6 +1313,48 @@ public void testSupervisorResetOneTaskSpecificOffsetsWithCheckpoints() throws In
     validateSupervisorStateAfterResetOffsets(supervisor, resetOffsets, 0);
   }
 
+  @Test
+  public void testGetActiveRealtimeSequencePrefixes() throws IOException
+  {
+    EasyMock.expect(spec.isSuspended()).andReturn(false);
+
+    replayAll();
+
+    final TestSeekableStreamSupervisor supervisor = new TestSeekableStreamSupervisor();
+
+    // Spin off two active tasks with each task serving one partition.
+    supervisor.getIoConfig().setTaskCount(3);
+    supervisor.start();
+    supervisor.addTaskGroupToActivelyReadingTaskGroup(
+        supervisor.getTaskGroupIdForPartition("0"),
+        ImmutableMap.of("0", "5"),
+        Optional.absent(),
+        Optional.absent(),
+        ImmutableSet.of("task1"),
+        ImmutableSet.of()
+    );
+
+    supervisor.addTaskGroupToActivelyReadingTaskGroup(
+        supervisor.getTaskGroupIdForPartition("1"),
+        ImmutableMap.of("1", "6"),
+        Optional.absent(),
+        Optional.absent(),
+        ImmutableSet.of("task2"),
+        ImmutableSet.of()
+    );
+
+    supervisor.addTaskGroupToPendingCompletionTaskGroup(
+        supervisor.getTaskGroupIdForPartition("2"),
+        ImmutableMap.of("2", "100"),
+        Optional.absent(),
+        Optional.absent(),
+        ImmutableSet.of("task3"),
+        ImmutableSet.of()
+    );
+
+    Assert.assertEquals(3, supervisor.getActiveRealtimeSequencePrefixes().size());
+  }
+
   @Test
   public void testSupervisorResetSpecificOffsetsTasksWithCheckpoints() throws InterruptedException, IOException
   {