# 下载任务切换到后台下载中断

## 问题现象

下载任务正常启动，但是应用切换到后台后下载中断。

## 定位过程

根据HiLog日志，进行问题定位。

1. 搜索关键字FOREGROUND、BACKGROUND，如果出现了类似日志且附近有"应用包名"的日志，则说明该应用此时进入了前台或后台，即故障发生点：

   ```txt
      I C01406/render_service/OHOS::RS: RSSurfaceRenderNodeDrawable::OnDraw name:BackgroundBlurView2, the number of total processedNodes: 2
      W C03301/'应用包名'/LIBUV: loop:394697872256, flag:complete, active reqs:100
      I C015B0/'应用包名'/NETSTACK: [http_exec.cpp:298] CURL_TRACE: type = 7, data = STATE: INIT => CONNECT handle f000
      I C01406/render_service/skia: RSHardwareThread::CommitAndReleaseLayers task execute, SurfaceName: [RCDBottomSurfaceNode7030861463553, RCDTopSurfaceNode7030861463553, DisplayNode]
      I C015B0/'应用包名'/NETSTACK: [http_exec.cpp:298] CURL_TRACE: type = 7, data = STATE: CONNECT => CONNECTING handle f000
      I C015B0/'应用包名'/NETSTACK: [http_exec.cpp:298] CURL_TRACE: type = 7, data = STATE: CONNECTING => PROTOCONNECT handle f000
      I C015B0/'应用包名'/NETSTACK: [http_exec.cpp:298] CURL_TRACE: type = 7, data = STATE: PROTOCONNECT => DO handle f000
      W C01719/hiview/ffrt: 73810:DevFFTSSchedClose:116 close ffts sched device fd:4
      I C015B0/'应用包名'/NETSTACK: [http_exec.cpp:298] CURL_TRACE: type = 7, data = STATE: DO => DID handle f000
      W C01719/useriam/ffrt: 24152:FFRTQosApplyForOther:244 tid 59557, Operation not permitted, ret:-1, eno:1
      I C015B0/'应用包名'/NETSTACK: [http_exec.cpp:298] CURL_TRACE: type = 7, data = STATE: DID => PERFORMING handle f000
      E C01719/hiview/ffrt: 73811:DevFFTSOpen:73 open ffts device error, fd= -1, errno=12.
   ```

2. 在故障位置附近发现报错信息为：

   ```txt
       W C01719/hiview/ffrt: 73810:DevFFTSSchedClose:116 close ffts sched device fd:4 code:1001
       W C01719/useriam/ffrt: 24152:FFRTQosApplyForOther:244 tid 59557, Operation not permitted, ret:-1, eno:1
       E C01719/hiview/ffrt: 73811:DevFFTSOpen:73 open ffts device error, fd= -1, errno=12.
   ```

   日志显示：
   * 尝试关闭FFRT设备时失败（fd:4）。
   * FFRTQosApplyForOther报错"Operation not permitted"。
   * DevFFTSOpen失败，返回errno=12（ENOMEM，内存不足）。
3. 且故障点附近日志显示LiveEventBus频繁调用，多个下载任务同时进行。

   ```txt
      -------> LiveEventBusCore get <------
      [LiveEventBus]    postValue:[object Object] observer:[object Object] owner:3
      -------> LiveEventBusCore get <------
      [LiveEventBus]    message received: [object Object]
      -------> LiveEventBusCore get <------
      [LiveEventBus]    postValue:[object Object] observer:[object Object] owner:3
      -------> LiveEventBusCore get <------
      [LiveEventBus]    message received: [object Object]
      -------> LiveEventBusCore get <------
      [LiveEventBus]    postValue:[object Object] observer:[object Object] owner:3
      -------> LiveEventBusCore get <------
      [LiveEventBus]    message received: [object Object]
      -------> LiveEventBusCore get <------
      [LiveEventBus]    postValue:[object Object] observer:[object Object] owner:3
   ```

## 分析结论

应用未考虑高负载下的处理逻辑进行优化，高并发下载导致手机内存耗尽，下载线程崩溃暂停。

## 修改建议

对高并发下的下载场景进行优化，使用TaskPool提供的多任务优先级调度机制，根据业务需求合理设置优先级。参考[应用并发设计](https://developer.huawei.com/consumer/cn/doc/best-practices/bpta-app-concurrency-design)。