[移动开发]Android P update_engine分析（六）-- PostinstallRunnerAction的工作

上篇已经分析过了InstallPlanAction、DownloadAction、FileSystemVerityAction 三个Action的工作内容，还有一个PostinstallRunnerAction的在这篇中继续分析，每个Action都是从PerformAction开始的：

void PostinstallRunnerAction::PerformAction() {
  CHECK(HasInputObject());
  install_plan_ = GetInputObject();
  //确认下是否需要powerwash_, 如果有设置，一般在ApplyPayload的那个hashKey中会带有POWERWASH的key值
  if (install_plan_.powerwash_required) {
    if (hardware_->SchedulePowerwash()) {
      powerwash_scheduled_ = true;
    } else {
      return CompletePostinstall(ErrorCode::kPostinstallPowerwashError);
    }
  }

  // 初始化所有升级分区的权重
  partition_weight_.resize(install_plan_.partitions.size());
  total_weight_ = 0;
  for (size_t i = 0; i < install_plan_.partitions.size(); ++i) {
    partition_weight_[i] = install_plan_.partitions[i].run_postinstall;
    total_weight_ += partition_weight_[i];
  }
  accumulated_weight_ = 0;
  ReportProgress(0);

  PerformPartitionPostinstall();
}

上面主要做判断是否需要powerwash_，一般的升级方式不需要powerwash，然后初始升级分区的权重，同时保存这些权重值，之后开始分区升级：

void PostinstallRunnerAction::PerformPartitionPostinstall() {
  //判断是否在升级过程中
  if (!install_plan_.run_post_install) {
    LOG(INFO) << "Skipping post-install according to install plan.";
    return CompletePostinstall(ErrorCode::kSuccess);
  }
  //判断下载路么为空
  if (install_plan_.download_url.empty()) {
    LOG(INFO) << "Skipping post-install during rollback";
    return CompletePostinstall(ErrorCode::kSuccess);
  }

  //检查当前分区是否需要升级（升级文件大小与当前分区比较以及当前分区是否需要升级）
  while (current_partition_ < install_plan_.partitions.size() &&
         !install_plan_.partitions[current_partition_].run_postinstall) {
    VLOG(1) << "Skipping post-install on partition "
            << install_plan_.partitions[current_partition_].name;
    current_partition_++;
  }
  //当前要升给的分区等于分区数据，表示已经遍历完一遍分区了
  if (current_partition_ == install_plan_.partitions.size())
    return CompletePostinstall(ErrorCode::kSuccess);
  //获取当前分区，同时挂载当前需要升级分区，挂载到/postinstall 目录下
  const InstallPlan::Partition& partition =
      install_plan_.partitions[current_partition_];

  const string mountable_device =
      utils::MakePartitionNameForMount(partition.target_path);
  if (mountable_device.empty()) {
    LOG(ERROR) << "Cannot make mountable device from " << partition.target_path;
    return CompletePostinstall(ErrorCode::kPostinstallRunnerError);
  }

  // Perform post-install for the current_partition_ partition. At this point we
  // need to call CompletePartitionPostinstall to complete the operation and
  // cleanup.
#ifdef __ANDROID__
  fs_mount_dir_ = "/postinstall";
#else   // __ANDROID__
  base::FilePath temp_dir;
  TEST_AND_RETURN(base::CreateNewTempDirectory("au_postint_mount", &temp_dir));
  fs_mount_dir_ = temp_dir.value();
#endif  // __ANDROID__

  //再次确认，挂载路径是不是已经被挂载了
  if (utils::IsMountpoint(fs_mount_dir_)) {
    LOG(INFO) << "Found previously mounted filesystem at " << fs_mount_dir_;
    utils::UnmountFilesystem(fs_mount_dir_);
  }
  //将升级包分区设备赋值
  base::FilePath postinstall_path(partition.postinstall_path);
  if (postinstall_path.IsAbsolute()) {
    LOG(ERROR) << "Invalid absolute path passed to postinstall, use a relative"
                  "path instead: "
               << partition.postinstall_path;
    return CompletePostinstall(ErrorCode::kPostinstallRunnerError);
  }
  //设置挂载点的绝对路径，并检查
  string abs_path =
      base::FilePath(fs_mount_dir_).Append(postinstall_path).value();
  if (!base::StartsWith(
          abs_path, fs_mount_dir_, base::CompareCase::SENSITIVE)) {
    LOG(ERROR) << "Invalid relative postinstall path: "
               << partition.postinstall_path;
    return CompletePostinstall(ErrorCode::kPostinstallRunnerError);
  }

#ifdef __ANDROID__
  // In Chromium OS, the postinstall step is allowed to write to the block
  // device on the target image, so we don't mark it as read-only and should
  // be read-write since we just wrote to it during the update.

  // Mark the block device as read-only before mounting for post-install.
  //在挂载前，将要升级包的设备设置为只读
  if (!utils::SetBlockDeviceReadOnly(mountable_device, true)) {
    return CompletePartitionPostinstall(
        1, "Error marking the device " + mountable_device + " read only.");
  }
#endif  // __ANDROID__

  if (!utils::MountFilesystem(mountable_device,
                              fs_mount_dir_,
                              MS_RDONLY,
                              partition.filesystem_type,
                              constants::kPostinstallMountOptions)) {
    return CompletePartitionPostinstall(
        1, "Error mounting the device " + mountable_device);
  }

  LOG(INFO) << "Performing postinst (" << partition.postinstall_path << " at "
            << abs_path << ") installed on device " << partition.target_path
            << " and mountable device " << mountable_device;

  // Logs the file format of the postinstall script we are about to run. This
  // will help debug when the postinstall script doesn't match the architecture
  // of our build.
  LOG(INFO) << "Format file for new " << partition.postinstall_path
            << " is: " << utils::GetFileFormat(abs_path);

  //组合command：abs_path target_slot kPostinstallStatusFd 
  vector<string> command = {abs_path};
#ifdef __ANDROID__
  // In Brillo and Android, we pass the slot number and status fd.
  command.push_back(std::to_string(install_plan_.target_slot));
  command.push_back(std::to_string(kPostinstallStatusFd));
#else
  // Chrome OS postinstall expects the target rootfs as the first parameter.
  command.push_back(partition.target_path);
#endif  // __ANDROID__
//执行command指令，错误信息输出kRedirectStderrToStdout，输出管道使用kPostinstallStatusFd
  current_command_ = Subprocess::Get().ExecFlags(
      command,
      Subprocess::kRedirectStderrToStdout,
      {kPostinstallStatusFd},
      base::Bind(&PostinstallRunnerAction::CompletePartitionPostinstall,
                 base::Unretained(this)));
  // Subprocess::Exec should never return a negative process id.
  CHECK_GE(current_command_, 0);

  if (!current_command_) {
    CompletePartitionPostinstall(1, "Postinstall didn't launch");
    return;
  }

  //查看kPostinstallStatusFd是否可以打开
  progress_fd_ =
      Subprocess::Get().GetPipeFd(current_command_, kPostinstallStatusFd);
  int fd_flags = fcntl(progress_fd_, F_GETFL, 0) | O_NONBLOCK;
  if (HANDLE_EINTR(fcntl(progress_fd_, F_SETFL, fd_flags)) < 0) {
    PLOG(ERROR) << "Unable to set non-blocking I/O mode on fd " << progress_fd_;
  }
//监听progress_fd_, 从这里面读出升级的状态和进度
  progress_task_ = MessageLoop::current()->WatchFileDescriptor(
      FROM_HERE,
      progress_fd_,
      MessageLoop::WatchMode::kWatchRead,
      true,
      base::Bind(&PostinstallRunnerAction::OnProgressFdReady,
                 base::Unretained(this)));
}

上面执行partition.postinstall_path的指令，然后从执行的过程中，获取状态和进度，但从生成的update.zip包看查看，没有postinstall_path这个执行脚本。所以这一步在android升级中是没有做任何事情的。
下篇我们将重点的研究DownloadAction 里的工作和下载的流程，AB升级的实际分区升级的工作也是在这个里面进行的。