OTA A/B 分区升级 update_engine简介

近期猛然发现公司的项目都已经换成了AB升级，AB升级之前一直有所了解，只是一直都没有去仔细查看过其具体升级流程，这两天抽空捋了捋，简单整理下。

AB升级（谷歌官网叫法无缝更新）是自android7.0开始新增的一种android设备升级方式，只是这种方式对设备存储要求高，所以之前没有怎么被国内手机厂商采用。
 

简而言之：系统同时存在两套system分区，一套处于休眠状态不可使用，一套处于使用状态，两者通过slot的概念来做区分，在设备启动引导阶段通过特殊标记位确定启动哪个system，当有可用升级版本时候，客户端将升级包下载下来，或者将下载地址请求下来，然后通过update_engine将当前没有在使用的一套system升级到最新版本，然后修改启动标志位，在下次启动的时候，就进入升级到最新版本的那套system系统，这样做的好处就是省去了recovery升级过程的耗时，规避了recovery升级过程中发生意外中断导致设备无法开机的风险。现将其主要逻辑做简单整理归纳
 

AB升级的主要实现过程在udate_engine中，主要代码集中在/system/update_engine路径下

主要分为两部分：

1.update_engine的初始化过程

2.java层调用升级接口执行升级过程

1.update_engine的初始化过程

首先查看目录下的Android.bp，这里只列举重点内容

// update_engine (type: executable)
// ========================================================
// update_engine daemon.
cc_binary {
    name: “update_engine”,
    defaults: [
        “ue_defaults“,
        “libupdate_engine_android_exports“,
    ],
 
    static_libs: [“libupdate_engine_android“],
    required: [
        “cacerts_google“,
        “otacerts“,
    ],
 
    srcs: [“main.cc“],
    init_rc: [“update_engine.rc“],
}

update_engine.rc内容如下：

service update_engine /system/bin/update_engine —logtostderr —logtofile –foreground
    class late_start
    user root
    group root system wakelock inet cache media_rw
    writepid /dev/cpuset/system–background/tasks /dev/blkio/background/tasks
    disabled
 
on property:ro.boot.slot_suffix=*
    enable update_engine

update_engine服务在init进程中启动，启动后运行main.cc中的main函数：

int main(int argc, char** argv) {
  ……运行UpdateEngineDaemon，这里用了c++11的新特性默认构造函数
  chromeos_update_engine::UpdateEngineDaemon update_engine_daemon;
  int exit_code = update_engine_daemon.Run();
 
  chromeos_update_engine::Subprocess::Get().FlushBufferedLogsAtExit();
 
  LOG(INFO) << “A/B Update Engine terminating with exit code ” << exit_code;
  return exit_code;
}

UpdateEngineDaemon 继承自 brillo::Daemon （externallibbrillobrillodaemons）
会运行到UpdateEngineDaemon::OnInit中：

systemupdate_enginedaemon.cc

int UpdateEngineDaemon::OnInit() {
  
  subprocess_.Init(this);
 
  int exit_code = Daemon::OnInit();
  if (exit_code != EX_OK)
    return exit_code;
 
#if USE_BINDER
  android::BinderWrapper::Create();
  binder_watcher_.Init();
#endif  // USE_BINDER
//这里初始化daemon_state_，因为是USE_BINDER，所以初始化结果是DaemonStateAndroid
#if USE_OMAHA  
  RealSystemState* real_system_state = new RealSystemState();
  daemon_state_.reset(real_system_state);
  LOG_IF(ERROR, !real_system_state->Initialize())
      << “Failed to initialize system state.”;
#else  // !USE_OMAHA
  DaemonStateAndroid* daemon_state_android = new DaemonStateAndroid();
  daemon_state_.reset(daemon_state_android);
  LOG_IF(ERROR, !daemon_state_android->Initialize())
      << “Failed to initialize system state.”;
#endif  // USE_OMAHA
//这里初始化binder_service_，最终将BinderUpdateEngineBrilloService注册到ServiceManager中，服务名android.os.UpdateEngineService
#if USE_BINDER
  // Create the Binder Service.
#if USE_OMAHA
  binder_service_ = new BinderUpdateEngineBrilloService{real_system_state};
#else   // !USE_OMAHA
  binder_service_ = new BinderUpdateEngineAndroidService{
      daemon_state_android->service_delegate()};
#endif  // USE_OMAHA
  auto binder_wrapper = android::BinderWrapper::Get();
  if (!binder_wrapper->RegisterService(binder_service_->ServiceName(),
                                       binder_service_)) {
    LOG(ERROR) << “Failed to register binder service.”;
  }
 
  daemon_state_->AddObserver(binder_service_.get());
#endif  // USE_BINDER
 
#if USE_DBUS
  // Create the DBus service.
  dbus_adaptor_.reset(new UpdateEngineAdaptor(real_system_state));
  daemon_state_->AddObserver(dbus_adaptor_.get());
 
  dbus_adaptor_->RegisterAsync(base::Bind(&amp;UpdateEngineDaemon::OnDBusRegistered,
                                          base::Unretained(this)));
  LOG(INFO) << “Waiting for DBus object to be registered.”;
#else   //开始执行升级
  daemon_state_->StartUpdater();
#endif  // USE_DBUS
  return EX_OK;
}

OnInit方法中执行的主要操作就是于binder服务关联起来，将BinderUpdateEngineBrilloService以android.os.UpdateEngineService的名称注册到serviceManager中，
而BinderUpdateEngineBrilloService的执行又是放到了UpdateAttempterAndroid中，一些主要的执行类关系图如下：
 

其主要流程时序图如下：

 进入daemon_state_->StartUpdater()方法，systemupdate_enginedaemon_state_android.h如下：

class DaemonStateAndroid : public DaemonStateInterface {
 public:
  DaemonStateAndroid() = default;
  ~DaemonStateAndroid() override = default;
 
  bool Initialize();
 
  // DaemonStateInterface overrides.
  bool StartUpdater() override;
  void AddObserver(ServiceObserverInterface* observer) override;
  void RemoveObserver(ServiceObserverInterface* observer) override;
 
  const std::set<ServiceObserverInterface*>&amp; service_observers() override {
    return service_observers_;
  }
 
  // Return a pointer to the service delegate.
  ServiceDelegateAndroidInterface* service_delegate();
 
 protected:
  std::set<ServiceObserverInterface*> service_observers_;
 
  // Interface for the boot control functions.
  std::unique_ptr<BootControlInterface> boot_control_;
 
  // Interface for the hardware functions.
  std::unique_ptr<HardwareInterface> hardware_;
 
  // Interface for persisted store.
  std::unique_ptr<PrefsInterface> prefs_;
 
  // The main class handling the updates.
  std::unique_ptr<UpdateAttempterAndroid> update_attempter_;
 
  // OpenSSLWrapper and CertificateChecker used for checking changes in SSL
  // certificates.
  OpenSSLWrapper openssl_wrapper_;
  std::unique_ptr<CertificateChecker> certificate_checker_;
};

systemupdate_enginedaemon_state_android.cc中部分方法如下：

bool DaemonStateAndroid::StartUpdater() {
  //update_attempter_的初始化在Initialize方法中，在UpdateEngineDaemon::OnInit被调用
  update_attempter_->Init();
  return true;
}
 
bool DaemonStateAndroid::Initialize() {
  boot_control_ = boot_control::CreateBootControl();
  if (!boot_control_) {
    LOG(WARNING) << “Unable to create BootControl instance, using stub “
                 << “instead. All update attempts will fail.”;
    boot_control_.reset(new BootControlStub());
  }
 
  hardware_ = hardware::CreateHardware();
  if (!hardware_) {
    LOG(ERROR) << “Error initializing the HardwareInterface.”;
    return false;
  }
 
  LOG_IF(INFO, !hardware_->IsNormalBootMode()) << “Booted in dev mode.”;
  LOG_IF(INFO, !hardware_->IsOfficialBuild()) << “Booted non–official build.”;
 
  // Initialize prefs.
  base::FilePath non_volatile_path;
  // TODO(deymo): Fall back to in-memory prefs if there‘s no physical directory
  // available.
  if (!hardware_->GetNonVolatileDirectory(&amp;non_volatile_path)) {
    LOG(ERROR) << “Failed to get a non-volatile directory.”;
    return false;
  }
  Prefs* prefs = new Prefs();
  prefs_.reset(prefs);
  if (!prefs->Init(non_volatile_path.Append(kPrefsSubDirectory))) {
    LOG(ERROR) << “Failed to initialize preferences.”;
    return false;
  }
  // The CertificateChecker singleton is used by the update attempter.
  certificate_checker_.reset(
      new CertificateChecker(prefs_.get(), &amp;openssl_wrapper_));
  certificate_checker_->Init();
  //UpdateAttempter初始化
  update_attempter_.reset(new UpdateAttempterAndroid(
      this, prefs_.get(), boot_control_.get(), hardware_.get()));
  return true;
}

systemupdate_engineupdate_attempter_android.h中定义了升级过程对java层通过binder提供的全部接口

进入UpdateAttempterAndroid的Init方法中：

void UpdateAttempterAndroid::Init() {
  // In case of update_engine restart without a reboot we need to restore the
  // reboot needed state.
  if (UpdateCompletedOnThisBoot()) {
    SetStatusAndNotify(UpdateStatus::UPDATED_NEED_REBOOT);
  } else {
    SetStatusAndNotify(UpdateStatus::IDLE);
    UpdatePrefsAndReportUpdateMetricsOnReboot();
#ifdef _UE_SIDELOAD
    LOG(INFO) << “Skip ScheduleCleanupPreviousUpdate in sideload because “
              << “ApplyPayload will call it later.”;
#else
    ScheduleCleanupPreviousUpdate();
#endif
  }
}

这里没有过多工作，就是判定状态然后发出通知

2.java层调用升级接口执行升级过程

在UpdateEngine.java中调用applyPayload方法：

public void applyPayload(String url, long offset, long size, String[] headerKeyValuePairs) {
        try {
            mUpdateEngine.applyPayload(url, offset, size, headerKeyValuePairs);
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }
 
public UpdateEngine() {
        mUpdateEngine = IUpdateEngine.Stub.asInterface(
                ServiceManager.getService(UPDATE_ENGINE_SERVICE));
}

这里的mUpdateEngine就是BinderUpdateEngineBrilloService服务，最终执行在UpdateAttempterAndroid中：

systemupdate_engineupdate_attempter_android.cc

bool UpdateAttempterAndroid::ApplyPayload(
    const string&amp; payload_url,
    int64_t payload_offset,
    int64_t payload_size,
    const vector<string>&amp; key_value_pair_headers,
    brillo::ErrorPtr* error) {
    //检查状态，获取传入的参数组
  if (status_ == UpdateStatus::UPDATED_NEED_REBOOT) {
    return LogAndSetError(
        error, FROM_HERE, “An update already applied, waiting for reboot”);
  }
  if (processor_->IsRunning()) {
    return LogAndSetError(
        error, FROM_HERE, “Already processing an update, cancel it first.”);
  }
  DCHECK(status_ == UpdateStatus::IDLE);
 
  std::map<string, string> headers;
  if (!ParseKeyValuePairHeaders(key_value_pair_headers, &amp;headers, error)) {
    return false;
  }
 
  string payload_id = GetPayloadId(headers);
 
  //初始化InstallPlan
  install_plan_ = InstallPlan();
 
  install_plan_.download_url = payload_url;
  install_plan_.version = “”;
  base_offset_ = payload_offset;
  InstallPlan::Payload payload;
  payload.size = payload_size;
  if (!payload.size) {
    if (!base::StringToUint64(headers[kPayloadPropertyFileSize],
                              &amp;payload.size)) {
      payload.size = 0;
    }
  }
  if (!brillo::data_encoding::Base64Decode(headers[kPayloadPropertyFileHash],
                                           &amp;payload.hash)) {
    LOG(WARNING) << “Unable to decode base64 file hash: “
                 << headers[kPayloadPropertyFileHash];
  }
  if (!base::StringToUint64(headers[kPayloadPropertyMetadataSize],
                            &payload.metadata_size)) {
    payload.metadata_size = 0;
  }
  // The |payload.type| is not used anymore since minor_version 3.
  payload.type = InstallPayloadType::kUnknown;
  install_plan_.payloads.push_back(payload);
 
  // The |public_key_rsa| key would override the public key stored on disk.
  install_plan_.public_key_rsa = “”;
 
  install_plan_.hash_checks_mandatory = hardware_->IsOfficialBuild();
  install_plan_.is_resume = !payload_id.empty() &&
                            DeltaPerformer::CanResumeUpdate(prefs_, payload_id);
  if (!install_plan_.is_resume) {
    // No need to reset dynamic_partititon_metadata_updated. If previous calls
    // to AllocateSpaceForPayload uses the same payload_id, reuse preallocated
    // space. Otherwise, DeltaPerformer re-allocates space when the payload is
    // applied.
    if (!DeltaPerformer::ResetUpdateProgress(
            prefs_,
            false /* quick */,
            true /* skip_dynamic_partititon_metadata_updated */)) {
      LOG(WARNING) << “Unable to reset the update progress.”;
    }
    if (!prefs_->SetString(kPrefsUpdateCheckResponseHash, payload_id)) {
      LOG(WARNING) << “Unable to save the update check response hash.”;
    }
  }
  install_plan_.source_slot = GetCurrentSlot();
  install_plan_.target_slot = GetTargetSlot();
 
  install_plan_.powerwash_required =
      GetHeaderAsBool(headers[kPayloadPropertyPowerwash], false);
 
  install_plan_.switch_slot_on_reboot =
      GetHeaderAsBool(headers[kPayloadPropertySwitchSlotOnReboot], true);
 
  install_plan_.run_post_install =
      GetHeaderAsBool(headers[kPayloadPropertyRunPostInstall], true);
 
  // Skip writing verity if we’re resuming and verity has already been written.
  install_plan_.write_verity = true;
  if (install_plan_.is_resume && prefs_->Exists(kPrefsVerityWritten)) {
    bool verity_written = false;
    if (prefs_->GetBoolean(kPrefsVerityWritten, &verity_written) &&
        verity_written) {
      install_plan_.write_verity = false;
    }
  }
  NetworkId network_id = kDefaultNetworkId;
  if (!headers[kPayloadPropertyNetworkId].empty()) {
    if (!base::StringToUint64(headers[kPayloadPropertyNetworkId],
                              &network_id)) {
      return LogAndSetError(
          error,
          FROM_HERE,
          “Invalid network_id: ” + headers[kPayloadPropertyNetworkId]);
    }
    if (!network_selector_->SetProcessNetwork(network_id)) {
      return LogAndSetError(
          error,
          FROM_HERE,
          “Unable to set network_id: ” + headers[kPayloadPropertyNetworkId]);
    }
  }
  LOG(INFO) << “Using this install plan:”;
  install_plan_.Dump();
·//fetcher用来给到DownloadAction执行下载
  HttpFetcher* fetcher = nullptr;
  if (FileFetcher::SupportedUrl(payload_url)) {
    DLOG(INFO) << “Using FileFetcher for file URL.”;
    fetcher = new FileFetcher();
  } else {
#ifdef _UE_SIDELOAD
    LOG(FATAL) << “Unsupported sideload URI: ” << payload_url;
#else
    LibcurlHttpFetcher* libcurl_fetcher =
        new LibcurlHttpFetcher(&proxy_resolver_, hardware_);
    libcurl_fetcher->set_server_to_check(ServerToCheck::kDownload);
    fetcher = libcurl_fetcher;
#endif  // _UE_SIDELOAD
  }
  // Setup extra headers.
  if (!headers[kPayloadPropertyAuthorization].empty())
    fetcher->SetHeader(“Authorization”, headers[kPayloadPropertyAuthorization]);
  if (!headers[kPayloadPropertyUserAgent].empty())
    fetcher->SetHeader(“User-Agent”, headers[kPayloadPropertyUserAgent]);
  //构建执行action序列
  BuildUpdateActions(fetcher);
  SetStatusAndNotify(UpdateStatus::UPDATE_AVAILABLE);
  UpdatePrefsOnUpdateStart(install_plan_.is_resume);
  //执行action序列中的action
  ScheduleProcessingStart();
  return true;
}

整个升级的执行过程都在ApplyPayload方法中进行了，先是检查了参数和当前升级状态，然后构建了install_plan_，它的作用就是打印信息，将
部分升级关键信息打印出来，然后初始化了fetcher，后面的DownloadAction会用到，最后就是组建action执行序列和执行序列中的action
 

void UpdateAttempterAndroid::BuildUpdateActions(HttpFetcher* fetcher) {
  CHECK(!processor_->IsRunning());
  processor_->set_delegate(this);
 
  // Actions:
  auto update_boot_flags_action =
      std::make_unique<UpdateBootFlagsAction>(boot_control_);
  auto cleanup_previous_update_action =
      boot_control_->GetDynamicPartitionControl()
          ->GetCleanupPreviousUpdateAction(boot_control_, prefs_, this);
  auto install_plan_action = std::make_unique<InstallPlanAction>(install_plan_);
  auto download_action =
      std::make_unique<DownloadAction>(prefs_,
                                       boot_control_,
                                       hardware_,
                                       nullptr,  // system_state, not used.
                                       fetcher,  // passes ownership
                                       true /* interactive */);
  download_action->set_delegate(this);
  download_action->set_base_offset(base_offset_);
  auto filesystem_verifier_action =
      std::make_unique<FilesystemVerifierAction>();
  auto postinstall_runner_action =
      std::make_unique<PostinstallRunnerAction>(boot_control_, hardware_);
  postinstall_runner_action->set_delegate(this);
 
  // Bond them together. We have to use the leaf–types when calling
  // BondActions().
  BondActions(install_plan_action.get(), download_action.get());
  BondActions(download_action.get(), filesystem_verifier_action.get());
  BondActions(filesystem_verifier_action.get(),
              postinstall_runner_action.get());
 
  processor_->EnqueueAction(std::move(update_boot_flags_action));
  processor_->EnqueueAction(std::move(cleanup_previous_update_action));
  processor_->EnqueueAction(std::move(install_plan_action));
  processor_->EnqueueAction(std::move(download_action));
  processor_->EnqueueAction(std::move(filesystem_verifier_action));
  processor_->EnqueueAction(std::move(postinstall_runner_action));
}

构建5个执行action，并且设置好执行顺序和关联，上一次执行完成的结果会通过管道传递到下一个action中。
这里看下最重要的一个action，postinstall_runner_action::PerformPartitionPostinstall

void PostinstallRunnerAction::PerformPartitionPostinstall() {
  if (install_plan_.download_url.empty()) {
    LOG(INFO) << “Skipping post-install during rollback“;
    return CompletePostinstall(ErrorCode::kSuccess);
  }
 
  // Skip all the partitions that don‘t have a post-install step.
  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_++;
  }
  if (current_partition_ == install_plan_.partitions.size())
    return CompletePostinstall(ErrorCode::kSuccess);
  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__
  // Double check that the fs_mount_dir is not busy with a previous mounted
  // filesystem from a previous crashed postinstall step.
  if (utils::IsMountpoint(fs_mount_dir_)) {
    LOG(INFO) << “Found previously mounted filesystem at ” << fs_mount_dir_;
    utils::UnmountFilesystem(fs_mount_dir_);
  }
  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);
  }
  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.
  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);
  // Runs the postinstall script asynchronously to free up the main loop while
  // it’s running.
  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__
 
  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;
  }
 
  // Monitor the status file descriptor.
  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_;
  }
 
  progress_task_ = MessageLoop::current()->WatchFileDescriptor(
      FROM_HERE,
      progress_fd_,
      MessageLoop::WatchMode::kWatchRead,
      true,
      base::Bind(&PostinstallRunnerAction::OnProgressFdReady,
                 base::Unretained(this)));
}

这里代码逻辑很多，但是之前很多都是做的各种状态检测，升级是通过执行命令

Subprocess::Get().ExecFlags(
      command,
      Subprocess::kRedirectStderrToStdout,
      {kPostinstallStatusFd},
      base::Bind(&PostinstallRunnerAction::CompletePartitionPostinstall,
                 base::Unretained(this)));

最后进入systemupdate_enginecommonsubprocess.cc中：

bool LaunchProcess(const vector<string>& cmd,
                   uint32_t flags,
                   const vector<int>& output_pipes,
                   brillo::Process* proc) {
  for (const string& arg : cmd)
    proc->AddArg(arg);
  proc->SetSearchPath((flags & Subprocess::kSearchPath) != 0);
 
  // Create an environment for the child process with just the required PATHs.
  std::map<string, string> env;
  for (const char* key : {“LD_LIBRARY_PATH”, “PATH”}) {
    const char* value = getenv(key);
    if (value)
      env.emplace(key, value);
  }
 
  for (const int fd : output_pipes) {
    proc->RedirectUsingPipe(fd, false);
  }
  proc->SetCloseUnusedFileDescriptors(true);
  proc->RedirectUsingPipe(STDOUT_FILENO, false);
  proc->SetPreExecCallback(base::Bind(&SetupChild, env, flags));
 
  return proc->Start();
}

cmd参数就是一开始传过来的command，这里用到了brillo/process.cc来执行命令，这里不再继续跟进了，update_engine大致的升级流程到这里就完成了，但还是有很多方面暂时没有涉及，一些详细的过程也没有去更进一步的去分析跟进，留待以后有时间或者遇到相关问题再去做进一步分析。
 

文件转载：

update_engine简介

代码007普通

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