Android A/B system - update_engine

Posted 2022-06-11 Dufre.WC

android A/B System的升级过程是怎样的？理解这一过程就需要理解update_engine的逻辑。

首先来看Android.mk，这里截取了一些关键信息

• 入口函数：main.cc
• 依赖的c文件/动态库/静态库

# update_engine (type: executable)
# ========================================================
# update_engine daemon.
include $(CLEAR_VARS)
LOCAL_MODULE := update_engine
LOCAL_MODULE_CLASS := EXECUTABLES
LOCAL_REQUIRED_MODULES := \\
    cacerts_google
LOCAL_CPP_EXTENSION := .cc
LOCAL_CFLAGS := $(ue_common_cflags)
LOCAL_CPPFLAGS := $(ue_common_cppflags)
LOCAL_LDFLAGS := $(ue_common_ldflags)
LOCAL_C_INCLUDES := \\
    $(ue_common_c_includes)
LOCAL_SHARED_LIBRARIES := \\
    $(ue_common_shared_libraries)
LOCAL_STATIC_LIBRARIES := \\
    $(ue_common_static_libraries)
LOCAL_SRC_FILES := \\
    main.cc

# 依赖的c文件/动态库/静态库
ue_common_c_includes := \\
    $(LOCAL_PATH)/client_library/include \\
    system
ue_common_shared_libraries := \\
    libbrillo-stream \\
    libbrillo \\
    libchrome
ue_common_static_libraries := \\
    libgtest_prod \\

首先来看main.cc，main.cc主要做了这些事：

• log相关
• 初始化crc-32 table
• 调用update_engine_daemon.Run()

/system/update_engine/main.cc

int main(int argc, char** argv) 
  DEFINE_bool(logtofile, false, "Write logs to a file in log_dir.");
  DEFINE_bool(logtostderr, false,
              "Write logs to stderr instead of to a file in log_dir.");
  DEFINE_bool(foreground, false,
              "Don't daemon()ize; run in foreground.");

  chromeos_update_engine::Terminator::Init();
  brillo::FlagHelper::Init(argc, argv, "Chromium OS Update Engine");

  // We have two logging flags "--logtostderr" and "--logtofile"; and the logic
  // to choose the logging destination is:
  // 1. --logtostderr --logtofile -> logs to both
  // 2. --logtostderr             -> logs to system debug
  // 3. --logtofile or no flags   -> logs to file
  bool log_to_system = FLAGS_logtostderr;
  bool log_to_file = FLAGS_logtofile || !FLAGS_logtostderr;
  chromeos_update_engine::SetupLogging(log_to_system, log_to_file);
  if (!FLAGS_foreground)
    PLOG_IF(FATAL, daemon(0, 0) == 1) << "daemon() failed";

  LOG(INFO) << "Chrome OS Update Engine starting";

  // xz-embedded requires to initialize its CRC-32 table once on startup.
  xz_crc32_init();

  // Ensure that all written files have safe permissions.
  // This is a mask, so we _block_ all permissions for the group owner and other
  // users but allow all permissions for the user owner. We allow execution
  // for the owner so we can create directories.
  // Done _after_ log file creation.
  umask(S_IRWXG | S_IRWXO);

  chromeos_update_engine::UpdateEngineDaemon update_engine_daemon;
  int exit_code = update_engine_daemon.Run();

  LOG(INFO) << "Chrome OS Update Engine terminating with exit code "
            << exit_code;
  return exit_code;

在/system/update_engine/daemon.h没有找到Run()函数，所以去找父类。

/system/update_engine/daemon.h

class UpdateEngineDaemon : public brillo::Daemon

可以看到这里Run()函数，首先执行了OnInit()函数。
/external/libbrillo/brillo/daemons/daemon.cc

int Daemon::Run() 
  int exit_code = OnInit();
  if (exit_code != EX_OK)
    return exit_code;

  message_loop_.Run();

  OnShutdown(&exit_code_);

  // base::RunLoop::QuitClosure() causes the message loop to quit
  // immediately, even if pending tasks are still queued.
  // Run a secondary loop to make sure all those are processed.
  // This becomes important when working with D-Bus since dbus::Bus does
  // a bunch of clean-up tasks asynchronously when shutting down.
  while (message_loop_.RunOnce(false /* may_block */)) 

  return exit_code_;

OnInit()函数做了两件事：

• Binder初始化（创建/注册）
• 调用DaemonStateAndroid的函数
  • AddObserver()
  • StartUpdater()
  • Initialized()

/system/update_engine/daemon.cc

int UpdateEngineDaemon::OnInit() 
  // Register the |subprocess_| singleton with this Daemon as the signal
  // handler.
  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

#if USE_OMAHA
  // Initialize update engine global state but continue if something fails.
  // TODO(deymo): Move the daemon_state_ initialization to a factory method
  // avoiding the explicit re-usage of the |bus| instance, shared between
  // D-Bus service and D-Bus client calls.
  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

#if USE_BINDER
  // Create the Binder Service.
#if USE_OMAHA
  binder_service_ = new BinderUpdateEngineBrilloServicereal_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(&UpdateEngineDaemon::OnDBusRegistered,
                                          base::Unretained(this)));
  LOG(INFO) << "Waiting for DBus object to be registered.";
#else  // !USE_DBUS
  daemon_state_->StartUpdater();
#endif  // USE_DBUS
  return EX_OK;


#if USE_DBUS
void UpdateEngineDaemon::OnDBusRegistered(bool succeeded) 
  if (!succeeded) 
    LOG(ERROR) << "Registering the UpdateEngineAdaptor";
    QuitWithExitCode(1);
    return;
  

  // Take ownership of the service now that everything is initialized. We need
  // to this now and not before to avoid exposing a well known DBus service
  // path that doesn't have the service it is supposed to implement.
  if (!dbus_adaptor_->RequestOwnership()) 
    LOG(ERROR) << "Unable to take ownership of the DBus service, is there "
               << "other update_engine daemon running?";
    QuitWithExitCode(1);
    return;
  
  daemon_state_->StartUpdater();

#endif  // USE_DBUS

  // namespace chromeos_update_engine

来看DaemonStateAndroid这个类做了什么，首先来看Initialize()这个函数，从名字可以看出做了一些初始化的工作，包括boot_control_,hardware_,ceritficate_checker_,update_attempter_。boot_control_可以看Android A/B System Updates，是google提供的接口，在启动的过程中会用到，每个芯片厂商自己去实现这部分的内容；hardware_与硬件相关；ceritficate_checker_,update_attempter_是UpdateAttempter会使用到，这是A/B升级的核心。

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 intializing 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(&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(), &openssl_wrapper_));
  certificate_checker_->Init();

  // Initialize the UpdateAttempter before the UpdateManager.
  update_attempter_.reset(new UpdateAttempterAndroid(
      this, prefs_.get(), boot_control_.get(), hardware_.get()));

  return true;

再来看StartUpdater()做了什么，它继续调用了update_attempter_->Init()，所以还要往下看。
/system/update_engine/daemon_state_android.cc

bool DaemonStateAndroid::StartUpdater() 
  // The DaemonState in Android is a passive daemon. It will only start applying
  // an update when instructed to do so from the exposed binder API.
  update_attempter_->Init();
  return true;

/system/update_engine/update_attempter_android.cc

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();
  

Init()函数。它分为两种情况：

• 如果升级已经完成，设置UPDATED_NEED_REBOOT状态，表示升级已经完成，需要重启。
• 否则，设置IDLE状态，并且更新并检查版本信息。

/system/update_engine/update_attempter_android.cc

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();
  

SetStatusAndNotify()函数先获取payload.bin（升级包中的一个核心文件）的大小，然后更新的情况发出（这里是观察者模式，所有的观察者都能接收到信息）。

/system/update_engine/update_attempter_android.cc

void UpdateAttempterAndroid::SetStatusAndNotify(UpdateStatus status) 
  status_ = status;
  size_t payload_size =
      install_plan_.payloads.empty() ? 0 : install_plan_.payloads[0].size;
  UpdateEngineStatus status_to_send = .status = status_,
                                       .progress = download_progress_,
                                       .new_size_bytes = payload_size;

  for (auto observer : daemon_state_->service_observers()) 
    observer->SendStatusUpdate(status_to_send);
  
  last_notify_time_ = TimeTicks::Now();

这里的每个观察者对应一个客户端，客户端接收到了更新信息。
/system/update_engine/update_engine_client_android.cc

void UpdateAttempterAndroid::SetStatusAndNotify(UpdateStatus status) 
  status_ = status;
  size_t payload_size =
      install_plan_.payloads.empty() ? 0 : install_plan_.payloads[0].size;
  UpdateEngineStatus status_to_send = .status = status_,
                                       .progress = download_progress_,
                                       .new_size_bytes = payload_size;

  for (auto observer : daemon_state_->service_observers()) 
    observer->SendStatusUpdate(status_to_send);
  
  last_notify_time_ = TimeTicks::Now();

这一篇就分析到这，下一篇继续分析，以下是这一篇涉及到的流程图和类图，可以对照着看。

Sequence：

Class：