Android A/B system - update_engine(一)

Android A/B System的升級過程是怎樣的？理解這一過程就需要理解update_engine的邏輯。

首先來看Android.mk，這裏截取了一些關鍵信息

• 入口函數：main.cc
• 依賴的c文件/動態庫/靜態庫

# 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

# 依賴的c文件/動態庫/靜態庫
ue_common_c_includes := \
    $(LOCAL_PATH)/client_library/include \
    system
ue_common_shared_libraries := \
    libbrillo-stream \
    libbrillo \
    libchrome
ue_common_static_libraries := \
    libgtest_prod \

首先來看main.cc，main.cc主要做了這些事：

• log相關
• 初始化crc-32 table
• 調用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沒有找到Run()函數，所以去找父類。

/system/update_engine/daemon.h

class UpdateEngineDaemon : public brillo::Daemon{}

可以看到這裏Run()函數，首先執行了OnInit()函數。
/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()函數做了兩件事：

• Binder初始化（創建/註冊）
• 調用DaemonStateAndroid的函數
  • 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 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(&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

來看DaemonStateAndroid這個類做了什麼，首先來看Initialize()這個函數，從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，在啓動的過程中會用到，每個芯片廠商自己去實現這部分的內容；hardware_與硬件相關；ceritficate_checker_,update_attempter_是UpdateAttempter會使用到，這是A/B升級的核心。

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;
}

再來看StartUpdater()做了什麼，它繼續調用了update_attempter_->Init()，所以還要往下看。
/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()函數。它分爲兩種情況：

• 如果升級已經完成，設置UPDATED_NEED_REBOOT狀態，表示升級已經完成，需要重啓。
• 否則，設置IDLE狀態，並且更新並檢查版本信息。

/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()函數先獲取payload.bin（升級包中的一個核心文件）的大小，然後更新的情況發出（這裏是觀察者模式，所有的觀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();
}

這裏的每個觀察者對應一個客戶端，客戶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();
}

這一篇就分析到這，下一篇繼續分析，以下是這一篇涉及到的流程圖和類圖，可以對照着看。

Sequence：

Class：