Android Update Engine分析（一）Makefile
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写完《Android AB System OTA分析》系列后打算一口气将Update Engine也写了的，但一直由于各种借口，最终没有完成。后来6月份的时候陆陆续续读了Update Engine部分代码，记了点笔记，本打算等彻底读完再分享的，但按照目前的进度不知道读完是哪一天，所以先将笔记贴在这里，如果我的这几篇笔记能对您阅读或理解Update Engine机制有一丝帮助，那花时间整理也是值得的，由于个人水平有限，如果发现错误，恳请指出，我会更正以免误导别人。
《Android A/B System OTA分析》主要从功能上分析A/B系统的特点、设置和操作，Update Engine分析深入A/B系统的底层实现，所以相对于前者，需要更加深入代码。
本文涉及的Android代码版本：android‐7.1.1_r23 (NMF27D)
Android Update Engine模块涉及的代码较多，这里主要以system/update_engine目录的代码为主，其它部分代码会编译为库的形式供system/update_engine下的代码调用。
如果只看update_engine目录，用find命令统计下：
ygu@stbszx-bld-5:/public/android/src/system/update_engine$ find . -type f | wc -l416ygu@stbszx-bld-5:/public/android/src/system/update_engine$ find . -type f -iname '*.cc' | wc -l 186这个目录下共有416个文件，除去makefile，资源文件，头文件等，仅.cc后缀的文件就有186个，虽然少了一大半，但是面对这么多文件从哪里着手，显然也是个问题。
分析复杂的代码，makefile是最好的入手点，分析makefile的好处有很多，最主要一条是对项目的组织结构有一个大方向的认识，代码入口点在哪里，由哪些模块构成，哪些模块有依赖关系，预定义了哪些条件编译的宏，哪些宏定义会传入到代码中等。分析整个Android工程如此，分析Update Engine模块也是如此。
对于Update Engine模块的makefile文件system/update_engine/Android.mk，初一看比较复杂。首先Android.mk里面定义的模块很多，各种模块加起来20+个，模块之间还有前后依赖；其次，Android.mk文件开始定义了一些需要求值的makefile变量，并根据变量的取值进行条件编译使得Makefile看起来更加复杂。如果先整理好条件变量的设置，再进行下一步的分析会比较简单。
1. Android.mk代码逐行分析
以下对system/update_engine/Androi.mk文件进行逐行分析。
这里分析makefile的主要目的是理清各模块的编译涉及的文件和模块间的关系，对于模块具体是如何编译的并不关心，具体的编译在单个模块分析时再深入介绍。所以这里的任务比较简单，分析也比较粗略，
第1~15行
版权申明，这里略过。
第16~68行
...LOCAL_PATH := $(my-dir)# Default values for the USE flags. Override these USE flags from your product# by setting BRILLO_USE_* values. Note that we define local variables like# local_use_* to prevent leaking our default setting for other packages.local_use_binder := $(if $(BRILLO_USE_BINDER),$(BRILLO_USE_BINDER),1)local_use_dbus := $(if $(BRILLO_USE_DBUS),$(BRILLO_USE_DBUS),0)local_use_hwid_override := \ $(if $(BRILLO_USE_HWID_OVERRIDE),$(BRILLO_USE_HWID_OVERRIDE),0)# "libcros" gates the LibCrosService exposed by the Chrome OS' chrome browser to# the system layer.local_use_libcros := $(if $(BRILLO_USE_LIBCROS),$(BRILLO_USE_LIBCROS),0)local_use_mtd := $(if $(BRILLO_USE_MTD),$(BRILLO_USE_MTD),0)local_use_power_management := \ $(if $(BRILLO_USE_POWER_MANAGEMENT),$(BRILLO_USE_POWER_MANAGEMENT),0)local_use_weave := $(if $(BRILLO_USE_WEAVE),$(BRILLO_USE_WEAVE),0)ue_common_cflags := \ -DUSE_BINDER=$(local_use_binder) \ -DUSE_DBUS=$(local_use_dbus) \ -DUSE_HWID_OVERRIDE=$(local_use_hwid_override) \ -DUSE_LIBCROS=$(local_use_libcros) \ -DUSE_MTD=$(local_use_mtd) \ -DUSE_POWER_MANAGEMENT=$(local_use_power_management) \ -DUSE_WEAVE=$(local_use_weave) \ -D_FILE_OFFSET_BITS=64 \ -D_POSIX_C_SOURCE=199309L \ -Wa,--noexecstack \ -Wall \ -Werror \ -Wextra \ -Wformat=2 \ -Wno-psabi \ -Wno-unused-parameter \ -ffunction-sections \ -fstack-protector-strong \ -fvisibility=hiddenue_common_cppflags := \ -Wnon-virtual-dtor \ -fno-strict-aliasing \ -std=gnu++11ue_common_ldflags := \ -Wl,--gc-sectionsue_common_c_includes := \ $(LOCAL_PATH)/client_library/include \ external/gtest/include \ systemue_common_shared_libraries := \ libbrillo-stream \ libbrillo \ libchrome总体上看，上面的代码定义了以下两类模块相关的变量：
逻辑控制类，标识是否需要相应模块，用于条件编译
local_use_binder, local_use_dbus, local_use_hwid_override, local_use_libcros, local_use_mtd, local_use_power_management, local_use_weave编译参数类，存储cflags/cppflags/ldflags等参数，传递给工具链进行编译和连接
ue_common_cflags, ue_common_cppflags, ue_common_ldflags, ue_common_c_includes, ue_common_shared_libraries
在定义逻辑控制类变量时，代码中大量使用了makefile的if函数。还记得if函数吗？
if 函数语法如下：
$(if CONDITION,THEN-PART[,ELSE-PART])对于参数“CONDITION”，在函数执行时忽略其前导和结尾空字符并展开：
- 如果展开结果非空，则条件为真，就将第二个参数“THEN_PATR”作为函数的计算表达式，函数的返回值就是“THEN-PART”的计算结果；
- 如果展开结果为空，将第三个参数“ELSE-PART”作为函数的表达式，返回结果为表达式“ELSE-PART”的计算结果。详细的
if函数，可以参考make手册：GNU Make
这里分别检查以下变量是否定义：
- BRILLO_USE_BINDER
- BRILLO_USE_DBUS
- BRILLO_USE_HWID_OVERRIDE
- BRILLO_USE_LIBCROS
- BRILLO_USE_MTD
- BRILLO_USE_POWER_MANAGEMENT
- BRILLO_USE_WEAVE
我完全不记得也不知道哪里有定义过这些变量了，如果你跟我一样不记得或不知道，没关系，那就在代码根目录下用grep命令找找吧：
$ grep -rn "BRILLO_USE_" . --exclude-dir=out./system/update_engine/Android.mk:20:# by setting BRILLO_USE_* values. Note that we define local variables like./system/update_engine/Android.mk:22:local_use_binder := $(if $(BRILLO_USE_BINDER),$(BRILLO_USE_BINDER),1)./system/update_engine/Android.mk:23:local_use_dbus := $(if $(BRILLO_USE_DBUS),$(BRILLO_USE_DBUS),0)./system/update_engine/Android.mk:25: $(if $(BRILLO_USE_HWID_OVERRIDE),$(BRILLO_USE_HWID_OVERRIDE),0)./system/update_engine/Android.mk:28:local_use_libcros := $(if $(BRILLO_USE_LIBCROS),$(BRILLO_USE_LIBCROS),0)./system/update_engine/Android.mk:29:local_use_mtd := $(if $(BRILLO_USE_MTD),$(BRILLO_USE_MTD),0)./system/update_engine/Android.mk:31: $(if $(BRILLO_USE_POWER_MANAGEMENT),$(BRILLO_USE_POWER_MANAGEMENT),0)./system/update_engine/Android.mk:32:local_use_weave := $(if $(BRILLO_USE_WEAVE),$(BRILLO_USE_WEAVE),0)./external/libbrillo/Android.mk:16:# by setting BRILLO_USE_* values. Note that we define local variables like./external/libbrillo/Android.mk:18:local_use_dbus := $(if $(BRILLO_USE_DBUS),$(BRILLO_USE_DBUS),1)./external/libchrome/Android.mk:16:# by setting BRILLO_USE_* values. Note that we define local variables like./external/libchrome/Android.mk:18:local_use_dbus := $(if $(BRILLO_USE_DBUS),$(BRILLO_USE_DBUS),1)根据搜索结果，在我目前工程中，一共在3个文件中出现过BRILLO_USE_xxx变量，但这些地方都是对变量的引用而非设置。因此，可以肯定这些BRILLO_USE_xxx都未定义，其引用为空，进一步可以得到所有逻辑控制类变量都是取if函数的”[ELSE-PART]”的值，例如:
local_use_binder := $(if $(BRILLO_USE_BINDER),$(BRILLO_USE_BINDER),1)由于if的CONDITION中$(BRILLO_USE_BINDER)变量没有定义，所以取值为空，local_use_binder的取值是if函数的ELSE-PART部分，即1。
因此，上面所有变量取值如下：
local_use_binder = 1,local_use_dbus = 0,local_use_hwid_override = 0, local_use_libcros = 0, local_use_mtd = 0, local_use_power_management = 0, local_use_weave = 0拿到了这些控制变量的值，剩下的事情就简单多了。
对于编译参数类变量 ue_common_{cflags, cppflags, ldflags, c_includes, shared_libraries}，在具体的模块编译时会将其传递给编译器或链接器，这里不再展开分析。
第69~82行
#依赖于local_use_dbus的条件编译，如果定义了BRILLO_USE_DBUS，则进程间基于DBUS机制进行通信ifeq ($(local_use_dbus),1)# update_engine_client-dbus-proxies (from generate-dbus-proxies.gypi)# ========================================================include $(CLEAR_VARS)LOCAL_MODULE := update_engine_client-dbus-proxiesLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_SRC_FILES := \ dbus_bindings/dbus-service-config.json \ dbus_bindings/org.chromium.UpdateEngineInterface.dbus-xmlLOCAL_DBUS_PROXY_PREFIX := update_engineinclude $(BUILD_STATIC_LIBRARY)endif # local_use_dbus == 1由于”local_use_debus=0“，显然这里不会包含这个模块，不需要关心模块update_engine_client-dbus-proxies。
第83~112行
# update_metadata-protos (type: static_library)# ========================================================# Protobufs.ue_update_metadata_protos_exported_static_libraries := \ update_metadata-protosue_update_metadata_protos_exported_shared_libraries := \ libprotobuf-cpp-liteue_update_metadata_protos_src_files := \ update_metadata.proto# Build for the host.include $(CLEAR_VARS)LOCAL_MODULE := update_metadata-protosLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_IS_HOST_MODULE := truegenerated_sources_dir := $(call local-generated-sources-dir)LOCAL_EXPORT_C_INCLUDE_DIRS := $(generated_sources_dir)/proto/systemLOCAL_SRC_FILES := $(ue_update_metadata_protos_src_files)include $(BUILD_HOST_STATIC_LIBRARY)# Build for the target.include $(CLEAR_VARS)LOCAL_MODULE := update_metadata-protosLOCAL_MODULE_CLASS := STATIC_LIBRARIESgenerated_sources_dir := $(call local-generated-sources-dir)LOCAL_EXPORT_C_INCLUDE_DIRS := $(generated_sources_dir)/proto/systemLOCAL_SRC_FILES := $(ue_update_metadata_protos_src_files)include $(BUILD_STATIC_LIBRARY)以上代码定义了静态库模块update_metadata-protos，分别用于host和target环境。
第114~136行
ifeq ($(local_use_dbus),1)# update_engine-dbus-adaptor (from generate-dbus-adaptors.gypi)# ========================================================# Chrome D-Bus bindings.include $(CLEAR_VARS)LOCAL_MODULE := update_engine-dbus-adaptorLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_SRC_FILES := \ dbus_bindings/org.chromium.UpdateEngineInterface.dbus-xmlinclude $(BUILD_STATIC_LIBRARY)# update_engine-dbus-libcros-client (from generate-dbus-proxies.gypi)# ========================================================include $(CLEAR_VARS)LOCAL_MODULE := update_engine-dbus-libcros-clientLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_SRC_FILES := \ dbus_bindings/org.chromium.LibCrosService.dbus-xmlLOCAL_DBUS_PROXY_PREFIX := libcrosinclude $(BUILD_STATIC_LIBRARY)endif # local_use_dbus == 1由于”local_use_dbus=0“，所以这里的模块也不用管了。
第137~227行
# libpayload_consumer (type: static_library)# ========================================================# The payload application component and common dependencies.ue_libpayload_consumer_exported_static_libraries := \ update_metadata-protos \ libxz-host \ libbz \ $(ue_update_metadata_protos_exported_static_libraries)ue_libpayload_consumer_exported_shared_libraries := \ libcrypto-host \ $(ue_update_metadata_protos_exported_shared_libraries)ue_libpayload_consumer_src_files := \ common/action_processor.cc \ common/boot_control_stub.cc \ common/clock.cc \ common/constants.cc \ common/cpu_limiter.cc \ common/error_code_utils.cc \ common/hash_calculator.cc \ common/http_common.cc \ common/http_fetcher.cc \ common/file_fetcher.cc \ common/hwid_override.cc \ common/multi_range_http_fetcher.cc \ common/platform_constants_android.cc \ common/prefs.cc \ common/subprocess.cc \ common/terminator.cc \ common/utils.cc \ payload_consumer/bzip_extent_writer.cc \ payload_consumer/delta_performer.cc \ payload_consumer/download_action.cc \ payload_consumer/extent_writer.cc \ payload_consumer/file_descriptor.cc \ payload_consumer/file_writer.cc \ payload_consumer/filesystem_verifier_action.cc \ payload_consumer/install_plan.cc \ payload_consumer/payload_constants.cc \ payload_consumer/payload_verifier.cc \ payload_consumer/postinstall_runner_action.cc \ payload_consumer/xz_extent_writer.ccifeq ($(HOST_OS),linux)# Build for the host.include $(CLEAR_VARS)LOCAL_MODULE := libpayload_consumerLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ external/e2fsprogs/libLOCAL_STATIC_LIBRARIES := \ update_metadata-protos \ $(ue_libpayload_consumer_exported_static_libraries) \ $(ue_update_metadata_protos_exported_static_libraries)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libpayload_consumer_exported_shared_libraries) \ $(ue_update_metadata_protos_exported_shared_libraries)LOCAL_SRC_FILES := $(ue_libpayload_consumer_src_files)include $(BUILD_HOST_STATIC_LIBRARY)endif # HOST_OS == linux# Build for the target.include $(CLEAR_VARS)LOCAL_MODULE := libpayload_consumerLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ external/e2fsprogs/libLOCAL_STATIC_LIBRARIES := \ update_metadata-protos \ $(ue_libpayload_consumer_exported_static_libraries:-host=) \ $(ue_update_metadata_protos_exported_static_libraries)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libpayload_consumer_exported_shared_libraries:-host=) \ $(ue_update_metadata_protos_exported_shared_libraries)LOCAL_SRC_FILES := $(ue_libpayload_consumer_src_files)include $(BUILD_STATIC_LIBRARY)以上代码定义了静态库模块libpayload_consumer，分别用于host和target环境。
由于ue_libpayload_consumer_exported_static_libraries包含了update_metadata-protos，所以明白了为什么需要编译update_metadata-protos模块。
第228~424行
ifdef BRILLO# libupdate_engine (type: static_library)# ========================================================# The main daemon static_library with all the code used to check for updates# with Omaha and expose a DBus daemon.ue_libupdate_engine_exported_c_includes := \ $(LOCAL_PATH)/include \ external/cros/system_api/dbusue_libupdate_engine_exported_static_libraries := \ libpayload_consumer \ update_metadata-protos \ update_engine-dbus-adaptor \ update_engine-dbus-libcros-client \ update_engine_client-dbus-proxies \ libbz \ libfs_mgr \ $(ue_libpayload_consumer_exported_static_libraries) \ $(ue_update_metadata_protos_exported_static_libraries)ue_libupdate_engine_exported_shared_libraries := \ libdbus \ libbrillo-dbus \ libchrome-dbus \ libmetrics \ libshill-client \ libexpat \ libbrillo-policy \ libhardware \ libcurl \ libcutils \ libssl \ $(ue_libpayload_consumer_exported_shared_libraries) \ $(ue_update_metadata_protos_exported_shared_libraries)ifeq ($(local_use_binder),1)ue_libupdate_engine_exported_shared_libraries += \ libbinder \ libbinderwrapper \ libbrillo-binder \ libutilsendif # local_use_binder == 1ifeq ($(local_use_weave),1)ue_libupdate_engine_exported_shared_libraries += \ libbinderwrapper \ libbrillo-binder \ libweavedendif # local_use_weave == 1include $(CLEAR_VARS)LOCAL_MODULE := libupdate_engineLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_EXPORT_C_INCLUDE_DIRS := $(ue_libupdate_engine_exported_c_includes)LOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ $(ue_libupdate_engine_exported_c_includes) \ bootable/recoveryLOCAL_STATIC_LIBRARIES := \ libpayload_consumer \ update_metadata-protos \ update_engine-dbus-adaptor \ update_engine-dbus-libcros-client \ update_engine_client-dbus-proxies \ $(ue_libupdate_engine_exported_static_libraries:-host=) \ $(ue_libpayload_consumer_exported_static_libraries:-host=) \ $(ue_update_metadata_protos_exported_static_libraries)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libupdate_engine_exported_shared_libraries:-host=) \ $(ue_libpayload_consumer_exported_shared_libraries:-host=) \ $(ue_update_metadata_protos_exported_shared_libraries)LOCAL_SRC_FILES := \ boot_control_android.cc \ certificate_checker.cc \ common_service.cc \ connection_manager.cc \ daemon.cc \ dbus_service.cc \ hardware_android.cc \ image_properties_android.cc \ libcros_proxy.cc \ libcurl_http_fetcher.cc \ metrics.cc \ metrics_utils.cc \ omaha_request_action.cc \ omaha_request_params.cc \ omaha_response_handler_action.cc \ p2p_manager.cc \ payload_state.cc \ proxy_resolver.cc \ real_system_state.cc \ shill_proxy.cc \ update_attempter.cc \ update_manager/boxed_value.cc \ update_manager/chromeos_policy.cc \ update_manager/default_policy.cc \ update_manager/evaluation_context.cc \ update_manager/policy.cc \ update_manager/real_config_provider.cc \ update_manager/real_device_policy_provider.cc \ update_manager/real_random_provider.cc \ update_manager/real_shill_provider.cc \ update_manager/real_system_provider.cc \ update_manager/real_time_provider.cc \ update_manager/real_updater_provider.cc \ update_manager/state_factory.cc \ update_manager/update_manager.cc \ update_status_utils.cc \ utils_android.cc \ weave_service_factory.ccifeq ($(local_use_binder),1)LOCAL_AIDL_INCLUDES += $(LOCAL_PATH)/binder_bindingsLOCAL_SRC_FILES += \ binder_bindings/android/brillo/IUpdateEngine.aidl \ binder_bindings/android/brillo/IUpdateEngineStatusCallback.aidl \ binder_service_brillo.cc \ parcelable_update_engine_status.ccendif # local_use_binder == 1ifeq ($(local_use_weave),1)LOCAL_SRC_FILES += \ weave_service.ccendif # local_use_weave == 1ifeq ($(local_use_libcros),1)LOCAL_SRC_FILES += \ chrome_browser_proxy_resolver.ccendif # local_use_libcros == 1include $(BUILD_STATIC_LIBRARY)else # !defined(BRILLO)...以上if-else中间的代码定义了BRILLO平台上的libupdate_engine库模块，由于我们是非BRILLO平台，但并不需要我们去关心。不管里面是神马，写了神马，定义了神马，we don't care。
下面这部分才是非BRILLO平台的libupdate_engine_android库：
ifneq ($(local_use_binder),1)$(error USE_BINDER is disabled but is required in non-Brillo devices.)endif # local_use_binder == 1# libupdate_engine_android (type: static_library)# ========================================================# The main daemon static_library used in Android (non-Brillo). This only has a# loop to apply payloads provided by the upper layer via a Binder interface.ue_libupdate_engine_android_exported_static_libraries := \ libpayload_consumer \ libfs_mgr \ $(ue_libpayload_consumer_exported_static_libraries)ue_libupdate_engine_android_exported_shared_libraries := \ $(ue_libpayload_consumer_exported_shared_libraries) \ libandroid \ libbinder \ libbinderwrapper \ libbrillo-binder \ libcutils \ libcurl \ libhardware \ libssl \ libutilsinclude $(CLEAR_VARS)LOCAL_MODULE := libupdate_engine_androidLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ bootable/recovery#TODO(deymo): Remove external/cros/system_api/dbus once the strings are moved# out of the DBus interface.LOCAL_C_INCLUDES += \ external/cros/system_api/dbusLOCAL_STATIC_LIBRARIES := \ $(ue_libupdate_engine_android_exported_static_libraries:-host=)LOCAL_SHARED_LIBRARIES += \ $(ue_common_shared_libraries) \ $(ue_libupdate_engine_android_exported_shared_libraries:-host=)LOCAL_AIDL_INCLUDES := $(LOCAL_PATH)/binder_bindingsLOCAL_SRC_FILES += \ binder_bindings/android/os/IUpdateEngine.aidl \ binder_bindings/android/os/IUpdateEngineCallback.aidl \ binder_service_android.cc \ boot_control_android.cc \ certificate_checker.cc \ daemon.cc \ daemon_state_android.cc \ hardware_android.cc \ libcurl_http_fetcher.cc \ network_selector_android.cc \ proxy_resolver.cc \ update_attempter_android.cc \ update_status_utils.cc \ utils_android.ccinclude $(BUILD_STATIC_LIBRARY)endif # !defined(BRILLO)以上代码定义了target上编译其它模块需要的libupdate_engine_android静态库模块。
有一点值得注意的是，其模块的LOCAL_C_INCLUDES变量竟然有对传统recovery目录的引用bootable/recovery：
LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ bootable/recovery第425~467行
# update_engine (type: executable)# ========================================================# update_engine daemon.include $(CLEAR_VARS)LOCAL_MODULE := update_engineLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_REQUIRED_MODULES := \ bspatch \ cacerts_googleifeq ($(local_use_weave),1)LOCAL_REQUIRED_MODULES += updater.jsonendif # local_use_weave == 1LOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_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_SRC_FILES := \ main.ccifdef BRILLOLOCAL_C_INCLUDES += \ $(ue_libupdate_engine_exported_c_includes)LOCAL_STATIC_LIBRARIES := \ libupdate_engine \ $(ue_libupdate_engine_exported_static_libraries:-host=)LOCAL_SHARED_LIBRARIES += \ $(ue_libupdate_engine_exported_shared_libraries:-host=)else # !defined(BRILLO)LOCAL_STATIC_LIBRARIES := \ libupdate_engine_android \ $(ue_libupdate_engine_android_exported_static_libraries:-host=)LOCAL_SHARED_LIBRARIES += \ $(ue_libupdate_engine_android_exported_shared_libraries:-host=)endif # !defined(BRILLO)LOCAL_INIT_RC := update_engine.rcinclude $(BUILD_EXECUTABLE)以上代码定义了生成target环境可执行应用update_engine的规则，其源码很简单，就只有一个main.cc。
这里的update_engine应用是Update Engine服务端的守护进程，通过binder方式向客户端提供服务。
库依赖方面，对于BRILLO平台和非BRILLO平台，update_engine依赖于不同的静态和动态库。
对于我们这里分析的非BRILLO平台，依赖于以下静态和共享库：
LOCAL_STATIC_LIBRARIES := \ libupdate_engine_android \ $(ue_libupdate_engine_android_exported_static_libraries:-host=)LOCAL_SHARED_LIBRARIES += \ $(ue_libupdate_engine_android_exported_shared_libraries:-host=)并有一个相应的init rc脚本：update_engine.rc
第468~535行
# update_engine_sideload (type: executable)# ========================================================# A static binary equivalent to update_engine daemon that installs an update# from a local file directly instead of running in the background.include $(CLEAR_VARS)LOCAL_MODULE := update_engine_sideloadLOCAL_FORCE_STATIC_EXECUTABLE := trueLOCAL_MODULE_PATH := $(TARGET_RECOVERY_ROOT_OUT)/sbinLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_REQUIRED_MODULES := \ bspatch_recoveryLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := \ $(ue_common_cflags) \ -D_UE_SIDELOADLOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ bootable/recovery#TODO(deymo): Remove external/cros/system_api/dbus once the strings are moved# out of the DBus interface.LOCAL_C_INCLUDES += \ external/cros/system_api/dbusLOCAL_SRC_FILES := \ boot_control_android.cc \ hardware_android.cc \ network_selector_stub.cc \ proxy_resolver.cc \ sideload_main.cc \ update_attempter_android.cc \ update_status_utils.cc \ utils_android.ccLOCAL_STATIC_LIBRARIES := \ libfs_mgr \ libpayload_consumer \ update_metadata-protos \ $(ue_libpayload_consumer_exported_static_libraries:-host=) \ $(ue_update_metadata_protos_exported_static_libraries)# We add the static versions of the shared libraries since we are forcing this# binary to be a static binary, so we also need to include all the static# library dependencies of these static libraries.LOCAL_STATIC_LIBRARIES += \ $(ue_common_shared_libraries) \ libcutils \ libcrypto_static \ $(ue_update_metadata_protos_exported_shared_libraries) \ libevent \ libmodpb64 \ liblogifeq ($(strip $(PRODUCT_STATIC_BOOT_CONTROL_HAL)),)# No static boot_control HAL defined, so no sideload support. We use a fake# boot_control HAL to allow compiling update_engine_sideload for test purposes.ifeq ($(strip $(AB_OTA_UPDATER)),true)$(warning No PRODUCT_STATIC_BOOT_CONTROL_HAL configured but AB_OTA_UPDATER is \true, no update sideload support.)endif # AB_OTA_UPDATER == trueLOCAL_SRC_FILES += \ boot_control_recovery_stub.ccelse # PRODUCT_STATIC_BOOT_CONTROL_HAL != ""LOCAL_STATIC_LIBRARIES += \ $(PRODUCT_STATIC_BOOT_CONTROL_HAL)endif # PRODUCT_STATIC_BOOT_CONTROL_HAL != ""include $(BUILD_EXECUTABLE)以上代码定义了生成target上可执行模块update_engine_sideload的规则。
从LOCAL_MODULE_PATH := $(TARGET_RECOVERY_ROOT_OUT)/sbin看，生成的文件位于recovery/bin目录下，显然这个应用是在recovery系统下才有的。
从名字中包含sideload看，跟recovery系统中的sideload升级方式有关。实际上在recovery系统下，没有了update_engine的服务端进程，所有升级都通过一个可执行的update_engine_sideload搞定。
另外，这个模块规则的最后指出，如果平台没有单独定义boot_controlHAL的静态库实现，即PRODUCT_STATIC_BOOT_CONTROL_HAL，默认会编译boot_control_recovery_stub.cc文件来替代，但后者其实是个空文件，此时update_engine_sideload并不真正支持sideload方式，而只能用于测试用途。
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# libupdate_engine_client (type: shared_library)# ========================================================include $(CLEAR_VARS)LOCAL_MODULE := libupdate_engine_clientLOCAL_CFLAGS := \ -Wall \ -Werror \ -Wno-unused-parameter \ -DUSE_DBUS=$(local_use_dbus) \ -DUSE_BINDER=$(local_use_binder)LOCAL_CLANG := trueLOCAL_CPP_EXTENSION := .cc# TODO(deymo): Remove "external/cros/system_api/dbus" when dbus is not used.LOCAL_C_INCLUDES := \ $(LOCAL_PATH)/client_library/include \ external/cros/system_api/dbus \ system \ external/gtest/includeLOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/client_library/includeLOCAL_SHARED_LIBRARIES := \ libchrome \ libbrilloLOCAL_SRC_FILES := \ client_library/client.cc \ update_status_utils.cc# We can only compile support for one IPC mechanism. If both "binder" and "dbus"# are defined, we prefer binder.ifeq ($(local_use_binder),1)LOCAL_AIDL_INCLUDES := $(LOCAL_PATH)/binder_bindingsLOCAL_SHARED_LIBRARIES += \ libbinder \ libbrillo-binder \ libutilsLOCAL_SRC_FILES += \ binder_bindings/android/brillo/IUpdateEngine.aidl \ binder_bindings/android/brillo/IUpdateEngineStatusCallback.aidl \ client_library/client_binder.cc \ parcelable_update_engine_status.ccelse # local_use_binder != 1LOCAL_STATIC_LIBRARIES := \ update_engine_client-dbus-proxiesLOCAL_SHARED_LIBRARIES += \ libchrome-dbus \ libbrillo-dbusLOCAL_SRC_FILES += \ client_library/client_dbus.ccendif # local_use_binder == 1include $(BUILD_SHARED_LIBRARY)以上定义了生成静态库libupdate_engine_client的规则，从字面看，这个静态库应该是用于update_engine的客户端的。
第587~625行
# update_engine_client (type: executable)# ========================================================# update_engine console client.include $(CLEAR_VARS)LOCAL_MODULE := update_engine_clientLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_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)ifdef BRILLOLOCAL_SHARED_LIBRARIES += \ libupdate_engine_clientLOCAL_SRC_FILES := \ update_engine_client.cc \ common/error_code_utils.ccelse # !defined(BRILLO)#TODO(deymo): Remove external/cros/system_api/dbus once the strings are moved# out of the DBus interface.LOCAL_C_INCLUDES += \ external/cros/system_api/dbusLOCAL_SHARED_LIBRARIES += \ libbinder \ libbinderwrapper \ libbrillo-binder \ libutilsLOCAL_AIDL_INCLUDES := $(LOCAL_PATH)/binder_bindingsLOCAL_SRC_FILES := \ binder_bindings/android/os/IUpdateEngine.aidl \ binder_bindings/android/os/IUpdateEngineCallback.aidl \ common/error_code_utils.cc \ update_engine_client_android.cc \ update_status_utils.ccendif # !defined(BRILLO)include $(BUILD_EXECUTABLE)以上定义了生成target可执行应用update_engine_client，这个是Android自带的uploade_engine客户端demo应用，实际各Android设备产商会开发自己的Update Engine客户端应用。
第626~714行
# libpayload_generator (type: static_library)# ========================================================# server-side code. This is used for delta_generator and unittests but not# for any client code.ue_libpayload_generator_exported_static_libraries := \ libpayload_consumer \ update_metadata-protos \ liblzma \ $(ue_libpayload_consumer_exported_static_libraries) \ $(ue_update_metadata_protos_exported_static_libraries)ue_libpayload_generator_exported_shared_libraries := \ libext2fs-host \ $(ue_libpayload_consumer_exported_shared_libraries) \ $(ue_update_metadata_protos_exported_shared_libraries)ue_libpayload_generator_src_files := \ payload_generator/ab_generator.cc \ payload_generator/annotated_operation.cc \ payload_generator/blob_file_writer.cc \ payload_generator/block_mapping.cc \ payload_generator/bzip.cc \ payload_generator/cycle_breaker.cc \ payload_generator/delta_diff_generator.cc \ payload_generator/delta_diff_utils.cc \ payload_generator/ext2_filesystem.cc \ payload_generator/extent_ranges.cc \ payload_generator/extent_utils.cc \ payload_generator/full_update_generator.cc \ payload_generator/graph_types.cc \ payload_generator/graph_utils.cc \ payload_generator/inplace_generator.cc \ payload_generator/payload_file.cc \ payload_generator/payload_generation_config.cc \ payload_generator/payload_signer.cc \ payload_generator/raw_filesystem.cc \ payload_generator/tarjan.cc \ payload_generator/topological_sort.cc \ payload_generator/xz_android.ccifeq ($(HOST_OS),linux)# Build for the host.include $(CLEAR_VARS)LOCAL_MODULE := libpayload_generatorLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := $(ue_common_c_includes)LOCAL_STATIC_LIBRARIES := \ libpayload_consumer \ update_metadata-protos \ liblzma \ $(ue_libpayload_consumer_exported_static_libraries) \ $(ue_update_metadata_protos_exported_static_libraries)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libpayload_generator_exported_shared_libraries) \ $(ue_libpayload_consumer_exported_shared_libraries) \ $(ue_update_metadata_protos_exported_shared_libraries)LOCAL_SRC_FILES := $(ue_libpayload_generator_src_files)include $(BUILD_HOST_STATIC_LIBRARY)endif # HOST_OS == linux# Build for the target.include $(CLEAR_VARS)LOCAL_MODULE := libpayload_generatorLOCAL_MODULE_CLASS := STATIC_LIBRARIESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := $(ue_common_c_includes)LOCAL_STATIC_LIBRARIES := \ libpayload_consumer \ update_metadata-protos \ liblzma \ $(ue_libpayload_consumer_exported_static_libraries:-host=) \ $(ue_update_metadata_protos_exported_static_libraries)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libpayload_generator_exported_shared_libraries:-host=) \ $(ue_libpayload_consumer_exported_shared_libraries:-host=) \ $(ue_update_metadata_protos_exported_shared_libraries)LOCAL_SRC_FILES := $(ue_libpayload_generator_src_files)include $(BUILD_STATIC_LIBRARY)以上定义了生成libpayload_generator静态库的两条规则，分别用于host和target。
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# delta_generator (type: executable)# ========================================================# server-side delta generator.ue_delta_generator_src_files := \ payload_generator/generate_delta_main.ccifeq ($(HOST_OS),linux)# Build for the host.include $(CLEAR_VARS)LOCAL_MODULE := delta_generatorLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := $(ue_common_c_includes)LOCAL_STATIC_LIBRARIES := \ libpayload_consumer \ libpayload_generator \ $(ue_libpayload_consumer_exported_static_libraries) \ $(ue_libpayload_generator_exported_static_libraries)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libpayload_consumer_exported_shared_libraries) \ $(ue_libpayload_generator_exported_shared_libraries)LOCAL_SRC_FILES := $(ue_delta_generator_src_files)include $(BUILD_HOST_EXECUTABLE)endif # HOST_OS == linux# Build for the target.include $(CLEAR_VARS)LOCAL_MODULE := delta_generatorLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := $(ue_common_c_includes)LOCAL_STATIC_LIBRARIES := \ libpayload_consumer \ libpayload_generator \ $(ue_libpayload_consumer_exported_static_libraries:-host=) \ $(ue_libpayload_generator_exported_static_libraries:-host=)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libpayload_consumer_exported_shared_libraries:-host=) \ $(ue_libpayload_generator_exported_shared_libraries:-host=)LOCAL_SRC_FILES := $(ue_delta_generator_src_files)include $(BUILD_EXECUTABLE)以上定义了生成可执行应用delta_generator的规则，分别对应于host和target。
第767~976行
# TODO(deymo): Enable the unittest binaries in non-Brillo builds once the DBus# dependencies are removed or placed behind the USE_DBUS flag.ifdef BRILLO# Private and public keys for unittests.# ========================================================# Generate a module that installs a prebuilt private key and a module that# installs a public key generated from the private key.## $(1): The path to the private key in pem format.define ue-unittest-keys $(eval include $(CLEAR_VARS)) \ $(eval LOCAL_MODULE := ue_$(1).pem) \ $(eval LOCAL_MODULE_CLASS := ETC) \ $(eval $(ifeq $(BRILLO), 1, LOCAL_MODULE_TAGS := eng)) \ $(eval LOCAL_SRC_FILES := $(1).pem) \ $(eval LOCAL_MODULE_PATH := \ $(TARGET_OUT_DATA_NATIVE_TESTS)/update_engine_unittests) \ $(eval LOCAL_MODULE_STEM := $(1).pem) \ $(eval include $(BUILD_PREBUILT)) \ \ $(eval include $(CLEAR_VARS)) \ $(eval LOCAL_MODULE := ue_$(1).pub.pem) \ $(eval LOCAL_MODULE_CLASS := ETC) \ $(eval $(ifeq $(BRILLO), 1, LOCAL_MODULE_TAGS := eng)) \ $(eval LOCAL_MODULE_PATH := \ $(TARGET_OUT_DATA_NATIVE_TESTS)/update_engine_unittests) \ $(eval LOCAL_MODULE_STEM := $(1).pub.pem) \ $(eval include $(BUILD_SYSTEM)/base_rules.mk) \ $(eval $(LOCAL_BUILT_MODULE) : $(LOCAL_PATH)/$(1).pem ; \ openssl rsa -in $$< -pubout -out $$@)endef$(call ue-unittest-keys,unittest_key)$(call ue-unittest-keys,unittest_key2)# Sample images for unittests.# ========================================================# Generate a prebuilt module that installs a sample image from the compressed# sample_images.tar.bz2 file used by the unittests.## $(1): The filename in the sample_images.tar.bz2define ue-unittest-sample-image $(eval include $(CLEAR_VARS)) \ $(eval LOCAL_MODULE := ue_unittest_$(1)) \ $(eval LOCAL_MODULE_CLASS := EXECUTABLES) \ $(eval $(ifeq $(BRILLO), 1, LOCAL_MODULE_TAGS := eng)) \ $(eval LOCAL_MODULE_PATH := \ $(TARGET_OUT_DATA_NATIVE_TESTS)/update_engine_unittests/gen) \ $(eval LOCAL_MODULE_STEM := $(1)) \ $(eval include $(BUILD_SYSTEM)/base_rules.mk) \ $(eval $(LOCAL_BUILT_MODULE) : \ $(LOCAL_PATH)/sample_images/sample_images.tar.bz2 ; \ tar -jxf $$< -C $$(dir $$@) $$(notdir $$@) && touch $$@)endef$(call ue-unittest-sample-image,disk_ext2_1k.img)$(call ue-unittest-sample-image,disk_ext2_4k.img)$(call ue-unittest-sample-image,disk_ext2_4k_empty.img)$(call ue-unittest-sample-image,disk_ext2_unittest.img)# Zlib Fingerprint# ========================================================include $(CLEAR_VARS)LOCAL_MODULE := zlib_fingerprintLOCAL_MODULE_CLASS := ETCLOCAL_MODULE_PATH := $(TARGET_OUT_DATA_NATIVE_TESTS)/update_engine_unittestsLOCAL_PREBUILT_MODULE_FILE := $(TARGET_OUT_COMMON_GEN)/zlib_fingerprintinclude $(BUILD_PREBUILT)# test_http_server (type: executable)# ========================================================# Test HTTP Server.include $(CLEAR_VARS)LOCAL_MODULE := test_http_serverifdef BRILLO LOCAL_MODULE_TAGS := engendifLOCAL_MODULE_PATH := $(TARGET_OUT_DATA_NATIVE_TESTS)/update_engine_unittestsLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_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_SRC_FILES := \ common/http_common.cc \ test_http_server.ccinclude $(BUILD_EXECUTABLE)# update_engine_unittests (type: executable)# ========================================================# Main unittest file.include $(CLEAR_VARS)LOCAL_MODULE := update_engine_unittestsifdef BRILLO LOCAL_MODULE_TAGS := engendifLOCAL_REQUIRED_MODULES := \ ue_unittest_disk_ext2_1k.img \ ue_unittest_disk_ext2_4k.img \ ue_unittest_disk_ext2_4k_empty.img \ ue_unittest_disk_ext2_unittest.img \ ue_unittest_key.pem \ ue_unittest_key.pub.pem \ ue_unittest_key2.pem \ ue_unittest_key2.pub.pem \ zlib_fingerprintLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_CPP_EXTENSION := .ccLOCAL_CLANG := trueLOCAL_CFLAGS := $(ue_common_cflags)LOCAL_CPPFLAGS := $(ue_common_cppflags)LOCAL_LDFLAGS := $(ue_common_ldflags)LOCAL_C_INCLUDES := \ $(ue_common_c_includes) \ $(ue_libupdate_engine_exported_c_includes)LOCAL_STATIC_LIBRARIES := \ libupdate_engine \ libpayload_generator \ libbrillo-test-helpers \ libgmock \ libgtest \ libchrome_test_helpers \ $(ue_libupdate_engine_exported_static_libraries:-host=) \ $(ue_libpayload_generator_exported_static_libraries:-host=)LOCAL_SHARED_LIBRARIES := \ $(ue_common_shared_libraries) \ $(ue_libupdate_engine_exported_shared_libraries:-host=) \ $(ue_libpayload_generator_exported_shared_libraries:-host=)LOCAL_SRC_FILES := \ certificate_checker_unittest.cc \ common/action_pipe_unittest.cc \ common/action_processor_unittest.cc \ common/action_unittest.cc \ common/cpu_limiter_unittest.cc \ common/fake_prefs.cc \ common/file_fetcher_unittest.cc \ common/hash_calculator_unittest.cc \ common/http_fetcher_unittest.cc \ common/hwid_override_unittest.cc \ common/mock_http_fetcher.cc \ common/prefs_unittest.cc \ common/subprocess_unittest.cc \ common/terminator_unittest.cc \ common/test_utils.cc \ common/utils_unittest.cc \ common_service_unittest.cc \ connection_manager_unittest.cc \ fake_shill_proxy.cc \ fake_system_state.cc \ metrics_utils_unittest.cc \ omaha_request_action_unittest.cc \ omaha_request_params_unittest.cc \ omaha_response_handler_action_unittest.cc \ p2p_manager_unittest.cc \ payload_consumer/bzip_extent_writer_unittest.cc \ payload_consumer/delta_performer_integration_test.cc \ payload_consumer/delta_performer_unittest.cc \ payload_consumer/download_action_unittest.cc \ payload_consumer/extent_writer_unittest.cc \ payload_consumer/file_writer_unittest.cc \ payload_consumer/filesystem_verifier_action_unittest.cc \ payload_consumer/postinstall_runner_action_unittest.cc \ payload_consumer/xz_extent_writer_unittest.cc \ payload_generator/ab_generator_unittest.cc \ payload_generator/blob_file_writer_unittest.cc \ payload_generator/block_mapping_unittest.cc \ payload_generator/cycle_breaker_unittest.cc \ payload_generator/delta_diff_utils_unittest.cc \ payload_generator/ext2_filesystem_unittest.cc \ payload_generator/extent_ranges_unittest.cc \ payload_generator/extent_utils_unittest.cc \ payload_generator/fake_filesystem.cc \ payload_generator/full_update_generator_unittest.cc \ payload_generator/graph_utils_unittest.cc \ payload_generator/inplace_generator_unittest.cc \ payload_generator/payload_file_unittest.cc \ payload_generator/payload_generation_config_unittest.cc \ payload_generator/payload_signer_unittest.cc \ payload_generator/tarjan_unittest.cc \ payload_generator/topological_sort_unittest.cc \ payload_generator/zip_unittest.cc \ payload_state_unittest.cc \ update_attempter_unittest.cc \ update_manager/boxed_value_unittest.cc \ update_manager/chromeos_policy_unittest.cc \ update_manager/evaluation_context_unittest.cc \ update_manager/generic_variables_unittest.cc \ update_manager/prng_unittest.cc \ update_manager/real_config_provider_unittest.cc \ update_manager/real_device_policy_provider_unittest.cc \ update_manager/real_random_provider_unittest.cc \ update_manager/real_shill_provider_unittest.cc \ update_manager/real_system_provider_unittest.cc \ update_manager/real_time_provider_unittest.cc \ update_manager/real_updater_provider_unittest.cc \ update_manager/umtest_utils.cc \ update_manager/update_manager_unittest.cc \ update_manager/variable_unittest.cc \ testrunner.ccifeq ($(local_use_libcros),1)LOCAL_SRC_FILES += \ chrome_browser_proxy_resolver_unittest.ccendif # local_use_libcros == 1include $(BUILD_NATIVE_TEST)endif # BRILLO从一开始的注释看，以上定义了BRILLO平台的一些单元测试的东东，目前是非BRILLO平台，暂不打算去关心里面到底做了什么，还是那句话，不管你的makefile有多长多复杂，we don't care。
第977~985行
# Weave schema files# ========================================================include $(CLEAR_VARS)LOCAL_MODULE := updater.jsonLOCAL_MODULE_CLASS := ETCLOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/weaved/traitsLOCAL_SRC_FILES := weaved/traits/$(LOCAL_MODULE)include $(BUILD_PREBUILT)定义了预编译的模块updater.json，目前我还不清楚这个模块到底是做什么用途的，知道的大神来指点下。
第986~998行
# Update payload signing public key.# ========================================================ifdef BRILLOinclude $(CLEAR_VARS)LOCAL_MODULE := brillo-update-payload-keyLOCAL_MODULE_CLASS := ETCLOCAL_MODULE_PATH := $(TARGET_OUT_ETC)/update_engineLOCAL_MODULE_STEM := update-payload-key.pub.pemLOCAL_SRC_FILES := update_payload_key/brillo-update-payload-key.pub.pemLOCAL_BUILT_MODULE_STEM := update_payload_key/brillo-update-payload-key.pub.peminclude $(BUILD_PREBUILT)endif # BRILLO定义了预编译规则，用于复制BRILLO平台的公钥，我也不打算去关心了，制作升级包时可能会使用到这个公钥，具体分析升级包制作时再说吧。
第999到1013行
# Brillo update payload generation script# ========================================================ifeq ($(HOST_OS),linux)include $(CLEAR_VARS)LOCAL_SRC_FILES := scripts/brillo_update_payloadLOCAL_MODULE := brillo_update_payloadLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_IS_HOST_MODULE := trueLOCAL_MODULE_TAGS := optionalLOCAL_REQUIRED_MODULES := \ delta_generator \ shflagsinclude $(BUILD_PREBUILT)endif # HOST_OS == linux这里定义了预编译规则，用于复制脚本brillo_update_payload，但是我很不理解的是，为啥不将这个应用包含到ifdef BRILLO宏中去呢？
2. Android.mk模块总结
盘点一下Android.mk中生成的各模块，针对非BRILLO平台主要有以下4类：
# 静态库模块STATIC_LIBRARIES: update_metadata-protos (host, target) libpayload_consumer (host, target) libupdate_engine_android (target) libpayload_generator (host, target)# 可执行模块EXECUTABLES: update_engine (target) update_engine_sideload (target) update_engine_client (target) delta_generator (host, target)# 共享库模块SHARED_LIBRARIES: libupdate_engine_client (target)# 预编译模块PREBUILT: updater.json (target) brillo_update_payload (host)从各模块规则的库依赖规则看，简化后从可执行应用开始的各模块依赖关系如下（仅列举了模块内部相关的库依赖，未列出对非update_engine模块的库依赖，各级箭头表示依赖关系）：
update_engine (target) --> libupdate_engine_android --> libpayload_consumer --> update_metadata-protosupdate_engine_sideload (target) --> update_engine_sideload --> update_metadata-protosupdate_engine_client (target)delta_generator (host) --> libpayload_generator --> libpayload_consumer --> update_metadata-protos!!! 注意了，实际上只有在BRILLO平台上，
update_engine_client才会依赖于libupdate_engine_client：update_engine_client (target) --> libupdate_engine_client
可执行应用后面的括号表示该应用运行的环境，target或host。
总体上，共生成了4可执行个应用，具体为android主系统使用的的服务端update_engine和客户端update_engine_client, recovery系统使用的update_engine_sideload，以及host上的升级包工具delta_generator。这4个可执行应用，部分依赖于4个静态库（update_metadata-protos, libpayload_consumer, libupdate_engine_android, libpayload_generator）和1个共享库（libupdate_engine_client）。
3. 模块对Update Engine文件的依赖
啰嗦一点，再将上面的各个可执行应用或库库文件目标的依赖详细列举出来，如下（没有列举依赖的非Update Engine的库）：
update_metadata-protos (STATIC_LIBRARIES) --> update_metadata.protolibpayload_consumer (STATIC_LIBRARIES) --> common/action_processor.cc common/boot_control_stub.cc common/clock.cc common/constants.cc common/cpu_limiter.cc common/error_code_utils.cc common/hash_calculator.cc common/http_common.cc common/http_fetcher.cc common/file_fetcher.cc common/hwid_override.cc common/multi_range_http_fetcher.cc common/platform_constants_android.cc common/prefs.cc common/subprocess.cc common/terminator.cc common/utils.cc payload_consumer/bzip_extent_writer.cc payload_consumer/delta_performer.cc payload_consumer/download_action.cc payload_consumer/extent_writer.cc payload_consumer/file_descriptor.cc payload_consumer/file_writer.cc payload_consumer/filesystem_verifier_action.cc payload_consumer/install_plan.cc payload_consumer/payload_constants.cc payload_consumer/payload_verifier.cc payload_consumer/postinstall_runner_action.cc payload_consumer/xz_extent_writer.cclibupdate_engine_android (STATIC_LIBRARIES) --> binder_bindings/android/os/IUpdateEngine.aidl binder_bindings/android/os/IUpdateEngineCallback.aidl binder_service_android.cc boot_control_android.cc certificate_checker.cc daemon.cc daemon_state_android.cc hardware_android.cc libcurl_http_fetcher.cc network_selector_android.cc proxy_resolver.cc update_attempter_android.cc update_status_utils.cc utils_android.ccupdate_engine (EXECUTABLES) --> main.ccupdate_engine_sideload (EXECUTABLES) --> boot_control_android.cc hardware_android.cc network_selector_stub.cc proxy_resolver.cc sideload_main.cc update_attempter_android.cc update_status_utils.cc utils_android.cc boot_control_recovery_stub.ccupdate_engine_client (EXECUTABLES) --> binder_bindings/android/os/IUpdateEngine.aidl binder_bindings/android/os/IUpdateEngineCallback.aidl common/error_code_utils.cc update_engine_client_android.cc update_status_utils.cclibpayload_generator (SHARED_LIBRARIES) --> payload_generator/ab_generator.cc payload_generator/annotated_operation.cc payload_generator/blob_file_writer.cc payload_generator/block_mapping.cc payload_generator/bzip.cc payload_generator/cycle_breaker.cc payload_generator/delta_diff_generator.cc payload_generator/delta_diff_utils.cc payload_generator/ext2_filesystem.cc payload_generator/extent_ranges.cc payload_generator/extent_utils.cc payload_generator/full_update_generator.cc payload_generator/graph_types.cc payload_generator/graph_utils.cc payload_generator/inplace_generator.cc payload_generator/payload_file.cc payload_generator/payload_generation_config.cc payload_generator/payload_signer.cc payload_generator/raw_filesystem.cc payload_generator/tarjan.cc payload_generator/topological_sort.cc payload_generator/xz_android.ccdelta_generator (EXECUTABLES) --> payload_generator/generate_delta_main.cc细分到目标对库和文件的依赖后，看起好好像没有那么漫无目的、无从下手的感觉了。
基于上面提出的目标和文件依赖关系，后续可以自顶向下或自底向上对代码进行分析：
- 自顶向下从顶层代码入手，向下分析各层模块，直到最底层的实现，好处是对代码容易有全局观，坏处是开始对底层实现不清楚。
- 自底向上从最底层的小模块开始，层层向上分析，直到最上层的应用逻辑，好处是一开始就了解代码的底层实现，坏处是容易陷入到各个模块中，没有全局观，弄不清楚各模块的关系。
后续从升级场景入手，先分析较简单的客户端update_engine_client，再分析代码复杂的服务端update_engine_client。
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