Android 7.0 init.rc的一点改变

在Android 7之前的版本中，系统Native服务，不管它们的可执行文件位于系统什么位置都定义在根分区的init.*.rc文件中。这造成init＊.rc文件臃肿庞大，给维护带来了一些不便，而且其中定义的一些服务的二进制文件根本不存在。

但在Android 7.0中，对该机制做了一些改变 。

1. 单一的init＊.rc，被拆分，服务根据其二进制文件的位置（／system，／vendor，／odm）定义到对应分区的etc／init目录中，每个服务一个rc文件。与该服务相关的触发器、操作等也定义在同一rc文件中。
   
   • ／system/etc/init，包含系统核心服务的定义，如SurfaceFlinger、MediaServer、Logcatd等。
   • /vendor/etc/init， SOC厂商针对SOC核心功能定义的一些服务。比如高通、MTK某一款SOC的相关的服务。
   • ／odm/etc/init，OEM/ODM厂商如小米、华为、OPP其产品所使用的外设以及差异化功能相关的服务。

这样的目录结构拆分，也与Android产品的开发流程相吻合，减轻了维护的负担。下图为Android7.0 模拟器／system/etc/init中定义的服务。

david@daviddeMBP:~$ adb shell
generic_x86:/ $ su
generic_x86:/ # cd /system/etc/init
generic_x86:/system/etc/init # ls
atrace.rc       gatekeeperd.rc mediacodec.rc     rild.rc
audioserver.rc  init-debug.rc  mediadrmserver.rc servicemanager.rc
bootanim.rc     installd.rc    mediaextractor.rc surfaceflinger.rc
bootstat.rc     keystore.rc    mediaserver.rc    uncrypt.rc
cameraserver.rc lmkd.rc        mtpd.rc           vdc.rc
debuggerd.rc    logcatd.rc     netd.rc           vold.rc
drmserver.rc    logd.rc        perfprofd.rc
dumpstate.rc    mdnsd.rc       racoon.rc
generic_x86:/system/etc/init #

2.提供一个编译宏LOCAL_INIT_RC用于将服务相关的RC文件编译到相应位置。这能确保服务定义和服务的可执行文件同时存在，避免了之前出现的服务对应的可执行程序不存在的问题。
例如logcatd的Android.mk中，通过LOCAL_INIT_RC将logcatd对应的logcatd.rc编译到/system/etc/init目录中。

#system/core/logcat/Android.mk
include $(CLEAR_VARS)

LOCAL_MODULE := logpersist.start
LOCAL_MODULE_TAGS := debug
LOCAL_MODULE_CLASS := EXECUTABLES
LOCAL_INIT_RC := logcatd.rc
LOCAL_MODULE_PATH := $(bin_dir)
LOCAL_SRC_FILES := logpersist
ALL_TOOLS := logpersist.start logpersist.stop logpersist.cat
LOCAL_POST_INSTALL_CMD := $(hide) $(foreach t,$(filter-out $(LOCAL_MODULE),$(ALL_TOOLS)),ln -sf $(LOCAL_MODULE) $(TARGET_OUT)/bin/$(t);)
include $(BUILD_PREBUILT)

3.在init执行mount_all指令挂载分区时，会加载这些目录中的rc文件，并在合适的时机运行这些服务和操作。下面位Android7.0模拟器的log。

[    0.912389] init: init first stage started!
－－－－－－－－－加载根分区的rc文件
[    0.996447] init: waitpid failed: No child processes
[    0.996558] init: (Loading properties from /default.prop took 0.00s.)
[    0.997274] init: (Parsing /init.environ.rc took 0.00s.)
[    0.997457] init: (Parsing /init.usb.rc took 0.00s.)
[    0.997561] init: (Parsing /init.ranchu.rc took 0.00s.)
[    0.997823] init: (Parsing /init.usb.configfs.rc took 0.00s.)
[    0.997876] init: (Parsing /init.zygote32.rc took 0.00s.)
[    0.997895] init: (Parsing /init.rc took 0.00s.)
[    1.121126] init: Starting service 'ueventd'...
[    1.121422] init: Waiting for /dev/.coldboot_done...

-----------挂载分区，加载附加rc文件
[    1.384113] EXT4-fs (vdc): Ignoring removed nomblk_io_submit option
[    1.386155] EXT4-fs (vdc): mounted filesystem with ordered data mode. Opts: nomblk_io_submit,errors=panic
[    1.386165] SELinux: initialized (dev vdc, type ext4), uses xattr
[    1.386179] fs_mgr: __mount(source=/dev/block/vdc,target=/data,type=ext4)=0
[    1.387397] init: (Parsing /system/etc/init/atrace.rc took 0.00s.)
[    1.387762] init: (Parsing /system/etc/init/audioserver.rc took 0.00s.)
[    1.388078] init: (Parsing /system/etc/init/bootanim.rc took 0.00s.)
[    1.388424] init: (Parsing /system/etc/init/bootstat.rc took 0.00s.)
[    1.388805] init: (Parsing /system/etc/init/cameraserver.rc took 0.00s.)
[    1.389125] init: (Parsing /system/etc/init/debuggerd.rc took 0.00s.)
[    1.389443] init: (Parsing /system/etc/init/drmserver.rc took 0.00s.)
[    1.389781] init: (Parsing /system/etc/init/dumpstate.rc took 0.00s.)
[    1.392698] init: (Parsing /system/etc/init/gatekeeperd.rc took 0.00s.)
[    1.393060] init: (Parsing /system/etc/init/init-debug.rc took 0.00s.)
[    1.393361] init: (Parsing /system/etc/init/installd.rc took 0.00s.)
[    1.393674] init: (Parsing /system/etc/init/keystore.rc took 0.00s.)
[    1.393992] init: (Parsing /system/etc/init/lmkd.rc took 0.00s.)

相关内容可以参考android 7.0 system/etc/init/readme.txt.