Android9.0版本Vold服务源码分析

Vold: Volume Daemon,用于管理和控制Android平台外部存储设备的后台进程,这些管理和控制,包括SD卡的插拔事件检测/SD卡挂载/卸载/格式化等.

9.0以前framework java层(StorageManagerService)和native层(Vold)的通信是socket,9.0以后使用binder通信.本文通过SD卡挂载流程,分析binder在vold服务中的使用.

vold服务在开机的时候会启动.定义于system/vold/vold.rc文件中：
service vold /system/bin/vold \
        --blkid_context=u:r:blkid:s0 --blkid_untrusted_context=u:r:blkid_untrusted:s0 \
        --fsck_context=u:r:fsck:s0 --fsck_untrusted_context=u:r:fsck_untrusted:s0
    class core
    ioprio be 2
    writepid /dev/cpuset/foreground/tasks
    shutdown critical
    group reserved_disk

被init进程启动后，将调用system/vold/main.cpp中的main函数,然后初始化VolumeManager,NetlinkManager等等.初始化Vold服务.

在看下StorageManagerService的初始化.StorageManagerService由SystemServer启动，我们简单看看它的启动流程：

//调用SystemServiceManager的startService 
mSystemServiceManager.startService(STORAGE_MANAGER_SERVICE_CLASS);

最终的实现如下:
public void startService(@NonNull final SystemService service) {
        // Register it.
        mServices.add(service);
        // Start it.
        long time = SystemClock.elapsedRealtime();
        try {
            //调用服务的onStart方法.
            service.onStart();
        } catch (RuntimeException ex) {
            throw new RuntimeException("Failed to start service " + service.getClass().getName()
                    + ": onStart threw an exception", ex);
        }
        warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
    }

        //StorageManagerService的onStart方法如下:
        @Override
        public void onStart() {
            //创建StorageManagerService
            mStorageManagerService = new StorageManagerService(getContext());
            //将StorageManagerService注册到ServiceManager中
            publishBinderService("mount", mStorageManagerService);
            //调用StorageManagerService的start方法
            mStorageManagerService.start();
        }

        private void start() {
            connect();
        }

    //主要建立和Vold的通信,获取mVold对象,与Vold进程通信.
    private void connect() {
        IBinder binder = ServiceManager.getService("storaged");
        if (binder != null) {
            try {
                //为binder对象设置死亡代理。当binder死亡后重新建立连接.
                binder.linkToDeath(new DeathRecipient() {
                    @Override
                    public void binderDied() {
                        Slog.w(TAG, "storaged died; reconnecting");
                        mStoraged = null;
                        connect();
                    }
                }, 0);
            } catch (RemoteException e) {
                binder = null;
            }
        }

        if (binder != null) {
            mStoraged = IStoraged.Stub.asInterface(binder);
        } else {
            Slog.w(TAG, "storaged not found; trying again");
        }

        binder = ServiceManager.getService("vold");
        if (binder != null) {
            try {
                //为binder对象设置死亡代理。当binder死亡后重新建立连接.
                binder.linkToDeath(new DeathRecipient() {
                    @Override
                    public void binderDied() {
                        Slog.w(TAG, "vold died; reconnecting");
                        mVold = null;
                        connect();
                    }
                }, 0);
            } catch (RemoteException e) {
                binder = null;
            }
        }

        if (binder != null) {
            //通过binder机制,获取mVold对象(mVold就是IVold对象,这是一个代理类,具体实现在VoldNativeService.cpp),从而和Vold进程进行通信.
            mVold = IVold.Stub.asInterface(binder);
            try {
                //mListener是IVoldListener的具体实现,当vold进程中相关事件改变通过IVoldListener这个代理,通知StorageManagerService处理.
                mVold.setListener(mListener);
            } catch (RemoteException e) {
                mVold = null;
                Slog.w(TAG, "vo