Android 存储设备管理 -- MountService

转自:http://blog.csdn.net/new_abc/article/details/7400740

在android usb挂载分析----vold启动,我们的vold模块已经启动了,通信的机制也已经建立起来了,接下来我们分析一下MountService的启动,也就是我们FrameWork层的启动,首先看下其大概流程:


MountService的启动在SystemServer.java中,有如下代码:

 try {
                /*
                 * NotificationManagerService is dependant on MountService,
                 * (for media / usb notifications) so we must start MountService first.
                 */
                Slog.i(TAG, "Mount Service");
                ServiceManager.addService("mount", new MountService(context));
            } catch (Throwable e) {
                Slog.e(TAG, "Failure starting Mount Service", e);
            }

这里new 了一个MountService,并把service添加到了ServiceManager,

我们先看下MountService的声明:

/**
 * MountService implements back-end services for platform storage
 * management.
 * @hide - Applications should use android.os.storage.StorageManager
 * to access the MountService.
 */
class MountService extends IMountService.Stub
        implements INativeDaemonConnectorCallbacks, Watchdog.Monitor {

    private static final boolean LOCAL_LOGD = false;
    private static final boolean DEBUG_UNMOUNT = true;
    private static final boolean DEBUG_EVENTS = true;
    private static final boolean DEBUG_OBB = false;

    // Disable this since it messes up long-running cryptfs operations.
    private static final boolean WATCHDOG_ENABLE = false;

我们看下MountService的构造函数:

    /**
     * Constructs a new MountService instance
     *
     * @param context  Binder context for this service
     */
    public MountService(Context context) {
        mContext = context;

        // XXX: This will go away soon in favor of IMountServiceObserver
        mPms = (PackageManagerService) ServiceManager.getService("package");//获取包管理服务

        mContext.registerReceiver(mBroadcastReceiver,
                new IntentFilter(Intent.ACTION_BOOT_COMPLETED), null, null);//注册广播接收器

        mHandlerThread = new HandlerThread("MountService");//处理消息
        mHandlerThread.start();
        mHandler = new MountServiceHandler(mHandlerThread.getLooper());

        // Add OBB Action Handler to MountService thread.
        mObbActionHandler = new ObbActionHandler(mHandlerThread.getLooper());

        /*
         * Vold does not run in the simulator, so pretend the connector thread
         * ran and did its thing.
         */
        if ("simulator".equals(SystemProperties.get("ro.product.device"))) {
            mReady = true;
            mUmsEnabling = true;
            return;
        }

        /*
         * Create the connection to vold with a maximum queue of twice the
         * amount of containers we'd ever expect to have. This keeps an
         * "asec list" from blocking a thread repeatedly.
         */
        mConnector = new NativeDaemonConnector(this, "vold",
                PackageManagerService.MAX_CONTAINERS * 2, VOLD_TAG);
        mReady = false;
        Thread thread = new Thread(mConnector, VOLD_TAG);
        thread.start();
    }

注:这里是2.3的代码,在4.0上,做了修改。添加了storage_list.xml文件,配合readStorageList()函数。如下

        resources = context.getResources();
        readStorageList(resources);

        if (mPrimaryVolume != null) {
            mExternalStoragePath = mPrimaryVolume.getPath();
            mEmulateExternalStorage = mPrimaryVolume.isEmulated();
            if (mEmulateExternalStorage) {
                Slog.d(TAG, "using emulated external storage");
                mVolumeStates.put(mExternalStoragePath, Environment.MEDIA_MOUNTED);
            }
        }

        // XXX: This will go away soon in favor of IMountServiceObserver
        mPms = (PackageManagerService) ServiceManager.getService("package");

        IntentFilter filter = new IntentFilter();
        filter.addAction(Intent.ACTION_BOOT_COMPLETED);
        filter.addAction("android.intent.action.ACTION_BOOT_IPO");
        filter.addAction("android.action.AMS_SYSTEM_READY");//power off alarm
        filter.addAction(OMADM_USB_ENABLE);
        filter.addAction(OMADM_USB_DISABLE);
        filter.addAction(OMADM_SD_FORMAT);
        filter.addAction(Intent.ACTION_LOCALE_CHANGED);
        filter.addAction(Intent.ACTION_PRE_BOOT_COMPLETED);
        // don't bother monitoring USB if mass storage is not supported on our primary volume
        if (mPrimaryVolume != null && mPrimaryVolume.allowMassStorage()) {
            filter.addAction(UsbManager.ACTION_USB_STATE);
        }
        mContext.registerReceiver(mBroadcastReceiver, filter, null, null);

在readStorageList()里,会解析storage_list.xml文件,获得android:mountPoint,android:storageDescription,android:removable,android:allowMassStorage(标志该块分区是否支持UMS功能),android:primary属性。通过这些属性创建StorageVolume对象,添加到mVolumes陈列。

有一篇文单,介绍怎么挂载U盘,提到了这个文件,详见http://blog.csdn.net/goleftgoright/article/details/8011647


在MountServic构造函数里还new 了一个NativeDaemonConnector,注意这里传递了一个"vold"字符串,跟我们在vold启动的时候传给CommandListener是一样的。

接下来调用 thread.start()启动线程,我们看下它的run函数

public void run() {

        while (true) {
            try {
                listenToSocket();
            } catch (Exception e) {
                Slog.e(TAG, "Error in NativeDaemonConnector", e);
                SystemClock.sleep(5000);
            }
        }
    }
在循环中调用listenToSocket函数,看下这个函数

    private void listenToSocket() throws IOException {
        LocalSocket socket = null;

        try {
            socket = new LocalSocket();
            LocalSocketAddress address = new LocalSocketAddress(mSocket,   //这里mSocket=“vold"
                    LocalSocketAddress.Namespace.RESERVED);              //注意这里的RESERVED

            socket.connect(address);              //连接到vold模块监听的套接字处
            mCallbacks.onDaemonConnected();       //实现在MountService中

            InputStream inputStream = socket.getInputStream();
            mOutputStream = socket.getOutputStream();

            byte[] buffer = new byte[BUFFER_SIZE];
            int start = 0;

            while (true) {
                int count = inputStream.read(buffer, start, BUFFER_SIZE - start); //读取消息
                if (count < 0) break;

                // Add our starting point to the count and reset the start.
                count += start;
                start = 0;

                for (int i = 0; i < count; i++) {
                    if (buffer[i] == 0) {
                        String event = new String(buffer, start, i - start);
                        if (LOCAL_LOGD) Slog.d(TAG, String.format("RCV <- {%s}", event));

                        String[] tokens = event.split(" ");
                        try {
                            int code = Integer.parseInt(tokens[0]);

                            if (code >= ResponseCode.UnsolicitedInformational) {
                                try {
                                    if (!mCallbacks.onEvent(code, event, tokens)) {//实现在MountService中
                                        Slog.w(TAG, String.format(
                                                "Unhandled event (%s)", event));
                                    }
                                } catch (Exception ex) {
                                    Slog.e(TAG, String.format(
                                            "Error handling '%s'", event), ex);
                                }
                            }
                            try {
                                mResponseQueue.put(event);
                            } catch (InterruptedException ex) {
                                Slog.e(TAG, "Failed to put response onto queue", ex);
                            }
                        } catch (NumberFormatException nfe) {
                            Slog.w(TAG, String.format("Bad msg (%s)", event));
                        }
                        start = i + 1;
                    }
                }

                // We should end at the amount we read. If not, compact then
                // buffer and read again.
                if (start != count) {
                    final int remaining = BUFFER_SIZE - start;
                    System.arraycopy(buffer, start, buffer, 0, remaining);
                    start = remaining;
                } else {
                    start = 0;
                }
            }
        } catch (IOException ex) {
            Slog.e(TAG, "Communications error", ex);
            throw ex;
        } finally {
            synchronized (this) {
                if (mOutputStream != null) {
                    try {
                        mOutputStream.close();
                    } catch (IOException e) {
                        Slog.w(TAG, "Failed closing output stream", e);
                    }
                    mOutputStream = null;
                }
            }

            try {
                if (socket != null) {
                    socket.close();
                }
            } catch (IOException ex) {
                Slog.w(TAG, "Failed closing socket", ex);
            }
        }
    }

onDaemonConnected的实现在MountServices吕,将向下下发volume list消息 获取到了磁盘的标签,挂载点与状态,调用connect函数连接到vold模块,connetc最终调用native函数connectLocal进行连接工作,我们看下他的jni层代码,最后调用的:

int socket_local_client_connect(int fd, const char *name, int namespaceId, 
        int type)
{
    struct sockaddr_un addr;
    socklen_t alen;
    size_t namelen;
    int err;

    err = socket_make_sockaddr_un(name, namespaceId, &addr, &alen);

    if (err < 0) {
        goto error;
    }

    if(connect(fd, (struct sockaddr *) &addr, alen) < 0) {
        goto error;
    }

    return fd;

error:
    return -1;
}

/** 
 * connect to peer named "name"
 * returns fd or -1 on error
 */

我们再跟进socket_make_sockaddr_un函数,这时namespaceId传的ANDROID_SOCKET_NAMESPACE_RESERVED,所以会执行下面几句:

 case ANDROID_SOCKET_NAMESPACE_RESERVED:
            namelen = strlen(name) + strlen(ANDROID_RESERVED_SOCKET_PREFIX);
            /* unix_path_max appears to be missing on linux */
            if (namelen > sizeof(*p_addr) 
                    - offsetof(struct sockaddr_un, sun_path) - 1) {
                goto error;
            }

            strcpy(p_addr->sun_path, ANDROID_RESERVED_SOCKET_PREFIX);  //  ANDROID_RESERVED_SOCKET_PREFIX="/dev/socket/"
            strcat(p_addr->sun_path, name);
        break;

注意在前面 connect  函数中的套接字的构造,使用了AF_LOCAL:

int socket_local_client(const char *name, int namespaceId, int type)
{
    int s;

    s = socket(AF_LOCAL, type, 0);
    if(s < 0) return -1;

    if ( 0 > socket_local_client_connect(s, name, namespaceId, type)) {
        close(s);
        return -1;
    }

    return s;
}
这样,就建立了一条从FrameWork层到vold层的通信链路,后面FrameWork层就等待Vold发送消息过来了。。。

FrameWork层的通信也ok了,就可以等待U盘挂载了。。

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