在Android6.0上给外置SD卡增加一个路径

看了前面的博客大家都发现，android6.0上vold对外置sd卡挂载都采用在storage增加一个sd卡的uuid的名字，作为文件名。

但是之前我们系统中很多系统应用都是直接写死的外置sd卡路径，是storage/sdcard1，那我们如何尽量少修改应用，并且在平台上左比较少的改动增加一个storage/scard1的外置sd卡路径呢。

符号链接：

我们熟悉之前的代码就知道：
在PublicVolume的doMount的最后我们增加一个linkExternal函数，把外置sd卡的路径在storage/sdcard1做了一符号链接。

status_t PublicVolume::doMount() {
    // TODO: expand to support mounting other filesystems
    readMetadata();

    if (mFsType != "vfat") {
        LOG(ERROR) << getId() << " unsupported filesystem " << mFsType;
        return -EIO;
    }

    if (vfat::Check(mDevPath)) {
        LOG(ERROR) << getId() << " failed filesystem check";
        return -EIO;
    }

    // Use UUID as stable name, if available
    std::string stableName = getId();
    if (!mFsUuid.empty()) {
        stableName = mFsUuid;
    }

    mRawPath = StringPrintf("/mnt/media_rw/%s", stableName.c_str());

    mFuseDefault = StringPrintf("/mnt/runtime/default/%s", stableName.c_str());
    mFuseRead = StringPrintf("/mnt/runtime/read/%s", stableName.c_str());
    mFuseWrite = StringPrintf("/mnt/runtime/write/%s", stableName.c_str());

    setInternalPath(mRawPath);
    if (getMountFlags() & MountFlags::kVisible) {
        setPath(StringPrintf("/storage/%s", stableName.c_str()));
    } else {
        setPath(mRawPath);
    }

    if (fs_prepare_dir(mRawPath.c_str(), 0700, AID_ROOT, AID_ROOT) ||
            fs_prepare_dir(mFuseDefault.c_str(), 0700, AID_ROOT, AID_ROOT) ||
            fs_prepare_dir(mFuseRead.c_str(), 0700, AID_ROOT, AID_ROOT) ||
            fs_prepare_dir(mFuseWrite.c_str(), 0700, AID_ROOT, AID_ROOT)) {
        PLOG(ERROR) << getId() << " failed to create mount points";
        return -errno;
    }

    if (vfat::Mount(mDevPath, mRawPath, false, false, false,
            AID_MEDIA_RW, AID_MEDIA_RW, 0007, true)) {
        PLOG(ERROR) << getId() << " failed to mount " << mDevPath;
        return -EIO;
    }

    if (getMountFlags() & MountFlags::kPrimary) {
        initAsecStage();
    }

    if (!(getMountFlags() & MountFlags::kVisible)) {
        // Not visible to apps, so no need to spin up FUSE
        return OK;
    }

    dev_t before = GetDevice(mFuseWrite);

    if (!(mFusePid = fork())) {
        if (getMountFlags() & MountFlags::kPrimary) {
            if (execl(kFusePath, kFusePath,
                    "-u", "1023", // AID_MEDIA_RW
                    "-g", "1023", // AID_MEDIA_RW
                    "-U", std::to_string(getMountUserId()).c_str(),
                    "-w",
                    mRawPath.c_str(),
                    stableName.c_str(),
                    NULL)) {
                PLOG(ERROR) << "Failed to exec";
            }
        } else {
            if (execl(kFusePath, kFusePath,
                    "-u", "1023", // AID_MEDIA_RW
                    "-g", "1023", // AID_MEDIA_RW
                    "-U", std::to_string(getMountUserId()).c_str(),
                    mRawPath.c_str(),
                    stableName.c_str(),
                    NULL)) {
                PLOG(ERROR) << "Failed to exec";
            }
        }

        LOG(ERROR) << "FUSE exiting";
        _exit(1);
    }

    if (mFusePid == -1) {
        PLOG(ERROR) << getId() << " failed to fork";
        return -errno;
    }

    while (before == GetDevice(mFuseWrite)) {
        LOG(VERBOSE) << "Waiting for FUSE to spin up...";
        usleep(50000); // 50ms
    }

	linkExternal();

    return OK;
}

void PublicVolume::linkExternal() {
    std::string source(getPath());

    std::string target("/storage/sdcard1");
    if (TEMP_FAILURE_RETRY(unlink(target.c_str()))) {
        if (errno != ENOENT) {
            LOG(VERBOSE) << "Failed to unlink " << target.c_str() << strerror(errno);
        }
    }
    LOG(DEBUG) << "Linking " << source << " to " << target;
    if (TEMP_FAILURE_RETRY(symlink(source.c_str(), target.c_str()))) {
		LOG(ERROR) << "Failed to link " << source.c_str() << " to "
			<< target.c_str() <<  strerror(errno);
    }
}

至于为什么放在doMount的最后因为，启动system/bin/sdcard来进行fuse操作，而在fuse中创建storage下的sd卡的uuid的文件名作为fuse路径。

而我们也正是把这个fuse路径，创建软链接。尽管是在两个进程中，但是从逻辑上还是应该放在后面。

这样就创建了storage/sdcard1这个符号链接，作为外置sd卡的路径。