UserManager 调用 createUser 成功后，会广播消息 Intent.ACTION_USER_ADDED。
StorageManagerSerive 会收到这个通知，然后通知 vold 服务。
if (Intent.ACTION_USER_ADDED.equals(action)) {
final UserManager um = mContext.getSystemService(UserManager.class);
final int userSerialNumber = um.getUserSerialNumber(userId);
mConnector.execute("volume", "user_added", userId, userSerialNumber);
}
system\vold\CommandListener.cpp
} else if (cmd == "user_added" && argc > 3) {
// user_added [user] [serial]
return sendGenericOkFail(cli, vm->onUserAdded(atoi(argv[2]), atoi(argv[3])));
} else if (cmd == "user_removed" && argc > 2) {
// user_removed [user]
return sendGenericOkFail(cli, vm->onUserRemoved(atoi(argv[2])));
} else if (cmd == "user_started" && argc > 2) {
// user_started [user]
return sendGenericOkFail(cli, vm->onUserStarted(atoi(argv[2])));
} else if (cmd == "user_stopped" && argc > 2) {
// user_stopped [user]
return sendGenericOkFail(cli, vm->onUserStopped(atoi(argv[2])));
当这个用户start时，就会调用 VolumeManager 的 onUserStarted 函数：
int VolumeManager::onUserStarted(userid_t userId) {
// Note that sometimes the system will spin up processes from Zygote
// before actually starting the user, so we're okay if Zygote
// already created this directory.
std::string path(StringPrintf("%s/%d", kPathUserMount, userId));
fs_prepare_dir(path.c_str(), 0755, AID_ROOT, AID_ROOT);
mStartedUsers.insert(userId);
if (mPrimary) {
linkPrimary(userId);
}
return 0;
}
fs_prepare_dir就负责创建这个用户的sd卡专有目录。
