多用户管理UserManager

uid = userId * 100000  + appId
SharedAppGid = 40000   + appId

UserHandle OWNER = new UserHandle(USER_OWNER); // 0
UserHandle ALL = new UserHandle(USER_ALL); // -1
UserHandle CURRENT = new UserHandle(USER_CURRENT); // -2
UserHandle CURRENT_OR_SELF = new UserHandle(USER_CURRENT_OR_SELF); // -3

USER      PID   PPID  VSIZE  RSS   WCHAN              PC  NAME
root      1     0     2108   1068  SyS_epoll_ 0000000000 S /init
root      2     0     0      0       kthreadd 0000000000 S kthreadd
root      275   1     13276  1640  hrtimer_na 0000000000 S /system/bin/vold
root      319   1     4148   904   SyS_epoll_ 0000000000 S /system/bin/lmkd
system    320   1     3848   1072  binder_thr 0000000000 S /system/bin/servicemanager
system    321   1     138396 10324 SyS_epoll_ 0000000000 S /system/bin/surfaceflinger
root      331   1     4472   792   __skb_recv 0000000000 S /system/bin/debuggerd64
radio     333   1     71240  4928  hrtimer_na 0000000000 S /system/bin/rild
drm       334   1     20128  2048  binder_thr 0000000000 S /system/bin/drmserver
media     336   1     122404 9536  binder_thr 0000000000 S /system/bin/mediaserver
root      338   1     3880   736   unix_strea 0000000000 S /system/bin/installd
root      359   1     1670784 40732 poll_sched 0000000000 S zygote64
system    1586  359   1880668 134868 SyS_epoll_ 0000000000 S system_server
radio     3341  359   1371948 55264 SyS_epoll_ 0000000000 S com.android.phone
media_rw  3409  275   19676  13828 inotify_re 0000000000 S /system/bin/sdcard
u0_a23    4152  359   1301552 27184 SyS_epoll_ 0000000000 S com.android.incallui
u0_a89    10920 360   1118036 129224 SyS_epoll_ 0000000000 S com.tencent.mobileqq
u0_a94    15481 360   939456 52004 SyS_epoll_ 0000000000 S com.sina.weibo
..

public static void formatUid(StringBuilder sb, int uid) {
     if (uid < Process.FIRST_APPLICATION_UID) {
         sb.append(uid);
     } else {
         sb.append('u');
         sb.append(getUserId(uid));
         final int appId = getAppId(uid);
         if (appId >= Process.FIRST_ISOLATED_UID && appId <= Process.LAST_ISOLATED_UID) {
             sb.append('i');
             sb.append(appId - Process.FIRST_ISOLATED_UID);
         } else if (appId >= Process.FIRST_APPLICATION_UID) {
             sb.append('a');
             sb.append(appId - Process.FIRST_APPLICATION_UID);
         } else {
             sb.append('s');
             sb.append(appId);
         }
     }
 }

u0i20 = 0 * 100000 + (99000 + 20) = 99020    
u1a30 = 1 * 100000 + (10000 + 30) = 110030    
u2s1001 = 2 * 100000 +      1001  = 201001  

//用户启动中
public final static int STATE_BOOTING = 0;
//用户正常运行状态
public final static int STATE_RUNNING = 1;
//用户正在停止中
public final static int STATE_STOPPING = 2;
//用户处于关闭状态
public final static int STATE_SHUTDOWN = 3;

STATE_BOOTING -> STATE_RUNNING -> STATE_STOPPING -> STATE_SHUTDOWN.

public PackageManagerService(...) {
    ...
    //【见小节2.2】
    sUserManager = new UserManagerService(context, this, mInstallLock, mPackages);
    ...
}

UserManagerService(Context context, PackageManagerService pm,
        Object installLock, Object packagesLock) {
    this(context, pm, installLock, packagesLock,
            Environment.getDataDirectory(),
            new File(Environment.getDataDirectory(), "user"));
}

private UserManagerService(Context context, PackageManagerService pm,
       Object installLock, Object packagesLock,
       File dataDir, File baseUserPath) {
   mContext = context;
   mPm = pm;
   mInstallLock = installLock;
   mPackagesLock = packagesLock;
   mHandler = new MainHandler();
   synchronized (mInstallLock) {
       synchronized (mPackagesLock) {
            //创建目录/data/system/users
           mUsersDir = new File(dataDir, USER_INFO_DIR);
           mUsersDir.mkdirs();
           //创建目录/data/system/users/0
           File userZeroDir = new File(mUsersDir, "0");
           userZeroDir.mkdirs();
           FileUtils.setPermissions(mUsersDir.toString(),
                   FileUtils.S_IRWXU|FileUtils.S_IRWXG
                   |FileUtils.S_IROTH|FileUtils.S_IXOTH,
                   -1, -1);
           //mUserListFile文件路径为/data/system/users/userlist.xml
           mUserListFile = new File(mUsersDir, USER_LIST_FILENAME);
           initDefaultGuestRestrictions();
           //解析userlist.xml文件
           readUserListLocked();
           sInstance = this;
       }
   }    }

public boolean bindService(Intent service, ServiceConnection conn,
        int flags) {
    warnIfCallingFromSystemProcess();
    //此处Process.myUserHandle获取的是获取当前进程uid所属的userId来创建UserHandle对象。
    return bindServiceCommon(service, conn, flags, Process.myUserHandle());
}

public ComponentName startService(Intent service) {
    warnIfCallingFromSystemProcess();
    //mUser也是通过Process.myUserHandle()方法获取
    return startServiceCommon(service, mUser);
}

public void sendBroadcast(Intent intent) {
    warnIfCallingFromSystemProcess();
    String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
    try {
        intent.prepareToLeaveProcess();
        ActivityManagerNative.getDefault().broadcastIntent(
                mMainThread.getApplicationThread(), intent, resolvedType, null,
                Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false, false,
                getUserId());
    } catch (RemoteException e) {
        throw new RuntimeException("Failure from system", e);
    }
}

int handleIncomingUser(int callingPid, int callingUid, int userId, boolean allowAll,
        int allowMode, String name, String callerPackage) {
    final int callingUserId = UserHandle.getUserId(callingUid);
    if (callingUserId == userId) {
        return userId;
    }
    
    // userId转换【见下文】
    int targetUserId = unsafeConvertIncomingUser(userId);

    if (callingUid != 0 && callingUid != Process.SYSTEM_UID) {
        final boolean allow;
        ... //对于非system uid，则会进行各种权限检查。
    }
    
    if (!allowAll && targetUserId < 0) {
        throw new IllegalArgumentException(...);
    }
    
    //检查shell权限
    if (callingUid == Process.SHELL_UID && targetUserId >= UserHandle.USER_OWNER) {
        if (mUserManager.hasUserRestriction(UserManager.DISALLOW_DEBUGGING_FEATURES,
                targetUserId)) {
            throw new SecurityException(...);
        }
    }
    return targetUserId;
}

int unsafeConvertIncomingUser(int userId) {
    return (userId == UserHandle.USER_CURRENT || userId == UserHandle.USER_CURRENT_OR_SELF)
            ? mCurrentUserId : userId;
}