/*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
    *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package org.apache.hadoop.hbase.security.access;
  
  import java.io.ByteArrayInputStream;
  import java.io.DataInput;
  import java.io.DataInputStream;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  import java.util.TreeMap;
  import java.util.TreeSet;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.AuthUtil;
  import org.apache.hadoop.hbase.Cell;
  import org.apache.hadoop.hbase.CellUtil;
  import org.apache.hadoop.hbase.HColumnDescriptor;
  import org.apache.hadoop.hbase.HConstants;
  import org.apache.hadoop.hbase.HTableDescriptor;
  import org.apache.hadoop.hbase.NamespaceDescriptor;
  import org.apache.hadoop.hbase.TableName;
  import org.apache.hadoop.hbase.Tag;
  import org.apache.hadoop.hbase.TagType;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.client.Connection;
  import org.apache.hadoop.hbase.client.ConnectionFactory;
  import org.apache.hadoop.hbase.client.Delete;
  import org.apache.hadoop.hbase.client.Get;
  import org.apache.hadoop.hbase.client.Put;
  import org.apache.hadoop.hbase.client.Result;
  import org.apache.hadoop.hbase.client.ResultScanner;
  import org.apache.hadoop.hbase.client.Scan;
  import org.apache.hadoop.hbase.client.Table;
  import org.apache.hadoop.hbase.exceptions.DeserializationException;
  import org.apache.hadoop.hbase.filter.CompareFilter.CompareOp;
  import org.apache.hadoop.hbase.filter.QualifierFilter;
  import org.apache.hadoop.hbase.filter.RegexStringComparator;
  import org.apache.hadoop.hbase.master.MasterServices;
  import org.apache.hadoop.hbase.protobuf.ProtobufUtil;
  import org.apache.hadoop.hbase.protobuf.generated.AccessControlProtos;
  import org.apache.hadoop.hbase.regionserver.BloomType;
  import org.apache.hadoop.hbase.regionserver.InternalScanner;
  import org.apache.hadoop.hbase.regionserver.Region;
  import org.apache.hadoop.hbase.security.User;
  import org.apache.hadoop.hbase.util.Bytes;
  import org.apache.hadoop.hbase.util.Pair;
  import org.apache.hadoop.io.Text;
  
  import com.google.common.collect.ArrayListMultimap;
  import com.google.common.collect.ListMultimap;
  import com.google.common.collect.Lists;
  
  /**
   * Maintains lists of permission grants to users and groups to allow for
   * authorization checks by {@link AccessController}.
   *
   * <p>
   * Access control lists are stored in an "internal" metadata table named
   * {@code _acl_}. Each table's permission grants are stored as a separate row,
   * keyed by the table name. KeyValues for permissions assignments are stored
   * in one of the formats:
   * <pre>
   * Key                      Desc
   * --------                 --------
   * user                     table level permissions for a user [R=read, W=write]
   * group                    table level permissions for a group
   * user,family              column family level permissions for a user
   * group,family             column family level permissions for a group
   * user,family,qualifier    column qualifier level permissions for a user
   * group,family,qualifier   column qualifier level permissions for a group
   * </pre>
   * <p>
   * All values are encoded as byte arrays containing the codes from the
   * org.apache.hadoop.hbase.security.access.TablePermission.Action enum.
   * </p>
   */
 @InterfaceAudience.Private
 public class AccessControlLists {
   /** Internal storage table for access control lists */
   public static final TableName ACL_TABLE_NAME =
       TableName.valueOf(NamespaceDescriptor.SYSTEM_NAMESPACE_NAME_STR, "acl");
   public static final byte[] ACL_GLOBAL_NAME = ACL_TABLE_NAME.getName();
   /** Column family used to store ACL grants */
   public static final String ACL_LIST_FAMILY_STR = "l";
   public static final byte[] ACL_LIST_FAMILY = Bytes.toBytes(ACL_LIST_FAMILY_STR);
   /** KV tag to store per cell access control lists */
   public static final byte ACL_TAG_TYPE = TagType.ACL_TAG_TYPE;
 
   public static final char NAMESPACE_PREFIX = '@';
 
   /**
    * Delimiter to separate user, column family, and qualifier in
    * _acl_ table info: column keys */
   public static final char ACL_KEY_DELIMITER = ',';
 
   private static final Log LOG = LogFactory.getLog(AccessControlLists.class);
 
   /**
    * Create the ACL table
    * @param master
    * @throws IOException
    */
   static void createACLTable(MasterServices master) throws IOException {
     /** Table descriptor for ACL table */
     final HTableDescriptor ACL_TABLEDESC = new HTableDescriptor(ACL_TABLE_NAME)
       .addFamily(new HColumnDescriptor(ACL_LIST_FAMILY)
         .setMaxVersions(1)
         .setInMemory(true)
         .setBlockCacheEnabled(true)
         .setBlocksize(8 * 1024)
         .setBloomFilterType(BloomType.NONE)
         .setScope(HConstants.REPLICATION_SCOPE_LOCAL)
         // Set cache data blocks in L1 if more than one cache tier deployed; e.g. this will
         // be the case if we are using CombinedBlockCache (Bucket Cache).
         .setCacheDataInL1(true));
     master.createSystemTable(ACL_TABLEDESC);
   }
 
   /**
    * Stores a new user permission grant in the access control lists table.
    * @param conf the configuration
    * @param userPerm the details of the permission to be granted
    * @param t acl table instance. It is closed upon method return
    * @throws IOException in the case of an error accessing the metadata table
    */
   static void addUserPermission(Configuration conf, UserPermission userPerm, Table t)
       throws IOException {
     Permission.Action[] actions = userPerm.getActions();
     byte[] rowKey = userPermissionRowKey(userPerm);
     Put p = new Put(rowKey);
     byte[] key = userPermissionKey(userPerm);
 
     if ((actions == null) || (actions.length == 0)) {
       String msg = "No actions associated with user '" + Bytes.toString(userPerm.getUser()) + "'";
       LOG.warn(msg);
       throw new IOException(msg);
     }
 
     byte[] value = new byte[actions.length];
     for (int i = 0; i < actions.length; i++) {
       value[i] = actions[i].code();
     }
     p.addImmutable(ACL_LIST_FAMILY, key, value);
     if (LOG.isDebugEnabled()) {
       LOG.debug("Writing permission with rowKey "+
           Bytes.toString(rowKey)+" "+
           Bytes.toString(key)+": "+Bytes.toStringBinary(value)
       );
     }
     if (t == null) {
       try (Connection connection = ConnectionFactory.createConnection(conf)) {
         try (Table table = connection.getTable(ACL_TABLE_NAME)) {
           table.put(p);
         }
       }
     } else {
       try {
         t.put(p);
       } finally {
         t.close();
       }
     }
   }
 
   /**
    * Removes a previously granted permission from the stored access control
    * lists.  The {@link TablePermission} being removed must exactly match what
    * is stored -- no wildcard matching is attempted.  Ie, if user "bob" has
    * been granted "READ" access to the "data" table, but only to column family
    * plus qualifier "info:colA", then trying to call this method with only
    * user "bob" and the table name "data" (but without specifying the
    * column qualifier "info:colA") will have no effect.
    *
    * @param conf the configuration
    * @param userPerm the details of the permission to be revoked
    * @param t acl table
    * @throws IOException if there is an error accessing the metadata table
    */
   static void removeUserPermission(Configuration conf, UserPermission userPerm, Table t)
       throws IOException {
     if (null == userPerm.getActions()) {
       removePermissionRecord(conf, userPerm, t);
     } else {
       // Get all the global user permissions from the acl table
       List<UserPermission> permsList = getUserPermissions(conf, userPermissionRowKey(userPerm));
       List<Permission.Action> remainingActions = new ArrayList<>();
       List<Permission.Action> dropActions = Arrays.asList(userPerm.getActions());
       for (UserPermission perm : permsList) {
         // Find the user and remove only the requested permissions
         if (Bytes.toString(perm.getUser()).equals(Bytes.toString(userPerm.getUser()))) {
           for (Permission.Action oldAction : perm.getActions()) {
             if (!dropActions.contains(oldAction)) {
               remainingActions.add(oldAction);
             }
           }
           if (!remainingActions.isEmpty()) {
             perm.setActions(remainingActions.toArray(new Permission.Action[remainingActions.size()]));
             addUserPermission(conf, perm, t);
           } else {
             removePermissionRecord(conf, userPerm, t);
           }
           break;
         }
       }
     }
     if (LOG.isDebugEnabled()) {
       LOG.debug("Removed permission "+ userPerm.toString());
     }
   }
 
   private static void removePermissionRecord(Configuration conf, UserPermission userPerm, Table t)
       throws IOException {
     Delete d = new Delete(userPermissionRowKey(userPerm));
     d.addColumns(ACL_LIST_FAMILY, userPermissionKey(userPerm));
     if (t == null) {
       try (Connection connection = ConnectionFactory.createConnection(conf)) {
         try (Table table = connection.getTable(ACL_TABLE_NAME)) {
           table.delete(d);
         }
       }
     } else {
       try {
         t.delete(d);
       } finally {
         t.close();
       }
     }
   }
 
   /**
    * Remove specified table from the _acl_ table.
    */
   static void removeTablePermissions(Configuration conf, TableName tableName, Table t)
       throws IOException{
     Delete d = new Delete(tableName.getName());
 
     if (LOG.isDebugEnabled()) {
       LOG.debug("Removing permissions of removed table "+ tableName);
     }
     try {
       t.delete(d);
     } finally {
       t.close();
     }
   }
 
   /**
    * Remove specified namespace from the acl table.
    */
   static void removeNamespacePermissions(Configuration conf, String namespace, Table t)
       throws IOException{
     Delete d = new Delete(Bytes.toBytes(toNamespaceEntry(namespace)));
 
     if (LOG.isDebugEnabled()) {
       LOG.debug("Removing permissions of removed namespace "+ namespace);
     }
 
     try {
       t.delete(d);
     } finally {
       t.close();
     }
   }
 
   /**
    * Remove specified table column from the acl table.
    */
   static void removeTablePermissions(TableName tableName, byte[] column, Table table,
       boolean closeTable) throws IOException{
 
     if (LOG.isDebugEnabled()) {
       LOG.debug("Removing permissions of removed column " + Bytes.toString(column) +
                 " from table "+ tableName);
     }
     Scan scan = new Scan();
     scan.addFamily(ACL_LIST_FAMILY);
 
     String columnName = Bytes.toString(column);
     scan.setFilter(new QualifierFilter(CompareOp.EQUAL, new RegexStringComparator(
         String.format("(%s%s%s)|(%s%s)$",
             ACL_KEY_DELIMITER, columnName, ACL_KEY_DELIMITER,
             ACL_KEY_DELIMITER, columnName))));
 
     Set<byte[]> qualifierSet = new TreeSet<byte[]>(Bytes.BYTES_COMPARATOR);
     ResultScanner scanner = null;
     try {
       scanner = table.getScanner(scan);
       for (Result res : scanner) {
         for (byte[] q : res.getFamilyMap(ACL_LIST_FAMILY).navigableKeySet()) {
           qualifierSet.add(q);
         }
       }
 
       if (qualifierSet.size() > 0) {
         Delete d = new Delete(tableName.getName());
         for (byte[] qualifier : qualifierSet) {
           d.addColumns(ACL_LIST_FAMILY, qualifier);
         }
         table.delete(d);
       }
     }  finally {
       if (scanner != null) scanner.close();
       if (closeTable) table.close();
     }
   }
 
   static void removeTablePermissions(Configuration conf, TableName tableName, byte[] column,
       Table t) throws IOException {
     if (LOG.isDebugEnabled()) {
       LOG.debug("Removing permissions of removed column " + Bytes.toString(column) +
           " from table "+ tableName);
     }
     removeTablePermissions(tableName, column, t, true);
   }
 
   static byte[] userPermissionRowKey(UserPermission userPerm) {
     byte[] row;
     if(userPerm.hasNamespace()) {
       row = Bytes.toBytes(toNamespaceEntry(userPerm.getNamespace()));
     } else if(userPerm.isGlobal()) {
       row = ACL_GLOBAL_NAME;
     } else {
       row = userPerm.getTableName().getName();
     }
     return row;
   }
 
   /**
    * Build qualifier key from user permission:
    *  username
    *  username,family
    *  username,family,qualifier
    */
   static byte[] userPermissionKey(UserPermission userPerm) {
     byte[] qualifier = userPerm.getQualifier();
     byte[] family = userPerm.getFamily();
     byte[] key = userPerm.getUser();
 
     if (family != null && family.length > 0) {
       key = Bytes.add(key, Bytes.add(new byte[]{ACL_KEY_DELIMITER}, family));
       if (qualifier != null && qualifier.length > 0) {
         key = Bytes.add(key, Bytes.add(new byte[]{ACL_KEY_DELIMITER}, qualifier));
       }
     }
 
     return key;
   }
 
   /**
    * Returns {@code true} if the given region is part of the {@code _acl_}
    * metadata table.
    */
   static boolean isAclRegion(Region region) {
     return ACL_TABLE_NAME.equals(region.getTableDesc().getTableName());
   }
 
   /**
    * Returns {@code true} if the given table is {@code _acl_} metadata table.
    */
   static boolean isAclTable(HTableDescriptor desc) {
     return ACL_TABLE_NAME.equals(desc.getTableName());
   }
 
   /**
    * Loads all of the permission grants stored in a region of the {@code _acl_}
    * table.
    *
    * @param aclRegion
    * @return a map of the permissions for this table.
    * @throws IOException
    */
   static Map<byte[], ListMultimap<String,TablePermission>> loadAll(Region aclRegion)
     throws IOException {
 
     if (!isAclRegion(aclRegion)) {
       throw new IOException("Can only load permissions from "+ACL_TABLE_NAME);
     }
 
     Map<byte[], ListMultimap<String, TablePermission>> allPerms =
         new TreeMap<byte[], ListMultimap<String, TablePermission>>(Bytes.BYTES_RAWCOMPARATOR);
 
     // do a full scan of _acl_ table
 
     Scan scan = new Scan();
     scan.addFamily(ACL_LIST_FAMILY);
 
     InternalScanner iScanner = null;
     try {
       iScanner = aclRegion.getScanner(scan);
 
       while (true) {
         List<Cell> row = new ArrayList<Cell>();
 
         boolean hasNext = iScanner.next(row);
         ListMultimap<String,TablePermission> perms = ArrayListMultimap.create();
         byte[] entry = null;
         for (Cell kv : row) {
           if (entry == null) {
             entry = CellUtil.cloneRow(kv);
           }
           Pair<String,TablePermission> permissionsOfUserOnTable =
               parsePermissionRecord(entry, kv);
           if (permissionsOfUserOnTable != null) {
             String username = permissionsOfUserOnTable.getFirst();
             TablePermission permissions = permissionsOfUserOnTable.getSecond();
             perms.put(username, permissions);
           }
         }
         if (entry != null) {
           allPerms.put(entry, perms);
         }
         if (!hasNext) {
           break;
         }
       }
     } finally {
       if (iScanner != null) {
         iScanner.close();
       }
     }
 
     return allPerms;
   }
 
   /**
    * Load all permissions from the region server holding {@code _acl_},
    * primarily intended for testing purposes.
    */
   static Map<byte[], ListMultimap<String,TablePermission>> loadAll(
       Configuration conf) throws IOException {
     Map<byte[], ListMultimap<String,TablePermission>> allPerms =
         new TreeMap<byte[], ListMultimap<String,TablePermission>>(Bytes.BYTES_RAWCOMPARATOR);
 
     // do a full scan of _acl_, filtering on only first table region rows
 
     Scan scan = new Scan();
     scan.addFamily(ACL_LIST_FAMILY);
 
     ResultScanner scanner = null;
     // TODO: Pass in a Connection rather than create one each time.
     try (Connection connection = ConnectionFactory.createConnection(conf)) {
       try (Table table = connection.getTable(ACL_TABLE_NAME)) {
         scanner = table.getScanner(scan);
         try {
           for (Result row : scanner) {
             ListMultimap<String,TablePermission> resultPerms = parsePermissions(row.getRow(), row);
             allPerms.put(row.getRow(), resultPerms);
           }
         } finally {
           if (scanner != null) scanner.close();
         }
       }
     }
 
     return allPerms;
   }
 
   static ListMultimap<String, TablePermission> getTablePermissions(Configuration conf,
         TableName tableName) throws IOException {
     return getPermissions(conf, tableName != null ? tableName.getName() : null, null);
   }
 
   static ListMultimap<String, TablePermission> getNamespacePermissions(Configuration conf,
         String namespace) throws IOException {
     return getPermissions(conf, Bytes.toBytes(toNamespaceEntry(namespace)), null);
   }
 
   /**
    * Reads user permission assignments stored in the <code>l:</code> column
    * family of the first table row in <code>_acl_</code>.
    *
    * <p>
    * See {@link AccessControlLists class documentation} for the key structure
    * used for storage.
    * </p>
    */
   static ListMultimap<String, TablePermission> getPermissions(Configuration conf,
       byte[] entryName, Table t) throws IOException {
     if (entryName == null) entryName = ACL_GLOBAL_NAME;
 
     // for normal user tables, we just read the table row from _acl_
     ListMultimap<String, TablePermission> perms = ArrayListMultimap.create();
     Get get = new Get(entryName);
     get.addFamily(ACL_LIST_FAMILY);
     Result row = null;
     if (t == null) {
       try (Connection connection = ConnectionFactory.createConnection(conf)) {
         try (Table table = connection.getTable(ACL_TABLE_NAME)) {
           row = table.get(get);
         }
       }
     } else {
       row = t.get(get);
     }
 
     if (!row.isEmpty()) {
       perms = parsePermissions(entryName, row);
     } else {
       LOG.info("No permissions found in " + ACL_TABLE_NAME + " for acl entry "
           + Bytes.toString(entryName));
     }
 
     return perms;
   }
 
   /**
    * Returns the currently granted permissions for a given table as a list of
    * user plus associated permissions.
    */
   static List<UserPermission> getUserTablePermissions(
       Configuration conf, TableName tableName) throws IOException {
     return getUserPermissions(conf, tableName == null ? null : tableName.getName());
   }
 
   static List<UserPermission> getUserNamespacePermissions(
       Configuration conf, String namespace) throws IOException {
     return getUserPermissions(conf, Bytes.toBytes(toNamespaceEntry(namespace)));
   }
 
   static List<UserPermission> getUserPermissions(
       Configuration conf, byte[] entryName)
   throws IOException {
     ListMultimap<String,TablePermission> allPerms = getPermissions(
       conf, entryName, null);
 
     List<UserPermission> perms = new ArrayList<UserPermission>();
 
     if(isNamespaceEntry(entryName)) {  // Namespace
       for (Map.Entry<String, TablePermission> entry : allPerms.entries()) {
         UserPermission up = new UserPermission(Bytes.toBytes(entry.getKey()),
           entry.getValue().getNamespace(), entry.getValue().getActions());
         perms.add(up);
       }
     } else {  // Table
       for (Map.Entry<String, TablePermission> entry : allPerms.entries()) {
         UserPermission up = new UserPermission(Bytes.toBytes(entry.getKey()),
             entry.getValue().getTableName(), entry.getValue().getFamily(),
             entry.getValue().getQualifier(), entry.getValue().getActions());
         perms.add(up);
       }
     }
     return perms;
   }
 
   private static ListMultimap<String, TablePermission> parsePermissions(
       byte[] entryName, Result result) {
     ListMultimap<String, TablePermission> perms = ArrayListMultimap.create();
     if (result != null && result.size() > 0) {
       for (Cell kv : result.rawCells()) {
 
         Pair<String,TablePermission> permissionsOfUserOnTable =
             parsePermissionRecord(entryName, kv);
 
         if (permissionsOfUserOnTable != null) {
           String username = permissionsOfUserOnTable.getFirst();
           TablePermission permissions = permissionsOfUserOnTable.getSecond();
           perms.put(username, permissions);
         }
       }
     }
     return perms;
   }
 
   private static Pair<String, TablePermission> parsePermissionRecord(
       byte[] entryName, Cell kv) {
     // return X given a set of permissions encoded in the permissionRecord kv.
     byte[] family = CellUtil.cloneFamily(kv);
 
     if (!Bytes.equals(family, ACL_LIST_FAMILY)) {
       return null;
     }
 
     byte[] key = CellUtil.cloneQualifier(kv);
     byte[] value = CellUtil.cloneValue(kv);
     if (LOG.isDebugEnabled()) {
       LOG.debug("Read acl: kv ["+
                 Bytes.toStringBinary(key)+": "+
                 Bytes.toStringBinary(value)+"]");
     }
 
     // check for a column family appended to the key
     // TODO: avoid the string conversion to make this more efficient
     String username = Bytes.toString(key);
 
     //Handle namespace entry
     if(isNamespaceEntry(entryName)) {
       return new Pair<String, TablePermission>(username,
           new TablePermission(Bytes.toString(fromNamespaceEntry(entryName)), value));
     }
 
     //Handle table and global entry
     //TODO global entry should be handled differently
     int idx = username.indexOf(ACL_KEY_DELIMITER);
     byte[] permFamily = null;
     byte[] permQualifier = null;
     if (idx > 0 && idx < username.length()-1) {
       String remainder = username.substring(idx+1);
       username = username.substring(0, idx);
       idx = remainder.indexOf(ACL_KEY_DELIMITER);
       if (idx > 0 && idx < remainder.length()-1) {
         permFamily = Bytes.toBytes(remainder.substring(0, idx));
         permQualifier = Bytes.toBytes(remainder.substring(idx+1));
       } else {
         permFamily = Bytes.toBytes(remainder);
       }
     }
 
     return new Pair<String,TablePermission>(username,
         new TablePermission(TableName.valueOf(entryName), permFamily, permQualifier, value));
   }
 
   /**
    * Writes a set of permissions as {@link org.apache.hadoop.io.Writable} instances
    * and returns the resulting byte array.
    *
    * Writes a set of permission [user: table permission]
    */
   public static byte[] writePermissionsAsBytes(ListMultimap<String, TablePermission> perms,
       Configuration conf) {
     return ProtobufUtil.prependPBMagic(ProtobufUtil.toUserTablePermissions(perms).toByteArray());
   }
 
   /**
    * Reads a set of permissions as {@link org.apache.hadoop.io.Writable} instances
    * from the input stream.
    */
   public static ListMultimap<String, TablePermission> readPermissions(byte[] data,
       Configuration conf)
   throws DeserializationException {
     if (ProtobufUtil.isPBMagicPrefix(data)) {
       int pblen = ProtobufUtil.lengthOfPBMagic();
       try {
         AccessControlProtos.UsersAndPermissions.Builder builder =
           AccessControlProtos.UsersAndPermissions.newBuilder();
         ProtobufUtil.mergeFrom(builder, data, pblen, data.length - pblen);
         return ProtobufUtil.toUserTablePermissions(builder.build());
       } catch (IOException e) {
         throw new DeserializationException(e);
       }
     } else {
       ListMultimap<String,TablePermission> perms = ArrayListMultimap.create();
       try {
         DataInput in = new DataInputStream(new ByteArrayInputStream(data));
         int length = in.readInt();
         for (int i=0; i<length; i++) {
           String user = Text.readString(in);
           List<TablePermission> userPerms =
             (List)HbaseObjectWritableFor96Migration.readObject(in, conf);
           perms.putAll(user, userPerms);
         }
       } catch (IOException e) {
         throw new DeserializationException(e);
       }
       return perms;
     }
   }
 
   public static boolean isNamespaceEntry(String entryName) {
     return entryName != null && entryName.charAt(0) == NAMESPACE_PREFIX;
   }
 
   public static boolean isNamespaceEntry(byte[] entryName) {
     return entryName != null && entryName.length !=0 && entryName[0] == NAMESPACE_PREFIX;
   }
 
   public static String toNamespaceEntry(String namespace) {
      return NAMESPACE_PREFIX + namespace;
    }
 
    public static String fromNamespaceEntry(String namespace) {
      if(namespace.charAt(0) != NAMESPACE_PREFIX)
        throw new IllegalArgumentException("Argument is not a valid namespace entry");
      return namespace.substring(1);
    }
 
    public static byte[] toNamespaceEntry(byte[] namespace) {
      byte[] ret = new byte[namespace.length+1];
      ret[0] = NAMESPACE_PREFIX;
      System.arraycopy(namespace, 0, ret, 1, namespace.length);
      return ret;
    }
 
    public static byte[] fromNamespaceEntry(byte[] namespace) {
      if(namespace[0] != NAMESPACE_PREFIX) {
        throw new IllegalArgumentException("Argument is not a valid namespace entry: " +
            Bytes.toString(namespace));
      }
      return Arrays.copyOfRange(namespace, 1, namespace.length);
    }
 
    public static List<Permission> getCellPermissionsForUser(User user, Cell cell)
        throws IOException {
      // Save an object allocation where we can
      if (cell.getTagsLength() == 0) {
        return null;
      }
      List<Permission> results = Lists.newArrayList();
      Iterator<Tag> tagsIterator = CellUtil.tagsIterator(cell.getTagsArray(), cell.getTagsOffset(),
         cell.getTagsLength());
      while (tagsIterator.hasNext()) {
        Tag tag = tagsIterator.next();
        if (tag.getType() == ACL_TAG_TYPE) {
          // Deserialize the table permissions from the KV
          // TODO: This can be improved. Don't build UsersAndPermissions just to unpack it again,
          // use the builder
          AccessControlProtos.UsersAndPermissions.Builder builder = 
            AccessControlProtos.UsersAndPermissions.newBuilder();
          ProtobufUtil.mergeFrom(builder, tag.getBuffer(), tag.getTagOffset(), tag.getTagLength());
          ListMultimap<String,Permission> kvPerms =
            ProtobufUtil.toUsersAndPermissions(builder.build());
          // Are there permissions for this user?
          List<Permission> userPerms = kvPerms.get(user.getShortName());
          if (userPerms != null) {
            results.addAll(userPerms);
          }
          // Are there permissions for any of the groups this user belongs to?
          String groupNames[] = user.getGroupNames();
          if (groupNames != null) {
            for (String group : groupNames) {
              List<Permission> groupPerms = kvPerms.get(AuthUtil.toGroupEntry(group));
              if (results != null) {
                results.addAll(groupPerms);
              }
            }
          }
        }
      }
      return results;
    }
 }