Neglected to add new file in HDFS-5514 (daryn)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1548167 13f79535-47bb-0310-9956-ffa450edef68

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystemLock.java

@@ -0,0 +1,61 @@
+
+/**
+ * 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.hdfs.server.namenode;
+
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+import com.google.common.annotations.VisibleForTesting;
+
+/**
+ * Mimics a ReentrantReadWriteLock so more sophisticated locking capabilities
+ * are possible.
+ */
+class FSNamesystemLock implements ReadWriteLock {
+  @VisibleForTesting
+  protected ReentrantReadWriteLock coarseLock;
+  
+  FSNamesystemLock(boolean fair) {
+    this.coarseLock = new ReentrantReadWriteLock(fair);
+  }
+  
+  @Override
+  public Lock readLock() {
+    return coarseLock.readLock();
+  }
+  
+  @Override
+  public Lock writeLock() {
+    return coarseLock.writeLock();
+  }
+  
+  public int getReadHoldCount() {
+    return coarseLock.getReadHoldCount();
+  }
+  
+  public int getWriteHoldCount() {
+    return coarseLock.getWriteHoldCount();
+  }
+  
+  public boolean isWriteLockedByCurrentThread() {
+    return coarseLock.isWriteLockedByCurrentThread();
+  }
+}