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1   /**
2    *
3    * Licensed to the Apache Software Foundation (ASF) under one
4    * or more contributor license agreements.  See the NOTICE file
5    * distributed with this work for additional information
6    * regarding copyright ownership.  The ASF licenses this file
7    * to you under the Apache License, Version 2.0 (the
8    * "License"); you may not use this file except in compliance
9    * with the License.  You may obtain a copy of the License at
10   *
11   *     http://www.apache.org/licenses/LICENSE-2.0
12   *
13   * Unless required by applicable law or agreed to in writing, software
14   * distributed under the License is distributed on an "AS IS" BASIS,
15   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16   * See the License for the specific language governing permissions and
17   * limitations under the License.
18   */
19  
20  package org.apache.hadoop.hbase.util;
21  
22  import java.util.concurrent.atomic.AtomicLong;
23  
24  /**
25   * A simple barrier that can be used by classes that need to wait for some operations to
26   * finish before stopping/closing/etc. forever.
27   */
28  public class DrainBarrier {
29    /**
30     * Contains the number of outstanding operations, as well as flags.
31     * Initially, the number of operations is 1. Each beginOp increments, and endOp decrements it.
32     * beginOp does not proceed when it sees the draining flag. When stop is called, it atomically
33     * decrements the number of operations (the initial 1) and sets the draining flag. If stop did
34     * the decrement to zero, that means there are no more operations outstanding, so stop is done.
35     * Otherwise, stop blocks, and the endOp that decrements the count to 0 unblocks it.
36     */
37    private final AtomicLong valueAndFlags = new AtomicLong(inc(0));
38    private final static long DRAINING_FLAG = 0x1;
39    private final static int FLAG_BIT_COUNT = 1;
40  
41    /**
42     * Tries to start an operation.
43     * @return false iff the stop is in progress, and the operation cannot be started.
44     */
45    public boolean beginOp() {
46      long oldValAndFlags;
47      do {
48        oldValAndFlags = valueAndFlags.get();
49        if (isDraining(oldValAndFlags)) return false;
50      } while (!valueAndFlags.compareAndSet(oldValAndFlags, inc(oldValAndFlags)));
51      return true;
52    }
53  
54    /**
55     * Ends the operation. Unblocks the blocked caller of stop, if necessary.
56     */
57    @edu.umd.cs.findbugs.annotations.SuppressWarnings(value="NN_NAKED_NOTIFY",
58        justification="First, we do change the state before notify, 2nd, it doesn't even matter")
59    public void endOp() {
60      long oldValAndFlags;
61      do {
62        oldValAndFlags = valueAndFlags.get();
63        long unacceptableCount = isDraining(oldValAndFlags) ? 0 : 1;
64        if (getValue(oldValAndFlags) == unacceptableCount) {
65          throw new AssertionError("endOp called without corresponding beginOp call ("
66            + "the current count is " + unacceptableCount + ")");
67        }
68      } while (!valueAndFlags.compareAndSet(oldValAndFlags, dec(oldValAndFlags)));
69      if (getValue(oldValAndFlags) == 1) {
70        synchronized (this) { this.notifyAll(); }
71      }
72    }
73  
74    /**
75     * Blocks new operations from starting, waits for the current ones to drain.
76     * If someone already called it, returns immediately, which is currently unavoidable as
77     * most of the users stop and close things right and left, and hope for the best.
78     * stopAndWaitForOpsOnce asserts instead.
79     * @throws InterruptedException the wait for operations has been interrupted.
80     */
81    public void stopAndDrainOps() throws InterruptedException {
82      stopAndDrainOps(true);
83    }
84  
85    /**
86     * Blocks new operations from starting, waits for the current ones to drain.
87     * Can only be called once.
88     * @throws InterruptedException the wait for operations has been interrupted.
89     */
90    public void stopAndDrainOpsOnce() throws InterruptedException {
91      stopAndDrainOps(false);
92    }
93  
94    /**
95     * @param ignoreRepeatedCalls If this is true and somebody already called stop, this method
96     *                            will return immediately if true; if this is false and somebody
97     *                            already called stop, it will assert.
98     */
99    // Justification for warnings - wait is not unconditional, and contrary to what WA_NOT_IN_LOOP
100   // description says we are not waiting on multiple conditions.
101   @edu.umd.cs.findbugs.annotations.SuppressWarnings({"UW_UNCOND_WAIT", "WA_NOT_IN_LOOP"})
102   private void stopAndDrainOps(boolean ignoreRepeatedCalls) throws InterruptedException {
103     long oldValAndFlags;
104     do {
105       oldValAndFlags = valueAndFlags.get();
106       if (isDraining(oldValAndFlags)) {
107         if (ignoreRepeatedCalls) return;
108         throw new AssertionError("stopAndWaitForOpsOnce called more than once");
109       }
110     } while (!valueAndFlags.compareAndSet(oldValAndFlags, dec(oldValAndFlags) | DRAINING_FLAG));
111     if (getValue(oldValAndFlags) == 1) return; // There were no operations outstanding.
112     synchronized (this) { this.wait(); }
113   }
114 
115   // Helper methods.
116   private static final boolean isDraining(long valueAndFlags) {
117     return (valueAndFlags & DRAINING_FLAG) == DRAINING_FLAG;
118   }
119 
120   private static final long getValue(long valueAndFlags) {
121     return valueAndFlags >> FLAG_BIT_COUNT;
122   }
123 
124   private static final long inc(long valueAndFlags) {
125     return valueAndFlags + (1 << FLAG_BIT_COUNT); // Not checking for overflow.
126   }
127 
128   private static final long dec(long valueAndFlags) {
129     return valueAndFlags - (1 << FLAG_BIT_COUNT); // Negative overflow checked outside.
130   }
131 }