Spill can fail with bad call to Random
--------------------------------------

                 Key: HADOOP-6766
                 URL: https://issues.apache.org/jira/browse/HADOOP-6766
             Project: Hadoop Common
          Issue Type: Bug
          Components: fs
    Affects Versions: 0.20.2
            Reporter: Peter Arthur Ciccolo
            Priority: Minor


java.lang.IllegalArgumentException: n must be positive
        at java.util.Random.nextInt(Random.java:250)
        at 
org.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.confChanged(LocalDirAllocator.java:243)
        at 
org.apache.hadoop.fs.LocalDirAllocator$AllocatorPerContext.getLocalPathForWrite(LocalDirAllocator.java:289)
        at 
org.apache.hadoop.fs.LocalDirAllocator.getLocalPathForWrite(LocalDirAllocator.java:124)
        at 
org.apache.hadoop.mapred.MapOutputFile.getSpillFileForWrite(MapOutputFile.java:107)
        at 
org.apache.hadoop.mapred.MapTask$MapOutputBuffer.sortAndSpill(MapTask.java:1221)
        at 
org.apache.hadoop.mapred.MapTask$MapOutputBuffer.flush(MapTask.java:1129)
        at 
org.apache.hadoop.mapred.MapTask$NewOutputCollector.close(MapTask.java:549)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:623)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:305)
        at org.apache.hadoop.mapred.Child.main(Child.java:159)

confChanged assumes that the list of dirs it creates 
(LocalDirAllocator.java:215) has at least one element in it by the end of the 
function. If, for each local dir, either the conditional on line 221 is false, 
or the call to DiskChecker.checkDir() throws an exception, this assumption will 
not hold. In this case, dirIndexRandomizer.nextInt() is called on the number of 
elements in dirs, which is 0. Since dirIndexRandomizer (195) is an instance of 
Random(), it needs a positive (non-zero) argument to nextInt().

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