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https://issues.apache.org/jira/browse/HDFS-4058?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Eli Collins updated HDFS-4058:
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    Attachment: hdfs-4058.txt

Thanks Andy and ATM. I've updated the code with a comment per your suggestion.  

It's hard to write a new test for this one because to reproduce it both the 
threads *and* the compilersInProgress entry set needs to be out of order. Ie 
the test would need to eg change the code to iterate over the futures in 
reverse order.
                
> DirectoryScanner may fail with IOOB if the directory scanning threads return 
> out of volume order
> ------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-4058
>                 URL: https://issues.apache.org/jira/browse/HDFS-4058
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: data-node
>    Affects Versions: 2.0.0-alpha
>            Reporter: Eli Collins
>            Assignee: Eli Collins
>         Attachments: hdfs-4058.txt, hdfs-4058.txt
>
>
> The DirectoryScanner may fail with an IOOB if the directory scanning threads 
> return out of volume order (ie volume scanner #3 returns before volume 
> scanner #2). This is because it's using an ArrayList and ArrayList#add(index, 
> element) throws IOOB if idx >= size, and size is only increased as elements 
> are added, therefore adds have to be done in index order. Since we know the 
> size when we create the ArrayList let's just use an array (perhaps an 
> ArrayList was used originally because someone thought array creation called 
> the default constructor?).

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