Andrew Wang created HDFS-4688:
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             Summary: DFSClient should not allow multiple concurrent creates 
for the same file
                 Key: HDFS-4688
                 URL: https://issues.apache.org/jira/browse/HDFS-4688
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 2.0.3-alpha, 3.0.0
            Reporter: Andrew Wang
            Assignee: Andrew Wang


Credit to Harsh for tracing down most of this.

If a DFSClient does create with overwrite multiple times on the same file, we 
can get into bad states. The exact failure mode depends on the state of the 
file, but at the least one DFSOutputStream will "win" over the others, leading 
to data loss in the sense that data written to the other DFSOutputStreams will 
be lost. While this is perhaps okay because of overwrite semantics, we've also 
seen other cases where the DFSClient loops indefinitely on close and blocks get 
marked as corrupt. This is not okay.

One fix for this is adding some locking to DFSClient which prevents a user from 
opening multiple concurrent output streams to the same path.

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