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https://issues.apache.org/jira/browse/HDFS-11583?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16051089#comment-16051089
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Karan Mehta commented on HDFS-11583:
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Thank you [~ajisakaa] and [~iwasakims]!
> Parent spans are not initialized to NullScope for every DFSPacket
> -----------------------------------------------------------------
>
> Key: HDFS-11583
> URL: https://issues.apache.org/jira/browse/HDFS-11583
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: tracing
> Affects Versions: 2.7.1
> Reporter: Karan Mehta
> Assignee: Masatake Iwasaki
> Fix For: 2.7.4
>
> Attachments: HDFS-11583-branch-2.7.001.patch,
> HDFS-11583-branch-2.7.002.patch, HDFS-11583-branch-2.7.003.patch
>
>
> The issue was found while working with PHOENIX-3752.
> Each packet received by the {{run()}} method of {{DataStreamer}} class, uses
> the {{parents}} field of the {{DFSPacket}} to create a new {{dataStreamer}}
> span, which in turn creates a {{writeTo}} span as its child span. The parents
> field is initialized when the packet is added to the {{dataQueue}} and the
> value is initialized from the {{ThreadLocal}}. This is how HTrace handles
> spans.
> A {{TraceScope}} is created and initialized to {{NullScope}} before the loop
> which runs till the point when the stream is closed.
> Consider the following scenario, when the {{dataQueue}} contains multiple
> packets, only the first of which has a tracing enabled. The scope is
> initialized to the {{dataStreamer}} scope and a {{writeTo}} span is created
> as its child, which gets closed once the packet is sent out to a remote
> datanode. Before {{writeTo}} span is started, the {{dataStreamer}} scope is
> detached. So calling the close method on it doesn't do anything at the end of
> loop.
> The second iteration will be using the stale value of the {{scope}} variable
> with a DFSPacket on which tracing is not enabled. This results in generation
> of an orphan {{writeTo}} spans which are being delivered to the
> {{SpanReceiver}} as registered in the TraceFramework. This may result in
> unlimited number of spans being generated and sent out to receiver.
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