Karan Mehta created HDFS-11583:
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Summary: Parent spans not initialized for every DFSPacket
Key: HDFS-11583
URL: https://issues.apache.org/jira/browse/HDFS-11583
Project: Hadoop HDFS
Issue Type: Bug
Reporter: Karan Mehta
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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