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https://issues.apache.org/jira/browse/TEZ-3974?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16570380#comment-16570380
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Jaume M commented on TEZ-3974:
------------------------------
Sure, to reproduce this the following step where taken:
* Create a DAG with two mappers and one reducer:
A B
\ /
C
* Purposely throw a RuntimeException in B in the close method of the processor.
This accomplishes the empty events being sent but the task not finishing
correctly. The RuntimeException is only thrown once, the second attempt
succeeds.
* Tweak tez.shuffle-vertex-manager.min-src-fraction,
tez.shuffle-vertex-manager.max-src-fraction so C is scheduled even though A, B
may not have finished yet.
* We see A start, B start, A finish, B throw the exception, C start, C finish,
B finish. So C finishes before B which shouldn't happen.
After the proposed path C finishes after B.
> Tez: Correctness regression of TEZ-955 in TEZ-2937
> --------------------------------------------------
>
> Key: TEZ-3974
> URL: https://issues.apache.org/jira/browse/TEZ-3974
> Project: Apache Tez
> Issue Type: Bug
> Reporter: Gopal V
> Assignee: Jaume M
> Priority: Critical
> Attachments: TEZ-3974.1.patch
>
>
> TEZ-2937 might have introduced a race condition for Tez output events, along
> with TEZ-2237
> {code}
> // Close the Outputs.
> for (OutputSpec outputSpec : outputSpecs) {
> String destVertexName = outputSpec.getDestinationVertexName();
> initializedOutputs.remove(destVertexName);
> List<Event> closeOutputEvents =
> ((LogicalOutputFrameworkInterface)outputsMap.get(destVertexName)).close();
> sendTaskGeneratedEvents(closeOutputEvents,
> EventProducerConsumerType.OUTPUT, taskSpec.getVertexName(),
> destVertexName, taskSpec.getTaskAttemptID());
> }
> // Close the Processor.
> processorClosed = true;
> processor.close();
> {code}
> As part of TEZ-2237, the outputs send empty events when the output is closed
> without being started (which happens in task init failures).
> These events are obsoleted when a task fails and this happens in the AM, but
> not before the dispatcher looks at them.
> Depending on the timing, the empty events can escape obsoletion & be sent to
> a downstream task.
> This gets marked as a SKIPPED event in the downstream task, which means that
> further obsoletion events sent to the downstream task is ignored (because a
> zero byte fetch is not repeated on node failure).
> So the downstream task can exit without actually waiting for the retry of the
> failed task and cause silent dataloss in case where the retry succeeds in
> another attempt.
> So if processor.close() throws an exception, this introduce a race condition
> and if the AM is too fast, we end up with correctness issues.
> This was originally reported in TEZ-955
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