Gopal V created TEZ-3974:
----------------------------
Summary: 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
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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