Sahil Takiar created IMPALA-9113:
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Summary: Queries can hang if an impalad is killed after a query
has FINISHED
Key: IMPALA-9113
URL: https://issues.apache.org/jira/browse/IMPALA-9113
Project: IMPALA
Issue Type: Bug
Components: Backend, Clients
Reporter: Sahil Takiar
Assignee: Sahil Takiar
There is a race condition in the query coordination code that could cause
queries to hang indefinitely in an un-cancellable state if an impalad crashes
after the query has transitioned to the FINISHED state, but before all backends
have completed.
The issue occurs if:
* A query produces all results
* A client issues a fetch request to read all of those results
* The client fetch request fetches all available rows (e.g. eos is hit)
* {{Coordinator::GetNext}} then calls
{{SetNonErrorTerminalState(ExecState::RETURNED_RESULTS)}} which eventually
calls {{WaitForBackends()}}
* {{WaitForBackends()}} will block until all backends have completed
* One of the impalads running the query crashes, and thus never reports
success for the query fragment it was running
* The {{WaitForBackends()}} call will then block indefinitely
* Any attempt to cancel the query fails because the original fetch request
that drove the {{WaitForBackends()}} call has acquired the
{{ClientRequestState}} lock, which thus prevents any cancellation from
occurring.
Implementing IMPALA-6984 should theoretically fix because as soon as eos is
hit, it would call {{CancelBackends()}} rather than {{WaitForBackends()}}.
Another solution would be to add a timeout to the {{WaitForBackends()}} so that
it returns after the timeout is hit, this would force the fetch request to
return 0 rows with {{hasMoreRows=true}}, and unblock any cancellation threads.
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