Christopher L. Shannon created AMQ-6293:
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Summary: Queue destination statistics can be inaccurate with a
purge
Key: AMQ-6293
URL: https://issues.apache.org/jira/browse/AMQ-6293
Project: ActiveMQ
Issue Type: Bug
Components: Broker
Affects Versions: 5.13.3
Reporter: Christopher L. Shannon
Assignee: Christopher L. Shannon
Recently I have been seeing some negative message counts from the
destinationStatistics message counter of a Queue. The dequeue counts also seem
to be wrong as well. This is hard to reproduce as it happens rarely, but
running a purge while there are consumers online acking seems to be an
occasional cause.
The removeMessage method in a Queue can be called more than once for the same
message if an ack comes in at the same time as a purge. (This is because
messages are prefetched so they can be acked even while they are purged) Taking
a closer look, the broker handles duplicate calls ok in most cases but there
are two areas that need to be fixed.
First, there is a race condition in the dropMessage() method in Queue. It
checks if the message is dropped before continuing with the drop. This is not
thread safe because the two methods calls are not synchronized together. I was
able to show negative counts by introducing a small amount of latency between
those two calls during a test. There needs to be some synchronization done
here to prevent the counts from getting out of sync.
Second, the dequeue counter needs to be moved out of the removeMessage() method
and to the dropMessage() method so it can be protected by the same sync and
only incremented when a message is dropped to prevent duplicate counts.
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