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https://issues.apache.org/jira/browse/QPID-8536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17367666#comment-17367666
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ASF GitHub Bot commented on QPID-8536:
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asfgit closed pull request #93:
URL: https://github.com/apache/qpid-broker-j/pull/93
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> [Broker-J] Incorrect check of maximum open connections
> ------------------------------------------------------
>
> Key: QPID-8536
> URL: https://issues.apache.org/jira/browse/QPID-8536
> Project: Qpid
> Issue Type: Bug
> Components: Broker-J
> Reporter: Marek Laca
> Priority: Minor
> Labels: Broker, Connections, Java, limit, port
>
> The AMQP port has the limit of maximum open connections. But this limit is
> not checked correctly. The AMQP port has two methods
> AmqpPort::canAcceptNewConnection and AmqpPort::incrementConnectionCount.
> Suppose that the port has the limit 5 connection and clients open 7
> connections at the same time:
> # 7 parallel connections is open by IO-threads.
> # All threads call the AmqpPort::canAcceptNewConnection method and every
> thread receive the positive answer because any connection has not been
> registered yet.
> # All threads call the AmqpPort::incrementConnectionCount methods. Hence the
> connection count is increased to 7 what breaks the limits.
> # All 7 connections are accepted in spite of breaking limit.
> The every thread can see the intermediate state of the counter when the limit
> is checked by another thread but the counter has not been incremented yet.
> The solution is the combination of the steps 'check' and 'increment' into
> single atomic operation when every thread has the exclusive access to the
> counter. The counter can be observed only in two states: 'before check' or
> 'after incrementation'.
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