messages may be stranded when selectors are used
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Key: QPID-601
URL: https://issues.apache.org/jira/browse/QPID-601
Project: Qpid
Issue Type: Bug
Affects Versions: M2, M2.1, M3
Reporter: Rafael H. Schloming
ConcurrentSelectorDeliveryManager = CSDM
PDQ = PreDeliveryQueue
pPDQ = populatePreDeliveryQueue
Messages may be stranded as a result of the way selectors are implemented. When
a consumer subscribes to a queue using a filter, CSDM.pPDQ is called to move
messages from CSDM._messages into the PDQ for that subscription. The pPDQ
method does this by calling _messages.iterator() and placing matching messages
into the PDQ of the subscription. The result of _messages.iterator() is not
guaranteed to reflect concurrent changes made to _messages, and so a publish
may end up causing a message that matches the subscription to be placed onto
_messages. This message is then stranded there despite the fact that the
subscription could accept the message because nothing ever reexamines the
_messages queue after the initial pPDQ when the consumer subscribes.
One potential fix for this issue would be to make the pPDQ method block
messages from being pushed onto the _messages queue while it operates. This
would work, however it would introduce undesirable synchronization for all
publishers attempting to place messages on the queue.
So far I can't think of a proper fix for this issue that doesn't involve some
substantial changes in the data structures for queuing and delivering messages.
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