Comments for page "52 - Queue busy when trying to consume messages."

Re: Errors in sample code

Fri, 27 Feb 2009 14:55:16 +0000

Pieter,

Thanks for the quick fix on this and the tip about the sample code. I appreciate your help.

Kelly

Errors in sample code

Fri, 27 Feb 2009 14:05:26 +0000

@mclaughlin77:

There is an error in the sample code that I'll point out in case you're using this in real apps:

 incoming_content = amq_content_basic_new(); incoming_content = amq_client_session_basic_arrived(session);

This will cause a memory leak. The first statement is not needed. The arrived() method automatically creates a new content to return to the caller.

Fixed

Fri, 27 Feb 2009 14:02:03 +0000

The problem was due to a race condition in amq_server_channel_consume().

You can fix it by patching amq_server_channel_agent.c:2083:

 ... if (self->credit == 0) self->credit = 1; amq_queue_consume (queue, consumer, self->active, nowait); ...

The line 'amq_queue_consume (queue, consumer, self->active, nowait);' must be moved to after the channel credit calculation.

What was happening was that (on fast multicore systems) the amq_queue_consume() call was starting to dispatch messages before the channel was given credit, and so the dispatch failed, with a "busy" message (which normally indicates that the channel has used up all its credit due to an overloaded network).

This patch will be in the next 1.3 and 1.4 releases.