Handle generic data structures in priority queue info callback

[noxdafox]

The priority queue implementation makes assumptions over the
data format returned by the underlying backing queue
info callback.

This causes priority queues to crash if used together with other
modules implementing the rabbit_backing_queue behaviour and
returning additional data with the info callback.

As we cannot predict the format of the data the backing queue
info callback could return, we just replace it with the undefined
keyword.

Signed-off-by: Matteo Cafasso noxdafox@gmail.com

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Further Comments

Notice this when investigating an issue in my plugin.

If a module implementing the rabbit_backing_queue behaviour wants to enrich the information returned by the info callback (more particularly, the :backing_queue_status information) will end up crashing the queue if used together with priorities.

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[michaelklishin]

Thank you!

[lukebakken]

Hi @noxdafox - do you have an example of a crash with a stack trace or a set of steps to reproduce a crash?

[noxdafox]

Hello @lukebakken, you can find the stack trace and the steps to reproduce it in the issue I linked in the comment above.

[lukebakken]

@noxdafox - rather than changing behavior here, could you pass a compatible term here instead, if the queue is a priority queue?

I'd rather keep the "crash on unexpected / malformatted" data behavior here.

[noxdafox]

I don't think that's the correct way to handle the problem.

The rabbit_backing_queue behaviour is currently not setting any restriction to the format of the data the info callback must return. Therefore, claiming that the data is "unexpected or malformatted" is incorrect.

This implies that any implementation of the behaviour is supposed to keep that into account. Nevertheless, the priority-queue implementation is enforcing a data format and by doing that it violates the API contract. Encouraging this violations severely hinders the scalability of the design.

We can in fact imagine what would happen if there would be more than one or two implementations of the rabbit_backing_queue behaviour. A developer should check all the quirks of the other implementations to cope with his/her own.

A better solution would indeed consist in defining a data format for the info callback which all other implementations could adhere to. This would also facilitate the folding/unfolding the priority-queue implementation is doing as it could rely on such format restrictions.

I am open towards such design and willing to commit to it. But I'd rather not delve into the priority-queue logic to find a way around to fit my data into. At that point, the current workaround I implemented (not reporting any info if x-max-priority is set) is more than enough.

[lukebakken]

@noxdafox good enough explanation for me.