[mosquitto-dev] queue sizes by client?

[Robert Linden <r.linden@xxxxxxxxx>]

Hello!

Since this is my first message to the list: thanks to Roger and all contributors for writing/providing mosquitto! :)

In an old message to this list from 2015 (https://www.eclipse.org/lists/mosquitto-dev/msg00861.html) Roger gave the
following answer to a question about queue sizes:

> > 4) Can we check the queue size with a SYS query?
> 
> No. Essentially anything on a client level you can't get information on.

I didn't find any newer message or information about that, but it seems to still be the case.
Since we have a problem with messages getting dropped we would really like to know which clients
have full queues, or basically as much about the status as possible, in order to debug our problems.
Maybe also to delete obsolete old sessions which are wasting space.

So my questions are:

1. is this information not available just because it wasn't implemented or there is no standardized way to do it?
2. are there explicit reasons why it would be a bad idea to have something like $SYS/client/<id>/messages/stored give the 
   current number of stored message for this client?
3. would that be useful to anybody else?
4. would it be possible/easy for us to implement ourselves? So far we only had a very superficial look at the code.
   (the general process for contributing is clear)
5. is there a better way to get information like that, maybe by looking at persisted files?

Thanks & all the best,
rob