[cups] Handling of "Installable Options" in user applications

[Michael Weghorn]

Hi Brian,

On 2015-09-04 18:16, Brian Potkin wrote:

> Tea4CUPS is worth looking at. The user can set as many options as she
> wishes but a script will ensure the ones set for the queue will not be
> overridden. The Debian Wiki might help.

Thank you for that tip. I had a look at Tea4CUPS and it looks like a 
really useful tool for various purposes.

As far as I understand it so far, the way to use it to prevent users 
from setting wrong values for "installable options" is similar to the 
scenario described in the section "Enforcing number-up=2 Printing" in 
the Debian wiki:

1) Set up a second virtual queue (which the users will send print jobs 
to).
2) Create a Tea4CUPS prehook for that qeue which modifies the printing 
options so that the system-wide settings for "installable options" are 
sent to the "real queue", leave other options as they are.

This would make sure that wrongly set options are always replaced.
What I see as a problem here (in addition to a somewhat increased 
complexity) is that the CUPS filter chain would be processed twice for 
each print job (the first time for the "virtual queue", the second time 
for the "real queue"). While this is not a real problem for small print 
jobs, it approximately doubles the time needed for large print jobs 
where processing the filter chain takes a lot of time (we have some 
cases where a single run takes more than half an hour).
This is because Tea4CUPS (as I understand it after reading the Debian 
wiki article) only comes into action after the CUPS filter chain has 
been processed. When processing the CUPS filter chain for the first 
time, the user-given options are used for all filters - which means that 
the potentially wrong options for the installable options are taken into 
account and thus make it into the file that would be sent to the 
printer. Only after the second run of the filter chain (which is 
triggered by the Tea4CUPS script), the file can safely be processed by 
the real CUPS backend.

Or do I miss something here and there is a better way of doing it?

What might also be a bit confusing is that the printer options are still 
shown in the printing dialog and can be changed by the users (though 
they are not applied in the end), but that is probably not a big 
problem. The most important thing is to avoid problems with the real 
hardware.

Best regards,
Michael