[cups.general] Debugging the filter chain?

[Johannes Meixner]

Hello,

On Oct 19 17:26 William Pietri wrote (shortened):
> Since I'm pretty ignorant of CUPS, I don't know how much sense these 
> suggestions make, but if I could wave a magic wand, these would have been 
> nice:
>
>  1. In debug logging, printing the full arguments of all filters called.

See "man 7 filter" and "man 7 backend":
All filters and the backend are called with the same command line
arguments which are logged as "argv[0]"..."argv[6]" and with the
same environment variables which are logged as "envp[0]"...

>  2. In regular logging, calling out sending a zero-byte payload to the
>     printer as an error.
>  3. The ability to get a little information on what's getting passed
>     between filters, so that I can tell where the failure is. E.g.,
>     logging how many bytes each filter takes in and puts out.
>  4. Getting dumps of the contents at each stage in the filter chain,
>     so I could inspect things.

What the cupsd does when it runs the filters is that it forks
for each filter and for the backend a separated process where
- only the first filter gets its input as file named via argv[6]
- all others get the input via stdin via pipe from the predecessor
- only the backend sends its output directly to its recipient (printer)
- all others provide the output via stdout to a pipe to the successor
- all their stderr go to the cupsd which forwards it to the log

Therefore it is not possible for the cupsd to inspect the data
which is forwarded via pipes from one filter to the next
(or to the backend) so that the cupsd cannot log how many bytes
each filter takes in and puts out or even dump the contents.
This would require that also all data of all filters is sent
via the cupsd which results probably a huge loss of performance
(a few bytes stderr but megabytes up to gigabytes of payload).


Kind Regards
Johannes Meixner
-- 
SUSE LINUX Products GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany
AG Nuernberg, HRB 16746, GF: Markus Rex