I had some thoughts about running the daemons on-demand.
Principally, we want the following:
When a user opens the print dialog, avavhi-daemon, cups, and
cups-browsed should be started (can get called explicitly), as
cups-browsed will get D-Bus signals from avahi-daemon when printers
appear or disappear and it will send requests to cupsd to create or
remove print queues pointing to these printers. The dialog is now
supposed to watch the print queues appearing and to update the
user-visible list appropriately, until the user clicks a printer. Then
printer capabilities should be polled via IPP (using libcups) and if the
user clicks "Print", the job is sent to CUPS (also using libcups).
When the dialog is closed, we check whether other instances of the
dialog (or other avahi-daemon-using apps, how do we identify them
without having an explicit list?) are still running and if not, we kill
avahi-daemon. cups-browsed and cupsd keep running, we modify
cups-browsed that it keeps running when avahi-daemon appears and
disappears and let it remove the job-less (empty) Avahi-discovered
queues if avahi-daemon disappears.cupsd closes by itself 30 sec after
there are no jobs any more and cups-browsed closes by itself if it runs
30 sec without having any cups-browsed-generated print queues, meaning
that all but the queue which is printuing the user# s job get removed
when avahi-daemon is killed by the dialog closing and the remaining
queue gets removed when the job finishes, if within 30 seconds no new
print dialog gets opened )which starts avahi-daemon) cups-browsed
closes. These non-permanent (auto-closing) modes of cups-browsed and
cupsd we are triggered by a special command line option which we will
introduce to the daemons and use it when calling from the print dialog
)or when calling cupsd socket-triggered by Upstart).
So we will modify cups-browsed:
- Keep running independent of avahi-daemon running, simply remove the
empty avahi-detected queues if avahi-daemon disappears and recover to
avahi use if avahi-deamon reappears.
- Command line option to stop if 30 sec without cups-browsed-created queues.
and cupsd:
- Support for Upstart-induced startup:
https://bugs.launchpad.net/ubuntu/+source/cups/+bug/1276713
- Command line option for stopping when 30 sec without jobs and without
shared printers (default when Upstart-socket-induced).
WDYT? Should we go this way?
Till
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