"lpstat -l -e" fails with Avahi

[tillkamppeter]

I create a CUPS queue on my locally installed CUPS and share it (turn sharing for CUPS on and also mark the printer itself shared). On remote machines it gets visible by that, as expected.

Now I call cupsEnumDests() like this:

  cupsEnumDests(CUPS_DEST_FLAGS_NONE, 1000, NULL, 0, 0,
                (cups_dest_cb_t)add_dest_cb, dest_list);

to let the callback function add_dest_cb() add all printers reported to dest_list.
In the end the list contains all permanent queues which I have created and it also contains the IPP printers discovered in the network (advertsed via DNS-SD).

This is all OK, but cupsEnumDests() also discovers the shared printers on my local CUPS. So if I have created a queue named printer and shared it and the host name of my local machine is my_cups_server, cupsEnumDests() also lists a discovered printer making up a temporary queue named printer_my_cups_server and when printing to it this queue gets created with the device URI ipps://my_cups_server:631/printers/printer making my CUPS daemon passing on a job to this queue to itself, into the queue printer.

cupsEnumDests() (or cupsd if it is doing the "dirty work" for this function) needs a filter to exclude temporary queues for shared
local (from the same cupsd) printers.

[michaelrsweet]

@tillkamppeter Can you provide the output of lpstat -v and lpstat -e?

[tillkamppeter]

Here we go:

$ lpstat -v
device for commandtops-test: usb://HP/OfficeJet%20Pro%208730?serial=CN783F60W1&interface=1
device for Printer: ipps://HP%20OfficeJet%20Pro%208730%20%5B08C229%5D._ipps._tcp.local/
device for second: /dev/null
$ lpstat -e
commandtops-test
Printer
second
$ lpstat -l -e
commandtops-test permanent ipp://localhost/printers/commandtops-test usb://HP/OfficeJet%20Pro%208730?serial=CN783F60W1&interface=1
Printer permanent ipp://localhost/printers/Printer ipps://HP%20OfficeJet%20Pro%208730%20%5B08C229%5D._ipps._tcp.local/
second permanent ipp://localhost/printers/second file:/dev/null
$

lpstat -e does not reveal this problem, unfortunately (another bug?), but right after I run a program which uses cupsEnumDests() to list all CUPS queues including the potential ones for discovered printers and then exercises the queue of each listed printer, I get:

$ lpstat -v
device for commandtops-test: usb://HP/OfficeJet%20Pro%208730?serial=CN783F60W1&interface=1
device for commandtops_test_till_x1nano: ipps://till-x1nano.local:631/printers/commandtops-test
device for Printer: ipps://HP%20OfficeJet%20Pro%208730%20%5B08C229%5D._ipps._tcp.local/
device for Printer_till_x1nano: ipps://till-x1nano.local:631/printers/Printer
device for second: /dev/null
device for second_till_x1nano: ipps://till-x1nano.local:631/printers/second

The potential queues are now physically created and therefore listed by lpstat -v. This means that cupsEnumDests()sees 6 queues, the 3 ones physically created in the first place and 3 additional ones pointing to the 3 first ones. Note that my machine's host name is till-x1nano.local and my CUPS here is an unsnapped standard CUPS running on port 631 (and also on /run/cups/cups.sock. All the three CUPS printers are shared.

[tillkamppeter]

This is the call of cupsEnumDests() which I am using:

  cupsEnumDests(CUPS_DEST_FLAGS_NONE, 1000, NULL, 0, 0,
		(cups_dest_cb_t)add_dest_cb, dest_list);

This is the callback function add_dest_cb():

int
add_dest_cb(dest_list_t *user_data, unsigned flags, cups_dest_t *dest)
{
  if (flags & CUPS_DEST_FLAGS_REMOVED)
    /* Remove destination from array */
    user_data->num_dests =
      cupsRemoveDest(dest->name, dest->instance, user_data->num_dests,
		     &(user_data->dests));
  else {
    /* Add destination to array... */
    user_data->num_dests =
      cupsCopyDest(dest, user_data->num_dests,
		   &(user_data->dests));
    if (dest->is_default)
      user_data->default_dest =
	cupsGetDest(dest->name, dest->instance, user_data->num_dests,
		    user_data->dests);
  }
  return (1);
}

[tillkamppeter]

lpstat -e uses cupsGetDests(), lpstat -v uses a CUPS_GET_PRINTERS IPP request.

[michaelrsweet]

OK, I think the Avahi rejection of local queues isn't working like it does with mDNSResponder. I will add a UUID array to reject identical UUIDs.

[tillkamppeter]

UUID array, exactly what I also do in cups-browsed ...

[tillkamppeter]

Also reported on Launchpad for Ubuntu.