Note that I have renamed the slots of the CUPS Snap from printing and admin to cups and cups-control, to do it the standard way as slots have the same names as their respective plugs, as of the discussion here.
I have updated the sample command lines in my initial posting appropriately. If you have already installed the CUPS Snap, please update your CUPS Snap via
sudo snap refresh --edge cups
ubuntu20:~$ sudo snap connect cups:cups-internal cups:cups-control
error: snap "cups" has no plug named "cups-internal"
ubuntu20:~$ snap connections cups
Interface Plug Slot Notes
avahi-control cups:avahi-control :avahi-control -
cups - cups:printing -
cups cups:cups - -
cups-control - cups:admin -
cups-control cups:cups-control - -
home cups:home :home -
network cups:network :network -
network-bind cups:network-bind :network-bind -
network-manager-observe cups:network-manager-observe :network-manager-observe manual
raw-usb cups:raw-usb :raw-usb -
system-files cups:etc-cups :system-files -
@adasiko Seems that the auto-build on GitHub commits got stopped as they have changed the service. I must find out how to do it under the new service.
No service change, there was an outage on the build servers. The servers are back and I have re-triggered a build of the Snap. Please do
sudo snap refresh --edge cups
again to get the Snap version with my described changes.
The last needed interface connections are now approved for auto-connection or not needed any more, so for everyone who installs the CUPS Snap from the Snap Store now, no manual interface connections are needed any more!
I have also updated the initial posting appropriately.
Hi, thx for working on this!
I have one question: is it possible to configure snapd cupsd daemon similarly how one can edit /etc/cups/cupsd.conf on distro package?
Yes, but you have to edit /var/snap/cups/common/etc/cups/cupsd.conf instead and after doing so, restart CUPS with
sudo snap restart cups
It’s still running after reboot (without snap CUPS package).
adasiko@ubuntu20:~$ systemctl status cups
● cups.service - CUPS Scheduler
Loaded: loaded (/lib/systemd/system/cups.service; disabled; vendor preset: enabled)
Active: active (running) since Thu 2021-03-11 22:11:47 +07; 17s ago
TriggeredBy: ● cups.socket
you’d prbably also want to stop/disable cups.socket here …
The CUPS Snap has a new feature now:
If you share a print queue it is advertised as AirPrint and as Mopria printer. So you can print now from iOS (iPhone, iPad, …) devices and also from Mopria clients (Windows, or Mopria app installed on phone).
As usual, refresh the CUPS Snap via
sudo snap refresh --edge cups
It’s really hard to kill it permanently 
sudo systemctl disable cups-browsed
sudo systemctl stop cups-browsed
sudo systemctl disable cups
sudo systemctl disable cups.socket
sudo systemctl stop cups
reboot
> systemctl status cups
● cups.service - CUPS Scheduler
Loaded: loaded (/lib/systemd/system/cups.service; disabled; vendor preset: enabled)
Active: active (running) since Sat 2021-03-13 18:14:51 +07; 17s ago
TriggeredBy: ● cups.socket
Docs: man:cupsd(8)
Main PID: 829 (cupsd)
Tasks: 1 (limit: 4654)
Memory: 6.4M
CGroup: /system.slice/cups.service
└─829 /usr/sbin/cupsd -l
мар 13 18:14:51 ubuntu20 systemd[1]: Started CUPS Scheduler.
мар 13 18:14:51 ubuntu20 /hpfax[844]: [844]: error: Failed to create /var/spool/cups/tmp/.hplip
Is it something trigger by connected printer?
> lsusb | grep HP
Bus 001 Device 008: ID 03f0:002a HP, Inc LaserJet P110
@adasiko @ogra probably the safest (but not the correctest) method for testing, at least for the time being until we found the correct way is:
sudo chmod -x /usr/sbin/cupsd
sudo chmod -x /usr/sbin/cups-browsed
sudo systemctl stop cups-browsed
sudo systemctl stop cups
Preventing starting CUPS either manually or via a dependency:
sudo systemctl mask cups
PS: unmask for reverse operation.
@adasiko Does this mean that for switching from the system’s CUPS to the Snap I have to run
sudo systemctl mask cups-browsed
sudo systemctl stop cups-browsed
sudo systemctl mask cups
sudo systemctl stop cups
and that is all to do before installing/starting the CUPS Snap?
Think that some sort of it…
I have HP P1102 over USB in Raspberry Pi with Ubuntu Server 20.04. And Ubuntu Desktop 20.04 in same local network.
Printer had not been found automatically. I added it manually over web interface (just one click in founded list).
Description: HP LaserJet Professional p1102s Printer
Location:
Driver: Local Raw Printer (grayscale, 2-sided printing)
Connection: dnssd://HP_LaserJet_Professional_P1102%20%40%20ubuntu._ipp._tcp.local/cups?uuid=a24df837-8136-34ed-5ed3-92078c855bc5
Defaults: job-sheets=none, none media=unknown
Sent test page from web interface and
Unable to locate printer “ubuntu.local”
Is it some sort of DNS problems? Or may be it has some sort of “bad” drivers (proprietary HP plugins)?
PS: printer works fine from many devices
@adasiko, first, where did you install the CUPS Snap? On the Raspberry Pi? On your Ubuntu Desktop? On both? Which web interface did you use? The one of the CUPS running on your Raspberry Pi? On your desktop? Or of the PostScript Printer Application on your Raspberry Pi?
Note that with the CUPS Snap and the PosyScript Printer Application no proprietary HPLIP drivers or plugins are involved, but if I check on a standard, classic setup with CUPS and HPLIP, your printer model requires the proprietary plugin from HP:
$ lpinfo -m | grep -i P1102
foo2zjs:0/ppd/foo2zjs/HP-LaserJet_Pro_P1102.ppd HP LaserJet Pro P1102 Foomatic/foo2zjs-z2 (recommended)
foo2zjs:0/ppd/foo2zjs/HP-LaserJet_Pro_P1102w.ppd HP LaserJet Pro P1102w Foomatic/foo2zjs-z2 (recommended)
hplip:0/ppd/hplip/HP/hp-laserjet_professional_p1102.ppd HP LaserJet Professional p1102, hpcups 3.21.2, requires proprietary plugin
hplip:1/ppd/hplip/HP/hp-laserjet_professional_p1102.ppd HP LaserJet Professional p1102, hpcups 3.21.2, requires proprietary plugin
hplip:2/ppd/hplip/HP/hp-laserjet_professional_p1102.ppd HP LaserJet Professional p1102, hpcups 3.21.2, requires proprietary plugin
drv:///hpcups.drv/hp-laserjet_professional_p1102.ppd HP LaserJet Professional p1102, hpcups 3.21.2, requires proprietary plugin
hplip:0/ppd/hplip/HP/hp-laserjet_professional_p1102s.ppd HP LaserJet Professional p1102s, hpcups 3.12.2, requires proprietary plugin
hplip:0/ppd/hplip/HP/hp-laserjet_professional_p1102w.ppd HP LaserJet Professional p1102w, hpcups 3.21.2, requires proprietary plugin
drv:///hpcups.drv/hp-laserjet_professional_p1102w.ppd HP LaserJet Professional p1102w, hpcups 3.21.2, requires proprietary plugin
$
Therefore it will not work with the PostScript Printer Application (it is not a PostScript printer). What you have to check is whether it simply works as a driverless (AirPrint, IPP Everywhere, Mopria) printer. In this case the CUPS Snap would pick it up fully automatically.
Could you run the command
driverless
on the Raspberry Pi, where you have connected the printer to USB and post the output here?
Raspberry Pi has casual CUPS from DEB repository. And proprietary plugin installed by hp-setup.
CUPS from SNAP on another Desktop Ubuntu (x64 PC).
From Raspberry Pi CUPS web interface:
HP LaserJet Professional p1102, hpcups 3.20.3, requires proprietary plugin
It’s proprietary plugin version 3.20.3 https://developers.hp.com/hp-linux-imaging-and-printing/binary_plugin.html
And I think that nothing driverless things on Raspberry Pi 
> driverless -v
DEBUG: Started ippfind (PID 2834)
DEBUG: Started post-processing (PID 2835)
DEBUG: PID 2834 (ippfind) stopped with status 1!
DEBUG: PID 2835 (Post-processing) exited with no errors.
I don’t know how AirPrint works. But I can print from my iPhone by default.
@adasiko, first test whether you printer works on the Raspberry Pi. Print a file from a terminal on the Pi or print a test page from the Pi’s CUPS web interface. Make sure that your Pi shares the printer (CUPS must share printers in general and each printer needs to be shared individually). Probably this works already, your iPhone talking with the CUPS of the RaspBerry Pi. AirPrint as a server works on Ubuntu’s CUPS already for several years (as distro patch as CUPS upstream was maintained by Apple) but this support was rather rudimentary, not allowing to set options on the iPhone. Newer CUPS (2.4.x) which is maintained by OpenPrinting and not by Apple has full AirPrint support (for example the CUPS Snap).
If the Pi and your client machine running the CUPS Snap are in the same subnet, the CUPS Snap should see the printers on the Pi and automatically create a queue.
Your manual queue should principally work, the problem you mention seems to be a DNS problem. Can you log into the Pi via ssh ubuntu.local? Does a browser on your client display the Pi’s web interface via http://ubuntu.local:631/?