"cups" interface merged into snapd - Additional steps to complete

snapd
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Also thanks to @cachio for his testing work and finding several bugs concerning the cups interface in snapd and the CUPS Snap.

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Update: snapd 2.55.2 is NOT in the stable channel. Seems that it got accidentally promoted and withdrawn right after. Therefore I have commented out the assumes: [snapd2.55] line in the snapcraft.yaml of the CUPS Snap now.

snapd 2.55.2 made it into the Stable channel and I have added the assumes: [snapd2.55] line to the snapcraft.yaml of the CUPS Snap, so the TODO List above is completed.

The Pull Request to fix the bug of CUPS_SERVER always being set to /var/cups/cups.sock regardless of cups plugged or not which I mentioned two posts earlier here got merged and you get the fix included if you upgrade your snapd to the one on the Edge channel:

sudo snap refresh --edge snapd

(Must be 2.55.2+git188.ge21669c or later, check with snap version or snap list)

This fixes the bug for both the CUPS Snap and Snaps plugging cups-control. My earlier assumptions that the latter are not covered was wrong.

I have tested the fix intensively (see my comments in the PR starting here) and with the workaround described below everything is working as designed!

So principally we are ready to go with the cups interface, but there is still a little problem when updating to the fixed snapd. If a Snap, especially the CUPS Snap has the environment variable set and after that we update snapd and restart the CUPS Snap, the variable stays set. There is some kind of cache pertinent through restart of the Snap.

To clean up this, once having updated snapd to the Edge version, run

sudo /usr/lib/snapd/snap-discard-ns cups

and the variable gets immediately cleared so that you can use the CUPS Snap and its cups interface correctly from now on.

Also all your Snaps which plug cups-control need this clean-up as otherwise you cannot print from these Snaps. Note that there are several, as cups-control was the interface used up to now also if one just wants to print. Run the following command for each Snap:

sudo /usr/lib/snapd/snap-discard-ns SNAPNAME

replacing SNAPNAME by the name of the Snap to fix. Run

snap connections | grep cups-control

to find all your installed Snaps which plug cups-control.

You need to run snap-discard-ns on your Snaps only once and never again, only when you update from a still buggy snapd from version 2.55 on and older than 2.55.2+git188.ge21669c to a fixed version, 2.55.2+git188.ge21669c or newer.

So we hae now the following new TODO list:

To be done as soon as possible:

  • Create an automated way to get rid of the CUPS_SERVER environment variable wrongly set by a buggy snapd, for example let snapd (snap-exec) unset the CUPS_SERVER environment variable if it finds that the /var/cups/ directory is not bind-mounted meaning that cups is not plugged, so when not setting CUPS_SERVER=/var/cups/cups.sock unsetting CUPS_SERVER. Or do a snap-discard-ns on all installed Snaps every time snapd is updated.
  • Sort out the snapd 2.55.x release into the stable channel.

To be done during the coming weeks/months:

  • Let snapd directly auto-install the CUPS Snap when we are installing a Snap which plugs cups, without the currently used placeholder content interface. This way a snapper only needs to add cups to the plugs of an app so that it can print, nothing more. No ugly content interface entry needs to be added then.
  • Let snapd be able to set environment variables triggered by plugging a given interface (and remove them if the interface is not plugged any more). This way we can get rid of the current workaround of checking for the bind mount on /var/cups/ and we can set environment variables also for other interfaces, where the use of them cannot get detected that easily.

@mvo thanks a lot for your cooperation with me to debug the CUPS_SERVER environment variable issue, for posting the PR, and for adding the long-term TODOs to the current developemnt cycle, and thanks @cachio, @mardy, and @pstolowski for approving the PR so quickly.

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snapd 2.55.2 is NOT in the stable channel. Seems that it got accidentally promoted and withdrawn right after.

Perhaps it was withheld due to the bug I described in the previous post, perhaps due to other bugs. I do not know …

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snapd 2.55.2 got indeed withdrawn from the stable channel:

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Thanks, @sergiusens to make me aware of this.

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In version 2.55.2, for example, libreoffice does not list the printers configured in ubuntu. The problem was solved thanks to the regression of version 2.54.4. When the “cups” interface is merged with snapd and released in a new stable version of snap, will the apps list the configured printers? Thank you very much.

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@jovimaco this is due to a known bug. In addition, there is also another, minor known bug. Both will get hopefully fixed in 2.55.3, but currently snapd release manager @mvo is very busy to fix the bug which has stopped 2.55.2 and so I cannot provide further information. Note also that next week is Easter week and many of my colleagues are on vacation. I will post here when there is something new to test.

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I got news from @mvo: 2.55.3 with the fix for the bug of the CUPS_SERVER environment also set in the CUPS Snap and Snaps plugging cups-control is already released into the beta channel. @mvo will work on the bug of environment variables in Snaps persisting through restarts and updates in the beginning of next week.

@jovimaco, with snapd 2.55.3 installed LibreOffice should list your printers correctly and you should be able to print. Please test this by running the command:

sudo snap refresh --beta snapd

and then checking the print dialog of LibreOffice again. If you do not see your printers, run the command

sudo /usr/lib/snapd/snap-discard-ns libreoffice

Does it work correctly now?

If the problem occurs again for you, to get back into working state, do

sudo snap refresh --stable snapd
sudo /usr/lib/snapd/snap-discard-ns libreoffice

Please tell us your results. THanks.

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Did some quick tests with (like the ones in PR #11616) with snapd 2.55.3 from the beta channel installed and the CUPS_SERVER ennvironment variable now gets only set to /var/cups/cups.sock in Snaps plugging cups, not in any other Snap. So this bug is fixed with 2.55.3. So at least upgrading from snapd 2.54.x or earlier to 2.55.3 will get you a working cups interface.

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I did another test, also with snapd 2.55.3 from beta:

$ snap run --shell cups-admin-test-no-control.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
$ stat / /var/cups | grep Device
Device: 740h/1856d	Inode: 11789       Links: 21
Device: 10302h/66306d	Inode: 53813571    Links: 2
$ exit
exit
$ sudo snap disconnect cups-admin-test-no-control:cups
$ snap run --shell cups-admin-test-no-control.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER

$ stat / /var/cups | grep Device
Device: 740h/1856d	Inode: 11789       Links: 21
Device: 740h/1856d	Inode: 11669       Links: 2
$ exit
exit
$ sudo snap connect cups-admin-test-no-control:cups cups:cups
$ snap run --shell cups-admin-test-no-control.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
$ stat / /var/cups | grep Device
Device: 740h/1856d	Inode: 11789       Links: 21
Device: 10302h/66306d	Inode: 53813571    Links: 2
$ exit
exit
$

This shows that disconnecting the cups interface immediately removes the bind mount on /var/cups/ and also the CUPS_SERVER environment variable. Re-connecting immediately sets the CUPS_SERVER variable and does the bind mount. No persistence of the variable over interface disconnect/connect actions.

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Now I am doing an up/downgrade test (before starting I was on snapd 2.55.3, at the end of the tests of the previous post, also keep in mind that the Snap cups-admin-test plugs cups-control and the Snap cups-admin-test-no-control plugs cups):

$ sudo snap refresh --candidate snapd
2022-04-09T15:52:23+02:00 INFO Waiting for automatic snapd restart...
snapd (candidate) 2.55.2 from Canonical✓ refreshed
$ snap run --shell cups-admin-test-no-control.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
$ stat / /var/cups | grep Device
Device: 740h/1856d	Inode: 11789       Links: 21
Device: 10302h/66306d	Inode: 53813571    Links: 2
$ exit
exit
$ snap run --shell cups-admin-test.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER

$ stat / /var/cups | grep Device
Device: 740h/1856d	Inode: 11789       Links: 21
Device: 740h/1856d	Inode: 11669       Links: 2
$ exit
exit
$ sudo /usr/lib/snapd/snap-discard-ns cups-admin-test
$ snap run --shell cups-admin-test.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
$ stat / /var/cups | grep Device
Device: 740h/1856d	Inode: 11789       Links: 21
Device: 740h/1856d	Inode: 11669       Links: 2
$ exit
exit
$ snap run --shell cups.cupsd
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER

$ exit
exit
$ sudo /usr/lib/snapd/snap-discard-ns cups
$ snap run --shell cups.cupsd
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
$ exit
exit
$ sudo snap refresh --beta snapd
2022-04-09T15:56:58+02:00 INFO Waiting for automatic snapd restart...
snapd (beta) 2.55.3 from Canonical✓ refreshed
$ snap run --shell cups-admin-test.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
till@till-x1nano:/home/till/printing/openprinting/cups-snap/x$ exit
exit
$ sudo /usr/lib/snapd/snap-discard-ns cups-admin-test
$ snap run --shell cups-admin-test.lpstat
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER

$ exit
exit
$ snap run --shell cups.cupsd
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER
/var/cups/cups.sock
$ exit
exit
$ sudo /usr/lib/snapd/snap-discard-ns cups
$ snap run --shell cups.cupsd
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ echo $CUPS_SERVER

$ exit
exit
$

Here one sees that updating or downdating snapd does not update the environments of the installed Snaps, independent whether variables need to get set or removed. One gets the installed Snaps only up-to-date with the running snapd if one runs snap-discard-ns on each of them. So to get all problems solved with updating snapd and with the cups interface bug fix (and consistent updating in general) the snapd install/update process should run snap-discard-ns on each installed Snap.

In the above example I am on snapd 2.55.3, which sets the CUPS_SERVER environment variable correctly only in Snaps plugging cups, identified by the bind mount on /var/cups/. Now to investigate u/downgrade behavior of snapd, I downgrade to snapd 2.55.2 by switching into the candidate channel. This version wrongly sets the CUPS_SERVER environment variable on every Snap. So I now expect it to be set on all my test Snaps. This is not the case, it is set only on the cups-admin-test-no-control Snaps, as it gets done by snapd 2.55.3, but we are under 2.55.2 now. So I run snap-discard-ns on all my test Snaps now and only then I get the snapd 2.55.2 behavior of CUPS_SERVER being set in all Snaps.

Now I upgrade back to snapd 2.55.3 by switching to the beta channel again. I expect an immediate transition to the correct configuration of the CUPS_SERVER environment variable being set only in the Snap which plugs cups, but it is still set in the other Snaps. I again have to run snap-discard-ns on all my test Snaps to get their CUPS_SERVER settings correct according to snapd 2.55.3.

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@mvo, I hope the above test results help you to correct the update behavior of snapd.

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We have finally made it!

The “cups” interface is now live!

See the launch post with usage instructions, how it works, and everything you need to know.

The interface is principally working now but to make it perfect there will be done the following improvements on snapd in the next few months:

  • Let snapd directly auto-install the CUPS Snap when we are installing a Snap which plugs cups, without the currently used placeholder content interface. This way a snapper only needs to add cups to the plugs of an app so that it can print, nothing more. No ugly content interface entry needs to be added then.
  • Let snapd be able to set environment variables triggered by plugging a given interface (and remove them if the interface is not plugged any more). This way we can get rid of the current workaround of checking for the bind mount on /var/cups/ and we can set environment variables also for other interfaces, where the use of them cannot get detected that easily.

Thanks

@mvo thanks a lot for your cooperation with me to debug the CUPS_SERVER environment variable issue, for posting the PR, and for adding the long-term TODOs to the current developemnt cycle, and thanks @cachio, @mardy, and @pstolowski for approving the PR so quickly. Thanks again, especially to @ijohnson and @jamesh, but also to @pedronis, @alexmurray, @mardy, and Michael Sweet, for their contributions to get the interface designed and implemented.

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