Snap "lxd" assumes unsupported features: snapd2.39

spindles7 · May 16, 2020, 1:29pm

Not sure whether this is the right place to ask this question, but here goes:
On a new Debian Buster (10.4.0) clean install I installed snapd then tried to install lxd using the command line:

snap install lxd

This returns the error:
error: cannot perform the following tasks:

Mount snap “lxd” (15067) (snap “lxd” assumes unsupported features: snapd2.39 (try to update snapd and refresh the core snap))

The version of snapd installed with Buster is:
snap version
snap 2.37.4-1+b1
snapd 2.37.4-1+b1
series 16
debian 10
kernel 4.19.0-9-amd64

I am lost as to how to update snapd to 2.39 as implied, so any advice would be appreciated.

Many thanks,

Roy

zyga-snapd · May 16, 2020, 1:39pm

Hey

Try installing the “core” snap first. You can then install lxd normally.

spindles7 · May 16, 2020, 2:10pm

Thanks for the speedy reply. Yes, that fixed it. Perhaps the documentation needs to be updated to include this step?

vquest · July 19, 2020, 1:27pm

I had this same problem exactly and I had already installed snap first and then snapd.

What fixed the problem for me was to install the hello-world app and run it to check that snap was working.

After that, installing lxd worked.

Cheers

galgalesh · July 19, 2020, 6:15pm

@zyga-snapd

Do you know why this is needed? Should the documentation for installing snap on Debian be updated for this?

jamesh · July 20, 2020, 2:32am

On some distros (currently just Debian and Ubuntu), snapd supports a “re-exec” feature:

github.com

snapcore/snapd/blob/master/snapdtool/tool_linux.go#L153-L206


func ExecInSnapdOrCoreSnap() {
	// Which executable are we?
	exe, err := os.Readlink(selfExe)
	if err != nil {
		logger.Noticef("cannot read /proc/self/exe: %v", err)
		return
	}
	// Special case for snapd re-execing from 2.21. In this
	// version of snap/snapd we did set SNAP_REEXEC=0 when we
	// re-execed. In this case we need to unset the reExecKey to
	// ensure that subsequent run of snap/snapd (e.g. when using
	// classic confinement) will *not* prevented from re-execing.
	if strings.HasPrefix(exe, dirs.SnapMountDir) && !osutil.GetenvBool(reExecKey, true) {
		mustUnsetenv(reExecKey)
		return
	}
	// If we are asked not to re-execute use distribution packages. This is
	// "spiritual" re-exec so use the same environment variable to decide.

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What this means is that when the .deb packaged snap, snapd, etc tools are executed, they will check if the snapd or core snaps are installed and simply call exec() on the equivalent binary found there.

This means that you can be running a newer version of snapd than is delivered by the distro. Of course, this doesn’t happen until one of those two snaps is installed on the system.

zyga-snapd · July 20, 2020, 7:49am

As James explained below, snapd in Debian benefits from the re-execution system, where the core snap provides an updated copy of snapd itself.

The installation instructions should be amended to suggest installing core before installing another snap.

galgalesh · July 21, 2020, 12:07am

Fixed: Installing snap on Debian

The docs already suggested to first install the hello-world snap, but I wouldn’t follow that so I can’t blame other people for doing the same