Snap "lxd" assumes unsupported features: snapd2.39

jamesh · July 20, 2020, 2:32am

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

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.