/usr/lib/snapd unmounted in app + core18 + core (tooling) use case; possibly on core18 refresh

[Alberto Donato]

While developing on the maas snap I use "snap try" inside a (bionic) LXD container.

I often run rsync to copy updated files (see https://github.com/maas/maas/blob/master/Makefile#L781) and then "snap restart maas".

On some occasions, the snaps stops working. Both CLI commands and the daemon report "execv failed: No such file or directory" errors.

After some digging (thanks to Zyga) turns out that /usr/lib/snapd inside the snap is not mounted (so no snap-exec is found).

The "core18" snap (which maas uses as base) has recently refreshed (I was tracking edge), although I can't be sure that the issue started right after the refresh).

Attached are /proc/self/mountinfo outputs on the host, in the container and inside the snap mount namespace.

[Alberto Donato (ack)]

Here's the strace output when trying to run the CLI

$ sudo snap run --strace=--raw maas
execve("/snap/core/6531/usr/lib/snapd/snap-confine", ["/snap/core/6531/usr/lib/snapd/sn"..., "--base", "core18", "snap.maas.maas", "/usr/lib/snapd/snap-exec", "maas"], 0x7fff2a50abc0 /* 37 vars */) = -1 EACCES (Permission denied)
fstat(2, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
write(2, "/usr/bin/strace: exec: Permissio"..., 41/usr/bin/strace: exec: Permission denied
) = 41
getpid() = 21884
exit_group(1) = ?
+++ exited with 1 +++
error: exit status 1