snap try doesn't work: "cannot find installed snap"

[Kyle Fazzari]

It seems I can no longer use `snap try`. Or rather, running `snap try` succeeds, but then running any app looks like this:

$ my-snap-name.hello
cannot snap-exec: cannot read info for "my-snap-name": cannot find installed snap "my-snap-name" at revision x1: missing file /var/lib/snapd/snaps/my-snap-name_x1.snap

This snap is just a test I created in /tmp/foo:

$ ls -l /var/lib/snapd/snaps/my-snap-name_x1.snap
lrwxrwxrwx 1 root root 14 Oct 25 10:30 /var/lib/snapd/snaps/my-snap-name_x1.snap -> /tmp/foo/prime

$ mount | grep my-snap-name
/dev/mapper/ubuntu--vg-root on /snap/my-snap-name/x1 type ext4 (ro,relatime,errors=remount-ro,data=ordered,x-gdu.hide)
nsfs on /run/snapd/ns/my-snap-name.mnt type nsfs (rw)

$ snap --version
snap 2.35.4
snapd 2.35.4
series 16
ubuntu 18.04
kernel 4.15.0-36-generic

[Zygmunt Krynicki (zyga)]

I can reproduce this problem with any trivial snap that is placed in /tmp. Outside of /tmp things work great. Looking at why.

[Zygmunt Krynicki (zyga)]

I was surprised to see this fail since the snap from /tmp/test-snapd-sh is bind-mounted to /snap/test-snapd-sh/x1. Yet at runtime this happens:

zyga@fyke:~$ SNAPD_DEBUG=1 test-snapd-sh
[snip, unrelated stuff]
DEBUG: execv(/usr/lib/snapd/snap-exec, /usr/lib/snapd/snap-exec...)
DEBUG: argv[1] = test-snapd-sh
cannot snap-exec: cannot read info for "test-snapd-sh": cannot find installed snap "test-snapd-sh" at revision x1: missing file /var/lib/snapd/snaps/test-snapd-sh_x1.snap
zyga@fyke:~$ ls -ld /var/lib/snapd/snaps/test-snapd-sh_x1.snap
lrwxrwxrwx 1 root root 18 Oct 25 20:49 /var/lib/snapd/snaps/test-snapd-sh_x1.snap -> /tmp/test-snapd-sh

So the snap "file" in /var/lib/snapd/snaps is a symbolic link. Why is it being looked at by snap-exec? No idea yet, checking.

[Zygmunt Krynicki (zyga)]

This happens because snap.ReadInfo tries to compute the size of the snap by stat-ing the broken symlink in /var/lib/snapd/snaps/. I have fixed this in https://github.com/snapcore/snapd/pull/6048