qemu path goes orphan upon snap revision update

[Gábor Mészáros]

I had a VM lying around in stopped state for a while.
I've installed several new snaps of microstack, which basically rendered qemu to break.

It has a reference to an old (x1) version of microstack, whereas snapd only keeps the last 2 copies of the snap (in my case, x5 and x6).

When I tried to restart the whole microstack service chain, it failed as it couldn't find the qemu x1 binary (because it has been cleaned up by the snap GC).

Basically:
# systemctl status snap.microstack.libvirtd.service | grep x1
Dec 06 13:19:31 abulfeda libvirtd[9811]: Cannot check QEMU binary /snap/microstack/x1/bin/qemu-system-x86_64: No such file or directory

# grep -sr qemu qemu/instance-0000000*
qemu/instance-0000000b.xml: <emulator>/snap/microstack/x1/bin/qemu-system-x86_64</emulator>
qemu/instance-0000000b.xml: <driver name='qemu' type='qcow2' cache='none'/>
qemu/instance-0000000c.xml: <emulator>/snap/microstack/x6/bin/qemu-system-x86_64</emulator>
qemu/instance-0000000c.xml: <driver name='qemu' type='qcow2' cache='none'/>
qemu/instance-0000000d.xml: <emulator>/snap/microstack/x6/bin/qemu-system-x86_64</emulator>
qemu/instance-0000000d.xml: <driver name='qemu' type='qcow2' cache='none'/>

# ls /snap/microstack/
current x5 x6

[Pen Gale (pengale)]

Thank you for the bug report! I haven't been able to loop back around and reproduce this one yet (and it's going to be a fun one to test), but this definitely needs to be fixed ...