Stale dnsmasq.pid can prevent IP allocation due to PID clash

[jocado]

Describe the bug
If a stale dnsmasq.pid exists from a previous execution and another running process has the PID, instances fail to boot properly with the error:

launch failed: The following errors occurred:                                   
failed to determine IP address

PID allocation on systems that don't reboot often means this is unlikely, but if you reboot daily [ for instance ] , it becomes much more likely. I have seen it happen , so it is at least possible :)

To Reproduce
Reboot a system
If you are unlucky, and one of the running PIDs matches the contents of dnsmasq.pid, then dnsmasq will not start instances fail to launch.

To reliably reproduce:
Stop multipassd
echo {PID_OF_A_RUNNING_PROCESS} > /var/snap/multipass/common/data/multipassd/network/dnsmasq.pid
Start multipassd
Try and launch an instance

Expected behavior
dnsmasq.pid should either be purge on multipassd stop, or additional logic added to check for a process signature that matches dnsmaq fore assuming dnsmaqk is running.

Instances should still be able to launch. A non-technical user will find it hard to debug this issue, and it could the cuase of some other tickets I've seen:
#1653
#1584

Logs
Also see this in the service logs:

Sep 18 11:40:02 hostname multipassd[27492]: Looking for dnsmasq
Sep 18 11:40:02 hostname multipassd[27492]: Read pid "27405" from file "/var/snap/multipass/common/data/multipassd/network/dnsmasq.pid"
Sep 18 11:40:02 hostname multipassd[27492]: existing dnsmasq found with pid 27405

Additional info
Ubuntu 18.04.5

multipass 1.4.0
multipassd 1.4.0

[ricab]

Hi @jocado, thanks for the very useful report. Indeed something we overlooked and need to address!

[ricab]

A couple of updates after internal discussion:

• we're moving to lxd by default, where this won't be an issue, so reducing the priority to medium
• I will still try a quick fix and see if it works with confinement. If so, great. Otherwise we may end up dropping daemonization of dnsmasq entirely and deal with effects outside of snaps.

[jocado]

Thanks!

[dlbeck]

@ricab Hi, I am running into this issue when trying to launch. I am wondering if you guys found a "quick fix" for the issue. I am trying to develop on Ubuntu 20.04 inside of VirtualBox. Thanks