snap-discard-ns fails

[Ondrej Kubik]

snapd: 2.37.2
Ubuntu 18.04
core snap: 6405
kernel: 4.15.0-29-generic

Preconditions:
 Snap updated base from core16 to core18
 Snap has one or more services
 there is forked process dangling around

Expected result:
 snap refreshes to new revision using new core18 base

Actual result:
 snapd fails to identify forked process and unmount namespace

This is related to: https://bugs.launchpad.net/snappy/+bug/1819875

Associated logs:
https://paste.ubuntu.com/p/99Pz3wQ79P/
This shows where base is actually core16 while it should be core18
https://paste.ubuntu.com/p/7w6nRX4Y4J/
Offending process was process forked by the snap service
https://paste.ubuntu.com/p/7DmG45TYnz/
https://paste.ubuntu.com/p/fGPY9WNRrp/
https://paste.ubuntu.com/p/SkBMzTPpHX/
https://paste.ubuntu.com/p/4S3VJyJJCv/
After killing offending process manually
https://paste.ubuntu.com/p/g9SgydYsdq/