Cannot launch a GUI snap from the command line without a reboot

[Jean-Baptiste Lallement]

Zesty Desktop
Unity8 as a deb
ubuntu-calculator-app 2.3.1 (37) 479kB devmode
ubuntu-app-platform 1 (34)

Test Case:
Install the calculator snap, ubuntu-app-platform and connect the calculator to the platform
Launch the calculator from the command line

Actual result
It fails with the following message:
j-lallement@ubuntu-Inspiron-3138:~⟫ ubuntu-calculator-app
You need to connect this snap to the ubuntu app platform snap.

You can do this with those commands:
snap install ubuntu-app-platform
snap connect ubuntu-calculator-app:platform ubuntu-app-platform:platform

The user must reboot the machine and there is nothing telling him to do so.

Expected result
It runs successfully or at least the installation of the ubuntu-app-platform shows a message indicating that the system must be rebooted.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: unity8 8.15+17.04.20170216.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.10.0-8.10-generic 4.10.0-rc8
Uname: Linux 4.10.0-8-generic x86_64
ApportVersion: 2.20.4-0ubuntu2
Architecture: amd64
Date: Fri Feb 24 10:20:50 2017
InstallationDate: Installed on 2014-07-23 (946 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
ProcEnviron:
 TERM=screen-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: unity8
UpgradeStatus: Upgraded to zesty on 2016-11-15 (100 days ago)

[Pat McGowan (pat-mcgowan)]

I believe this is related to several snapd bugs on content interface handling which are currently being worked. See lp:11645731 for example.
There are workarounds in that bug. You can also disable, enable the app snap, connect again and it generally works.