unity8 should only allow launching once

[Omer Akram]

Currently if we start Unity8 from terminal on the phone, it starts a new instance. Instead the case should be handled a better way, either a second instance should not be allowed or the already running unity should be killed before.

[Michał Sawicz (saviq)]

I believe this should be handled via an upstart session - before starting a new instance, you "stop unity8" to make sure there isn't one running, and then launch it with whatever you need.

A --replace option I don't think is wanted / needed - it only makes sense for a split display server / window manager / compositor where you want to replace the window manager in the currently running display server. With Unity8 on Mir it's all going to be the same process, so the effect is the same as killing and replacing.

Unity8 should, though, bail out if there is an instance already running.

[Stephen M. Webb (bregma)]

> A --replace option I don't think is wanted / needed - it only makes
> sense for a split display server / window manager / compositor where you
> want to replace the window manager in the currently running display
> server. With Unity8 on Mir it's all going to be the same process, so the
> effect is the same as killing and replacing.

A --replace option makes perfect sense in the case that Unity is hung in a session. While it's true that a series of
command could be entered from the command line to shut down the old running Unity 8 session and start a new one, among
the faults with that approach are these.

(1) It requires an in-depth knowledge of the internal architecture of session management on the part of the poor
struggling user who is experiencing a hung session.

(2) Every extra step after the first is an extra step that isn't necessary and one more place things can go wrong in the
field -- and remember: if things can go wrong in the field, they will do so in a way that maximizes inconvenience.

(3) It is inconsistent with known published documented procedures already widely disseminated in the community.