snapd should allow snaps to define systemd unit alias names

[Simon Fels]

To allow us to replace services which where formerly provided by debs we need something in snapd which can add and Alias= field to the create systemd unit file.

This would allow us for example for the network-manager snap to replace the deb based version and all other service depending on the NetworkManager.service unit can still work as the snap service file now has an Alias=NetworkManager.service field.

See https://code.launchpad.net/~morphis/netplan/+git/netplan/+merge/306607 for some more details.

[Jamie Strandboge (jdstrand)]

How would we prevent a snap from replace another snap's name? Would this be a reserved declaration? Would that be supported in a snap declaration (ie, "this snap is allowed to alias NetworkManager, but not bluez)?

[Martin Pitt (pitti)]

Sprinkling a lot of hacky patches like https://code.launchpad.net/~morphis/netplan/+git/netplan/+merge/306607 into Ubuntu is not the right way forward -- if we want to provide plumbing services like NetworkManager, bluez, or cups as snaps, then they need to be able to assume the expected service unit and D-Bus .service files. Let's rather do this properly via a declarative alias than hardcoding dynamically generated (!) snap service names in unrelated code.

These snaps need to be manually reviewed anyway as they have a lot of privileges and provide central services to both the OS itself and to other snaps.