[enhancement] Please move /tmp/mir_socket to /run/mir_socket (or similar)

An exposed endpoint allowing "anyone" to connect to the system compositor is a bad idea wherever it.

It has been the plan for the system compositor not to create an endpoint on the filesystem and instead to pass a connection FD to the session compositors and clients. (Although we could specify a file to create on the command-line for debugging purposes.)

I think Robert Ansell started work on the system compositor code to pass the FD via dbus.

Surely not creating the FD at all is a better fix than just moving the fallback location? (/tmp is only used if =$XDG_RUNTIME_DIR is not set)

Notes:

A suitable FD can be created using connector->client_socket_fd() - there's been an an example in <mir>/examples/basic_server_configuration.cpp for over a year.

We are also using this approach with "trust sessions" (to allocate sockets before launching new participants in the trust session).

…cue lots of discussion…

Unless we change the way we start Unity8 (which I've yet to convince people we should), we - tragically - can't pass an fd to unity on startup.

What we *can* do is hand off an FD to logind, and have unity8 pull it from there. Keeping that fd away from non-Unity processes is then left as an exercise for AppArmour confinement profiles written by the security team.

Adding appropriate tasks.

Actually, those tasks are really for a separate bug. It's trivial to fix this issue, so I'll do so. And then open a separate bug.