lack of XDG_RUNTIME_DIR causes problems

[Antoine Martin]

Switching to another user account via su - otheruser may not create XDG_RUNTIME_DIR.
On some distros, logging in via ssh does not create it either.

Then when we try to create the sockets, /run/user/$UID does not exist and we end up not creating any local sockets... then xpra list shows nothing, and connecting via ssh also fails to locate the server.

[Antoine Martin]

Log sample as guest user via su:

Failed to create bus connection: No such file or directory
2020-11-08 19:37:05,642 Warning: failed to create script directory '/run/user/1001/xpra':
2020-11-08 19:37:05,643  [Errno 2] No such file or directory: '/run/user/1001/xpra'
2020-11-08 19:37:05,643  ($XDG_RUNTIME_DIR has not been created?)

...

2020-11-08 19:37:07,898 Warning: failed to create socket directory '/run/user/1001/xpra'
2020-11-08 19:37:07,898  [Errno 2] No such file or directory: '/run/user/1001/xpra'
2020-11-08 19:37:07,899 Warning: cannot create socket '/run/user/1001/xpra/localhost.localdomain-12':
2020-11-08 19:37:07,899  [Errno 2] No such file or directory
2020-11-08 19:37:07,899  ($XDG_RUNTIME_DIR has not been created?)

...

2020-11-08 19:37:07,899 Warning: no local server sockets,
2020-11-08 19:37:07,899  forward-xdg-open cannot be enabled
2020-11-08 19:37:07,899  non-embedded ssh connections will not be available

...

2020-11-08 19:37:08,011 Warning: the runtime directory '/run/user/1001' does not exist,
2020-11-08 19:37:08,011  cannot start a private pulseaudio server

[Antoine Martin]

Fixed in r27856.