Identifying the real display server

[Mattias Andrée]

On Fri, 10 Apr 2015 18:24:52 -0700
Thiago Macieira <thiago at kde.org> wrote:

> On Saturday 11 April 2015 00:33:50 Mattias Andrée wrote:
> > Is there another way to identify which display server is
> > actually running? If not, I propose we add the
> > environment variable REAL_DISPLAY. The value of
> > REAL_DISPLAY should be then name of the environment
> > variable used by the display server that is actually
> > running. For example if Wayland is running with an X
> > compatibility layer, the value of REAL_DISPLAY should
> > be WAYLAND_DISPLAY.
> 
> I'd recommend having one common variable that lists all
> the options, in the order of preference of the
> compositing server. Suppose it's a Wayland server with
> Mir and X compatibility layers and the X layer is better
> supported, we'd have:
> 
>  wayland:///run/user/1000/wayland-0|x11::0|mir:///run/user/1000/mir-0
> 
> I chose to use the pipe character (|) because it cannot
> appear unencoded in URLs, so you can easily tokenise and
> parse each item as a URL. The space character would have
> served just as well.
> 
> As for the name of the variable itself... I'll leave the
> bikeshed to others :-)
> 

List of preference is a good idea. But I think the
URL is overly complicated and redundant. The *DISPLAY
variable must still be set for each protocol.

I suggest the same specifications as my original
proposal with the addition that the value is a
space (\x20) separated list in order of preference.
Leading, duplicate and trailing spaces should be
allowed by are ignored.

The reason for using a space character as the
delimiter is that single ASCII character
delimiters is simplest to implement, it is
a character that never causes any problems
except for in once case that we can utilise,
shells do not support them in environment
variable names. Similarly, = is not supported
in variable environment names, but using it
as a delimiter is not as readable, and it
does not have the same nice property as
whitespace: $REAL_DISPLAY, without quotes,
in a shell is evaluated to one argument
display server listed in it.

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