py-sdl2 does not honor the DISPLAY environment variable from X Protocol

[drpeabody]

The DISPLAY Environment Variable is used by all the applications that know about the X Protocol to find which X Server to connect to. SDL2 Libraries in other languages use this variable properly and work fine in case of multiple X Servers etc. However PySDL2 does not work if there is a fully functional X Sever on DISPLAY=:1 instead of DISPLAY=:0

To test this, we can run a simple virtual X Server in the background:
$ Xvfb :1 -s '-screen 0 1920x1080x24 +iglx' &

Next we can see that SDL_Init() fails:

$ export DISPLAY=:1
$ python
>>> import sdl2
>>> sdl2.SDL_Init()
-1
>>> sdl2.SDL_GetError(sdl2.SDL_INIT_EVERYTHING)
'No available video device'

In contrast, SDL works when default DISPLAY is used:

$ Xvfb :0 -s '-screen 0 1920x1080x24 +iglx' &

Next we can see that SDL_Init() fails:

$ export DISPLAY=:0
$ python
>>> import sdl2
>>> sdl2.SDL_Init(sdl2.SDL_INIT_EVERYTHING)
0
>>> sdl2.SDL_GetError()
''

This issue was found after building SDL2 from source with various combinations of build flags and after a week of wondering why SDL2 could not find the X server, as the X server logs remained empty all the while. All Applications that use X Servers respect this environment variable as it allows applications to work in scenarios where different X servers configured with different settings need to be run at the same time.