glXCreateWindow() in mesa/src/glx/x11/ will call createDrawable() provided by dri backends. Right now the swrast implementation of that function (in mesa/src/glx/x11/drisw_glx.c) is quite like DRI implementation, which does not allow to have a new created drawable for GLX. Only DRI2 can work well. If the graphic driver does not support DRI2, or X client application is forced to run on swrast, there would be problems.

How to reproduce:

1, Find a Linux machine with GLX1.3 or higher supported
2, compile the attached test case with:
      gcc -o test_glXCreateWindow test_glXCreateWindow.c -lGL
3, run the application with:
      LIBGL_ALWAYS_SOFTWARE=yes ./test_glXCreateWindow

4, MESA will then report "failed to create drawable"