Gosu Window#gl segfaults with shader when deferring drawing.

gosu_shader_segfault.rb

# Failing case of Window#gl used with shaders.

require "opengl" # Gem must be "opengl" gem, not "ruby-opengl" gem
require 'gosu'

def media_path(file); File.expand_path "media/#{file}", File.dirname(__FILE__) end

class TestWindow < Gosu::Window
  def initialize
    super 640, 480, false

    @star = Gosu::Image.new(self, media_path("LargeStar.png"), true)

    # Vertex shader
    @vertex = glCreateShader GL_VERTEX_SHADER
    glShaderSource @vertex, <<-END
#version 110

varying vec2 var_TexCoord;

void main()
{
  gl_Position = ftransform();
  var_TexCoord = gl_MultiTexCoord0.xy;
}
    END

    glCompileShader @vertex

    #
    @fragment = glCreateShader GL_FRAGMENT_SHADER
    glShaderSource @fragment, <<-END
#version 110

uniform sampler2D in_Texture;
varying vec2 var_TexCoord;

void main()
{
  gl_FragColor = texture2D(in_Texture, var_TexCoord);
  gl_FragColor.g = 1.0;
}

    END

    glCompileShader @fragment

    # Shader program
    @program = glCreateProgram
    glAttachShader @program, @vertex
    glAttachShader @program, @fragment
    glLinkProgram @program
  end

  def draw_with_shader(x)
    glUseProgram @program
    glActiveTexture GL_TEXTURE0
    glBindTexture GL_TEXTURE_2D, @star.gl_tex_info.tex_name
    @star.draw x, 0, 0
    flush # Force drawing before we un-attach the shader.
    glUseProgram 0
  end

  def draw
    puts "Draw immediately, outside #gl (OK)"
    draw_with_shader 0

    gl do
      puts "Draw immediately in #gl (OK)"
      glEnable GL_BLEND
      draw_with_shader 200
    end

    gl 5 do
      puts "Draw deferred with #gl (SEGFAULT!!!)"
      glEnable GL_BLEND
      draw_with_shader 400
    end
  end
end

TestWindow.new.show