dispatch: Learn about glvnd

glvnd has the rather nice property that GLX and (desktop) GL are
available in separate libraries. As an example this would allow an EGL
app to use desktop GL without any X11 libraries. Fix up our dlopens to
prefer the glvnd libraries if available and fall back to the old ABI if
not.

Signed-off-by: Adam Jackson <ajax@redhat.com>

src/dispatch_common.c

@@ -178,6 +178,7 @@
 #elif defined(ANDROID)
 #define GLX_LIB "libGLESv2.so"
 #else
+#define GLVND_GLX_LIB "libGLX.so.1"
 #define GLX_LIB "libGL.so.1"
 #endif
 
@@ -193,6 +194,7 @@
 #define EGL_LIB "libEGL.so.1"
 #define GLES1_LIB "libGLESv1_CM.so.1"
 #define GLES2_LIB "libGLESv2.so.2"
+#define OPENGL_LIB "libOpenGL.so.1"
 #endif
 
 #ifdef __GNUC__
@@ -223,13 +225,18 @@ struct api {
     pthread_mutex_t mutex;
 #endif
 
-    /* dlopen() return value for libGL.so.1. */
+    /*
+     * dlopen() return value for the GLX API. This is libGLX.so.1 if the
+     * runtime is glvnd-enabled, else libGL.so.1
+     */
     void *glx_handle;
 
     /*
-     * dlopen() return value for OS X's GL library.
+     * dlopen() return value for the desktop GL library.
      *
-     * On linux, glx_handle is used instead.
+     * On Windows this is OPENGL32. On OSX this is classic libGL. On Linux
+     * this is either libOpenGL (if the runtime is glvnd-enabled) or
+     * classic libGL.so.1
      */
     void *gl_handle;
 
@@ -590,6 +597,10 @@ epoxy_egl_dlsym(const char *name)
 void *
 epoxy_conservative_glx_dlsym(const char *name, bool exit_if_fails)
 {
+    /* prefer the glvnd library if it exists */
+    if (!api.glx_handle)
+	get_dlopen_handle(&api.glx_handle, GLVND_GLX_LIB, false);
+
     return do_dlsym(&api.glx_handle, GLX_LIB, name, exit_if_fails);
 }
 
@@ -609,8 +620,18 @@ epoxy_gl_dlsym(const char *name)
                     "/System/Library/Frameworks/OpenGL.framework/Versions/Current/OpenGL",
                     name, true);
 #else
-    /* There's no library for desktop GL support independent of GLX. */
-    return epoxy_glx_dlsym(name);
+    void *sym;
+
+    if (!api.gl_handle)
+	get_dlopen_handle(&api.gl_handle, OPENGL_LIB, false);
+
+    if (api.gl_handle)
+	return do_dlsym(&api.gl_handle, NULL, name, true);
+
+    sym = do_dlsym(&api.glx_handle, GLX_LIB, name, true);
+    api.gl_handle = api.glx_handle; /* skip the dlopen next time */
+
+    return sym;
 #endif
 }