Bug fix #181: Add --xrender-sync{,-fence}

- Add --xrender-sync{,-fence} to deal with redraw lag issue on GLX
  backend. --xrender-sync-fence requires a sufficiently new xorg-server
  and libXext. NO_XSYNC=1 may be used to disable it at compile time.
  Thanks to tchebb for reporting and everybody else for testing. (#181)

- A bit code clean-up. Replace a few XSync() with XFlush() to minimize
  the latency.

5 files changed, 379 insertions, 51 deletions

diff --git a/common.h b/common.h
index e87651c9e..d243dc077 100644
--- a/common.h
+++ b/common.h
@@ -55,11 +55,19 @@
 // #define CONFIG_DBUS 1
 // Whether to enable condition support.
 // #define CONFIG_C2 1
+// Whether to enable X Sync support.
+// #define CONFIG_XSYNC 1
+// Whether to enable GLX Sync support.
+// #define CONFIG_GLX_XSYNC 1
 
 #if !defined(CONFIG_C2) && defined(DEBUG_C2)
 #error Cannot enable c2 debugging without c2 support.
 #endif
 
+#if (!defined(CONFIG_XSYNC) || !defined(CONFIG_VSYNC_OPENGL)) && defined(CONFIG_GLX_SYNC)
+#error Cannot enable GL sync without X Sync / OpenGL support.
+#endif
+
 #ifndef COMPTON_VERSION
 #define COMPTON_VERSION "unknown"
 #endif
@@ -89,6 +97,9 @@
 #include <X11/extensions/shape.h>
 #include <X11/extensions/Xrandr.h>
 #include <X11/extensions/Xdbe.h>
+#ifdef CONFIG_XSYNC
+#include <X11/extensions/sync.h>
+#endif
 
 #ifdef CONFIG_XINERAMA
 #include <X11/extensions/Xinerama.h>
@@ -338,6 +349,16 @@ enum {
 typedef struct _glx_texture glx_texture_t;
 
 #ifdef CONFIG_VSYNC_OPENGL
+#ifdef DEBUG_GLX_DEBUG_CONTEXT
+typedef GLXContext (*f_glXCreateContextAttribsARB) (Display *dpy,
+    GLXFBConfig config, GLXContext share_context, Bool direct,
+    const int *attrib_list);
+typedef void (*GLDEBUGPROC) (GLenum source, GLenum type,
+    GLuint id, GLenum severity, GLsizei length, const GLchar* message,
+    GLvoid* userParam);
+typedef void (*f_DebugMessageCallback) (GLDEBUGPROC, void *userParam);
+#endif
+
 typedef int (*f_WaitVideoSync) (int, int, unsigned *);
 typedef int (*f_GetVideoSync) (unsigned *);
 
@@ -352,6 +373,47 @@ typedef void (*f_ReleaseTexImageEXT) (Display *display, GLXDrawable drawable, in
 
 typedef void (*f_CopySubBuffer) (Display *dpy, GLXDrawable drawable, int x, int y, int width, int height);
 
+#ifdef CONFIG_GLX_SYNC
+// Looks like duplicate typedef of the same type is safe?
+typedef int64_t GLint64;
+typedef uint64_t GLuint64;
+typedef struct __GLsync *GLsync;
+
+#ifndef GL_SYNC_FLUSH_COMMANDS_BIT
+#define GL_SYNC_FLUSH_COMMANDS_BIT 0x00000001
+#endif
+
+#ifndef GL_TIMEOUT_IGNORED
+#define GL_TIMEOUT_IGNORED 0xFFFFFFFFFFFFFFFFull
+#endif
+
+#ifndef GL_ALREADY_SIGNALED
+#define GL_ALREADY_SIGNALED 0x911A
+#endif
+
+#ifndef GL_TIMEOUT_EXPIRED
+#define GL_TIMEOUT_EXPIRED 0x911B
+#endif
+
+#ifndef GL_CONDITION_SATISFIED
+#define GL_CONDITION_SATISFIED 0x911C
+#endif
+
+#ifndef GL_WAIT_FAILED
+#define GL_WAIT_FAILED 0x911D
+#endif
+
+typedef GLsync (*f_FenceSync) (GLenum condition, GLbitfield flags);
+typedef GLboolean (*f_IsSync) (GLsync sync);
+typedef void (*f_DeleteSync) (GLsync sync);
+typedef GLenum (*f_ClientWaitSync) (GLsync sync, GLbitfield flags,
+    GLuint64 timeout);
+typedef void (*f_WaitSync) (GLsync sync, GLbitfield flags,
+    GLuint64 timeout);
+typedef GLsync (*f_ImportSyncEXT) (GLenum external_sync_type,
+    GLintptr external_sync, GLbitfield flags);
+#endif
+
 #ifdef DEBUG_GLX_MARK
 typedef void (*f_StringMarkerGREMEDY) (GLsizei len, const void *string);
 typedef void (*f_FrameTerminatorGREMEDY) (void);
@@ -438,7 +500,7 @@ struct _win;
 typedef struct _c2_lptr c2_lptr_t;
 
 /// Structure representing all options.
-typedef struct {
+typedef struct _options_t {
   // === General ===
   /// The configuration file we used.
   char *config_file;
@@ -451,6 +513,11 @@ typedef struct {
   char *display_repr;
   /// The backend in use.
   enum backend backend;
+  /// Whether to sync X drawing to avoid certain delay issues with
+  /// GLX backend.
+  bool xrender_sync;
+  /// Whether to sync X drawing with X Sync fence.
+  bool xrender_sync_fence;
   /// Whether to avoid using stencil buffer under GLX backend. Might be
   /// unsafe.
   bool glx_no_stencil;
@@ -617,7 +684,7 @@ typedef struct {
 } options_t;
 
 /// Structure containing all necessary data for a compton session.
-typedef struct {
+typedef struct _session_t {
   // === Display related ===
   /// Display in use.
   Display *dpy;
@@ -650,6 +717,9 @@ typedef struct {
   Picture tgt_picture;
   /// Temporary buffer to paint to before sending to display.
   paint_t tgt_buffer;
+#ifdef CONFIG_XSYNC
+  XSyncFence tgt_buffer_fence;
+#endif
   /// DBE back buffer for root window. Used in DBE painting mode.
   XdbeBackBuffer root_dbe;
   /// Window ID of the window we register as a symbol.
@@ -783,6 +853,20 @@ typedef struct {
   f_ReleaseTexImageEXT glXReleaseTexImageProc;
   /// Pointer to glXCopySubBufferMESA function.
   f_CopySubBuffer glXCopySubBufferProc;
+#ifdef CONFIG_GLX_SYNC
+  /// Pointer to the glFenceSync() function.
+  f_FenceSync glFenceSyncProc;
+  /// Pointer to the glIsSync() function.
+  f_IsSync glIsSyncProc;
+  /// Pointer to the glDeleteSync() function.
+  f_DeleteSync glDeleteSyncProc;
+  /// Pointer to the glClientWaitSync() function.
+  f_ClientWaitSync glClientWaitSyncProc;
+  /// Pointer to the glWaitSync() function.
+  f_WaitSync glWaitSyncProc;
+  /// Pointer to the glImportSyncEXT() function.
+  f_ImportSyncEXT glImportSyncEXT;
+#endif
 #ifdef DEBUG_GLX_MARK
   /// Pointer to StringMarkerGREMEDY function.
   f_StringMarkerGREMEDY glStringMarkerGREMEDY;
@@ -850,6 +934,14 @@ typedef struct {
   /// Number of Xinerama screens.
   int xinerama_nscrs;
 #endif
+#ifdef CONFIG_XSYNC
+  /// Whether X Sync extension exists.
+  bool xsync_exists;
+  /// Event base number for X Sync extension.
+  int xsync_event;
+  /// Error base number for X Sync extension.
+  int xsync_error;
+#endif
   /// Whether X Render convolution filter exists.
   bool xrfilter_convolution_exists;
 
@@ -915,6 +1007,10 @@ typedef struct _win {
   winmode_t mode;
   /// Whether the window has been damaged at least once.
   bool damaged;
+#ifdef CONFIG_XSYNC
+  /// X Sync fence of drawable.
+  XSyncFence fence;
+#endif
   /// Whether the window was damaged after last paint.
   bool pixmap_damaged;
   /// Damage of the window.
@@ -1802,6 +1898,20 @@ free_all_damage_last(session_t *ps) {
     free_region(ps, &ps->all_damage_last[i]);
 }
 
+#ifdef CONFIG_XSYNC
+/**
+ * Free a XSync fence.
+ */
+static inline void
+free_fence(session_t *ps, XSyncFence *pfence) {
+  if (*pfence)
+    XSyncDestroyFence(ps->dpy, *pfence);
+  *pfence = None;
+}
+#else
+#define free_fence(ps, pfence) ((void) 0)
+#endif
+
 /**
  * Crop a rectangle by another rectangle.
  *
@@ -1927,6 +2037,11 @@ vsync_deinit(session_t *ps);
  */
 ///@{
 
+#ifdef CONFIG_GLX_SYNC
+void
+xr_glx_sync(session_t *ps, Drawable d, XSyncFence *pfence);
+#endif
+
 bool
 glx_init(session_t *ps, bool need_render);
 
@@ -2096,6 +2211,58 @@ glx_mark_frame(session_t *ps) {
 
 ///@}
 
+#ifdef CONFIG_XSYNC
+#define xr_sync(ps, d, pfence) xr_sync_(ps, d, pfence)
+#else
+#define xr_sync(ps, d, pfence) xr_sync_(ps, d)
+#endif
+
+/**
+ * Synchronizes a X Render drawable to ensure all pending painting requests
+ * are completed.
+ */
+static inline void
+xr_sync_(session_t *ps, Drawable d
+#ifdef CONFIG_XSYNC
+    , XSyncFence *pfence
+#endif
+    ) {
+  if (!ps->o.xrender_sync)
+    return;
+
+#ifdef CONFIG_XSYNC
+  if (ps->o.xrender_sync_fence && ps->xsync_exists) {
+    // TODO: If everybody just follows the rules stated in X Sync prototype,
+    // we need only one fence per screen, but let's stay a bit cautious right
+    // now
+    XSyncFence tmp_fence = None;
+    if (!pfence)
+      pfence = &tmp_fence;
+    assert(pfence);
+    if (!*pfence)
+      *pfence = XSyncCreateFence(ps->dpy, d, False);
+    if (*pfence) {
+      Bool triggered = False;
+      // The fence may fail to be created (e.g. because of died drawable)
+      assert(!XSyncQueryFence(ps->dpy, *pfence, &triggered) || !triggered);
+      XSyncTriggerFence(ps->dpy, *pfence);
+      XSyncAwaitFence(ps->dpy, pfence, 1);
+      assert(!XSyncQueryFence(ps->dpy, *pfence, &triggered) || triggered);
+    }
+    else {
+      printf_errf("(%#010lx): Failed to create X Sync fence.", d);
+    }
+    free_fence(ps, &tmp_fence);
+    if (*pfence)
+      XSyncResetFence(ps->dpy, *pfence);
+  }
+#endif
+  XSync(ps->dpy, False);
+#ifdef CONFIG_GLX_SYNC
+  xr_glx_sync(ps, d, pfence);
+#endif
+}
+
 /** @name DBus handling
  */
 ///@{
diff --git a/compton.c b/compton.c
index 645aa5c57..1efb05d62 100644
--- a/compton.c
+++ b/compton.c
@@ -489,6 +489,9 @@ win_build_shadow(session_t *ps, win *w, double opacity) {
   w->shadow_paint.pixmap = shadow_pixmap_argb;
   w->shadow_paint.pict = shadow_picture_argb;
 
+  // Sync it once and only once
+  xr_sync(ps, w->shadow_paint.pixmap, NULL);
+
   bool success = paint_bind_tex(ps, &w->shadow_paint, shadow_image->width, shadow_image->height, 32, true);
 
   XFreeGC(ps->dpy, gc);
@@ -1513,12 +1516,16 @@ win_paint_win(session_t *ps, win *w, XserverRegion reg_paint,
   if (!w->paint.pixmap && ps->has_name_pixmap) {
     set_ignore_next(ps);
     w->paint.pixmap = XCompositeNameWindowPixmap(ps->dpy, w->id);
+    if (w->paint.pixmap)
+      free_fence(ps, &w->fence);
   }
+
+  Drawable draw = w->paint.pixmap;
+  if (!draw)
+    draw = w->id;
+
   // XRender: Build picture
   if (bkend_use_xrender(ps) && !w->paint.pict) {
-    Drawable draw = w->paint.pixmap;
-    if (!draw)
-      draw = w->id;
     {
       XRenderPictureAttributes pa = {
         .subwindow_mode = IncludeInferiors,
@@ -1528,6 +1535,10 @@ win_paint_win(session_t *ps, win *w, XserverRegion reg_paint,
           CPSubwindowMode, &pa);
     }
   }
+
+  if (IsViewable == w->a.map_state)
+    xr_sync(ps, draw, &w->fence);
+
   // GLX: Build texture
   // Let glx_bind_pixmap() determine pixmap size, because if the user
   // is resizing windows, the width and height we get may not be up-to-date,
@@ -1948,11 +1959,13 @@ paint_all(session_t *ps, XserverRegion region, XserverRegion region_real, win *t
       else
         glFlush();
       glXWaitX();
+      assert(ps->tgt_buffer.pixmap);
+      xr_sync(ps, ps->tgt_buffer.pixmap, &ps->tgt_buffer_fence);
       paint_bind_tex_real(ps, &ps->tgt_buffer,
           ps->root_width, ps->root_height, ps->depth,
           !ps->o.glx_no_rebind_pixmap);
       // See #163
-      XSync(ps->dpy, False);
+      xr_sync(ps, ps->tgt_buffer.pixmap, &ps->tgt_buffer_fence);
       if (ps->o.vsync_use_glfinish)
         glFinish();
       else
@@ -2116,7 +2129,7 @@ map_win(session_t *ps, Window id) {
   }
 
   // Make sure the XSelectInput() requests are sent
-  XSync(ps->dpy, False);
+  XFlush(ps->dpy);
 
   // Update window mode here to check for ARGB windows
   win_determine_mode(ps, w);
@@ -2205,7 +2218,7 @@ finish_unmap_win(session_t *ps, win *w) {
     w->extents = None;
   }
 
-  free_paint(ps, &w->paint);
+  free_wpaint(ps, w);
   free_region(ps, &w->border_size);
   free_paint(ps, &w->shadow_paint);
 }
@@ -2219,6 +2232,11 @@ static void
 unmap_win(session_t *ps, win *w) {
   if (!w || IsUnmapped == w->a.map_state) return;
 
+  // One last synchronization
+  if (w->paint.pixmap)
+    xr_sync(ps, w->paint.pixmap, &w->fence);
+  free_fence(ps, &w->fence);
+
   // Set focus out
   win_set_focused(ps, w, false);
 
@@ -2654,7 +2672,7 @@ win_mark_client(session_t *ps, win *w, Window client) {
       determine_evmask(ps, client, WIN_EVMODE_CLIENT));
 
   // Make sure the XSelectInput() requests are sent
-  XSync(ps->dpy, False);
+  XFlush(ps->dpy);
 
   win_upd_wintype(ps, w);
 
@@ -3037,7 +3055,7 @@ configure_win(session_t *ps, XConfigureEvent *ce) {
 
     if (w->a.width != ce->width || w->a.height != ce->height
         || w->a.border_width != ce->border_width)
-      free_paint(ps, &w->paint);
+      free_wpaint(ps, w);
 
     if (w->a.width != ce->width || w->a.height != ce->height
         || w->a.border_width != ce->border_width) {
@@ -3097,7 +3115,7 @@ finish_destroy_win(session_t *ps, Window id) {
   for (prev = &ps->list; (w = *prev); prev = &w->next) {
     if (w->id == id && w->destroyed) {
 #ifdef DEBUG_EVENTS
-		printf_dbgf("(%#010lx \"%s\"): %p\n", id, w->name, w);
+      printf_dbgf("(%#010lx \"%s\"): %p\n", id, w->name, w);
 #endif
 
       finish_unmap_win(ps, w);
@@ -3188,10 +3206,10 @@ damage_win(session_t *ps, XDamageNotifyEvent *de) {
  * Xlib error handler function.
  */
 static int
-error(Display __attribute__((unused)) *dpy, XErrorEvent *ev) {
+xerror(Display __attribute__((unused)) *dpy, XErrorEvent *ev) {
   session_t * const ps = ps_g;
 
-  int o;
+  int o = 0;
   const char *name = "Unknown";
 
   if (should_ignore(ps, ev->serial)) {
@@ -3240,6 +3258,17 @@ error(Display __attribute__((unused)) *dpy, XErrorEvent *ev) {
   }
 #endif
 
+#ifdef CONFIG_XSYNC
+  if (ps->xsync_exists) {
+    o = ev->error_code - ps->xsync_error;
+    switch (o) {
+      CASESTRRET2(XSyncBadCounter);
+      CASESTRRET2(XSyncBadAlarm);
+      CASESTRRET2(XSyncBadFence);
+    }
+  }
+#endif
+
   switch (ev->error_code) {
     CASESTRRET2(BadAccess);
     CASESTRRET2(BadAlloc);
@@ -3771,18 +3800,27 @@ ev_name(session_t *ps, XEvent *ev) {
     CASESTRRET(Expose);
     CASESTRRET(PropertyNotify);
     CASESTRRET(ClientMessage);
-    default:
-      if (isdamagenotify(ps, ev))
-        return "Damage";
+  }
 
-      if (ps->shape_exists && ev->type == ps->shape_event) {
-        return "ShapeNotify";
-      }
+  if (isdamagenotify(ps, ev))
+    return "Damage";
 
-      sprintf(buf, "Event %d", ev->type);
+  if (ps->shape_exists && ev->type == ps->shape_event)
+    return "ShapeNotify";
 
-      return buf;
+#ifdef CONFIG_XSYNC
+  if (ps->xsync_exists) {
+    int o = ev->type - ps->xsync_event;
+    switch (o) {
+      CASESTRRET(CounterNotify);
+      CASESTRRET(AlarmNotify);
+    }
   }
+#endif
+
+  sprintf(buf, "Event %d", ev->type);
+
+  return buf;
 }
 
 static Window
@@ -5464,6 +5502,8 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
     { "unredir-if-possible-delay", required_argument, NULL, 309 },
     { "write-pid-path", required_argument, NULL, 310 },
     { "vsync-use-glfinish", no_argument, NULL, 311 },
+    { "xrender-sync", no_argument, NULL, 312 },
+    { "xrender-sync-fence", no_argument, NULL, 313 },
     // Must terminate with a NULL entry
     { NULL, 0, NULL, 0 },
   };
@@ -5714,6 +5754,8 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
         ps->o.write_pid_path = mstrcpy(optarg);
         break;
       P_CASEBOOL(311, vsync_use_glfinish);
+      P_CASEBOOL(312, xrender_sync);
+      P_CASEBOOL(313, xrender_sync_fence);
       default:
         usage(1);
         break;
@@ -5761,6 +5803,9 @@ get_cfg(session_t *ps, int argc, char *const *argv, bool first_pass) {
   if (ps->o.blur_background_frame)
     ps->o.blur_background = true;
 
+  if (ps->o.xrender_sync_fence)
+    ps->o.xrender_sync = true;
+
   // Other variables determined by options
 
   // Determine whether we need to track focus changes
@@ -6478,7 +6523,7 @@ redir_stop(session_t *ps) {
     // If we don't destroy them here, looks like the resources are just
     // kept inaccessible somehow
     for (win *w = ps->list; w; w = w->next)
-      free_paint(ps, &w->paint);
+      free_wpaint(ps, w);
 
     XCompositeUnredirectSubwindows(ps->dpy, ps->root, CompositeRedirectManual);
     // Unmap overlay window
@@ -6852,7 +6897,7 @@ session_init(session_t *ps_old, int argc, char **argv) {
     }
   }
 
-  XSetErrorHandler(error);
+  XSetErrorHandler(xerror);
   if (ps->o.synchronize) {
     XSynchronize(ps->dpy, 1);
   }
@@ -6907,11 +6952,6 @@ session_init(session_t *ps_old, int argc, char **argv) {
     exit(1);
   }
 
-  // Query X Shape
-  if (XShapeQueryExtension(ps->dpy, &ps->shape_event, &ps->shape_error)) {
-    ps->shape_exists = true;
-  }
-
   // Build a safe representation of display name
   {
     char *display_repr = DisplayString(ps->dpy);
@@ -6936,6 +6976,32 @@ session_init(session_t *ps_old, int argc, char **argv) {
   // Second pass
   get_cfg(ps, argc, argv, false);
 
+  // Query X Shape
+  if (XShapeQueryExtension(ps->dpy, &ps->shape_event, &ps->shape_error)) {
+    ps->shape_exists = true;
+  }
+
+  if (ps->o.xrender_sync_fence) {
+#ifdef CONFIG_XSYNC
+    // Query X Sync
+    if (XSyncQueryExtension(ps->dpy, &ps->xsync_event, &ps->xsync_error)) {
+      // TODO: Fencing may require version >= 3.0?
+      int major_version_return = 0, minor_version_return = 0;
+      if (XSyncInitialize(ps->dpy, &major_version_return, &minor_version_return))
+        ps->xsync_exists = true;
+    }
+    if (!ps->xsync_exists) {
+      printf_errf("(): X Sync extension not found. No X Sync fence sync is "
+          "possible.");
+      exit(1);
+    }
+#else
+    printf_errf("(): X Sync support not compiled in. --xrender-sync-fence"
+        "can't work.");
+    exit(1);
+#endif
+  }
+
   // Query X RandR
   if ((ps->o.sw_opti && !ps->o.refresh_rate) || ps->o.xinerama_shadow_crop) {
     if (XRRQueryExtension(ps->dpy, &ps->randr_event, &ps->randr_error))
@@ -7219,6 +7285,7 @@ session_destroy(session_t *ps) {
     ps->tgt_picture = None;
   else
     free_picture(ps, &ps->tgt_picture);
+  free_fence(ps, &ps->tgt_buffer_fence);
 
   free_picture(ps, &ps->root_picture);
   free_paint(ps, &ps->tgt_buffer);
diff --git a/compton.h b/compton.h
index 22ee195dd..a4e838efb 100644
--- a/compton.h
+++ b/compton.h
@@ -276,6 +276,15 @@ free_paint(session_t *ps, paint_t *ppaint) {
 }
 
 /**
+ * Free w->paint.
+ */
+static inline void
+free_wpaint(session_t *ps, win *w) {
+  free_paint(ps, &w->paint);
+  free_fence(ps, &w->fence);
+}
+
+/**
  * Destroy all resources in a <code>struct _win</code>.
  */
 static inline void
@@ -883,7 +892,7 @@ static void
 damage_win(session_t *ps, XDamageNotifyEvent *de);
 
 static int
-error(Display *dpy, XErrorEvent *ev);
+xerror(Display *dpy, XErrorEvent *ev);
 
 static void
 expose_root(session_t *ps, XRectangle *rects, int nrects);
diff --git a/opengl.c b/opengl.c
index d557aea63..39aac33c2 100644
--- a/opengl.c
+++ b/opengl.c
@@ -10,6 +10,50 @@
 
 #include "opengl.h"
 
+#ifdef CONFIG_GLX_SYNC
+void
+xr_glx_sync(session_t *ps, Drawable d, XSyncFence *pfence) {
+  if (*pfence) {
+    // GLsync sync = ps->glFenceSyncProc(GL_SYNC_GPU_COMMANDS_COMPLETE, 0);
+    GLsync sync = ps->glImportSyncEXT(GL_SYNC_X11_FENCE_EXT, *pfence, 0);
+    XSync(ps->dpy, False);
+    glx_check_err(ps);
+    /* GLenum ret = ps->glClientWaitSyncProc(sync, GL_SYNC_FLUSH_COMMANDS_BIT,
+        1000);
+    assert(GL_CONDITION_SATISFIED == ret); */
+    ps->glWaitSyncProc(sync, 0, GL_TIMEOUT_IGNORED);
+    // ps->glDeleteSyncProc(sync);
+    // XSyncResetFence(ps->dpy, *pfence);
+  }
+  glx_check_err(ps);
+}
+#endif
+
+static inline GLXFBConfig
+get_fbconfig_from_visualinfo(session_t *ps, const XVisualInfo *visualinfo) {
+  int nelements = 0;
+  GLXFBConfig *fbconfigs = glXGetFBConfigs(ps->dpy, visualinfo->screen,
+      &nelements);
+  for (int i = 0; i < nelements; ++i) {
+    int visual_id = 0;
+    if (Success == glXGetFBConfigAttrib(ps->dpy, fbconfigs[i], GLX_VISUAL_ID, &visual_id)
+        && visual_id == visualinfo->visualid)
+      return fbconfigs[i];
+  }
+
+  return NULL;
+}
+
+#ifdef DEBUG_GLX_DEBUG_CONTEXT
+static void
+glx_debug_msg_callback(GLenum source, GLenum type,
+    GLuint id, GLenum severity, GLsizei length, const GLchar *message,
+    GLvoid *userParam) {
+  printf_dbgf("(): source 0x%04X, type 0x%04X, id %u, severity 0x%0X, \"%s\"\n",
+      source, type, id, severity, message);
+}
+#endif
+
 /**
  * Initialize OpenGL.
  */
@@ -56,7 +100,33 @@ glx_init(session_t *ps, bool need_render) {
 
   if (!ps->glx_context) {
     // Get GLX context
+#ifndef DEBUG_GLX_DEBUG_CONTEXT
     ps->glx_context = glXCreateContext(ps->dpy, pvis, None, GL_TRUE);
+#else
+    {
+      GLXFBConfig fbconfig = get_fbconfig_from_visualinfo(ps, pvis);
+      if (!fbconfig) {
+        printf_errf("(): Failed to get GLXFBConfig for root visual %#lx.",
+            pvis->visualid);
+        goto glx_init_end;
+      }
+
+      f_glXCreateContextAttribsARB p_glXCreateContextAttribsARB =
+        (f_glXCreateContextAttribsARB)
+        glXGetProcAddress((const GLubyte *) "glXCreateContextAttribsARB");
+      if (!p_glXCreateContextAttribsARB) {
+        printf_errf("(): Failed to get glXCreateContextAttribsARB().");
+        goto glx_init_end;
+      }
+
+      static const int attrib_list[] = {
+        GLX_CONTEXT_FLAGS_ARB, GLX_CONTEXT_DEBUG_BIT_ARB,
+        None
+      };
+      ps->glx_context = p_glXCreateContextAttribsARB(ps->dpy, fbconfig, NULL,
+          GL_TRUE, attrib_list);
+    }
+#endif
 
     if (!ps->glx_context) {
       printf_errf("(): Failed to get GLX context.");
@@ -68,6 +138,20 @@ glx_init(session_t *ps, bool need_render) {
       printf_errf("(): Failed to attach GLX context.");
       goto glx_init_end;
     }
+
+#ifdef DEBUG_GLX_DEBUG_CONTEXT
+    {
+      f_DebugMessageCallback p_DebugMessageCallback =
+        (f_DebugMessageCallback)
+        glXGetProcAddress((const GLubyte *) "glDebugMessageCallback");
+      if (!p_DebugMessageCallback) {
+        printf_errf("(): Failed to get glDebugMessageCallback(0.");
+        goto glx_init_end;
+      }
+      p_DebugMessageCallback(glx_debug_msg_callback, ps);
+    }
+#endif
+
   }
 
   // Ensure we have a stencil buffer. X Fixes does not guarantee rectangles
@@ -114,6 +198,27 @@ glx_init(session_t *ps, bool need_render) {
         goto glx_init_end;
       }
     }
+
+#ifdef CONFIG_GLX_SYNC
+    ps->glFenceSyncProc = (f_FenceSync)
+      glXGetProcAddress((const GLubyte *) "glFenceSync");
+    ps->glIsSyncProc = (f_IsSync)
+      glXGetProcAddress((const GLubyte *) "glIsSync");
+    ps->glDeleteSyncProc = (f_DeleteSync)
+      glXGetProcAddress((const GLubyte *) "glDeleteSync");
+    ps->glClientWaitSyncProc = (f_ClientWaitSync)
+      glXGetProcAddress((const GLubyte *) "glClientWaitSync");
+    ps->glWaitSyncProc = (f_WaitSync)
+      glXGetProcAddress((const GLubyte *) "glWaitSync");
+    ps->glImportSyncEXT = (f_ImportSyncEXT)
+      glXGetProcAddress((const GLubyte *) "glImportSyncEXT");
+    if (!ps->glFenceSyncProc || !ps->glIsSyncProc || !ps->glDeleteSyncProc
+        || !ps->glClientWaitSyncProc || !ps->glWaitSyncProc
+        || !ps->glImportSyncEXT) {
+      printf_errf("(): Failed to acquire GLX sync functions.");
+      goto glx_init_end;
+    }
+#endif
   }
 
   // Acquire FBConfigs
@@ -344,9 +449,7 @@ glx_init_blur(session_t *ps) {
   }
 
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 
   return true;
 #else
@@ -655,9 +758,7 @@ glx_bind_pixmap(session_t *ps, glx_texture_t **pptex, Pixmap pixmap,
   glBindTexture(ptex->target, 0);
   glDisable(ptex->target);
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 
   return true;
 }
@@ -680,9 +781,7 @@ glx_release_pixmap(session_t *ps, glx_texture_t *ptex) {
     ptex->glpixmap = 0;
   }
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 }
 
 /**
@@ -803,9 +902,7 @@ glx_paint_pre(session_t *ps, XserverRegion *preg) {
   glx_render_color(ps, 0, 0, ps->root_width, ps->root_height, 0, *preg, NULL);
 #endif
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 }
 
 /**
@@ -886,9 +983,7 @@ glx_set_clip(session_t *ps, XserverRegion reg, const reg_data_t *pcache_reg) {
 
   cxfree(rects_free);
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 }
 
 #define P_PAINTREG_START() \
@@ -1174,9 +1269,7 @@ glx_blur_dst_end:
     free_glx_bc(ps, pbc);
   }
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 
   return ret;
 }
@@ -1212,9 +1305,7 @@ glx_dim_dst(session_t *ps, int dx, int dy, int width, int height, float z,
   glColor4f(0.0f, 0.0f, 0.0f, 0.0f);
   glDisable(GL_BLEND);
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 
   return true;
 }
@@ -1412,9 +1503,7 @@ glx_render(session_t *ps, const glx_texture_t *ptex,
     glActiveTexture(GL_TEXTURE0);
   }
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 
   return true;
 }
@@ -1452,9 +1541,7 @@ glx_render_color(session_t *ps, int dx, int dy, int width, int height, int z,
   }
   glColor4f(0.0f, 0.0f, 0.0f, 0.0f);
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 }
 
 /**
@@ -1492,9 +1579,7 @@ glx_render_dots(session_t *ps, int dx, int dy, int width, int height, int z,
   }
   glColor4f(0.0f, 0.0f, 0.0f, 0.0f);
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 }
 
 /**
@@ -1524,9 +1609,7 @@ glx_swap_copysubbuffermesa(session_t *ps, XserverRegion reg) {
     }
   }
 
-#ifdef DEBUG_GLX_ERR
   glx_check_err(ps);
-#endif
 
   cxfree(rects);
 }
diff --git a/opengl.h b/opengl.h
index 564b7e20a..8628e36d3 100644
--- a/opengl.h
+++ b/opengl.h
@@ -59,6 +59,8 @@ glx_check_err_(session_t *ps, const char *func, int line) {
 }
 
 #define glx_check_err(ps) glx_check_err_(ps, __func__, __LINE__)
+#else
+#define glx_check_err(ps) ((void) 0)
 #endif
 
 /**