cogl: Add _cogl_xlib_{,un}trap_errors

This is similar to clutter_x11_{,un}trap_errors except that it stores
the previous trap state in a caller-allocated struct so that it can be
re-entrant.

Make _cogl_xlib_trap_errors re-entrant

(this will be squashed into an earlier commit)

clutter/cogl/cogl/winsys/cogl-context-winsys.c

@@ -94,6 +94,8 @@ _cogl_create_context_winsys (CoglContext *context)
 {
 #ifdef COGL_HAS_XLIB_SUPPORT
   context->winsys.event_filters = NULL;
+
+  context->winsys.trap_state = NULL;
 #endif
 
   _cogl_winsys_features_init (context);

clutter/cogl/cogl/winsys/cogl-context-winsys.h

@@ -24,11 +24,31 @@
 #ifndef __COGL_CONTEXT_WINSYS_H
 #define __COGL_CONTEXT_WINSYS_H
 
+#ifdef COGL_HAS_XLIB_SUPPORT
+#include <X11/Xlib.h>
+#endif
+
 typedef enum
 {
   COGL_WINSYS_FEATURE_STUB /* no features are defined yet */
 } CoglWinsysFeatureFlags;
 
+#ifdef COGL_HAS_XLIB_SUPPORT
+
+typedef struct _CoglXlibTrapState CoglXlibTrapState;
+
+struct _CoglXlibTrapState
+{
+  /* These values are intended to be internal to
+     _cogl_xlib_{un,}trap_errors but they need to be in the header so
+     that the struct can be allocated on the stack */
+  int (* old_error_handler) (Display *, XErrorEvent *);
+  int trapped_error_code;
+  CoglXlibTrapState *old_state;
+};
+
+#endif
+
 typedef struct
 {
   /* These are specific to winsys backends supporting Xlib. This
@@ -36,6 +56,9 @@ typedef struct
      to Xlib when Cogl gains a more complete winsys abstraction */
 #ifdef COGL_HAS_XLIB_SUPPORT
   GSList *event_filters;
+  /* Current top of the XError trap state stack. The actual memory for
+     these is expected to be allocated on the stack by the caller */
+  CoglXlibTrapState *trap_state;
 #endif
 
   /* Function pointers for winsys specific extensions */

clutter/cogl/cogl/winsys/cogl-xlib.c

@@ -129,3 +129,42 @@ _cogl_xlib_remove_filter (CoglXlibFilterFunc func,
         }
     }
 }
+
+static int
+error_handler (Display     *xdpy,
+               XErrorEvent *error)
+{
+  _COGL_GET_CONTEXT (ctxt, 0);
+
+  g_assert (ctxt->winsys.trap_state);
+
+  ctxt->winsys.trap_state->trapped_error_code = error->error_code;
+
+  return 0;
+}
+
+void
+_cogl_xlib_trap_errors (CoglXlibTrapState *state)
+{
+  _COGL_GET_CONTEXT (ctxt, NO_RETVAL);
+
+  state->trapped_error_code = 0;
+  state->old_error_handler = XSetErrorHandler (error_handler);
+
+  state->old_state = ctxt->winsys.trap_state;
+  ctxt->winsys.trap_state = state;
+}
+
+int
+_cogl_xlib_untrap_errors (CoglXlibTrapState *state)
+{
+  _COGL_GET_CONTEXT (ctxt, 0);
+
+  g_assert (state == ctxt->winsys.trap_state);
+
+  XSetErrorHandler (state->old_error_handler);
+
+  ctxt->winsys.trap_state = state->old_state;
+
+  return state->trapped_error_code;
+}

clutter/cogl/cogl/winsys/cogl-xlib.h

@@ -25,6 +25,9 @@
 #define __COGL_XLIB_H
 
 #include "cogl.h"
+#include "cogl-context-winsys.h"
+
+#include <X11/Xlib.h>
 
 typedef struct _CoglXlibFilterClosure CoglXlibFilterClosure;
 
@@ -34,4 +37,30 @@ struct _CoglXlibFilterClosure
   gpointer data;
 };
 
+/*
+ * _cogl_xlib_trap_errors:
+ * @state: A temporary place to store data for the trap.
+ *
+ * Traps every X error until _cogl_xlib_untrap_errors() called. You
+ * should allocate an uninitialised CoglXlibTrapState struct on the
+ * stack to pass to this function. The same pointer should later be
+ * passed to _cogl_xlib_untrap_errors(). Calls to
+ * _cogl_xlib_trap_errors() can be nested as long as
+ * _cogl_xlib_untrap_errors() is called with the corresponding state
+ * pointers in reverse order.
+ */
+void
+_cogl_xlib_trap_errors (CoglXlibTrapState *state);
+
+/*
+ * _cogl_xlib_untrap_errors:
+ * @state: The state that was passed to _cogl_xlib_trap_errors().
+ *
+ * Removes the X error trap and returns the current status.
+ *
+ * Return value: the trapped error code, or 0 for success
+ */
+int
+_cogl_xlib_untrap_errors (CoglXlibTrapState *state);
+
 #endif /* __COGL_XLIB_H */