drawin: Fix a segfault.

When a drawable has a pending redraw in a delayed call and the
drawin is GCed (which was impossible because it was broken
until the last commit), it crashes.

objects/drawable.c

@@ -193,6 +193,12 @@ static int
 luaA_drawable_refresh(lua_State *L)
 {
     drawable_t *drawable = luaA_checkudata(L, 1, &drawable_class);
+
+    /* GC race condition where the drawin is gone, but the drawable as a
+     * refresh in a delayed call */
+    if (!drawable->refresh_callback)
+        return 0;
+
     drawable->refreshed = true;
     (*drawable->refresh_callback)(drawable->refresh_data);
 
@@ -240,6 +246,12 @@ drawable_class_setup(lua_State *L)
                             NULL);
 }
 
+void drawable_invalidate(drawable_t *d)
+{
+    d->refresh_callback = NULL;
+    d->refresh_data = NULL;
+}
+
 /* @DOC_cobject_COMMON@ */
 
 // vim: filetype=c:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

objects/drawable.h

@@ -50,6 +50,7 @@ typedef struct drawable_t drawable_t;
 drawable_t *drawable_allocator(lua_State *, drawable_refresh_callback *, void *);
 void drawable_set_geometry(lua_State *, int, area_t);
 void drawable_class_setup(lua_State *);
+void drawable_invalidate(drawable_t *d);
 
 #endif
 // vim: filetype=c:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

objects/drawin.c

@@ -187,6 +187,7 @@ drawin_wipe(drawin_t *w)
         w->window = XCB_NONE;
     }
     /* No unref needed because we are being garbage collected */
+    drawable_invalidate(w->drawable);
     w->drawable = NULL;
 }