diff --git a/glib/gmain.c b/glib/gmain.c
index 18a7974aa..1b4260b11 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -958,6 +958,7 @@ g_source_set_dispose_function (GSource            *source,
 {
   g_return_if_fail (source != NULL);
   g_return_if_fail (source->priv->dispose == NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   source->priv->dispose = dispose;
 }
 
@@ -1220,6 +1221,8 @@ g_source_attach (GSource      *source,
 {
   guint result = 0;
 
+  g_return_val_if_fail (source != NULL, 0);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, 0);
   g_return_val_if_fail (source->context == NULL, 0);
   g_return_val_if_fail (!SOURCE_DESTROYED (source), 0);
   
@@ -1317,6 +1320,7 @@ g_source_destroy (GSource *source)
   GMainContext *context;
   
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   
   context = source->context;
   
@@ -1348,6 +1352,7 @@ g_source_get_id (GSource *source)
   guint result;
   
   g_return_val_if_fail (source != NULL, 0);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, 0);
   g_return_val_if_fail (source->context != NULL, 0);
 
   LOCK_CONTEXT (source->context);
@@ -1377,6 +1382,8 @@ g_source_get_id (GSource *source)
 GMainContext *
 g_source_get_context (GSource *source)
 {
+  g_return_val_if_fail (source != NULL, NULL);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, NULL);
   g_return_val_if_fail (source->context != NULL || !SOURCE_DESTROYED (source), NULL);
 
   return source->context;
@@ -1408,6 +1415,7 @@ g_source_add_poll (GSource *source,
   GMainContext *context;
   
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (fd != NULL);
   g_return_if_fail (!SOURCE_DESTROYED (source));
   
@@ -1444,6 +1452,7 @@ g_source_remove_poll (GSource *source,
   GMainContext *context;
   
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (fd != NULL);
   g_return_if_fail (!SOURCE_DESTROYED (source));
   
@@ -1494,7 +1503,9 @@ g_source_add_child_source (GSource *source,
   GMainContext *context;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (child_source != NULL);
+  g_return_if_fail (g_atomic_int_get (&child_source->ref_count) > 0);
   g_return_if_fail (!SOURCE_DESTROYED (source));
   g_return_if_fail (!SOURCE_DESTROYED (child_source));
   g_return_if_fail (child_source->context == NULL);
@@ -1555,7 +1566,9 @@ g_source_remove_child_source (GSource *source,
   GMainContext *context;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (child_source != NULL);
+  g_return_if_fail (g_atomic_int_get (&child_source->ref_count) > 0);
   g_return_if_fail (child_source->priv->parent_source == source);
   g_return_if_fail (!SOURCE_DESTROYED (source));
   g_return_if_fail (!SOURCE_DESTROYED (child_source));
@@ -1638,6 +1651,7 @@ g_source_set_callback_indirect (GSource              *source,
   GSourceCallbackFuncs *old_cb_funcs;
   
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (callback_funcs != NULL || callback_data == NULL);
 
   context = source->context;
@@ -1700,6 +1714,7 @@ g_source_set_callback (GSource        *source,
   GSourceCallback *new_callback;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
 
   TRACE (GLIB_SOURCE_SET_CALLBACK (source, func, data, notify));
 
@@ -1813,6 +1828,7 @@ g_source_set_priority (GSource  *source,
   GMainContext *context;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (source->priv->parent_source == NULL);
 
   context = source->context;
@@ -1836,6 +1852,7 @@ gint
 g_source_get_priority (GSource *source)
 {
   g_return_val_if_fail (source != NULL, 0);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, 0);
 
   return source->priority;
 }
@@ -1878,15 +1895,7 @@ g_source_set_ready_time (GSource *source,
   GMainContext *context;
 
   g_return_if_fail (source != NULL);
-  /* We deliberately don't check for ref_count > 0 here, because that
-   * breaks cancellable_source_cancelled() in GCancellable: it has no
-   * way to find out that the last-unref has happened until the
-   * finalize() function is called, but that's too late, because the
-   * ref_count already has already reached 0 before that time.
-   * However, priv is only poisoned (set to NULL) after finalize(),
-   * so we can use this as a simple guard against use-after-free.
-   * See https://bugzilla.gnome.org/show_bug.cgi?id=791754 */
-  g_return_if_fail (source->priv != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
 
   context = source->context;
 
@@ -1930,6 +1939,7 @@ gint64
 g_source_get_ready_time (GSource *source)
 {
   g_return_val_if_fail (source != NULL, -1);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, -1);
 
   return source->priv->ready_time;
 }
@@ -1951,6 +1961,7 @@ g_source_set_can_recurse (GSource  *source,
   GMainContext *context;
   
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
 
   context = source->context;
 
@@ -1979,6 +1990,7 @@ gboolean
 g_source_get_can_recurse (GSource  *source)
 {
   g_return_val_if_fail (source != NULL, FALSE);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, FALSE);
   
   return (source->flags & G_SOURCE_CAN_RECURSE) != 0;
 }
@@ -2015,6 +2027,7 @@ g_source_set_name (GSource    *source,
   GMainContext *context;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
 
   context = source->context;
 
@@ -2050,6 +2063,7 @@ const char *
 g_source_get_name (GSource *source)
 {
   g_return_val_if_fail (source != NULL, NULL);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, NULL);
 
   return source->name;
 }
@@ -2106,6 +2120,9 @@ GSource *
 g_source_ref (GSource *source)
 {
   g_return_val_if_fail (source != NULL, NULL);
+  /* We allow ref_count == 0 here to allow the dispose function to resurrect
+   * the GSource if needed */
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) >= 0, NULL);
 
   g_atomic_int_inc (&source->ref_count);
 
@@ -2171,6 +2188,8 @@ g_source_unref_internal (GSource      *source,
 
       if (source->source_funcs->finalize)
 	{
+          gint old_ref_count;
+
           /* Temporarily increase the ref count again so that GSource methods
            * can be called from finalize(). */
           g_atomic_int_inc (&source->ref_count);
@@ -2179,11 +2198,14 @@ g_source_unref_internal (GSource      *source,
 	  source->source_funcs->finalize (source);
 	  if (context)
 	    LOCK_CONTEXT (context);
-          g_atomic_int_add (&source->ref_count, -1);
+          old_ref_count = g_atomic_int_add (&source->ref_count, -1);
+          g_warn_if_fail (old_ref_count == 1);
 	}
 
       if (old_cb_funcs)
         {
+          gint old_ref_count;
+
           /* Temporarily increase the ref count again so that GSource methods
            * can be called from callback_funcs.unref(). */
           g_atomic_int_inc (&source->ref_count);
@@ -2194,7 +2216,8 @@ g_source_unref_internal (GSource      *source,
 
           if (context)
             LOCK_CONTEXT (context);
-          g_atomic_int_add (&source->ref_count, -1);
+          old_ref_count = g_atomic_int_add (&source->ref_count, -1);
+          g_warn_if_fail (old_ref_count == 1);
         }
 
       g_free (source->name);
@@ -2238,6 +2261,7 @@ void
 g_source_unref (GSource *source)
 {
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
 
   g_source_unref_internal (source, source->context, FALSE);
 }
@@ -2541,6 +2565,7 @@ g_source_add_unix_fd (GSource      *source,
   GPollFD *poll_fd;
 
   g_return_val_if_fail (source != NULL, NULL);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, NULL);
   g_return_val_if_fail (!SOURCE_DESTROYED (source), NULL);
 
   poll_fd = g_new (GPollFD, 1);
@@ -2594,6 +2619,7 @@ g_source_modify_unix_fd (GSource      *source,
   GPollFD *poll_fd;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (g_slist_find (source->priv->fds, tag));
 
   context = source->context;
@@ -2631,6 +2657,7 @@ g_source_remove_unix_fd (GSource  *source,
   GPollFD *poll_fd;
 
   g_return_if_fail (source != NULL);
+  g_return_if_fail (g_atomic_int_get (&source->ref_count) > 0);
   g_return_if_fail (g_slist_find (source->priv->fds, tag));
 
   context = source->context;
@@ -2679,6 +2706,7 @@ g_source_query_unix_fd (GSource  *source,
   GPollFD *poll_fd;
 
   g_return_val_if_fail (source != NULL, 0);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, 0);
   g_return_val_if_fail (g_slist_find (source->priv->fds, tag), 0);
 
   poll_fd = tag;
@@ -3109,6 +3137,8 @@ g_main_current_source (void)
 gboolean
 g_source_is_destroyed (GSource *source)
 {
+  g_return_val_if_fail (source != NULL, TRUE);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, TRUE);
   return SOURCE_DESTROYED (source);
 }
 
@@ -4509,6 +4539,8 @@ g_source_get_time (GSource *source)
   GMainContext *context;
   gint64 result;
 
+  g_return_val_if_fail (source != NULL, 0);
+  g_return_val_if_fail (g_atomic_int_get (&source->ref_count) > 0, 0);
   g_return_val_if_fail (source->context != NULL, 0);
 
   context = source->context;