[gnome-shell/gnome-3-36] st-bin: Disallow st_bin_set_child with already-parented children

[Florian Müllner <fmuellner src gnome org>]

commit f8afcd8b891da79dc7968455fe42e2a3eaecf6b2
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Tue Nov 24 17:34:08 2020 +0800

    st-bin: Disallow st_bin_set_child with already-parented children
    
    Not checking for this would result in `clutter_actor_add_child`
    failing, but StBin keeping a copy in `priv->child`. So later on,
    `st_bin_remove` would never be called on it and this assertion
    would fail and crash the whole shell:
    
    ```
    static void
    st_bin_destroy (ClutterActor *actor)
    {
      StBinPrivate *priv = st_bin_get_instance_private (ST_BIN (actor));
    
      if (priv->child)
        clutter_actor_destroy (priv->child);
      g_assert (priv->child == NULL);
    
    ```
    
    By disallowing spurious `st_bin_set_child` calls we now prevent StBin
    from entering such a corrupt state and the above assertion won't fail
    anymore.
    
    Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1507>
    (cherry picked from commit 244c266c9f59f1758024deb46a0418efe23243a8)

 src/st/st-bin.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/src/st/st-bin.c b/src/st/st-bin.c
index b9c17e59c7..d1fedb17a9 100644
--- a/src/st/st-bin.c
+++ b/src/st/st-bin.c
@@ -412,6 +412,19 @@ st_bin_set_child (StBin        *bin,
   if (priv->child == child)
     return;
 
+  if (child)
+    {
+      ClutterActor *parent = clutter_actor_get_parent (child);
+
+      if (parent)
+        {
+          g_warning ("%s: The provided 'child' actor %p already has a "
+                     "(different) parent %p and can't be made a child of %p.",
+                     G_STRFUNC, child, parent, bin);
+          return;
+        }
+    }
+
   if (priv->child)
     clutter_actor_remove_child (CLUTTER_ACTOR (bin), priv->child);