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>

diff --git a/src/st/st-bin.c b/src/st/st-bin.c
index 2eadd57ed0e637c1bce181714d88c36925aae4d2..e96768626fd72cac8397f98c930ab332f660da3c 100644 (file)
--- a/src/st/st-bin.c
+++ b/src/st/st-bin.c
@@ -358,6 +358,19 @@ 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);