[gnome-shell] st/theme: Unref CRStylesheet after removing old theme nodes

[Florian Müllner <fmuellner src gnome org>]

commit e06109c23ce2a6dd53d32c26a89ab46a952d7373
Author: Jonas Dreßler <verdre v0yd nl>
Date:   Thu Apr 30 22:23:52 2020 +0200

    st/theme: Unref CRStylesheet after removing old theme nodes
    
    Since we now remove all theme nodes on a stylesheet change (ie.
    StTheme's "custom-stylesheets-changed" signal) instead of only
    invalidating them, those nodes may not be accessed anymore as soon as
    "custom-stylesheets-changed" is emitted.
    
    It turned out though that when comparing them to the newly generated
    nodes in `st_widget_recompute_style()` using the
    `st_theme_node_paint/geometry_equal()` functions, the properties of the
    old nodes will still be accessed, causing a crash since the
    CRDeclarations are already freed.
    
    To fix that, keep the reference to the CRStylesheet, which owns the
    CRDeclarations used by the theme nodes, around a bit longer, so it's
    still possible to access the CRDeclarations inside the
    "custom-stylesheets-changed" signal handler. This allows us to compare
    the old theme nodes to the new ones since the CSS properties of both are
    still valid.
    
    Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2709

 src/st/st-theme.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/st/st-theme.c b/src/st/st-theme.c
index 0d2d2eeace..7ccf7b5bad 100644
--- a/src/st/st-theme.c
+++ b/src/st/st-theme.c
@@ -287,8 +287,10 @@ st_theme_unload_stylesheet (StTheme    *theme,
   theme->custom_stylesheets = g_slist_remove (theme->custom_stylesheets, stylesheet);
   g_hash_table_remove (theme->stylesheets_by_file, file);
   g_hash_table_remove (theme->files_by_stylesheet, stylesheet);
-  cr_stylesheet_unref (stylesheet);
+
   g_signal_emit (theme, signals[STYLESHEETS_CHANGED], 0);
+
+  cr_stylesheet_unref (stylesheet);
 }
 
 /**