[mutter] wayland/xdg-shell: Move popup role assignment behind checks

[Robert Mader <rmader src gnome org>]

commit 1c4a518cd36b9b3be5b7c0ef5d053b5f2bb3aa51
Author: Robert Mader <robert mader posteo de>
Date:   Wed Aug 26 22:55:40 2020 +0200

    wayland/xdg-shell: Move popup role assignment behind checks
    
    If we returned early in one of the checks but already assigned the
    surface role, we'd later run into a double-free and crash. Just do
    the checks at the beginning.
    
    Also add a missing return statement that was left out in commit
    88ff196fe3ca5 and tighten the parent surface check.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1415

 src/wayland/meta-wayland-xdg-shell.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)
---
diff --git a/src/wayland/meta-wayland-xdg-shell.c b/src/wayland/meta-wayland-xdg-shell.c
index 85eca7fd62..768b909acb 100644
--- a/src/wayland/meta-wayland-xdg-shell.c
+++ b/src/wayland/meta-wayland-xdg-shell.c
@@ -171,6 +171,10 @@ static MetaWaylandSurface *
 surface_from_xdg_surface_resource (struct wl_resource *resource)
 {
   MetaWaylandSurfaceRole *surface_role = wl_resource_get_user_data (resource);
+
+  if (!META_IS_WAYLAND_SURFACE_ROLE (surface_role))
+    return NULL;
+
   return meta_wayland_surface_role_get_surface (surface_role);
 }
 
@@ -1851,18 +1855,6 @@ xdg_surface_constructor_get_popup (struct wl_client   *client,
   MetaWaylandXdgPopup *xdg_popup;
   MetaWaylandXdgSurface *xdg_surface;
 
-  if (!meta_wayland_surface_assign_role (surface,
-                                         META_TYPE_WAYLAND_XDG_POPUP,
-                                         "shell-client", shell_client,
-                                         "xdg-surface-resource", xdg_surface_resource,
-                                         NULL))
-    {
-      wl_resource_post_error (xdg_wm_base_resource, XDG_WM_BASE_ERROR_ROLE,
-                              "wl_surface@%d already has a different role",
-                              wl_resource_get_id (surface->resource));
-      return;
-    }
-
   if (!parent_resource)
     {
       wl_resource_post_error (xdg_wm_base_resource,
@@ -1870,10 +1862,11 @@ xdg_surface_constructor_get_popup (struct wl_client   *client,
                               "Parent surface is null but Mutter does not yet "
                               "support specifying parent surfaces via other "
                               "protocols");
+      return;
     }
 
   parent_surface = surface_from_xdg_surface_resource (parent_resource);
-  if (!META_IS_WAYLAND_XDG_SURFACE (parent_surface->role))
+  if (!parent_surface || !META_IS_WAYLAND_XDG_SURFACE (parent_surface->role))
     {
       wl_resource_post_error (xdg_wm_base_resource,
                               XDG_WM_BASE_ERROR_INVALID_POPUP_PARENT,
@@ -1890,6 +1883,18 @@ xdg_surface_constructor_get_popup (struct wl_client   *client,
       return;
     }
 
+  if (!meta_wayland_surface_assign_role (surface,
+                                         META_TYPE_WAYLAND_XDG_POPUP,
+                                         "shell-client", shell_client,
+                                         "xdg-surface-resource", xdg_surface_resource,
+                                         NULL))
+    {
+      wl_resource_post_error (xdg_wm_base_resource, XDG_WM_BASE_ERROR_ROLE,
+                              "wl_surface@%d already has a different role",
+                              wl_resource_get_id (surface->resource));
+      return;
+    }
+
   xdg_popup = META_WAYLAND_XDG_POPUP (surface->role);
 
   xdg_popup->resource = wl_resource_create (client,