backend, steamcompmgr: fix use-after-free issue at exit

all: also release the xwayland_server_guard lock before calling pthread_exit() to prevent gamescope from hanging at exit

src/backend.cpp

@@ -16,34 +16,29 @@ namespace gamescope
     // IBackend
     /////////////
 
-    static IBackend *s_pBackend = nullptr;
+    static std::atomic<IBackend*> s_pBackend = nullptr;
 
     IBackend *IBackend::Get()
     {
         return s_pBackend;
     }
-
+
+
     bool IBackend::Set( IBackend *pBackend )
     {
-        if ( s_pBackend )
-        {
-            delete s_pBackend;
-            s_pBackend = nullptr;
-        }
-
         if ( pBackend )
         {
             s_pBackend = pBackend;
-            if ( !s_pBackend->Init() )
+            if ( !GetBackend()->Init() )
             {
-                delete s_pBackend;
-                s_pBackend = nullptr;
+								s_pBackend = nullptr;
                 return false;
             }
         }
 
         return true;
     }
+
 
     /////////////////
     // CBaseBackendFb
@@ -184,4 +179,4 @@ namespace gamescope
 
         GetBackend()->DumpDebugInfo();
     });
-}
+}

src/steamcompmgr.cpp

@@ -5896,7 +5896,7 @@ error(Display *dpy, XErrorEvent *ev)
 }
 
 [[noreturn]] static void
-steamcompmgr_exit(void)
+steamcompmgr_exit(std::optional<std::unique_lock<std::mutex>> lock = std::nullopt)
 {
 	g_ImageWaiter.Shutdown();
 
@@ -5936,7 +5936,9 @@ steamcompmgr_exit(void)
     wlserver_lock();
     wlserver_shutdown();
     wlserver_unlock(false);
-
+
+	if (lock)
+		lock->unlock();
 	pthread_exit(NULL);
 }
 
@@ -8042,7 +8044,7 @@ steamcompmgr_main(int argc, char **argv)
 		vblank = false;
 	}
 
-	steamcompmgr_exit();
+	steamcompmgr_exit(std::optional<std::unique_lock<std::mutex>> {std::in_place_t{}, std::move(xwayland_server_guard)} );
 }
 
 void steamcompmgr_send_frame_done_to_focus_window()