function: Always initialize callback return value

When callback_closure() exits early, for example due to being called
during GC, the return value would not be initialized. This value is
often non-zero. If the callback is a source func of an idle or a timeout
this would then get interpreted as G_SOURCE_CONTINUE and the same would
repeat in the next iteration. If this happens fast enough, this results
in the entire process being seemingly frozen while spamming the log with
error messages.

To fix this always initialize the return value to 0 or a comparable
neutral value.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/1868

1 files changed, 8 insertions, 10 deletions

diff --git a/gi/function.cpp b/gi/function.cpp
index cf94b39e..3fb87939 100644
--- a/gi/function.cpp
+++ b/gi/function.cpp
@@ -297,6 +297,14 @@ void GjsCallbackTrampoline::warn_about_illegal_js_callback(const char* when,
 void GjsCallbackTrampoline::callback_closure(GIArgument** args, void* result) {
     GITypeInfo ret_type;
 
+    // Fill in the result with some hopefully neutral value
+    g_callable_info_load_return_type(m_info, &ret_type);
+    if (g_type_info_get_tag(&ret_type) != GI_TYPE_TAG_VOID) {
+        GIArgument argument = {};
+        gjs_gi_argument_init_default(&ret_type, &argument);
+        set_return_ffi_arg_from_giargument(&ret_type, result, &argument);
+    }
+
     if (G_UNLIKELY(!is_valid())) {
         warn_about_illegal_js_callback(
             "during shutdown",
@@ -375,8 +383,6 @@ void GjsCallbackTrampoline::callback_closure(GIArgument** args, void* result) {
 
     JS::RootedValue rval(context);
 
-    g_callable_info_load_return_type(m_info, &ret_type);
-
     if (!callback_closure_inner(context, this_object, gobj, &rval, args,
                                 &ret_type, n_args, c_args_offset, result)) {
         if (!JS_IsExceptionPending(context)) {
@@ -399,14 +405,6 @@ void GjsCallbackTrampoline::callback_closure(GIArgument** args, void* result) {
                     descr.c_str(), m_info.ns(), m_info.name());
         }
 
-        // Fill in the result with some hopefully neutral value
-        if (g_type_info_get_tag(&ret_type) != GI_TYPE_TAG_VOID) {
-            GIArgument argument = {};
-            g_callable_info_load_return_type(m_info, &ret_type);
-            gjs_gi_argument_init_default(&ret_type, &argument);
-            set_return_ffi_arg_from_giargument(&ret_type, result, &argument);
-        }
-
         // If the callback has a GError** argument, then make a GError from the
         // value that was thrown. Otherwise, log it as "uncaught" (critical
         // instead of warning)