function: Do not allow returning a transfer-none string from JS callback

In order to marshal a string from JS into C, we have to create a temporary
UTF-8 string. That string cannot be returned as transfer-none, because
there is no obvious place to keep it alive so that it can live as long as
the C code needs it to. See #519 for more explanation.

This is currently "supported" but causes a memory leak. Instead, throw
when creating a callback or vfunc in JS that would otherwise have this
memory leak.

2 files changed, 17 insertions, 3 deletions

diff --git a/gi/function.cpp b/gi/function.cpp
index c9f8b1f3..e6672e05 100644
--- a/gi/function.cpp
+++ b/gi/function.cpp
@@ -679,6 +679,19 @@ bool GjsCallbackTrampoline::initialize() {
     g_assert(is_valid());
     g_assert(!m_closure);
 
+    GITypeInfo return_type;
+    g_callable_info_load_return_type(m_info, &return_type);
+    GITypeTag return_tag = g_type_info_get_tag(&return_type);
+    if (g_callable_info_get_caller_owns(m_info) == GI_TRANSFER_NOTHING &&
+        (return_tag == GI_TYPE_TAG_FILENAME ||
+         return_tag == GI_TYPE_TAG_UTF8)) {
+        gjs_throw(context(),
+                  "%s %s returns a transfer-none string. This is not supported "
+                  "(https://gitlab.gnome.org/GNOME/gjs/-/issues/519)",
+                  m_is_vfunc ? "VFunc" : "Callback", m_info.name());
+        return false;
+    }
+
     /* Analyze param types and directions, similarly to
      * init_cached_function_data */
     int n_param_types = g_callable_info_get_n_args(m_info);
diff --git a/installed-tests/js/testGIMarshalling.js b/installed-tests/js/testGIMarshalling.js
index 97f8d542..2140f67b 100644
--- a/installed-tests/js/testGIMarshalling.js
+++ b/installed-tests/js/testGIMarshalling.js
@@ -1258,9 +1258,10 @@ let VFuncTester = GObject.registerClass(class VFuncTester extends GIMarshallingT
         return i + 4;
     }
 
-    vfunc_method_str_arg_out_ret(s) {
-        return [`Called with ${s}`, 41];
-    }
+    // https://gitlab.gnome.org/GNOME/gjs/-/issues/519
+    // vfunc_method_str_arg_out_ret(s) {
+    //     return [`Called with ${s}`, 41];
+    // }
 
     vfunc_method_with_default_implementation(i) {
         this.int = i + 2;