Virtual function in an abstract type returning GObject pointer is not introspectible

Question

I'm playing around with libpeas and I stumbled upon a problem with the way gobject-introspection works.

In libpeas there is a type called PluginLoader (see here).

There is a method called create_extension that returns PeasExtension * which is a point to a GObject. When I add this type to gobject-introspection and I check the resulting GIR file for this method I can see that it's marked as introspectable="0":

<field name="create_extension" introspectable="0">
<callback name="create_extension" introspectable="0">
  <source-position filename="../libpeas/peas-plugin-loader.h"
                   line="60"/>
  <return-value>
    <type name="Extension" c:type="PeasExtension*"/>
  </return-value>
  <parameters>
    <parameter name="loader" transfer-ownership="none">
      <type name="PluginLoader" c:type="PeasPluginLoader*"/>
    </parameter>
    <parameter name="info" transfer-ownership="none">
      <type name="PluginInfo" c:type="PeasPluginInfo*"/>
    </parameter>
    <parameter name="ext_type" transfer-ownership="none">
      <type name="GType" c:type="GType"/>
    </parameter>
    <parameter name="n_parameters" transfer-ownership="none">
      <type name="guint" c:type="guint"/>
    </parameter>
    <parameter name="parameters" transfer-ownership="none">
      <type name="GObject.Parameter" c:type="GParameter*"/>
    </parameter>
  </parameters>
</callback>
</field>

When I change the return value to void * and recompile the introspectable="0" attribute is gone from the GIR file. If I change it to GObject * then the introspectable="0" attribute is still present.

It looks like it becomes non-introspectable when it returns an instance of GObject (a pointer). But I don't undestand why?

By reading the gobject introspection docs it looks like returning a GObject instance is a valid use case. Maybe it has something to do with that PluginLoader type is abstract and this is a virtual method?

Could any annotations help in making this object introspectable without changing the method signature?

Answer 1

With the help of Philip in the comments I've managed to resolve the issue.

There was actually a message in the output of g-ir-scanner saying:

../../source/libpeas/libpeas/peas-plugin-loader.h:91: Warning: Peas: peas_plugin_loader_create_extension: return value: Missing (transfer) annotation

By this message I understood that to make the virtual function create_extension introspectible I have to add annotations, but not at the function pointer in the PeasPluginLoaderClass definition, but rather next to accessor function peas_plugin_loader_create_extension.