gibaseinfo: Fix use of stack-allocated GIBaseInfos

Most of the code for handling stack-allocated infos was correct, it was
just missing code to initialise the `GTypeInstance` member.

Since `GTypeInstance` isn’t really designed for stack allocation, this
is a little hacky — it requires setting up the member within
`GTypeInstance` manually. It works, though.

The externally visible consequence of this, though, is that
stack-allocated `GIBaseInfo`s now need to be cleared when they’re
finished being used. This allows the `GTypeClass` ref to be dropped.

All users of the stack-allocated APIs in libgirepository will need to
adapt to this change.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Fixes: #3218

girepository/giarginfo.c

@@ -354,6 +354,9 @@ gi_arg_info_get_type_info (GIArgInfo *info)
  *
  * The initialized @type must not be referenced after @info is deallocated.
  *
+ * Once you are done with @type, it must be cleared using
+ * [method@GIRepository.BaseInfo.clear].
+ *
  * Since: 2.80
  */
 void

girepository/gibaseinfo.c

@@ -141,6 +141,9 @@ gi_base_info_class_init (GIBaseInfoClass *klass)
 static void
 gi_base_info_init (GIBaseInfo *self)
 {
+  /* Initialise a dynamically allocated #GIBaseInfo’s members.
+   *
+   * This function *must* be kept in sync with gi_info_init(). */
   g_atomic_ref_count_init (&self->ref_count);
 }
 
@@ -405,6 +408,20 @@ gi_info_init (GIRealInfo   *info,
 {
   memset (info, 0, sizeof (GIRealInfo));
 
+  /* Evil setup of a stack allocated #GTypeInstance. This is not something it’s
+   * really designed to do.
+   *
+   * This function *must* be kept in sync with gi_base_info_init(), which is
+   * the equivalent function for dynamically allocated types. */
+  info->parent_instance.g_class = g_type_class_ref (type);
+
+  /* g_type_create_instance() calls the #GInstanceInitFunc for each of the
+   * parent types, down to (and including) @type. We don’t need to do that, as
+   * #GIBaseInfo is fundamental so doesn’t have a parent type, the instance init
+   * function for #GIBaseInfo is gi_base_info_init() (which only sets the
+   * refcount, which we already do here), and subtypes of #GIBaseInfo don’t have
+   * instance init functions (see gi_base_info_type_register_static()). */
+
   /* Invalid refcount used to flag stack-allocated infos */
   info->ref_count = INVALID_REFCOUNT;
   info->typelib = typelib;
@@ -417,6 +434,34 @@ gi_info_init (GIRealInfo   *info,
   info->repository = repository;
 }
 
+/**
+ * gi_base_info_clear:
+ * @info: (type GIRepository.BaseInfo): a #GIBaseInfo
+ *
+ * Clears memory allocated internally by a stack-allocated
+ * [type@GIRepository.BaseInfo].
+ *
+ * This does not deallocate the [type@GIRepository.BaseInfo] struct itself.
+ *
+ * This must only be called on stack-allocated [type@GIRepository.BaseInfo]s.
+ * Use [method@GIRepository.BaseInfo.unref] for heap-allocated ones.
+ *
+ * Since: 2.80
+ */
+void
+gi_base_info_clear (void *info)
+{
+  GIBaseInfo *rinfo = (GIBaseInfo *) info;
+
+  g_return_if_fail (GI_IS_BASE_INFO (rinfo));
+
+  g_assert (rinfo->ref_count == INVALID_REFCOUNT);
+
+  GI_BASE_INFO_GET_CLASS (info)->finalize (rinfo);
+
+  g_type_class_unref (rinfo->parent_instance.g_class);
+}
+
 GIBaseInfo *
 gi_info_from_entry (GIRepository *repository,
                     GITypelib    *typelib,
@@ -559,6 +604,9 @@ gi_base_info_ref (void *info)
  * Decreases the reference count of @info. When its reference count
  * drops to 0, the info is freed.
  *
+ * This must not be called on stack-allocated [type@GIRepository.BaseInfo]s —
+ * use [method@GIRepository.BaseInfo.clear] for that.
+ *
  * Since: 2.80
  */
 void

girepository/gibaseinfo.h

@@ -94,6 +94,9 @@ GIBaseInfo *           gi_base_info_ref              (void         *info);
 GI_AVAILABLE_IN_ALL
 void                   gi_base_info_unref            (void         *info);
 
+GI_AVAILABLE_IN_ALL
+void                   gi_base_info_clear            (void         *info);
+
 GI_AVAILABLE_IN_ALL
 GIInfoType             gi_base_info_get_info_type    (GIBaseInfo   *info);
 

girepository/gicallableinfo.c

@@ -201,6 +201,9 @@ gi_callable_info_get_return_type (GICallableInfo *info)
  *
  * The initialized @type must not be referenced after @info is deallocated.
  *
+ * Once you are done with @type, it must be cleared using
+ * [method@GIRepository.BaseInfo.clear].
+ *
  * Since: 2.80
  */
 void
@@ -389,6 +392,9 @@ gi_callable_info_get_arg (GICallableInfo *info,
  *
  * The initialized @arg must not be referenced after @info is deallocated.
  *
+ * Once you are done with @arg, it must be cleared using
+ * [method@GIRepository.BaseInfo.clear].
+ *
  * Since: 2.80
  */
 void

girepository/girffi.c

@@ -220,6 +220,9 @@ gi_callable_info_get_ffi_arg_types (GICallableInfo *callable_info,
             default:
               g_assert_not_reached ();
           }
+
+        gi_base_info_clear (&arg_type);
+        gi_base_info_clear (&arg_info);
       }
 
     arg_types[n_invoke_args] = NULL;