girepository: Expose GITypeInfo and GIArgInfo as stack allocatable

There are a handful of APIs in libgirepository which are used on
performance-sensitive code paths in language bindings (such as looking
at arguments when doing function calls). Historically libgirepository
has provided a stack-allocated variant for them, which avoids returning
a newly allocated `GIBaseInfo`. Since moving to glib.git and porting to
`GTypeInstance`, that stack allocated version has been broken.

This commit fixes it, by exposing obfuscated stack allocatable versions
of `GITypeInfo` and `GIArgInfo`, which are the two `GIBaseInfo`
subtypes which can be returned by the stack allocation functions.

The commit includes unit tests for them.

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

Fixes: #3217

girepository/girepository-private.h

@@ -57,8 +57,15 @@ struct _GIBaseInfo
   uint32_t offset;
 
   uint32_t type_is_embedded : 1; /* Used by GITypeInfo */
+
+  /* A copy of GIBaseInfo is exposed publicly for stack-allocated derivatives
+   * such as GITypeInfo, so its size is now ABI. */
+  void *padding[6];
 };
 
+G_STATIC_ASSERT (sizeof (GIBaseInfo) == sizeof (GIBaseInfoStack));
+G_STATIC_ASSERT (G_ALIGNOF (GIBaseInfo) == G_ALIGNOF (GIBaseInfoStack));
+
 /* Subtypes */
 struct _GICallableInfo
 {
@@ -196,18 +203,15 @@ struct _GIFieldInfo
 void gi_field_info_class_init (gpointer g_class,
                                gpointer class_data);
 
-struct _GIArgInfo
+/* GIArgInfo is stack-allocatable so it can be used with
-{
+ * gi_callable_info_load_return_type() and gi_callable_info_load_arg(), so its
-  GIBaseInfo parent;
+ * definition is actually public in gitypes.h. */
-};
 
 void gi_arg_info_class_init (gpointer g_class,
                              gpointer class_data);
 
-struct _GITypeInfo
+/* GITypeInfo is stack-allocatable so it can be used with
-{
+ * gi_arg_info_load_type(), so its definition is actually public in gitypes.h. */
-  GIBaseInfo parent;
-};
 
 void gi_type_info_class_init (gpointer g_class,
                               gpointer class_data);

girepository/gitypes.h

@@ -41,6 +41,17 @@ G_BEGIN_DECLS
 typedef struct _GIBaseInfo GIBaseInfo;
 typedef struct _GIBaseInfoClass GIBaseInfoClass;
 
+typedef struct
+{
+  /*< private >*/
+  GTypeInstance parent_instance;
+
+  int dummy0;
+  void *dummy1[3];
+  uint32_t dummy2[2];
+  void *dummy3[6];
+} GIBaseInfoStack;
+
 /* Documented in gicallableinfo.c */
 typedef struct _GICallableInfo GICallableInfo;
 GI_AVAILABLE_IN_ALL GType gi_callable_info_get_type (void);
@@ -110,11 +121,23 @@ typedef struct _GIFieldInfo GIFieldInfo;
 GI_AVAILABLE_IN_ALL GType gi_field_info_get_type (void);
 
 /* Documented in giarginfo.c */
-typedef struct _GIArgInfo GIArgInfo;
+typedef struct
+{
+  /*< private >*/
+  GIBaseInfoStack parent;
+
+  void *padding[6];
+} GIArgInfo;
 GI_AVAILABLE_IN_ALL GType gi_arg_info_get_type (void);
 
 /* Documented in gitypeinfo.c */
-typedef struct _GITypeInfo GITypeInfo;
+typedef struct
+{
+  /*< private >*/
+  GIBaseInfoStack parent;
+
+  void *padding[6];
+} GITypeInfo;
 GI_AVAILABLE_IN_ALL GType gi_type_info_get_type (void);
 
 /* Documented in giunresolvedinfo.c */

girepository/tests/repository.c

@@ -152,6 +152,7 @@ test_repository_arg_info (RepositoryFixture *fx,
   GIFunctionInfo *method_info = NULL;
   GIArgInfo *arg_info = NULL;
   GITypeInfo *type_info = NULL;
+  GITypeInfo type_info_stack;
   unsigned int idx;
 
   g_test_summary ("Test retrieving GIArgInfos from a typelib");
@@ -188,7 +189,13 @@ test_repository_arg_info (RepositoryFixture *fx,
   g_assert_true (gi_type_info_is_pointer (type_info));
   g_assert_cmpint (gi_type_info_get_tag (type_info), ==, GI_TYPE_TAG_UTF8);
 
+  gi_arg_info_load_type (arg_info, &type_info_stack);
+  g_assert_true (gi_type_info_is_pointer (&type_info_stack) == gi_type_info_is_pointer (type_info));
+  g_assert_cmpint (gi_type_info_get_tag (&type_info_stack), ==, gi_type_info_get_tag (type_info));
+
+  gi_base_info_clear (&type_info_stack);
   g_clear_pointer (&type_info, gi_base_info_unref);
+
   g_clear_pointer (&arg_info, gi_base_info_unref);
   g_clear_pointer (&method_info, gi_base_info_unref);
   g_clear_pointer (&object_info, gi_base_info_unref);
@@ -237,9 +244,11 @@ test_repository_callable_info (RepositoryFixture *fx,
   GIFunctionInfo *method_info = NULL;
   GICallableInfo *callable_info;
   GITypeInfo *type_info = NULL;
+  GITypeInfo type_info_stack;
   GIAttributeIter iter = GI_ATTRIBUTE_ITER_INIT;
   const char *name, *value;
   GIArgInfo *arg_info = NULL;
+  GIArgInfo arg_info_stack;
 
   g_test_summary ("Test retrieving GICallableInfos from a typelib");
 
@@ -260,6 +269,12 @@ test_repository_callable_info (RepositoryFixture *fx,
   g_assert_nonnull (type_info);
   g_assert_true (gi_type_info_is_pointer (type_info));
   g_assert_cmpint (gi_type_info_get_tag (type_info), ==, GI_TYPE_TAG_VOID);
+
+  gi_callable_info_load_return_type (callable_info, &type_info_stack);
+  g_assert_true (gi_type_info_is_pointer (&type_info_stack) == gi_type_info_is_pointer (type_info));
+  g_assert_cmpint (gi_type_info_get_tag (&type_info_stack), ==, gi_type_info_get_tag (type_info));
+
+  gi_base_info_clear (&type_info_stack);
   g_clear_pointer (&type_info, gi_base_info_unref);
 
   /* This method has no attributes */
@@ -275,6 +290,12 @@ test_repository_callable_info (RepositoryFixture *fx,
 
   arg_info = gi_callable_info_get_arg (callable_info, 0);
   g_assert_nonnull (arg_info);
+
+  gi_callable_info_load_arg (callable_info, 0, &arg_info_stack);
+  g_assert_cmpint (gi_arg_info_get_direction (&arg_info_stack), ==, gi_arg_info_get_direction (arg_info));
+  g_assert_true (gi_arg_info_may_be_null (&arg_info_stack) == gi_arg_info_may_be_null (arg_info));
+
+  gi_base_info_clear (&arg_info_stack);
   g_clear_pointer (&arg_info, gi_base_info_unref);
 
   g_assert_cmpint (gi_callable_info_get_instance_ownership_transfer (callable_info), ==, GI_TRANSFER_NOTHING);