giunioninfo: Split success and return value for get_discriminator_offset()

Otherwise there’s no obvious suitable return value to return when the
union is *not* discriminated.

This is an API break, but libgirepository has not been in a stable
release yet, so that’s fine.

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

Helps: #3155

docs/reference/girepository/migrating-gi.md

@@ -72,5 +72,6 @@ your code if integer type warnings are enabled.
 | `g_type_info_get_array_length` | [method@GIRepository.TypeInfo.get_array_length_index] |
 | `g_typelib_new_from_*` | All replaced with `gi_typelib_new_from_bytes()` |
 | `GI_FUNCTION_THROWS` and `GI_VFUNC_THROWS` | [method@GIRepository.CallableInfo.can_throw_gerror] |
+| `g_union_info_get_discriminator_offset` | Split success and failure return values out into a new out-argument and return value |
 | `g_union_info_get_copy_function` | [method@GIRepository.UnionInfo.get_copy_function_name] |
 | `g_union_info_get_free_function` | [method@GIRepository.UnionInfo.get_free_function_name] |

girepository/girwriter.c

@@ -1283,7 +1283,7 @@ write_union_info (const char *ns,
       size_t offset;
       GITypeInfo *type;
 
-      offset = gi_union_info_get_discriminator_offset (info);
+      gi_union_info_get_discriminator_offset (info, &offset);
       type = gi_union_info_get_discriminator_type (info);
 
       xml_start_element (file, "discriminator");

girepository/giunioninfo.c

@@ -151,19 +151,30 @@ gi_union_info_is_discriminated (GIUnionInfo *info)
 /**
  * gi_union_info_get_discriminator_offset:
  * @info: a #GIUnionInfo
+ * @out_offset: (out) (optional): return location for the offset, in bytes, of
+ *   the discriminator
  *
- * Returns the offset of the discriminator field in the structure.
+ * Obtain the offset of the discriminator field within the structure.
  *
- * Returns: offset, in bytes, of the discriminator
+ * The union must be discriminated, or `FALSE` will be returned.
+ *
+ * Returns: `TRUE` if the union is discriminated
  * Since: 2.80
  */
-size_t
+gboolean
-gi_union_info_get_discriminator_offset (GIUnionInfo *info)
+gi_union_info_get_discriminator_offset (GIUnionInfo *info,
+                                        size_t      *out_offset)
 {
   GIRealInfo *rinfo = (GIRealInfo *)info;
   UnionBlob *blob = (UnionBlob *)&rinfo->typelib->data[rinfo->offset];
+  size_t discriminator_offset;
 
-  return blob->discriminator_offset;
+  discriminator_offset = (blob->discriminated) ? blob->discriminator_offset : 0;
+
+  if (out_offset != NULL)
+    *out_offset = discriminator_offset;
+
+  return blob->discriminated;
 }
 
 /**

girepository/giunioninfo.h

@@ -76,7 +76,8 @@ GI_AVAILABLE_IN_ALL
 gboolean         gi_union_info_is_discriminated         (GIUnionInfo *info);
 
 GI_AVAILABLE_IN_ALL
-size_t           gi_union_info_get_discriminator_offset (GIUnionInfo *info);
+gboolean         gi_union_info_get_discriminator_offset (GIUnionInfo *info,
+                                                         size_t      *out_offset);
 
 GI_AVAILABLE_IN_ALL
 GITypeInfo *     gi_union_info_get_discriminator_type   (GIUnionInfo *info);