GstCustomMeta: simplify API

Move the GstStructure field into public struct for direct access, that's
easier than having to call a function to get it. It is not an API/ABI
breakage to extend the public structure of a GstMeta because they are
always allocated by inside GStreamer. The structure is exposed already
by gst_custom_meta_get_structure() which does not return a copy/ref, so
it is locked into holding a GstStructure forever anyway.

Also add gst_meta_register_custom_simple() because most of the time only
a name is required, tags and transform functions are more niche
use-case.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5385>

girs/Gst-1.0.gir

@@ -10501,11 +10501,17 @@ this functionality yet.</doc>
       </function>
     </enumeration>
     <record name="CustomMeta" c:type="GstCustomMeta" version="1.20">
-      <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.h">Simple typing wrapper around #GstMeta</doc>
+      <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.h">Extra custom metadata. The @structure field is the same as returned by
+gst_custom_meta_get_structure().</doc>
       <source-position filename="../subprojects/gstreamer/gst/gstmeta.h"/>
       <field name="meta" writable="1">
+        <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.h">parent #GstMeta</doc>
         <type name="Meta" c:type="GstMeta"/>
       </field>
+      <field name="structure" writable="1">
+        <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.h">a #GstStructure containing custom metadata. (Since: 1.24)</doc>
+        <type name="Structure" c:type="GstStructure*"/>
+      </field>
       <method name="get_structure" c:identifier="gst_custom_meta_get_structure" version="1.20">
         <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">Retrieve the #GstStructure backing a custom meta, the structure's mutability
 is conditioned to the writability of the #GstBuffer @meta is attached to.</doc>
@@ -25863,6 +25869,21 @@ access metadata.</doc>
           </parameter>
         </parameters>
       </function>
+      <function name="register_custom_simple" c:identifier="gst_meta_register_custom_simple" version="1.24">
+        <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">Simplified version of gst_meta_register_custom(), with no tags and no
+transform function.</doc>
+        <source-position filename="../subprojects/gstreamer/gst/gstmeta.h"/>
+        <return-value transfer-ownership="none">
+          <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">a #GstMetaInfo that can be used to access metadata.</doc>
+          <type name="MetaInfo" c:type="const GstMetaInfo*"/>
+        </return-value>
+        <parameters>
+          <parameter name="name" transfer-ownership="none">
+            <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">the name of the #GstMeta implementation</doc>
+            <type name="utf8" c:type="const gchar*"/>
+          </parameter>
+        </parameters>
+      </function>
     </record>
     <bitfield name="MetaFlags" glib:type-name="GstMetaFlags" glib:get-type="gst_meta_flags_get_type" c:type="GstMetaFlags">
       <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.h">Extra metadata flags.</doc>
@@ -51649,6 +51670,21 @@ access metadata.</doc>
         </parameter>
       </parameters>
     </function>
+    <function name="meta_register_custom_simple" c:identifier="gst_meta_register_custom_simple" moved-to="Meta.register_custom_simple" version="1.24">
+      <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">Simplified version of gst_meta_register_custom(), with no tags and no
+transform function.</doc>
+      <source-position filename="../subprojects/gstreamer/gst/gstmeta.h"/>
+      <return-value transfer-ownership="none">
+        <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">a #GstMetaInfo that can be used to access metadata.</doc>
+        <type name="MetaInfo" c:type="const GstMetaInfo*"/>
+      </return-value>
+      <parameters>
+        <parameter name="name" transfer-ownership="none">
+          <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstmeta.c">the name of the #GstMeta implementation</doc>
+          <type name="utf8" c:type="const gchar*"/>
+        </parameter>
+      </parameters>
+    </function>
     <function name="mini_object_replace" c:identifier="gst_mini_object_replace" moved-to="MiniObject.replace">
       <doc xml:space="preserve" filename="../subprojects/gstreamer/gst/gstminiobject.c">Atomically modifies a pointer to point to a new mini-object.
 The reference count of @olddata is decreased and the reference count of

subprojects/gstreamer/gst/gstmeta.c

@@ -59,13 +59,6 @@ GQuark _gst_meta_transform_copy;
 GQuark _gst_meta_tag_memory;
 GQuark _gst_meta_tag_memory_reference;
 
-typedef struct
-{
-  GstCustomMeta meta;
-
-  GstStructure *structure;
-} GstCustomMetaImpl;
-
 typedef struct
 {
   GstMetaInfo info;
@@ -154,7 +147,7 @@ gst_meta_api_type_register (const gchar * api, const gchar ** tags)
 static gboolean
 custom_init_func (GstMeta * meta, gpointer params, GstBuffer * buffer)
 {
-  GstCustomMetaImpl *cmeta = (GstCustomMetaImpl *) meta;
+  GstCustomMeta *cmeta = (GstCustomMeta *) meta;
 
   cmeta->structure = gst_structure_new_empty (g_type_name (meta->info->type));
 
@@ -167,7 +160,7 @@ custom_init_func (GstMeta * meta, gpointer params, GstBuffer * buffer)
 static void
 custom_free_func (GstMeta * meta, GstBuffer * buffer)
 {
-  GstCustomMetaImpl *cmeta = (GstCustomMetaImpl *) meta;
+  GstCustomMeta *cmeta = (GstCustomMeta *) meta;
 
   gst_structure_set_parent_refcount (cmeta->structure, NULL);
   gst_structure_free (cmeta->structure);
@@ -177,16 +170,15 @@ static gboolean
 custom_transform_func (GstBuffer * transbuf, GstMeta * meta,
     GstBuffer * buffer, GQuark type, gpointer data)
 {
-  GstCustomMetaImpl *custom, *cmeta = (GstCustomMetaImpl *) meta;
+  GstCustomMeta *custom, *cmeta = (GstCustomMeta *) meta;
   GstMetaInfoImpl *info = (GstMetaInfoImpl *) meta->info;
 
   if (info->custom_transform_func)
-    return info->custom_transform_func (transbuf, (GstCustomMeta *) meta,
+    return info->custom_transform_func (transbuf, cmeta,
         buffer, type, data, info->custom_transform_user_data);
 
   if (GST_META_TRANSFORM_IS_COPY (type)) {
-    custom =
+    custom = (GstCustomMeta *) gst_buffer_add_meta (transbuf, meta->info, NULL);
-        (GstCustomMetaImpl *) gst_buffer_add_meta (transbuf, meta->info, NULL);
     gst_structure_set_parent_refcount (custom->structure, NULL);
     gst_structure_take (&custom->structure,
         gst_structure_copy (cmeta->structure));
@@ -215,7 +207,7 @@ gst_custom_meta_get_structure (GstCustomMeta * meta)
   g_return_val_if_fail (gst_meta_info_is_custom (((GstMeta *) meta)->info),
       NULL);
 
-  return ((GstCustomMetaImpl *) meta)->structure;
+  return meta->structure;
 }
 
 /**
@@ -233,7 +225,7 @@ gst_custom_meta_has_name (GstCustomMeta * meta, const gchar * name)
   g_return_val_if_fail (gst_meta_info_is_custom (((GstMeta *) meta)->info),
       FALSE);
 
-  return gst_structure_has_name (((GstCustomMetaImpl *) meta)->structure, name);
+  return gst_structure_has_name (meta->structure, name);
 }
 
 /**
@@ -281,7 +273,7 @@ gst_meta_register_custom (const gchar * name, const gchar ** tags,
     goto done;
 
   info = (GstMetaInfoImpl *) gst_meta_register (api, name,
-      sizeof (GstCustomMetaImpl),
+      sizeof (GstCustomMeta),
       custom_init_func, custom_free_func, custom_transform_func);
 
   if (!info)
@@ -298,6 +290,23 @@ done:
   return ret;
 }
 
+/**
+ * gst_meta_register_custom_simple:
+ * @name: the name of the #GstMeta implementation
+ *
+ * Simplified version of gst_meta_register_custom(), with no tags and no
+ * transform function.
+ *
+ * Returns: (transfer none): a #GstMetaInfo that can be used to access metadata.
+ * Since: 1.24
+ */
+const GstMetaInfo *
+gst_meta_register_custom_simple (const gchar * name)
+{
+  const gchar *tags[] = { NULL };
+  return gst_meta_register_custom (name, tags, NULL, NULL, NULL);
+}
+
 /**
  * gst_meta_info_is_custom:
  *

subprojects/gstreamer/gst/gstmeta.h

@@ -116,14 +116,26 @@ struct _GstMeta {
 };
 
 /**
- * GstCustomMeta:
+ * GstCustomMeta.structure:
  *
- * Simple typing wrapper around #GstMeta
+ * #GstStructure containing custom metadata.
+ *
+ * Since: 1.24
+ */
+
+/**
+ * GstCustomMeta:
+ * @meta: parent #GstMeta
+ * @structure: a #GstStructure containing custom metadata. (Since: 1.24)
+ *
+ * Extra custom metadata. The @structure field is the same as returned by
+ * gst_custom_meta_get_structure().
  *
  * Since: 1.20
  */
 typedef struct {
   GstMeta meta;
+  GstStructure *structure;
 } GstCustomMeta;
 
 #include <gst/gstbuffer.h>
@@ -267,6 +279,9 @@ const GstMetaInfo *  gst_meta_register_custom   (const gchar *name, const gchar
                                                  GstCustomMetaTransformFunction transform_func,
                                                  gpointer user_data, GDestroyNotify destroy_data);
 
+GST_API
+const GstMetaInfo *  gst_meta_register_custom_simple (const gchar *name);
+
 GST_API
 gboolean             gst_meta_info_is_custom    (const GstMetaInfo *info);