identity: Add a stats property

This is inspired by the stats on rtpjitterbuffer, it's useful
to be able to get some simple stats out of the pipeline without having
to write yet another pad probe.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/615>

docs/plugins/gst_plugins_cache.json

@@ -1218,6 +1218,18 @@
                         "type": "guint",
                         "writable": true
                     },
+                    "stats": {
+                        "blurb": "Statistics",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "application/x-identity-stats, num-bytes=(guint64)0, num-buffers=(guint64)0;",
+                        "mutable": "null",
+                        "readable": true,
+                        "type": "GstStructure",
+                        "writable": false
+                    },
                     "sync": {
                         "blurb": "Synchronize to pipeline clock",
                         "conditionally-available": false,

plugins/elements/gstidentity.c

@@ -95,7 +95,8 @@ enum
   PROP_CHECK_IMPERFECT_OFFSET,
   PROP_SIGNAL_HANDOFFS,
   PROP_DROP_ALLOCATION,
-  PROP_EOS_AFTER
+  PROP_EOS_AFTER,
+  PROP_STATS
 };
 
 
@@ -267,6 +268,36 @@ gst_identity_class_init (GstIdentityClass * klass)
       G_STRUCT_OFFSET (GstIdentityClass, handoff), NULL, NULL,
       NULL, G_TYPE_NONE, 1, GST_TYPE_BUFFER | G_SIGNAL_TYPE_STATIC_SCOPE);
 
+  /**
+   * GstIdentity:stats:
+
+   * Various statistics. This property returns a GstStructure
+   * with name application/x-identity-stats with the following fields:
+   *
+   * <itemizedlist>
+   * <listitem>
+   *   <para>
+   *   #guint64
+   *   <classname>&quot;num-buffers&quot;</classname>:
+   *   the number of buffers that passed through.
+   *   </para>
+   * </listitem>
+   * <listitem>
+   *   <para>
+   *   #guint64
+   *   <classname>&quot;num-bytes&quot;</classname>:
+   *   the number of bytes that passed through.
+   *   </para>
+   * </listitem>
+   * </itemizedlist>
+   *
+   * Since: 1.20
+   */
+  g_object_class_install_property (gobject_class, PROP_STATS,
+      g_param_spec_boxed ("stats", "Statistics",
+          "Statistics", GST_TYPE_STRUCTURE,
+          G_PARAM_READABLE | G_PARAM_STATIC_STRINGS));
+
   gobject_class->finalize = gst_identity_finalize;
 
   gst_element_class_set_static_metadata (gstelement_class,
@@ -786,6 +817,11 @@ gst_identity_transform_ip (GstBaseTransform * trans, GstBuffer * buf)
     GST_BUFFER_OFFSET_END (buf) = GST_CLOCK_TIME_NONE;
   }
 
+  GST_OBJECT_LOCK (trans);
+  identity->num_bytes += gst_buffer_get_size (buf);
+  identity->num_buffers++;
+  GST_OBJECT_UNLOCK (trans);
+
   return ret;
 
   /* ERRORS */
@@ -883,6 +919,20 @@ gst_identity_set_property (GObject * object, guint prop_id,
     gst_base_transform_set_passthrough (GST_BASE_TRANSFORM (identity), TRUE);
 }
 
+static GstStructure *
+gst_identity_create_stats (GstIdentity * identity)
+{
+  GstStructure *s;
+
+  GST_OBJECT_LOCK (identity);
+  s = gst_structure_new ("application/x-identity-stats",
+      "num-bytes", G_TYPE_UINT64, identity->num_bytes,
+      "num-buffers", G_TYPE_UINT64, identity->num_buffers, NULL);
+  GST_OBJECT_UNLOCK (identity);
+
+  return s;
+}
+
 static void
 gst_identity_get_property (GObject * object, guint prop_id, GValue * value,
     GParamSpec * pspec)
@@ -942,6 +992,9 @@ gst_identity_get_property (GObject * object, guint prop_id, GValue * value,
     case PROP_EOS_AFTER:
       g_value_set_int (value, identity->eos_after);
       break;
+    case PROP_STATS:
+      g_value_take_boxed (value, gst_identity_create_stats (identity));
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -1080,6 +1133,8 @@ gst_identity_change_state (GstElement * element, GstStateChange transition)
       GST_OBJECT_UNLOCK (identity);
       if (identity->sync)
         no_preroll = TRUE;
+      identity->num_bytes = 0;
+      identity->num_buffers = 0;
       break;
     case GST_STATE_CHANGE_PAUSED_TO_PLAYING:
       GST_OBJECT_LOCK (identity);

plugins/elements/gstidentity.h

@@ -83,6 +83,8 @@ struct _GstIdentity {
   gboolean       drop_allocation;
   gint           eos_after;
   gint           eos_after_counter;
+  guint64        num_bytes;
+  guint64        num_buffers;
 };
 
 struct _GstIdentityClass {