discoverer: Add a signal to retrieve serialized GstDiscovererInfo

This allows user to serialize the GstDiscovererInfo in other places
than the default folder, like a database when running web services for
examples.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3911>

subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer.c

@@ -172,6 +172,7 @@ enum
   SIGNAL_STARTING,
   SIGNAL_DISCOVERED,
   SIGNAL_SOURCE_SETUP,
+  SIGNAL_LOAD_SERIALIZED_INFO,
   LAST_SIGNAL
 };
 
@@ -215,6 +216,8 @@ static GVariant *gst_discoverer_info_to_variant_recurse (GstDiscovererStreamInfo
     * sinfo, GstDiscovererSerializeFlags flags);
 static GstDiscovererStreamInfo *_parse_discovery (GVariant * variant,
     GstDiscovererInfo * info);
+static GstDiscovererInfo *load_serialized_info (GstDiscoverer * dc,
+    gchar * uri);
 
 static void
 gst_discoverer_class_init (GstDiscovererClass * klass)
@@ -227,6 +230,8 @@ gst_discoverer_class_init (GstDiscovererClass * klass)
   gobject_class->set_property = gst_discoverer_set_property;
   gobject_class->get_property = gst_discoverer_get_property;
 
+  klass->load_serialize_info = load_serialized_info;
+
 
   /* properties */
   /**
@@ -323,6 +328,26 @@ gst_discoverer_class_init (GstDiscovererClass * klass)
       g_signal_new ("source-setup", G_TYPE_FROM_CLASS (klass),
       G_SIGNAL_RUN_LAST, G_STRUCT_OFFSET (GstDiscovererClass, source_setup),
       NULL, NULL, NULL, G_TYPE_NONE, 1, GST_TYPE_ELEMENT);
+
+  /**
+   * GstDiscoverer::load-serialized-info:
+   * @discoverer: the #GstDiscoverer
+   * @uri: THe URI to load the serialized info for
+   *
+   * Retrieves information about a URI from and external source of information,
+   * like a cache file. This is used by the discoverer to speed up the
+   * discovery.
+   *
+   * Returns: (nullable) (transfer full): The #GstDiscovererInfo representing
+   * @uri, or %NULL if no information
+   *
+   * Since: 1.24
+   */
+  gst_discoverer_signals[SIGNAL_LOAD_SERIALIZED_INFO] =
+      g_signal_new ("load-serialized-info", G_TYPE_FROM_CLASS (klass),
+      G_SIGNAL_RUN_LAST, G_STRUCT_OFFSET (GstDiscovererClass,
+          load_serialize_info), g_signal_accumulator_first_wins, NULL, NULL,
+      GST_TYPE_DISCOVERER_INFO, 1, G_TYPE_STRING);
 }
 
 static void
@@ -1836,40 +1861,53 @@ _get_info_from_cachefile (GstDiscoverer * dc, gchar * cachefile)
   return NULL;
 }
 
+static GstDiscovererInfo *
+load_serialized_info (GstDiscoverer * dc, gchar * uri)
+{
+  GstDiscovererInfo *res = NULL;
+
+  if (dc->priv->use_cache) {
+    gchar *cachefile = _serialized_info_get_path (dc, uri);
+
+    if (cachefile) {
+      res = _get_info_from_cachefile (dc, cachefile);
+      g_free (cachefile);
+    }
+  }
+
+  return res;
+}
+
 static gboolean
 _setup_locked (GstDiscoverer * dc)
 {
   GstStateChangeReturn ret;
   gchar *uri = (gchar *) dc->priv->pending_uris->data;
-  gchar *cachefile = NULL;
 
   dc->priv->pending_uris =
       g_list_delete_link (dc->priv->pending_uris, dc->priv->pending_uris);
 
-  if (dc->priv->use_cache) {
-    cachefile = _serialized_info_get_path (dc, uri);
-    if (cachefile)
-      dc->priv->current_info = _get_info_from_cachefile (dc, cachefile);
 
+  GST_DEBUG ("Setting up");
+
+  g_signal_emit (dc, gst_discoverer_signals[SIGNAL_LOAD_SERIALIZED_INFO], 0,
+      uri, &dc->priv->current_info);
   if (dc->priv->current_info) {
     /* Make sure the URI is exactly what the user passed in */
     g_free (dc->priv->current_info->uri);
     dc->priv->current_info->uri = uri;
 
-      dc->priv->current_info->cachefile = cachefile;
     dc->priv->processing = FALSE;
     dc->priv->target_state = GST_STATE_NULL;
 
     return TRUE;
   }
-  }
-
-  GST_DEBUG ("Setting up");
 
   /* Pop URI off the pending URI list */
   dc->priv->current_info =
       (GstDiscovererInfo *) g_object_new (GST_TYPE_DISCOVERER_INFO, NULL);
-  dc->priv->current_info->cachefile = cachefile;
+  if (dc->priv->use_cache)
+    dc->priv->current_info->cachefile = _serialized_info_get_path (dc, uri);
   dc->priv->current_info->uri = uri;
 
   /* set uri on uridecodebin */

subprojects/gst-plugins-base/gst-libs/gst/pbutils/gstdiscoverer.h

@@ -386,8 +386,22 @@ struct _GstDiscovererClass {
                                   const GError *err);
   void        (*source_setup)    (GstDiscoverer *discoverer,
                                   GstElement *source);
+  /**
+   * GstDiscovererClass::load_serialize_info:
+   * @dc: the #GstDiscoverer
+   * @uri: the uri to load the info from
+   *
+   * Loads the serialized info from the given uri.
+   *
+   * Returns: (transfer full): the #GstDiscovererInfo or %NULL if it could not be loaded
+   *
+   * Since: 1.24
+   */
+  GstDiscovererInfo *
+  (*load_serialize_info)          (GstDiscoverer *dc,
+                                   gchar *uri);
 
-  gpointer _reserved[GST_PADDING];
+  gpointer _reserved[GST_PADDING - 1];
 };
 
 GST_PBUTILS_API