protection: add function to filter system ids

gst_protection_filter_systems_by_available_decryptors() takes an array
of strings and returns a new array of strings filtered by the available
decryptors for them so the ones you get are the ones that you should be
able to decrypt.

https://bugzilla.gnome.org/show_bug.cgi?id=770107

docs/gst/gstreamer-sections.txt

@@ -2461,6 +2461,7 @@ GstProtectionMeta
 gst_buffer_add_protection_meta
 gst_buffer_get_protection_meta
 gst_protection_select_system
+gst_protection_filter_systems_by_available_decryptors
 GST_PROTECTION_SYSTEM_ID_CAPS_FIELD
 <SUBSECTION Standard>
 GST_PROTECTION_META_API_TYPE

gst/gstprotection.c

@@ -193,6 +193,66 @@ gst_protection_select_system (const gchar ** system_identifiers)
   return retval;
 }
 
+/**
+ * gst_protection_filter_systems_by_available_decryptors:
+ * @system_identifiers: (transfer none): A null terminated array of strings
+ * that contains the UUID values of each protection system that is to be
+ * checked.
+ *
+ * Iterates the supplied list of UUIDs and checks the GstRegistry for
+ * all the decryptors supporting one of the supplied UUIDs.
+ *
+ * Returns: (transfer full): A null terminated array containing all the
+ * @system_identifiers supported by the set of available decryptors, or %NULL
+ * if no matches were found.
+ *
+ * Since: 1.14
+ */
+gchar **
+gst_protection_filter_systems_by_available_decryptors (const gchar **
+    system_identifiers)
+{
+  GList *decryptors, *walk;
+  gchar **retval;
+  guint i = 0, decryptors_number;
+
+  decryptors =
+      gst_element_factory_list_get_elements (GST_ELEMENT_FACTORY_TYPE_DECRYPTOR,
+      GST_RANK_MARGINAL);
+
+  decryptors_number = g_list_length (decryptors);
+
+  GST_TRACE ("found %u decrytors", decryptors_number);
+
+  if (decryptors_number == 0)
+    return NULL;
+
+  retval = g_new (gchar *, decryptors_number + 1);
+
+  for (walk = decryptors; walk; walk = g_list_next (walk)) {
+    GstElementFactory *fact = (GstElementFactory *) walk->data;
+    const char *found_sys_id =
+        gst_protection_factory_check (fact, system_identifiers);
+
+    GST_DEBUG ("factory %s is valid for %s", GST_OBJECT_NAME (fact),
+        found_sys_id);
+
+    if (found_sys_id) {
+      retval[i++] = g_strdup (found_sys_id);
+    }
+  }
+  retval[i] = NULL;
+
+  if (retval[0] == NULL) {
+    g_free (retval);
+    retval = NULL;
+  }
+
+  gst_plugin_feature_list_free (decryptors);
+
+  return retval;
+}
+
 static const gchar *
 gst_protection_factory_check (GstElementFactory * fact,
     const gchar ** system_identifiers)

gst/gstprotection.h

@@ -71,5 +71,9 @@ GstProtectionMeta * gst_buffer_add_protection_meta (GstBuffer * buffer,
 GST_EXPORT
 const gchar * gst_protection_select_system (const gchar ** system_identifiers);
 
+GST_EXPORT
+gchar ** gst_protection_filter_systems_by_available_decryptors (
+    const gchar ** system_identifiers);
+
 G_END_DECLS
 #endif /* __GST_PROTECTION_META_H__ */

win32/common/libgstreamer.def

@@ -1078,6 +1078,7 @@ EXPORTS
 	gst_printerrln
 	gst_println
 	gst_progress_type_get_type
+	gst_protection_filter_systems_by_available_decryptors
 	gst_protection_meta_api_get_type
 	gst_protection_meta_get_info
 	gst_protection_select_system