Other usecases could be:
* caching device properties for hardware related sinks and sources
* caching connection parameters for stream sources (for servers)

I think it would be even okay to reset the cache when core is bumped. This way we avoid accumullation of cache data)

Sounds like a very good idea... this should probably be stored as a key-value list/array where the keys are normal strings and the values are GValues that we serialize to strings for storing inside the registry.

You mean a GstStructure :)

Err... yes, a serialized GstStructure containing no pointer/boxed GValues ;)

One more place where this is handy would we sub-registries. Try this;

GST_DEBUG="ladspa:4" gst-launch ladspa-sawtooth-fa-oa ! audioconvert ! autoaudiosink

Everytime one instantiates a wrapper plugin the first time it scans the plugins once again.

Not sure if this case could be handled with a (nested) GstStructure too (should be okay).

It can handled simillar to uri-handler iface in registry. Instead of GstElementFactory (in the uri-handler case) we would extend GstPluginFeature to have a GstStructure *extra_data; and add _set/get functions.

Plugins would need to access or produce this data in plugin_init().

GstPluginFeature *my_feature;
GstStructure *data;

/* getting the feature from the plugin itself would be useful */
my_feature = gst_registry_lookup_feature(
  gst_registry_get_default(), "my-plugin");

data =  gst_plugin_feature_get_extra_data (my_feature);
if(!data) {
  /* we are running in registry update */
  data = gather_data(...);
  gst_plugin_feature_set_extra_data (my_feature, data);
}

/* use the cache data */
register_elements (plugin, data);

Inside the binary registry this is eary to be implemented using:

gchar *gst_structure_to_string (const GstStructure *structure);
GstStructure *gst_structure_from_string(const gchar *string, gchar **end);

Created attachment 136067 [details] [review]
add cache_data to GstPlugin

Do it in GstPlugin and not GstPluginFeature. This way we don'T need to lookup the feature in plugin_init, which would not work anyway (is NULL).

Same patch as before, but make the returned GStStructure const and add new API to win32/def file.

commit ed88db818b64982adcdcc29e88ad476358a0757d
Author: Stefan Kost <ensonic@users.sf.net>
Date:   Sun Jun 7 23:46:54 2009 +0300

    registry: allow plugins to cache extra data in registry. Fixes #570233
    
    Add a GstStructure to GstPlugin. Plugins can retieve it in plugin_init and
    access the cached info or build the cache and store it there.