gstreamer/gst/autoplug/gstspider.c

/* GStreamer
 * Copyright (C) 2002 Erik Walthinsen <omega@cse.ogi.edu>
 *               2002 Wim Taymans <wtay@chello.be>
 *
 * gstspider.c: element to automatically link sinks and sources
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

/*
 * TODO:
 * - handle automatic removal of unneeded elements
 * - make the spider handle and send events (esp. new media)
 * - decide if we plug pads or elements, currently it's a mess
 * - allow unlinking
 * - implement proper saving/loading from xml
 * - implement a way to allow merging/splitting (aka tee)
 * - find ways to define which elements to use when plugging
 * - remove pads
 * - improve typefinding
 * - react to errors inside the pipeline
 * - implement more properties, change the current
 * - emit signals (most important: "NOT PLUGGABLE")
 * - implement something for reporting the state of the spider
 *   to allow easier debugging.
 *   (could be useful for bins in general)
 * - fix bugs
 * ...
 */

#ifdef HAVE_CONFIG_H
#  include "config.h"
#endif

#include "../gst-i18n-lib.h"
#include "gstspider.h"
#include "gstspideridentity.h"
#include "gstsearchfuncs.h"

static GstElementDetails gst_spider_details = GST_ELEMENT_DETAILS ("Spider",
    "Generic",
    "Automatically link sinks and sources",
    "Benjamin Otte <in7y118@public.uni-hamburg.de>");

GST_DEBUG_CATEGORY (gst_spider_debug);
#define GST_CAT_DEFAULT gst_spider_debug

/* signals and args */
enum
{
  /* FILL ME */
  LAST_SIGNAL
};

enum
{
  ARG_0,
  ARG_FACTORIES,
  /* FILL ME TOO */
};

/* generic templates */
static GstStaticPadTemplate spider_sink_factory =
GST_STATIC_PAD_TEMPLATE ("sink",
    GST_PAD_SINK,
    GST_PAD_ALWAYS,
    GST_STATIC_CAPS_ANY);
static GstStaticPadTemplate spider_src_factory =
GST_STATIC_PAD_TEMPLATE ("src_%d",
    GST_PAD_SRC,
    GST_PAD_REQUEST,
    GST_STATIC_CAPS_ANY);

/* standard GObject stuff */
static void gst_spider_class_init (GstSpiderClass * klass);
static void gst_spider_init (GstSpider * spider);
static void gst_spider_dispose (GObject * object);

/* element class functions */
static GstPad *gst_spider_request_new_pad (GstElement * element,
    GstPadTemplate * templ, const gchar * name);
static void gst_spider_set_property (GObject * object, guint prop_id,
    const GValue * value, GParamSpec * pspec);
static void gst_spider_get_property (GObject * object, guint prop_id,
    GValue * value, GParamSpec * pspec);

/* link functions */
static GstSpiderConnection *gst_spider_link_new (GstSpiderIdentity * src);
static void gst_spider_link_destroy (GstSpiderConnection * conn);
static void gst_spider_link_reset (GstSpiderConnection * conn, GstElement * to);
static void gst_spider_link_add (GstSpiderConnection * conn,
    GstElement * element);
static GstSpiderConnection *gst_spider_link_find (GstSpiderIdentity * src);
static GstSpiderConnection *gst_spider_link_get (GstSpiderIdentity * src);

/* autoplugging functions */
static GstElement *gst_spider_find_element_to_plug (GstElement * src,
    GstElementFactory * fac, GstPadDirection dir);
static GstPadLinkReturn gst_spider_plug (GstSpiderConnection * conn);
static GstPadLinkReturn gst_spider_plug_from_srcpad (GstSpiderConnection * conn,
    GstPad * srcpad);
/*static GstPadLinkReturn      gst_spider_plug_peers			(GstSpider *spider, GstPad *srcpad, GstPad *sinkpad); */
static GstPadLinkReturn gst_spider_create_and_plug (GstSpiderConnection * conn,
    GList * plugpath);

/* random functions */
static gchar *gst_spider_unused_elementname (GstBin * bin,
    const gchar * startwith);

/* debugging stuff
static void			print_spider_contents			(GstSpider *spider);
static void			print_spider_link			(GstSpiderConnection *conn); */

/* === variables === */
static GstElementClass *parent_class = NULL;

/* no signals yet
static guint gst_spider_signals[LAST_SIGNAL] = { 0 };*/

/* GObject and GStreamer init functions */
GType
gst_spider_get_type (void)
{
  static GType spider_type = 0;

  if (!spider_type) {
    static const GTypeInfo spider_info = {
      sizeof (GstSpiderClass),
      NULL,
      NULL,
      (GClassInitFunc) gst_spider_class_init,
      NULL,
      NULL,
      sizeof (GstSpider),
      0,
      (GInstanceInitFunc) gst_spider_init,
    };

    spider_type =
        g_type_register_static (GST_TYPE_BIN, "GstSpider", &spider_info, 0);
  }
  return spider_type;
}

static void
gst_spider_class_init (GstSpiderClass * klass)
{
  GObjectClass *gobject_class;
  GstElementClass *gstelement_class;

  gobject_class = (GObjectClass *) klass;
  gstelement_class = (GstElementClass *) klass;

  parent_class = g_type_class_ref (GST_TYPE_BIN);

  /* properties */
  g_object_class_install_property (G_OBJECT_CLASS (klass), ARG_FACTORIES,
      g_param_spec_pointer ("factories", "allowed factories",
          "allowed factories for autoplugging", G_PARAM_READWRITE));

  gobject_class->set_property = gst_spider_set_property;
  gobject_class->get_property = gst_spider_get_property;
  gobject_class->dispose = gst_spider_dispose;

  gst_element_class_add_pad_template (gstelement_class,
      gst_static_pad_template_get (&spider_sink_factory));
  gst_element_class_add_pad_template (gstelement_class,
      gst_static_pad_template_get (&spider_src_factory));
  gst_element_class_set_details (gstelement_class, &gst_spider_details);

  gstelement_class->request_new_pad =
      GST_DEBUG_FUNCPTR (gst_spider_request_new_pad);
}
static void
gst_spider_init (GstSpider * spider)
{
  /* use only elements which have sources and sinks and where the sinks have caps */
  /* FIXME: How do we handle factories that are added after the spider was constructed? */
  GList *list = gst_registry_pool_feature_list (GST_TYPE_ELEMENT_FACTORY);

  spider->factories = gst_autoplug_factories_filters_with_sink_caps (list);
  g_list_free (list);

  spider->links = NULL;

  spider->sink_ident = gst_spider_identity_new_sink ("sink_ident");
  gst_bin_add (GST_BIN (spider), GST_ELEMENT (spider->sink_ident));
  gst_element_add_ghost_pad (GST_ELEMENT (spider), spider->sink_ident->sink,
      "sink");

}

static void
gst_spider_dispose (GObject * object)
{
  GstSpider *spider;
  GList *list;

  spider = GST_SPIDER (object);
  g_list_free (spider->factories);
  spider->factories = NULL;

  for (list = spider->links; list; list = list->next) {
    GstSpiderConnection *conn = list->data;

    g_list_free (conn->path);
    g_free (conn);
  }
  g_list_free (spider->links);
  spider->links = NULL;

  ((GObjectClass *) parent_class)->dispose (object);
}
static GstPad *
gst_spider_request_new_pad (GstElement * element, GstPadTemplate * templ,
    const gchar * name)
{
  GstPad *returnpad;
  gchar *padname;
  GstSpiderIdentity *identity;
  GstSpider *spider;

  g_return_val_if_fail (templ != NULL, NULL);
  g_return_val_if_fail (GST_IS_PAD_TEMPLATE (templ), NULL);
  g_return_val_if_fail (GST_PAD_TEMPLATE_DIRECTION (templ) == GST_PAD_SRC,
      NULL);

  spider = GST_SPIDER (element);

  /* create an identity object, so we have a pad */
  padname = gst_spider_unused_elementname ((GstBin *) spider, "src_");
  identity = gst_spider_identity_new_src (padname);
  returnpad = identity->src;

  /* FIXME: use the requested name for the pad */

  gst_object_replace ((GstObject **) & returnpad->padtemplate,
      (GstObject *) templ);

  gst_bin_add (GST_BIN (element), GST_ELEMENT (identity));

  returnpad = gst_element_add_ghost_pad (element, returnpad, padname);
  g_free (padname);
  gst_spider_link_new (identity);
  GST_DEBUG ("successfully created requested pad %s:%s",
      GST_DEBUG_PAD_NAME (returnpad));

  return returnpad;
}

static void
gst_spider_set_property (GObject * object, guint prop_id, const GValue * value,
    GParamSpec * pspec)
{
  GstSpider *spider;
  GList *list;

  /* it's not null if we got it, but it might not be ours */
  g_return_if_fail (GST_IS_SPIDER (object));

  spider = GST_SPIDER (object);

  switch (prop_id) {
    case ARG_FACTORIES:
      list = (GList *) g_value_get_pointer (value);
      for (; list; list = list->next) {
        g_return_if_fail (list->data != NULL);
        g_return_if_fail (GST_IS_ELEMENT_FACTORY (list->data));
      }
      g_list_free (spider->factories);
      spider->factories = (GList *) g_value_get_pointer (value);
      break;
    default:
      break;
  }
}
static void
gst_spider_get_property (GObject * object, guint prop_id, GValue * value,
    GParamSpec * pspec)
{
  GstSpider *spider;

  /* it's not null if we got it, but it might not be ours */
  spider = GST_SPIDER (object);

  switch (prop_id) {
    case ARG_FACTORIES:
      g_value_set_pointer (value, spider->factories);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
  }
}

/* get a name for an element that isn't used yet */
static gchar *
gst_spider_unused_elementname (GstBin * bin, const gchar * startwith)
{
  gchar *name = g_strdup_printf ("%s%d", startwith, 0);
  guint i;

  for (i = 0; gst_bin_get_by_name (bin, name) != NULL;) {
    g_free (name);
    name = g_strdup_printf ("%s%d", startwith, ++i);
  }

  return name;
}
static void
gst_spider_link_sometimes (GstElement * src, GstPad * pad,
    GstSpiderConnection * conn)
{
  gulong signal_id = conn->signal_id;

  GST_INFO ("plugging from new sometimes pad %s:%s", GST_DEBUG_PAD_NAME (pad));
  /* try to autoplug the elements */
  if (gst_spider_plug_from_srcpad (conn, pad) != GST_PAD_LINK_REFUSED) {
    GST_DEBUG ("%s:%s was autoplugged to %s:%s, removing callback",
        GST_DEBUG_PAD_NAME (pad), GST_DEBUG_PAD_NAME (conn->src->sink));
    g_signal_handler_disconnect (src, signal_id);
    signal_id = 0;
  }
}

/* create a new link from those two elements */
static GstSpiderConnection *
gst_spider_link_new (GstSpiderIdentity * src)
{
  GstSpider *spider = GST_SPIDER (GST_OBJECT_PARENT (src));

  GstSpiderConnection *conn = g_new0 (GstSpiderConnection, 1);

  conn->src = src;
  conn->path = NULL;
  conn->current = (GstElement *) spider->sink_ident;
  spider->links = g_list_prepend (spider->links, conn);

  return conn;
}
static void
gst_spider_link_destroy (GstSpiderConnection * conn)
{
  GstSpider *spider = GST_SPIDER (GST_OBJECT_PARENT (conn->src));

  /* reset link to unplugged */
  gst_spider_link_reset (conn, (GstElement *) spider->sink_ident);
  g_free (conn);
}
static void
gst_spider_link_reset (GstSpiderConnection * conn, GstElement * to)
{
  GstSpider *spider = GST_SPIDER (GST_OBJECT_PARENT (conn->src));

  GST_DEBUG ("resetting link from %s to %s, currently at %s to %s",
      GST_ELEMENT_NAME (spider->sink_ident), GST_ELEMENT_NAME (conn->src),
      GST_ELEMENT_NAME (conn->current), GST_ELEMENT_NAME (to));
  while ((conn->path != NULL) && ((GstElement *) conn->path->data != to)) {
    gst_object_unref ((GstObject *) conn->path->data);
    conn->path = g_list_delete_link (conn->path, conn->path);
  }
  if (conn->path == NULL) {
    conn->current = (GstElement *) spider->sink_ident;
  } else {
    conn->current = to;
  }
}

/* add an element to the link */
static void
gst_spider_link_add (GstSpiderConnection * conn, GstElement * element)
{
  conn->path = g_list_prepend (conn->path, element);
  conn->current = element;
}

/* find the link from those two elements */
static GstSpiderConnection *
gst_spider_link_find (GstSpiderIdentity * src)
{
  GstSpider *spider = (GstSpider *) GST_OBJECT_PARENT (src);
  GList *list;

  for (list = spider->links; list; list = list->next) {
    GstSpiderConnection *conn = (GstSpiderConnection *) list->data;

    if (conn->src == src) {
      return conn;
    }
  }
  return NULL;
}

/* get a new link from those two elements
 * search first; if none is found, create a new one */
static GstSpiderConnection *
gst_spider_link_get (GstSpiderIdentity * src)
{
  GstSpiderConnection *ret;

  if ((ret = gst_spider_link_find (src)) != NULL) {
    return ret;
  }
  return gst_spider_link_new (src);
}

void
gst_spider_identity_plug (GstSpiderIdentity * ident)
{
  GstSpider *spider;
  const GList *padlist;
  GstPadDirection dir;
  GstSpiderConnection *conn;

  /* checks */
  g_return_if_fail (ident != NULL);
  g_return_if_fail (GST_IS_SPIDER_IDENTITY (ident));
  spider = GST_SPIDER (GST_ELEMENT_PARENT (ident));
  g_assert (spider != NULL);
  g_assert (GST_IS_SPIDER (spider));

  /* return if we're already plugged */
  if (ident->plugged)
    return;

  /* check if there is at least one element factory that can handle the
     identity's src caps */
  {
    GstCaps *src_caps = gst_pad_get_caps (ident->src);

    if (!gst_caps_is_empty (src_caps) && !gst_caps_is_any (src_caps)) {
      GList *factories;
      GstPadTemplate *padtemp;
      gboolean found = FALSE;

      factories = spider->factories;
      while (factories) {
        if ((padtemp =
                gst_autoplug_can_connect_src (factories->data, src_caps))) {
          GST_DEBUG ("can connect src to %s pad template: %" GST_PTR_FORMAT,
              GST_PLUGIN_FEATURE_NAME (factories->data),
              gst_pad_template_get_caps (padtemp));
          found = TRUE;
        }
        factories = factories->next;
      }
      if (!found) {
        const char *mime;

        mime = gst_structure_get_name (gst_caps_get_structure (src_caps, 0));

        GST_ELEMENT_ERROR (spider, STREAM, CODEC_NOT_FOUND,
            (_("There is no element present to handle the stream's mime type %s."), mime), (NULL));
        gst_caps_unref (src_caps);
        return;
      }
    }
    gst_caps_unref (src_caps);
  }

  /* get the direction of our ident */
  if (GST_PAD_PEER (ident->sink)) {
    if (GST_PAD_PEER (ident->src)) {
      /* Hey, the ident is linked on both sides */
      g_warning ("Trying to autoplug a linked element. Aborting...");
      return;
    } else {
      dir = GST_PAD_SINK;
    }
  } else {
    if (GST_PAD_PEER (ident->src)) {
      dir = GST_PAD_SRC;
    } else {
      /* the ident isn't linked on either side */
      g_warning ("Trying to autoplug an unlinked element. Aborting...");
      return;
    }
  }

  /* now iterate all possible pads and link when needed */
  padlist = GST_ELEMENT (spider)->pads;
  for (; padlist; padlist = padlist->next) {
    GstPad *otherpad;
    GstSpiderIdentity *peer;

    g_assert (GST_IS_PAD (padlist->data));
    otherpad = (GstPad *) GST_GPAD_REALPAD (padlist->data);
    peer = (GstSpiderIdentity *) GST_PAD_PARENT (otherpad);
    /* we only want to link to the other side */
    if (dir != GST_PAD_DIRECTION (otherpad)) {
      /* we only link to plugged in elements */
      if (peer->plugged == TRUE) {
        /* plug in the right direction */
        if (dir == GST_PAD_SINK) {
          conn = gst_spider_link_get (peer);
        } else {
          conn = gst_spider_link_get (ident);
        }
        if ((GstElement *) spider->sink_ident == conn->current) {
          gst_spider_plug (conn);
        }
      }
    }
  }

  ident->plugged = TRUE;
}

void
gst_spider_identity_unplug (GstSpiderIdentity * ident)
{
  GstSpider *spider = (GstSpider *) GST_OBJECT_PARENT (ident);
  GList *list;

  for (list = spider->links; list; list = list->next) {
    GstSpiderConnection *conn = list->data;

    if (conn->src == ident) {
      g_list_delete_link (spider->links, list);
      gst_spider_link_destroy (conn);
    }
  }
  ident->plugged = FALSE;
}

/* links src to sink using the elementfactories in plugpath
 * plugpath will be removed afterwards */
static GstPadLinkReturn
gst_spider_create_and_plug (GstSpiderConnection * conn, GList * plugpath)
{
  GstSpider *spider = (GstSpider *) GST_OBJECT_PARENT (conn->src);
  GList *endelements = NULL, *templist = NULL;
  GstElement *element;

  /* exit if plugging is already done */
  if ((GstElement *) conn->src == conn->current)
    return GST_PAD_LINK_DONE;

  /* try to shorten the list at the end and not duplicate link code */
  if (plugpath != NULL) {
    templist = g_list_last (plugpath);
    element = (GstElement *) conn->src;
    while ((plugpath != NULL)
        && (element =
            gst_spider_find_element_to_plug (element,
                (GstElementFactory *) plugpath->data, GST_PAD_SINK))) {
      GList *cur = templist;

      endelements = g_list_prepend (endelements, element);
      templist = g_list_previous (templist);
      g_list_delete_link (cur, cur);
    }
  }

  /* do the linking */
  while (conn->current != (GstElement *) (endelements ==
          NULL ? conn->src : endelements->data)) {
    /* get sink element to plug, src is conn->current */
    if (plugpath == NULL) {
      element =
          (GstElement *) (endelements == NULL ? conn->src : endelements->data);
    } else {
      element =
          gst_element_factory_create ((GstElementFactory *) plugpath->data,
          NULL);
      GST_DEBUG
          ("Adding element %s of type %s and syncing state with autoplugger",
          GST_ELEMENT_NAME (element), GST_PLUGIN_FEATURE_NAME (plugpath->data));
      gst_bin_add (GST_BIN (spider), element);
    }
    /* insert and link new element */
    if (gst_element_link (conn->current, element)) {
      gst_element_sync_state_with_parent (element);
    } else {
      /* check if the src has SOMETIMES templates. If so, link a callback */
      GList *templs = gst_element_get_pad_template_list (conn->current);

      /* remove element that couldn't be linked, if it wasn't the endpoint */
      if (element != (GstElement *) conn->src)
        gst_bin_remove (GST_BIN (spider), element);

      for (; templs; templs = templs->next) {
        GstPadTemplate *templ = (GstPadTemplate *) templs->data;

        if ((GST_PAD_TEMPLATE_DIRECTION (templ) == GST_PAD_SRC)
            && (GST_PAD_TEMPLATE_PRESENCE (templ) == GST_PAD_SOMETIMES)) {
          GST_DEBUG ("adding callback to link element %s to %s",
              GST_ELEMENT_NAME (conn->current), GST_ELEMENT_NAME (conn->src));
          conn->signal_id =
              g_signal_connect (G_OBJECT (conn->current), "new_pad",
              G_CALLBACK (gst_spider_link_sometimes), conn);
          g_list_free (plugpath);
          return GST_PAD_LINK_DELAYED;
        }
      }
      GST_DEBUG ("no chance to link element %s to %s",
          GST_ELEMENT_NAME (conn->current), GST_ELEMENT_NAME (conn->src));
      g_list_free (plugpath);
      return GST_PAD_LINK_REFUSED;
    }
    GST_DEBUG ("coupling %s and %s",
        GST_ELEMENT_NAME (element), GST_ELEMENT_NAME (conn->current));
    gst_spider_link_add (conn, element);
    if (plugpath != NULL)
      plugpath = g_list_delete_link (plugpath, plugpath);
  }

  /* ref all elements at the end */
  while (endelements) {
    gst_spider_link_add (conn, endelements->data);
    endelements = g_list_delete_link (endelements, endelements);
  }

  return GST_PAD_LINK_DONE;
}

/* checks, if src is already linked to an element from factory fac on direction dir */
static GstElement *
gst_spider_find_element_to_plug (GstElement * src, GstElementFactory * fac,
    GstPadDirection dir)
{
  GList *padlist = GST_ELEMENT_PADS (src);

  for (; padlist; padlist = padlist->next) {
    GstPad *pad = (GstPad *) GST_PAD_REALIZE (padlist->data);

    /* is the pad on the right side and is it linked? */
    if ((GST_PAD_DIRECTION (pad) == dir)
        && (pad = (GstPad *) (GST_RPAD_PEER (pad)))) {
      /* is the element the pad is linked to of the right type? */
      GstElement *element = GST_PAD_PARENT (pad);

      if (G_TYPE_FROM_INSTANCE (element) ==
          gst_element_factory_get_element_type (fac)) {
        return element;
      }
    }
  }

  return NULL;
}

/* try to establish the link */
static GstPadLinkReturn
gst_spider_plug (GstSpiderConnection * conn)
{
  GstSpider *spider = (GstSpider *) GST_OBJECT_PARENT (conn->src);

  if ((GstElement *) conn->src == conn->current)
    return GST_PAD_LINK_DONE;
  if ((GstElement *) spider->sink_ident == conn->current)
    return gst_spider_plug_from_srcpad (conn, spider->sink_ident->src);
  g_warning
      ("FIXME: autoplugging only possible from GstSpiderIdentity conn->sink yet (yep, that's technical)\n");
  return GST_PAD_LINK_REFUSED;
}

/* try to establish the link using this pad */
static GstPadLinkReturn
gst_spider_plug_from_srcpad (GstSpiderConnection * conn, GstPad * srcpad)
{
  GstElement *element;
  GList *plugpath;
  gboolean result = TRUE;
  GstSpider *spider = (GstSpider *) GST_OBJECT_PARENT (conn->src);
  GstElement *startelement = conn->current;
  GstCaps *caps1;
  GstCaps *caps2;

  g_assert ((GstElement *) GST_OBJECT_PARENT (srcpad) == conn->current);
  GST_DEBUG ("trying to plug from %s:%s to %s",
      GST_DEBUG_PAD_NAME (srcpad), GST_ELEMENT_NAME (conn->src));

  /* see if they match already */
  if (gst_pad_link (srcpad, conn->src->sink)) {
    GST_DEBUG ("%s:%s and %s:%s can link directly",
        GST_DEBUG_PAD_NAME (srcpad), GST_DEBUG_PAD_NAME (conn->src->sink));
    gst_pad_unlink (srcpad, conn->src->sink);
    gst_spider_create_and_plug (conn, NULL);
    return GST_PAD_LINK_OK;
  }

  /* find a path from src to sink */
  caps1 = gst_pad_get_caps (srcpad);
  caps2 = gst_pad_get_caps (conn->src->sink);
  plugpath = gst_autoplug_sp (caps1, caps2, spider->factories);
  gst_caps_unref (caps1);
  gst_caps_unref (caps2);

  /* prints out the path that was found for plugging */
  /* g_print ("found path from %s to %s:\n", GST_ELEMENT_NAME (conn->current), GST_ELEMENT_NAME (conn->src));
     templist = plugpath;
     while (templist)
     {
     g_print("%s\n", GST_OBJECT_NAME (templist->data));
     templist = g_list_next (templist);
     } */

  /* if there is no way to plug: return */
  if (plugpath == NULL) {
    GST_DEBUG ("no chance to plug from %s to %s",
        GST_ELEMENT_NAME (conn->current), GST_ELEMENT_NAME (conn->src));
    return GST_PAD_LINK_REFUSED;
  }
  GST_DEBUG ("found a link that needs %d elements", g_list_length (plugpath));

  /* now remove non-needed elements from the beginning of the path 
   * alter src to point to the new element where we need to start 
   * plugging and alter the plugpath to represent the elements, that must be plugged
   */
  element = conn->current;
  while ((plugpath != NULL)
      && (element =
          gst_spider_find_element_to_plug (element,
              (GstElementFactory *) plugpath->data, GST_PAD_SRC))) {
    gst_spider_link_add (conn, element);
    plugpath = g_list_delete_link (plugpath, plugpath);
  }

  GST_DEBUG ("%d elements must be inserted to establish the link",
      g_list_length (plugpath));
  /* create the elements and plug them */
  result = gst_spider_create_and_plug (conn, plugpath);

  /* reset the "current" element */
  if (result == GST_PAD_LINK_REFUSED) {
    gst_spider_link_reset (conn, startelement);
  }

  return result;
}

static gboolean
plugin_init (GstPlugin * plugin)
{
  GST_DEBUG_CATEGORY_INIT (gst_spider_debug, "spider", 0,
      "spider autoplugging element");

  if (!gst_element_register (plugin, "spider", GST_RANK_NONE, GST_TYPE_SPIDER))
    return FALSE;
  if (!gst_element_register (plugin, "spideridentity", GST_RANK_NONE,
          GST_TYPE_SPIDER_IDENTITY))
    return FALSE;

  return TRUE;
}

GST_PLUGIN_DEFINE (GST_VERSION_MAJOR,
    GST_VERSION_MINOR,
    "gstspider",
    "a 1:n autoplugger",
    plugin_init, VERSION, GST_LICENSE, GST_PACKAGE, GST_ORIGIN)