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/*
 * This file is part of the Nice GLib ICE library.
 *
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 * (C) 2006-2009 Collabora Ltd.
 *  Contact: Youness Alaoui
 * (C) 2006-2009 Nokia Corporation. All rights reserved.
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 *  Contact: Kai Vehmanen
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is the Nice GLib ICE library.
 *
 * The Initial Developers of the Original Code are Collabora Ltd and Nokia
 * Corporation. All Rights Reserved.
 *
 * Contributors:
 *   Dafydd Harries, Collabora Ltd.
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 *   Youness Alaoui, Collabora Ltd.
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 *   Kai Vehmanen, Nokia
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 *
 * Alternatively, the contents of this file may be used under the terms of the
 * the GNU Lesser General Public License Version 2.1 (the "LGPL"), in which
 * case the provisions of LGPL are applicable instead of those above. If you
 * wish to allow use of your version of this file only under the terms of the
 * LGPL and not to allow others to use your version of this file under the
 * MPL, indicate your decision by deleting the provisions above and replace
 * them with the notice and other provisions required by the LGPL. If you do
 * not delete the provisions above, a recipient may use your version of this
 * file under either the MPL or the LGPL.
 */
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/*
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 * @file component.c
 * @brief ICE component functions
 */

#ifdef HAVE_CONFIG_H
# include <config.h>
#endif

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#include <string.h>

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#include "debug.h"

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#include "component.h"
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#include "agent-priv.h"
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static void
component_schedule_io_callback (Component *component);
static void
component_deschedule_io_callback (Component *component);


/* Must *not* take the agent lock, since it’s called from within
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 * component_set_io_context(), which holds the Component’s I/O lock. */
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static void
socket_source_attach (SocketSource *socket_source, GMainContext *context)
{
  GSource *source;

  /* Create a source. */
  source = g_socket_create_source (socket_source->socket->fileno,
      G_IO_IN, NULL);
  g_source_set_callback (source, (GSourceFunc) component_io_cb,
      socket_source, NULL);

  /* Add the source. */
  nice_debug ("Attaching source %p (socket %p, FD %d) to context %p", source,
      socket_source->socket, g_socket_get_fd (socket_source->socket->fileno),
      context);

  g_assert (socket_source->source == NULL);
  socket_source->source = source;
  g_source_attach (source, context);
}

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static void
socket_source_detach (SocketSource *source)
{
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  nice_debug ("Detaching source %p (socket %p, FD %d) from context %p",
      source->source, source->socket,
      (source->socket->fileno != NULL) ?
          g_socket_get_fd (source->socket->fileno) : 0,
      (source->source != NULL) ? g_source_get_context (source->source) : 0);

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  if (source->source != NULL) {
    g_source_destroy (source->source);
    g_source_unref (source->source);
  }
  source->source = NULL;
}

static void
socket_source_free (SocketSource *source)
{
  nice_socket_free (source->socket);
  socket_source_detach (source);

  g_slice_free (SocketSource, source);
}

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Component *
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component_new (guint id, NiceAgent *agent, Stream *stream)
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{
  Component *component;

  component = g_slice_new0 (Component);
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  component->id = id;
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  component->state = NICE_COMPONENT_STATE_DISCONNECTED;
  component->restart_candidate = NULL;
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  component->tcp = NULL;
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  component->agent = agent;
  component->stream = stream;
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  g_mutex_init (&component->io_mutex);
  g_queue_init (&component->pending_io_messages);
  component->io_callback_id = 0;

  /* Start off with a fresh main context and all I/O paused. This
   * will be updated when nice_agent_attach_recv() or nice_agent_recv() are
   * called. */
  component_set_io_context (component, NULL);
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  component_set_io_callback (component, NULL, NULL, NULL, 0, NULL);
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  return component;
}

void
component_free (Component *cmp)
{
  GSList *i;
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  GList *item;
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  IOCallbackData *data;
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  for (i = cmp->local_candidates; i; i = i->next) {
    NiceCandidate *candidate = i->data;
    nice_candidate_free (candidate);
  }
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  for (i = cmp->remote_candidates; i; i = i->next) {
    NiceCandidate *candidate = i->data;
    nice_candidate_free (candidate);
  }
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  if (cmp->restart_candidate)
    nice_candidate_free (cmp->restart_candidate),
      cmp->restart_candidate = NULL;

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  for (i = cmp->incoming_checks; i; i = i->next) {
    IncomingCheck *icheck = i->data;
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    g_free (icheck->username);
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    g_slice_free (IncomingCheck, icheck);
  }
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  g_slist_free (cmp->local_candidates);
  g_slist_free (cmp->remote_candidates);
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  component_free_socket_sources (cmp);
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  g_slist_free (cmp->incoming_checks);
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  for (item = cmp->turn_servers; item; item = g_list_next (item)) {
    TurnServer *turn = item->data;
    g_free (turn->username);
    g_free (turn->password);
    g_slice_free (TurnServer, turn);
  }
  g_list_free (cmp->turn_servers);
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  if (cmp->selected_pair.keepalive.tick_source != NULL) {
    g_source_destroy (cmp->selected_pair.keepalive.tick_source);
    g_source_unref (cmp->selected_pair.keepalive.tick_source);
    cmp->selected_pair.keepalive.tick_source = NULL;
  }

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  if (cmp->tcp_clock) {
    g_source_destroy (cmp->tcp_clock);
    g_source_unref (cmp->tcp_clock);
    cmp->tcp_clock = NULL;
  }
  if (cmp->tcp) {
    pseudo_tcp_socket_close (cmp->tcp, TRUE);
    g_object_unref (cmp->tcp);
    cmp->tcp = NULL;
  }

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  while ((data = g_queue_pop_head (&cmp->pending_io_messages)) != NULL)
    io_callback_data_free (data);

  component_deschedule_io_callback (cmp);

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  if (cmp->ctx != NULL) {
    g_main_context_unref (cmp->ctx);
    cmp->ctx = NULL;
  }

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  g_mutex_clear (&cmp->io_mutex);

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  g_slice_free (Component, cmp);
}

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/*
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 * Finds a candidate pair that has matching foundation ids.
 *
 * @return TRUE if pair found, pointer to pair stored at 'pair'
 */
gboolean
component_find_pair (Component *cmp, NiceAgent *agent, const gchar *lfoundation, const gchar *rfoundation, CandidatePair *pair)
{
  GSList *i;
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  CandidatePair result;

  memset (&result, 0, sizeof(result));
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  for (i = cmp->local_candidates; i; i = i->next) {
    NiceCandidate *candidate = i->data;
    if (strncmp (candidate->foundation, lfoundation, NICE_CANDIDATE_MAX_FOUNDATION) == 0) {
      result.local = candidate;
      break;
    }
  }

  for (i = cmp->remote_candidates; i; i = i->next) {
    NiceCandidate *candidate = i->data;
    if (strncmp (candidate->foundation, rfoundation, NICE_CANDIDATE_MAX_FOUNDATION) == 0) {
      result.remote = candidate;
      break;
    }
  }

  if (result.local && result.remote) {
    result.priority = agent_candidate_pair_priority (agent, result.local, result.remote);
    if (pair)
      *pair = result;
    return TRUE;
  }

  return FALSE;
}

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/*
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 * Resets the component state to that of a ICE restarted
 * session.
 */
gboolean
component_restart (Component *cmp)
{
  GSList *i;

  for (i = cmp->remote_candidates; i; i = i->next) {
    NiceCandidate *candidate = i->data;

    /* note: do not remove the remote candidate that is
     *       currently part of the 'selected pair', see ICE
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     *       9.1.1.1. "ICE Restarts" (ID-19) */
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    if (candidate == cmp->selected_pair.remote) {
      if (cmp->restart_candidate)
	nice_candidate_free (cmp->restart_candidate);
      cmp->restart_candidate = candidate;
    }
    else 
      nice_candidate_free (candidate);
  }
  g_slist_free (cmp->remote_candidates),
    cmp->remote_candidates = NULL;

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  for (i = cmp->incoming_checks; i; i = i->next) {
    IncomingCheck *icheck = i->data;
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    g_free (icheck->username);
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    g_slice_free (IncomingCheck, icheck);
  }
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  g_slist_free (cmp->incoming_checks);
  cmp->incoming_checks = NULL;
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  /* note: component state managed by agent */

  return TRUE;
}
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/*
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 * Changes the selected pair for the component to 'pair'. Does not
 * emit the "selected-pair-changed" signal.
 */ 
void component_update_selected_pair (Component *component, const CandidatePair *pair)
{
  g_assert (component);
  g_assert (pair);
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  nice_debug ("setting SELECTED PAIR for component %u: %s:%s (prio:%"
      G_GUINT64_FORMAT ").", component->id, pair->local->foundation,
      pair->remote->foundation, pair->priority);
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  if (component->selected_pair.keepalive.tick_source != NULL) {
    g_source_destroy (component->selected_pair.keepalive.tick_source);
    g_source_unref (component->selected_pair.keepalive.tick_source);
    component->selected_pair.keepalive.tick_source = NULL;
  }

  memset (&component->selected_pair, 0, sizeof(CandidatePair));

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  component->selected_pair.local = pair->local;
  component->selected_pair.remote = pair->remote;
  component->selected_pair.priority = pair->priority;
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}
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/*
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 * Finds a remote candidate with matching address and 
 * transport.
 *
 * @return pointer to candidate or NULL if not found
 */
NiceCandidate *
component_find_remote_candidate (const Component *component, const NiceAddress *addr, NiceCandidateTransport transport)
{
  GSList *i;

  for (i = component->remote_candidates; i; i = i->next) {
    NiceCandidate *candidate = i->data;

    if (nice_address_equal(&candidate->addr, addr) &&
	candidate->transport == transport)
      return candidate;

  }
  
  return NULL;
}
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/*
 * Sets the desired remote candidate as the selected pair
 *
 * It will start sending on the highest priority pair available with
 * this candidate.
 */

NiceCandidate *
component_set_selected_remote_candidate (NiceAgent *agent, Component *component,
    NiceCandidate *candidate)
{
  NiceCandidate *local = NULL;
  NiceCandidate *remote = NULL;
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  guint64 priority = 0;
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  GSList *item = NULL;

  for (item = component->local_candidates; item; item = g_slist_next (item)) {
    NiceCandidate *tmp = item->data;
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    guint64 tmp_prio = 0;
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    if (tmp->transport != candidate->transport ||
	tmp->addr.s.addr.sa_family != candidate->addr.s.addr.sa_family ||
        tmp->type != NICE_CANDIDATE_TYPE_HOST)
      continue;

    tmp_prio = agent_candidate_pair_priority (agent, tmp, candidate);

    if (tmp_prio > priority) {
      priority = tmp_prio;
      local = tmp;
    }
  }

  if (local == NULL)
    return NULL;

  remote = component_find_remote_candidate (component, &candidate->addr,
      candidate->transport);

  if (!remote) {
    remote = nice_candidate_copy (candidate);
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    component->remote_candidates = g_slist_append (component->remote_candidates,
        remote);
    agent_signal_new_remote_candidate (agent, remote);
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  }

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  if (component->selected_pair.keepalive.tick_source != NULL) {
    g_source_destroy (component->selected_pair.keepalive.tick_source);
    g_source_unref (component->selected_pair.keepalive.tick_source);
    component->selected_pair.keepalive.tick_source = NULL;
  }

  memset (&component->selected_pair, 0, sizeof(CandidatePair));
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  component->selected_pair.local = local;
  component->selected_pair.remote = remote;
  component->selected_pair.priority = priority;

  return local;
}
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static gint
_find_socket_source (gconstpointer a, gconstpointer b)
{
  const SocketSource *source_a = a;
  const NiceSocket *socket_b = b;

  return (source_a->socket == socket_b) ? 0 : 1;
}

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/* This takes ownership of the socket.
 * It creates and attaches a source to the component’s context. */
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void
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component_attach_socket (Component *component, NiceSocket *socket)
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{
  GSList *l;
  SocketSource *socket_source;

  g_assert (component != NULL);
  g_assert (socket != NULL);

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  g_assert (component->ctx != NULL);

  /* Find an existing SocketSource in the component which contains @socket, or
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   * create a new one.
   *
   * In order for socket_sources_age to work properly, socket_sources must only
   * grow monotonically, or be entirely cleared. i.e. New SocketSources must be
   * prepended to socket_sources, and all other existing SocketSource must be
   * left untouched; *or* the whole of socket_sources must be cleared. If
   * socket_sources is cleared, age is reset to 0 and *must not* be incremented
   * again or the new sockets will not be picked up by ComponentSocket. This is
   * guaranteed by the fact that socket_sources is only cleared on disconnection
   * or discovery failure, which are both unrecoverable states.
   *
   * An empty socket_sources corresponds to age 0. */
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  l = g_slist_find_custom (component->socket_sources, socket,
          _find_socket_source);
  if (l != NULL) {
    socket_source = l->data;
  } else {
    socket_source = g_slice_new0 (SocketSource);
    socket_source->socket = socket;
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    socket_source->component = component;
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    component->socket_sources =
        g_slist_prepend (component->socket_sources, socket_source);
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    component->socket_sources_age++;
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  }

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  /* Create and attach a source */
  nice_debug ("Component %p (agent %p): Attach source (stream %u).",
      component, component->agent, component->stream->id);
  socket_source_attach (socket_source, component->ctx);
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}

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/* Reattaches socket handles of @component to the main context.
 *
 * Must *not* take the agent lock, since it’s called from within
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 * component_set_io_context(), which holds the Component’s I/O lock. */
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static void
component_reattach_all_sockets (Component *component)
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{
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  GSList *i;
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  for (i = component->socket_sources; i != NULL; i = i->next) {
    SocketSource *socket_source = i->data;
    nice_debug ("Reattach source %p.", socket_source->source);
    socket_source_detach (socket_source);
    socket_source_attach (socket_source, component->ctx);
  }
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}

/**
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 * component_detach_socket:
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 * @component: a #Component
 * @socket: the socket to detach the source for
 *
 * Detach the #GSource for the single specified @socket. Leave the socket itself
 * untouched.
 *
 * If the @socket doesn’t exist in this @component, do nothing.
 */
void
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component_detach_socket (Component *component, NiceSocket *socket)
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{
  GSList *l;
  SocketSource *socket_source;

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  nice_debug ("Detach socket %p.", socket);

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  /* Find the SocketSource for the socket. */
  l = g_slist_find_custom (component->socket_sources, socket,
          _find_socket_source);
  if (l == NULL)
    return;

  /* Detach the source. */
  socket_source = l->data;
  socket_source_detach (socket_source);
}

/*
 * Detaches socket handles of @component from the main context. Leaves the
 * sockets themselves untouched.
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 *
 * Must *not* take the agent lock, since it’s called from within
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 * component_set_io_context(), which holds the Component’s I/O lock.
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 */
void
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component_detach_all_sockets (Component *component)
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{
  GSList *i;

  for (i = component->socket_sources; i != NULL; i = i->next) {
    SocketSource *socket_source = i->data;
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    nice_debug ("Detach source %p, socket %p.", socket_source->source,
        socket_source->socket);
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    socket_source_detach (socket_source);
  }
}

void
component_free_socket_sources (Component *component)
{
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  nice_debug ("Free socket sources for component %p.", component);

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  g_slist_free_full (component->socket_sources,
      (GDestroyNotify) socket_source_free);
  component->socket_sources = NULL;
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  component->socket_sources_age = 0;
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}
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GMainContext *
component_dup_io_context (Component *component)
{
  return g_main_context_ref (component->ctx);
}

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/* If @context is %NULL, a fresh context is used, so component->ctx is always
 * guaranteed to be non-%NULL. */
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void
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component_set_io_context (Component *component, GMainContext *context)
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{
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  g_mutex_lock (&component->io_mutex);

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  if (component->ctx != context || component->ctx == NULL) {
    if (context == NULL)
      context = g_main_context_new ();
    else
      g_main_context_ref (context);

    component_detach_all_sockets (component);
    if (component->ctx != NULL)
      g_main_context_unref (component->ctx);
    component->ctx = context;
    component_reattach_all_sockets (component);
  }
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  g_mutex_unlock (&component->io_mutex);
}
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/* (func, user_data) and (recv_buf, recv_buf_len) are mutually exclusive.
 * At most one of the two must be specified; if both are NULL, the Component
 * will not receive any data (i.e. reception is paused). */
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void
component_set_io_callback (Component *component,
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    NiceAgentRecvFunc func, gpointer user_data,
    guint8 *recv_buf, gsize recv_buf_len,
    GError **error)
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{
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  g_assert (func == NULL || recv_buf == NULL);
  g_assert (recv_buf != NULL || recv_buf_len == 0);
  g_assert (error == NULL || *error == NULL);

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  g_mutex_lock (&component->io_mutex);
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  if (func != NULL) {
    component->io_callback = func;
    component->io_user_data = user_data;
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    component->recv_buf = NULL;
    component->recv_buf_len = 0;
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    component_schedule_io_callback (component);
  } else {
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    component->io_callback = NULL;
    component->io_user_data = NULL;
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    component->recv_buf = recv_buf;
    component->recv_buf_len = recv_buf_len;
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    component_deschedule_io_callback (component);
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  }
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  component->recv_buf_valid_len = 0;
  component->recv_buf_error = error;

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  g_mutex_unlock (&component->io_mutex);
}

gboolean
component_has_io_callback (Component *component)
{
  gboolean has_io_callback;

  g_mutex_lock (&component->io_mutex);
  has_io_callback = (component->io_callback != NULL);
  g_mutex_unlock (&component->io_mutex);

  return has_io_callback;
}

IOCallbackData *
io_callback_data_new (const guint8 *buf, gsize buf_len)
{
  IOCallbackData *data;

  data = g_slice_new0 (IOCallbackData);
  data->buf = g_memdup (buf, buf_len);
  data->buf_len = buf_len;
  data->offset = 0;

  return data;
}

void
io_callback_data_free (IOCallbackData *data)
{
  g_free (data->buf);
  g_slice_free (IOCallbackData, data);
}

/* This is called with the global agent lock released. It does not take that
 * lock, but does take the io_mutex. */
static gboolean
emit_io_callback_cb (gpointer user_data)
{
  Component *component = user_data;
  IOCallbackData *data;
  NiceAgentRecvFunc io_callback;
  gpointer io_user_data;
  guint stream_id, component_id;
  NiceAgent *agent;

  agent = component->agent;
  g_object_add_weak_pointer (G_OBJECT (agent), (gpointer *) &agent);

  stream_id = component->stream->id;
  component_id = component->id;

  g_mutex_lock (&component->io_mutex);

  /* The members of Component are guaranteed not to have changed since this
   * GSource was attached in component_emit_io_callback(). The Component’s agent
   * and stream are immutable after construction, as are the stream and
   * component IDs. The callback and its user data may have changed, but are
   * guaranteed to be non-%NULL at the start as the idle source is removed when
   * the callback is set to %NULL. They may become %NULL during the io_callback,
   * so must be re-checked every loop iteration. The data buffer is copied into
   * the #IOCallbackData closure.
   *
   * If the component is destroyed (which happens if the agent or stream are
   * destroyed) between attaching the GSource and firing it, the GSource is
   * detached in component_free() and this callback is never invoked. If the
   * agent is destroyed during an io_callback, its weak pointer will be
   * nullified. Similarly, the Component needs to be re-queried for after every
   * iteration, just in case the client has removed the stream in the
   * callback. */
  while (TRUE) {
    io_callback = component->io_callback;
    io_user_data = component->io_user_data;
    data = g_queue_peek_head (&component->pending_io_messages);

    if (data == NULL || io_callback == NULL)
      break;

    g_mutex_unlock (&component->io_mutex);

    io_callback (agent, stream_id, component_id,
        data->buf_len - data->offset, (gchar *) data->buf + data->offset,
        io_user_data);

    /* Check for the user destroying things underneath our feet. */
    if (agent == NULL ||
        !agent_find_component (agent, stream_id, component_id,
            NULL, &component)) {
      nice_debug ("%s: Agent or component destroyed.", G_STRFUNC);
      return G_SOURCE_REMOVE;
    }

    g_queue_pop_head (&component->pending_io_messages);
    io_callback_data_free (data);

    g_mutex_lock (&component->io_mutex);
  }

  component->io_callback_id = 0;
  g_mutex_unlock (&component->io_mutex);

  g_object_remove_weak_pointer (G_OBJECT (agent), (gpointer *) &agent);

  return G_SOURCE_REMOVE;
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}

/* This must be called with the agent lock *held*. */
void
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component_emit_io_callback (Component *component,
    const guint8 *buf, gsize buf_len)
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{
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  NiceAgent *agent;
  guint stream_id, component_id;
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  NiceAgentRecvFunc io_callback;
  gpointer io_user_data;

  g_assert (component != NULL);
  g_assert (buf != NULL);
  g_assert (buf_len > 0);

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  agent = component->agent;
  stream_id = component->stream->id;
  component_id = component->id;
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  g_mutex_lock (&component->io_mutex);
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  io_callback = component->io_callback;
  io_user_data = component->io_user_data;
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  g_mutex_unlock (&component->io_mutex);

  /* Allow this to be called with a NULL io_callback, since the caller can’t
   * lock io_mutex to check beforehand. */
  if (io_callback == NULL)
    return;
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  g_assert (NICE_IS_AGENT (agent));
  g_assert (stream_id > 0);
  g_assert (component_id > 0);
  g_assert (io_callback != NULL);

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  /* Only allocate a closure if the callback is being deferred to an idle
   * handler. */
  if (g_main_context_is_owner (component->ctx)) {
    /* Thread owns the main context, so invoke the callback directly. */
    agent_unlock ();
    io_callback (agent, stream_id,
        component_id, buf_len, (gchar *) buf, io_user_data);
    agent_lock ();
  } else {
    IOCallbackData *data;

    g_mutex_lock (&component->io_mutex);

    /* Slow path: Current thread doesn’t own the Component’s context at the
     * moment, so schedule the callback in an idle handler. */
    data = io_callback_data_new (buf, buf_len);
    g_queue_push_tail (&component->pending_io_messages,
        data);  /* transfer ownership */

    nice_debug ("%s: **WARNING: SLOW PATH**", G_STRFUNC);

    component_schedule_io_callback (component);

    g_mutex_unlock (&component->io_mutex);
  }
}

/* Note: Must be called with the io_mutex held. */
static void
component_schedule_io_callback (Component *component)
{
  GSource *source;
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  /* Already scheduled or nothing to schedule? */
  if (component->io_callback_id != 0 ||
      g_queue_is_empty (&component->pending_io_messages))
    return;

  /* Add the idle callback. If nice_agent_attach_recv() is called with a
   * NULL callback before this source is dispatched, the source will be
   * destroyed, but any pending data will remain in
   * component->pending_io_messages, ready to be picked up when a callback
   * is re-attached, or if nice_agent_recv() is called. */
  source = g_idle_source_new ();
  g_source_set_priority (source, G_PRIORITY_DEFAULT);
  g_source_set_callback (source, emit_io_callback_cb, component, NULL);
  component->io_callback_id = g_source_attach (source, component->ctx);
  g_source_unref (source);
}

/* Note: Must be called with the io_mutex held. */
static void
component_deschedule_io_callback (Component *component)
{
  /* Already descheduled? */
  if (component->io_callback_id == 0)
    return;
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  g_source_remove (component->io_callback_id);
  component->io_callback_id = 0;
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}
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/**
 * ComponentSource:
 *
 * This is a GSource which wraps a single Component and is dispatched whenever
 * any of its NiceSockets are dispatched, i.e. it proxies all poll() events for
 * every socket in the Component. It is designed for use by GPollableInputStream
 * and GPollableOutputStream, so that a Component can be incorporated into a
 * custom main context iteration.
 *
 * The callbacks dispatched by a ComponentSource have type GPollableSourceFunc.
 *
 * ComponentSource supports adding a GCancellable child source which will
 * additionally dispatch if a provided GCancellable is cancelled.
 *
 * Internally, ComponentSource adds a new GSocketSource for each socket in the
 * Component. Changes to the Component’s list of sockets are detected on each
 * call to component_source_prepare(), which compares a stored age with the
 * current age of the Component’s socket list — if the socket list has changed,
 * the age will have increased (indicating added sockets) or will have been
 * reset to 0 (indicating all sockets have been closed).
 */
typedef struct {
  GSource parent;

  GObject *pollable_stream;  /* owned */
  GIOCondition condition;

  Component *component;  /* unowned */
  guint component_socket_sources_age;
} ComponentSource;

static gboolean
component_source_prepare (GSource *source, gint *timeout_)
{
  ComponentSource *component_source = (ComponentSource *) source;
  gint age_diff;

  /* Needed due to accessing the Component. */
  agent_lock ();

  age_diff =
      component_source->component->socket_sources_age -
      component_source->component_socket_sources_age;

  /* If the age has changed, either:
   *  • a new socket has been *prepended* to component->socket_sources (and
   *    age_diff > 0); or
   *  • component->socket_sources has been emptied (and age_diff < 0).
   * We can’t remove any child sources without destroying them, so must
   * monotonically add new ones, or remove everything.
   *
   * Removing everything only happens on shutdown or failure, in which case
   * the ComponentSource itself can be destroyed, automatically destroying all
   * the child sources. */
  if (age_diff < 0) {
    g_source_destroy (source);
  } else if (age_diff > 0) {
    /* Add the new child sources. The difference between the two ages gives
     * the number of new child sources. */
    guint i;
    GSList *l;

    for (i = 0, l = component_source->component->socket_sources;
         i < (guint) age_diff && l != NULL;
         i++, l = l->next) {
      GSource *child_source;
      SocketSource *socket_source;

      socket_source = l->data;

      child_source = g_socket_create_source (socket_source->socket->fileno,
          component_source->condition, NULL);
      g_source_set_dummy_callback (child_source);
      g_source_add_child_source (source, child_source);
      g_source_unref (child_source);
    }
  }

  /* Update the age. */
  component_source->component_socket_sources_age =
      component_source->component->socket_sources_age;

  agent_unlock ();

  /* We can’t be sure if the ComponentSource itself needs to be dispatched until
   * poll() is called on all the child sources. */
  return FALSE;
}

static gboolean
component_source_dispatch (GSource *source, GSourceFunc callback,
    gpointer user_data)
{
  ComponentSource *component_source = (ComponentSource *) source;
  GPollableSourceFunc func = (GPollableSourceFunc) callback;

  return func (component_source->pollable_stream, user_data);
}

static void
component_source_finalize (GSource *source)
{
  ComponentSource *component_source = (ComponentSource *) source;

  g_object_unref (component_source->pollable_stream);
  component_source->pollable_stream = NULL;
}

static gboolean
component_source_closure_callback (GObject *pollable_stream, gpointer user_data)
{
  GClosure *closure = user_data;
  GValue param_value = G_VALUE_INIT;
  GValue result_value = G_VALUE_INIT;
  gboolean retval;

  g_value_init (&result_value, G_TYPE_BOOLEAN);
  g_value_init (&param_value, G_TYPE_OBJECT);
  g_value_set_object (&param_value, pollable_stream);

  g_closure_invoke (closure, &result_value, 1, &param_value, NULL);
  retval = g_value_get_boolean (&result_value);

  g_value_unset (&param_value);
  g_value_unset (&result_value);

  return retval;
}

static GSourceFuncs component_source_funcs = {
  component_source_prepare,
  NULL,  /* check */
  component_source_dispatch,
  component_source_finalize,
  (GSourceFunc) component_source_closure_callback,
};

/**
 * component_source_new:
 * @component: a #Component
 * @pollable_stream: a #GPollableInputStream or #GPollableOutputStream to pass
 * to dispatched callbacks
 * @condition: the I/O condition to poll for (e.g. %G_IO_IN or %G_IO_OUT)
 * @cancellable: (allow-none): a #GCancellable, or %NULL
 *
 * Create a new #ComponentSource, a type of #GSource which proxies poll events
 * from all sockets in the given @component.
 *
 * A callback function of type #GPollableSourceFunc must be connected to the
 * returned #GSource using g_source_set_callback(). @pollable_stream is passed
 * to all callbacks dispatched from the #GSource, and a reference is held on it
 * by the #GSource.
 *
 * The #GSource will automatically update to poll sockets as they’re added to
 * the @component (e.g. during peer discovery).
 *
 * Returns: (transfer full): a new #ComponentSource; unref with g_source_unref()
 */
GSource *
component_source_new (Component *component, GObject *pollable_stream,
    GIOCondition condition, GCancellable *cancellable)
{
  ComponentSource *component_source;

  g_assert (component != NULL);
  g_assert (G_IS_POLLABLE_INPUT_STREAM (pollable_stream) ||
            G_IS_POLLABLE_OUTPUT_STREAM (pollable_stream));

  component_source =
      (ComponentSource *)
          g_source_new (&component_source_funcs, sizeof (ComponentSource));
  g_source_set_name ((GSource *) component_source, "ComponentSource");

  component_source->component = component;
  component_source->component_socket_sources_age = 0;
  component_source->pollable_stream = g_object_ref (pollable_stream);
  component_source->condition = condition;

  /* Add a cancellable source. */
  if (cancellable != NULL) {
    GSource *cancellable_source;

    cancellable_source = g_cancellable_source_new (cancellable);
    g_source_set_dummy_callback (cancellable_source);
    g_source_add_child_source ((GSource *) component_source,
        cancellable_source);
    g_source_unref (cancellable_source);
  }

  return (GSource *) component_source;
}