func_lock.c

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/*
 * Asterisk -- An open source telephony toolkit.
 *
 * Copyright (C) 2007, Tilghman Lesher
 *
 * Tilghman Lesher <func_lock_2007@the-tilghman.com>
 *
 * See http://www.asterisk.org for more information about
 * the Asterisk project. Please do not directly contact
 * any of the maintainers of this project for assistance;
 * the project provides a web site, mailing lists and IRC
 * channels for your use.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License Version 2. See the LICENSE file
 * at the top of the source tree.
 */

/*! \file
 *
 * \brief Dialplan mutexes
 *
 * \author Tilghman Lesher <func_lock_2007@the-tilghman.com>
 *
 * \ingroup functions
 * 
 */

/*** MODULEINFO
   <support_level>core</support_level>
 ***/

#include "asterisk.h"

ASTERISK_FILE_VERSION(__FILE__, "$Revision: 328209 $")

#include <signal.h>

#include "asterisk/lock.h"
#include "asterisk/file.h"
#include "asterisk/channel.h"
#include "asterisk/pbx.h"
#include "asterisk/module.h"
#include "asterisk/linkedlists.h"
#include "asterisk/astobj2.h"
#include "asterisk/utils.h"

/*** DOCUMENTATION
   <function name="LOCK" language="en_US">
      <synopsis>
         Attempt to obtain a named mutex.
      </synopsis>
      <syntax>
         <parameter name="lockname" required="true" />
      </syntax>
      <description>
         <para>Attempts to grab a named lock exclusively, and prevents other channels from
         obtaining the same lock.  LOCK will wait for the lock to become available.
         Returns <literal>1</literal> if the lock was obtained or <literal>0</literal> on error.</para>
         <note><para>To avoid the possibility of a deadlock, LOCK will only attempt to
         obtain the lock for 3 seconds if the channel already has another lock.</para></note>
      </description>
   </function>
   <function name="TRYLOCK" language="en_US">
      <synopsis>
         Attempt to obtain a named mutex.
      </synopsis>
      <syntax>
         <parameter name="lockname" required="true" />
      </syntax>
      <description>
         <para>Attempts to grab a named lock exclusively, and prevents other channels
         from obtaining the same lock.  Returns <literal>1</literal> if the lock was 
         available or <literal>0</literal> otherwise.</para>
      </description>
   </function>
   <function name="UNLOCK" language="en_US">
      <synopsis>
         Unlocks a named mutex.
      </synopsis>
      <syntax>
         <parameter name="lockname" required="true" />
      </syntax>
      <description>
         <para>Unlocks a previously locked mutex. Returns <literal>1</literal> if the channel 
         had a lock or <literal>0</literal> otherwise.</para>
         <note><para>It is generally unnecessary to unlock in a hangup routine, as any locks 
         held are automatically freed when the channel is destroyed.</para></note>
      </description>
   </function>
 ***/



static AST_LIST_HEAD_STATIC(locklist, lock_frame);

static void lock_free(void *data);
static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan);
static int unloading = 0;
static pthread_t broker_tid = AST_PTHREADT_NULL;

static struct ast_datastore_info lock_info = {
   .type = "MUTEX",
   .destroy = lock_free,
   .chan_fixup = lock_fixup,
};

struct lock_frame {
   AST_LIST_ENTRY(lock_frame) entries;
   ast_mutex_t mutex;
   ast_cond_t cond;
   /*! count is needed so if a recursive mutex exits early, we know how many times to unlock it. */
   unsigned int count;
   /*! Container of requesters for the named lock */
   struct ao2_container *requesters;
   /*! who owns us */
   struct ast_channel *owner;
   /*! name of the lock */
   char name[0];
};

struct channel_lock_frame {
   AST_LIST_ENTRY(channel_lock_frame) list;
   /*! Need to save channel pointer here, because during destruction, we won't have it. */
   struct ast_channel *channel;
   struct lock_frame *lock_frame;
};

static void lock_free(void *data)
{
   AST_LIST_HEAD(, channel_lock_frame) *oldlist = data;
   struct channel_lock_frame *clframe;
   AST_LIST_LOCK(oldlist);
   while ((clframe = AST_LIST_REMOVE_HEAD(oldlist, list))) {
      /* Only unlock if we own the lock */
      if (clframe->channel == clframe->lock_frame->owner) {
         clframe->lock_frame->count = 0;
         clframe->lock_frame->owner = NULL;
      }
      ast_free(clframe);
   }
   AST_LIST_UNLOCK(oldlist);
   AST_LIST_HEAD_DESTROY(oldlist);
   ast_free(oldlist);
}

static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan)
{
   struct ast_datastore *lock_store = ast_channel_datastore_find(oldchan, &lock_info, NULL);
   AST_LIST_HEAD(, channel_lock_frame) *list;
   struct channel_lock_frame *clframe = NULL;

   if (!lock_store) {
      return;
   }
   list = lock_store->data;

   AST_LIST_LOCK(list);
   AST_LIST_TRAVERSE(list, clframe, list) {
      if (clframe->lock_frame->owner == oldchan) {
         clframe->lock_frame->owner = newchan;
      }
      /* We don't move requesters, because the thread stack is different */
      clframe->channel = newchan;
   }
   AST_LIST_UNLOCK(list);
}

static void *lock_broker(void *unused)
{
   struct lock_frame *frame;
   struct timespec forever = { 1000000, 0 };
   for (;;) {
      int found_requester = 0;

      /* Test for cancel outside of the lock */
      pthread_testcancel();
      AST_LIST_LOCK(&locklist);

      AST_LIST_TRAVERSE(&locklist, frame, entries) {
         if (ao2_container_count(frame->requesters)) {
            found_requester++;
            ast_mutex_lock(&frame->mutex);
            if (!frame->owner) {
               ast_cond_signal(&frame->cond);
            }
            ast_mutex_unlock(&frame->mutex);
         }
      }

      AST_LIST_UNLOCK(&locklist);
      pthread_testcancel();

      /* If there are no requesters, then wait for a signal */
      if (!found_requester) {
         nanosleep(&forever, NULL);
      } else {
         sched_yield();
      }
   }
   /* Not reached */
   return NULL;
}

static int ast_channel_hash_cb(const void *obj, const int flags)
{
   const struct ast_channel *chan = obj;
   return ast_str_case_hash(chan->name);
}

static int ast_channel_cmp_cb(void *obj, void *arg, int flags)
{
   struct ast_channel *chan = obj, *cmp_args = arg;
   return strcasecmp(chan->name, cmp_args->name) ? 0 : CMP_MATCH;
}

static int get_lock(struct ast_channel *chan, char *lockname, int try)
{
   struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
   struct lock_frame *current;
   struct channel_lock_frame *clframe = NULL;
   AST_LIST_HEAD(, channel_lock_frame) *list;
   int res = 0;
   struct timespec three_seconds = { .tv_sec = 3 };

   if (!lock_store) {
      ast_debug(1, "Channel %s has no lock datastore, so we're allocating one.\n", chan->name);
      lock_store = ast_datastore_alloc(&lock_info, NULL);
      if (!lock_store) {
         ast_log(LOG_ERROR, "Unable to allocate new datastore.  No locks will be obtained.\n");
         return -1;
      }

      list = ast_calloc(1, sizeof(*list));
      if (!list) {
         ast_log(LOG_ERROR, "Unable to allocate datastore list head.  %sLOCK will fail.\n", try ? "TRY" : "");
         ast_datastore_free(lock_store);
         return -1;
      }

      lock_store->data = list;
      AST_LIST_HEAD_INIT(list);
      ast_channel_datastore_add(chan, lock_store);
   } else
      list = lock_store->data;

   /* Lock already exists? */
   AST_LIST_LOCK(&locklist);
   AST_LIST_TRAVERSE(&locklist, current, entries) {
      if (strcmp(current->name, lockname) == 0) {
         break;
      }
   }

   if (!current) {
      if (unloading) {
         /* Don't bother */
         AST_LIST_UNLOCK(&locklist);
         return -1;
      }

      /* Create new lock entry */
      current = ast_calloc(1, sizeof(*current) + strlen(lockname) + 1);
      if (!current) {
         AST_LIST_UNLOCK(&locklist);
         return -1;
      }

      strcpy(current->name, lockname); /* SAFE */
      if ((res = ast_mutex_init(&current->mutex))) {
         ast_log(LOG_ERROR, "Unable to initialize mutex: %s\n", strerror(res));
         ast_free(current);
         AST_LIST_UNLOCK(&locklist);
         return -1;
      }
      if ((res = ast_cond_init(&current->cond, NULL))) {
         ast_log(LOG_ERROR, "Unable to initialize condition variable: %s\n", strerror(res));
         ast_mutex_destroy(&current->mutex);
         ast_free(current);
         AST_LIST_UNLOCK(&locklist);
         return -1;
      }
      if (!(current->requesters = ao2_container_alloc(1, ast_channel_hash_cb, ast_channel_cmp_cb))) {
         ast_mutex_destroy(&current->mutex);
         ast_cond_destroy(&current->cond);
         ast_free(current);
         AST_LIST_UNLOCK(&locklist);
         return -1;
      }
      AST_LIST_INSERT_TAIL(&locklist, current, entries);
   }
   AST_LIST_UNLOCK(&locklist);

   /* Found lock or created one - now find or create the corresponding link in the channel */
   AST_LIST_LOCK(list);
   AST_LIST_TRAVERSE(list, clframe, list) {
      if (clframe->lock_frame == current) {
         break;
      }
   }

   if (!clframe) {
      if (unloading) {
         /* Don't bother */
         AST_LIST_UNLOCK(list);
         return -1;
      }

      if (!(clframe = ast_calloc(1, sizeof(*clframe)))) {
         ast_log(LOG_ERROR, "Unable to allocate channel lock frame.  %sLOCK will fail.\n", try ? "TRY" : "");
         AST_LIST_UNLOCK(list);
         return -1;
      }

      clframe->lock_frame = current;
      clframe->channel = chan;
      AST_LIST_INSERT_TAIL(list, clframe, list);
   }
   AST_LIST_UNLOCK(list);

   /* If we already own the lock, then we're being called recursively.
    * Keep track of how many times that is, because we need to unlock
    * the same amount, before we'll release this one.
    */
   if (current->owner == chan) {
      current->count++;
      return 0;
   }

   /* Okay, we have both frames, so now we need to try to lock.
    *
    * Locking order: always lock locklist first.  We need the
    * locklist lock because the broker thread counts whether
    * there are requesters with the locklist lock held, and we
    * need to hold it, so that when we send our signal, below,
    * to wake up the broker thread, it definitely will see that
    * a requester exists at that point in time.  Otherwise, we
    * could add to the requesters after it has already seen that
    * that lock is unoccupied and wait forever for another signal.
    */
   AST_LIST_LOCK(&locklist);
   ast_mutex_lock(&current->mutex);
   /* Add to requester list */
   ao2_link(current->requesters, chan);
   pthread_kill(broker_tid, SIGURG);
   AST_LIST_UNLOCK(&locklist);

   if ((!current->owner) ||
      (!try && !(res = ast_cond_timedwait(&current->cond, &current->mutex, &three_seconds)))) {
      res = 0;
      current->owner = chan;
      current->count++;
   } else {
      res = -1;
   }
   /* Remove from requester list */
   ao2_unlink(current->requesters, chan);
   ast_mutex_unlock(&current->mutex);

   return res;
}

static int unlock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
{
   struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
   struct channel_lock_frame *clframe;
   AST_LIST_HEAD(, channel_lock_frame) *list;

   if (!lock_store) {
      ast_log(LOG_WARNING, "No datastore for dialplan locks.  Nothing was ever locked!\n");
      ast_copy_string(buf, "0", len);
      return 0;
   }

   if (!(list = lock_store->data)) {
      ast_debug(1, "This should NEVER happen\n");
      ast_copy_string(buf, "0", len);
      return 0;
   }

   /* Find item in the channel list */
   AST_LIST_LOCK(list);
   AST_LIST_TRAVERSE(list, clframe, list) {
      if (clframe->lock_frame && clframe->lock_frame->owner == chan && strcmp(clframe->lock_frame->name, data) == 0) {
         break;
      }
   }
   /* We never destroy anything until channel destruction, which will never
    * happen while this routine is executing, so we don't need to hold the
    * lock beyond this point. */
   AST_LIST_UNLOCK(list);

   if (!clframe) {
      /* We didn't have this lock in the first place */
      ast_copy_string(buf, "0", len);
      return 0;
   }

   if (--clframe->lock_frame->count == 0) {
      clframe->lock_frame->owner = NULL;
   }

   ast_copy_string(buf, "1", len);
   return 0;
}

static int lock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
{
   if (chan)
      ast_autoservice_start(chan);

   ast_copy_string(buf, get_lock(chan, data, 0) ? "0" : "1", len);

   if (chan)
      ast_autoservice_stop(chan);

   return 0;
}

static int trylock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
{
   if (chan)
      ast_autoservice_start(chan);

   ast_copy_string(buf, get_lock(chan, data, 1) ? "0" : "1", len);

   if (chan)
      ast_autoservice_stop(chan);

   return 0;
}

static struct ast_custom_function lock_function = {
   .name = "LOCK",
   .read = lock_read,
   .read_max = 2,
};

static struct ast_custom_function trylock_function = {
   .name = "TRYLOCK",
   .read = trylock_read,
   .read_max = 2,
};

static struct ast_custom_function unlock_function = {
   .name = "UNLOCK",
   .read = unlock_read,
   .read_max = 2,
};

static int unload_module(void)
{
   struct lock_frame *current;

   /* Module flag */
   unloading = 1;

   AST_LIST_LOCK(&locklist);
   while ((current = AST_LIST_REMOVE_HEAD(&locklist, entries))) {
      /* If any locks are currently in use, then we cannot unload this module */
      if (current->owner || ao2_container_count(current->requesters)) {
         /* Put it back */
         AST_LIST_INSERT_HEAD(&locklist, current, entries);
         AST_LIST_UNLOCK(&locklist);
         unloading = 0;
         return -1;
      }
      ast_mutex_destroy(&current->mutex);
      ao2_ref(current->requesters, -1);
      ast_free(current);
   }

   /* No locks left, unregister functions */
   ast_custom_function_unregister(&lock_function);
   ast_custom_function_unregister(&trylock_function);
   ast_custom_function_unregister(&unlock_function);

   pthread_cancel(broker_tid);
   pthread_kill(broker_tid, SIGURG);
   pthread_join(broker_tid, NULL);

   AST_LIST_UNLOCK(&locklist);

   return 0;
}

static int load_module(void)
{
   int res = ast_custom_function_register(&lock_function);
   res |= ast_custom_function_register(&trylock_function);
   res |= ast_custom_function_register(&unlock_function);
   ast_pthread_create_background(&broker_tid, NULL, lock_broker, NULL);
   return res;
}

AST_MODULE_INFO_STANDARD(ASTERISK_GPL_KEY, "Dialplan mutexes");