/*
* Helper function to find channels. It supports these modes:
*
* prev != NULL : get channel next in list after prev
* name != NULL : get channel with matching name
* name != NULL && namelen != 0 : get channel whose name starts with prefix
* exten != NULL : get channel whose exten or macroexten matches
* context != NULL && exten != NULL : get channel whose context or macrocontext
*
* It returns with the channel's lock held. If getting the individual lock fails,
* unlock and retry quickly up to 10 times, then give up.
*
* XXX Note that this code has cost O(N) because of the need to verify
* that the object is still on the global list.
*
* XXX also note that accessing fields (e.g. c->name in opbx_log())
* can only be done with the lock held or someone could delete the
* object while we work on it. This causes some ugliness in the code.
* Note that removing the first opbx_log() may be harmful, as it would
* shorten the retry period and possibly cause failures.
* We should definitely go for a better scheme that is deadlock-free.
*/
static struct opbx_channel *channel_find_locked(const struct opbx_channel *prev,
const char *name, const int namelen,
const char *context, const char *exten)
{
const char *msg = prev ? "deadlock" : "initial deadlock";
int retries, done;
struct opbx_channel *c;
for (retries = 0; retries < 10; retries++) {
opbx_mutex_lock(&chlock);
for (c = channels; c; c = c->next) {
if (!prev) {
/* want head of list */
if (!name && !exten)
break;
if (name) {
/* want match by full name */
if (!namelen) {
if (!strcasecmp(c->name, name))
break;
else
continue;
}
/* want match by name prefix */
if (!strncasecmp(c->name, name, namelen))
break;
} else if (exten) {
/* want match by context and exten */
if (context && (strcasecmp(c->context, context) &&
strcasecmp(c->macrocontext, context)))
continue;
/* match by exten */
if (strcasecmp(c->exten, exten) &&
strcasecmp(c->macroexten, exten))
continue;
else
break;
}
} else if (c == prev) { /* found, return c->next */
c = c->next;
break;
}
}
/* exit if chan not found or mutex acquired successfully */
done = (c == NULL) || (opbx_mutex_trylock(&c->lock) == 0);
/* this is slightly unsafe, as we _should_ hold the lock to access c->name */
if (!done && c)
opbx_log(LOG_DEBUG, "Avoiding %s for '%s'\n", msg, c->name);
opbx_mutex_unlock(&chlock);
if (done)
return c;
usleep(1);
}
/*
* c is surely not null, but we don't have the lock so cannot
* access c->name
*/
opbx_log(LOG_WARNING, "Avoided %s for '%p', %d retries!\n",
msg, c, retries);
return NULL;
}