Fix deadlock on presence state changes.

A deadlock was observed where three threads were competing for different
locks:

* One thread held the hints lock and was attempting to lock a specific
  hint.
* One thread was holding the specific hint's lock and was attempting to
  lock the contexts lock
* One thread was holding the contexts lock and attempting to lock the
  hints lock.

Clearly the second thread was doing the wrong thing here. The fix for
this is to make sure that the hint's lock is not held on presence state
changes. Something similar is already done (and commented about) for
device state changes.

ASTERISK-25362 #close
Reported by Mark Michelson

Change-Id: I15ec2416b92978a4c0c08273b2d46cb21aff97e2

main/pbx.c

@@ -11978,10 +11978,11 @@ static void presence_state_cb(void *unused, struct stasis_subscription *sub, str
 		struct ast_state_cb *state_cb;
 		const char *app;
 		char *parse;
-		SCOPED_AO2LOCK(lock, hint);
+		ao2_lock(hint);
 
 		if (!hint->exten) {
 			/* The extension has already been destroyed */
+			ao2_unlock(hint);
 			continue;
 		}
 
@@ -11989,16 +11990,19 @@ static void presence_state_cb(void *unused, struct stasis_subscription *sub, str
 		app = ast_get_extension_app(hint->exten);
 		if (ast_strlen_zero(app)) {
 			/* The hint does not monitor presence at all. */
+			ao2_unlock(hint);
 			continue;
 		}
 
 		ast_str_set(&hint_app, 0, "%s", app);
 		parse = parse_hint_presence(hint_app);
 		if (ast_strlen_zero(parse)) {
+			ao2_unlock(hint);
 			continue;
 		}
 		if (strcasecmp(parse, presence_state->provider)) {
 			/* The hint does not monitor the presence provider. */
+			ao2_unlock(hint);
 			continue;
 		}
 
@@ -12019,6 +12023,7 @@ static void presence_state_cb(void *unused, struct stasis_subscription *sub, str
 			((hint->last_presence_message && presence_state->message && !strcmp(hint->last_presence_message, presence_state->message)) || (!hint->last_presence_message && !presence_state->message))) {
 
 			/* this update is the same as the last, do nothing */
+			ao2_unlock(hint);
 			continue;
 		}
 
@@ -12028,6 +12033,14 @@ static void presence_state_cb(void *unused, struct stasis_subscription *sub, str
 		hint->last_presence_state = presence_state->state;
 		hint->last_presence_subtype = presence_state->subtype ? ast_strdup(presence_state->subtype) : NULL;
 		hint->last_presence_message = presence_state->message ? ast_strdup(presence_state->message) : NULL;
+		/*
+		 * (Copied from device_state_cb)
+		 *
+		 * NOTE: We cannot hold any locks while notifying
+		 * the watchers without causing a deadlock.
+		 * (conlock, hints, and hint)
+		 */
+		ao2_unlock(hint);
 
 		/* For general callbacks */
 		cb_iter = ao2_iterator_init(statecbs, 0);