cbus: fix inconsistency in endpoint creation

The notification of wait variable shall be done under
a bound mutex locked. Otherwise the results are not
guaranteed (see pthread manuals).

Thus when we create a new endpoint via cbus_endpoint_create
and there is an other thread which sleeps inside cpipe_create
we should notify the sleeper under cbus.mutex.

Fixes #4806

Reported-by: Alexander Turenko <alexander.turenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
(cherry picked from commit d6d69c9)

src/lib/core/cbus.c

@@ -234,7 +234,6 @@ cbus_endpoint_create(struct cbus_endpoint *endpoint, const char *name,
 	ev_async_start(endpoint->consumer, &endpoint->async);
 
 	rlist_add_tail(&cbus.endpoints, &endpoint->in_cbus);
-	tt_pthread_mutex_unlock(&cbus.mutex);
 	/*
 	 * Alert all waiting producers.
 	 *
@@ -243,6 +242,7 @@ cbus_endpoint_create(struct cbus_endpoint *endpoint, const char *name,
 	 * blocked on cond.
 	 */
 	tt_pthread_cond_broadcast(&cbus.cond);
+	tt_pthread_mutex_unlock(&cbus.mutex);
 	return 0;
 }