GUACAMOLE-1293: Do not re-acquire __users_lock while already held for…

… writing.

Per POSIX spec, the behavior of acquiring a read lock on a rwlock that's
already acquired for writing is undefined. From the documentation for
pthread_rwlock_rdlock():

"... Results are undefined if the calling thread holds a write lock on
rwlock at the time the call is made."

src/libguac/client.c

@@ -302,15 +302,15 @@ int guac_client_add_user(guac_client* client, guac_user* user, int argc, char**
         /* Update owner pointer if user is owner */
         if (user->owner)
             client->__owner = user;
-
-        /* Notify owner of user joining connection. */
-        else
-            guac_client_owner_notify_join(client, user);
 
     }
 
     pthread_rwlock_unlock(&(client->__users_lock));
 
+    /* Notify owner of user joining connection. */
+    if (retval == 0 && !user->owner)
+        guac_client_owner_notify_join(client, user);
+
     return retval;
 
 }
@@ -335,12 +335,12 @@ void guac_client_remove_user(guac_client* client, guac_user* user) {
     if (user->owner)
         client->__owner = NULL;
 
+    pthread_rwlock_unlock(&(client->__users_lock));
+
     /* Update owner of user having left the connection. */
-    else
+    if (!user->owner)
         guac_client_owner_notify_leave(client, user);
 
-    pthread_rwlock_unlock(&(client->__users_lock));
-
     /* Call handler, if defined */
     if (user->leave_handler)
         user->leave_handler(user);