Fix crash on double creation of listeners

When the listeners were created twice, MaxScale would crash when the users
were loaded. This is caused by the fact that the DCB for the listener is
NULL for failed listeners.

server/core/service.c

@@ -170,7 +170,6 @@ SERVICE* service_alloc(const char *name, const char *router)
     service->stats.n_failed_starts = 0;
     service->state = SERVICE_STATE_ALLOC;
     spinlock_init(&service->spin);
-    spinlock_init(&service->users_table_spin);
 
     spinlock_acquire(&service_spin);
     service->next = allServices;
@@ -209,7 +208,12 @@ service_isvalid(SERVICE *service)
 
 static inline void close_port(SERV_LISTENER *port)
 {
-    port->service->state = SERVICE_STATE_FAILED;
+    if (port->service->state == SERVICE_STATE_ALLOC)
+    {
+        /** The service failed when it was being allocated */
+        port->service->state = SERVICE_STATE_FAILED;
+    }
+
     if (port->listener)
     {
         dcb_close(port->listener);
@@ -499,9 +503,59 @@ int serviceInitialize(SERVICE *service)
     return listeners;
 }
 
+/**
+ * @brief Remove a failed listener
+ *
+ * This should only be called when a newly created listener fails to start.
+ *
+ * @note The service spinlock must be held when this function is called.
+ *
+ * @param service Service where @c port points to
+ * @param port Port to remove
+ */
+void serviceRemoveListener(SERVICE *service, SERV_LISTENER *port)
+{
+
+    if (service->ports == port)
+    {
+        service->ports = service->ports->next;
+    }
+    else
+    {
+        SERV_LISTENER *prev = service->ports;
+        SERV_LISTENER *current = service->ports->next;
+
+        while (current)
+        {
+            if (current == port)
+            {
+                prev->next = current->next;
+                break;
+            }
+            prev = current;
+            current = current->next;
+        }
+    }
+}
+
 bool serviceLaunchListener(SERVICE *service, SERV_LISTENER *port)
 {
-    return serviceStartPort(service, port);
+    ss_dassert(service->state != SERVICE_STATE_FAILED);
+    bool rval = true;
+
+    spinlock_acquire(&service->spin);
+
+    if (serviceStartPort(service, port) == 0)
+    {
+        /** Failed to start the listener */
+        serviceRemoveListener(service, port);
+        listener_free(port);
+        rval = false;
+    }
+
+    spinlock_release(&service->spin);
+
+    return rval;
 }
 
 bool serviceStopListener(SERVICE *service, const char *name)
@@ -692,8 +746,8 @@ void service_free(SERVICE *service)
  * @return Created listener or NULL on error
  */
 SERV_LISTENER* serviceCreateListener(SERVICE *service, const char *name, const char *protocol,
-                   const char *address, unsigned short port, const char *authenticator,
-                   const char *options, SSL_LISTENER *ssl)
+                                     const char *address, unsigned short port, const char *authenticator,
+                                     const char *options, SSL_LISTENER *ssl)
 {
     SERV_LISTENER *proto = listener_alloc(service, name, protocol, address,
                                           port, authenticator, options, ssl);
@@ -719,7 +773,7 @@ SERV_LISTENER* serviceCreateListener(SERVICE *service, const char *name, const c
  * @return      True if the protocol/port is already part of the service
  */
 bool serviceHasListener(SERVICE *service, const char *protocol,
-                       const char* address, unsigned short port)
+                        const char* address, unsigned short port)
 {
     SERV_LISTENER *proto;
 
@@ -1545,7 +1599,7 @@ int service_refresh_users(SERVICE *service)
 {
     int ret = 1;
 
-    if (spinlock_acquire_nowait(&service->users_table_spin))
+    if (spinlock_acquire_nowait(&service->spin))
     {
         time_t now = time(NULL);
 
@@ -1571,7 +1625,7 @@ int service_refresh_users(SERVICE *service)
             for (SERV_LISTENER *port = service->ports; port; port = port->next)
             {
                 /** Load the authentication users before before starting the listener */
-                if (port->listener->authfunc.loadusers)
+                if (port->listener && port->listener->authfunc.loadusers)
                 {
                     switch (port->listener->authfunc.loadusers(port))
                     {
@@ -1594,7 +1648,7 @@ int service_refresh_users(SERVICE *service)
             }
         }
 
-        spinlock_release(&service->users_table_spin);
+        spinlock_release(&service->spin);
     }
 
     return ret;
@@ -1911,12 +1965,12 @@ void service_destroy_instances(void)
         /* Call destroyInstance hook for routers */
         if (svc->router->destroyInstance && svc->router_instance)
         {
-           svc->router->destroyInstance(svc->router_instance);
+            svc->router->destroyInstance(svc->router_instance);
         }
         if (svc->n_filters)
         {
             FILTER_DEF **filters = svc->filters;
-            for (int i=0; i < svc->n_filters; i++)
+            for (int i = 0; i < svc->n_filters; i++)
             {
                 if (filters[i]->obj->destroyInstance && filters[i]->filter)
                 {