MXS-2122: Close listener socket on destruction

Even though directly closing the socket is not very neat in the
architectural sense of things, it allows the best of both worlds: the
socket is instantly closed and is open for reuse while the listener struct
is still available as a reference.

This change needs to be revised when the listeners are refactored into
separate objects.

Updated documentation to reflect the change in behavior.

Documentation/REST-API/Resources-Service.md

@@ -323,9 +323,6 @@ all the listeners pointing to the service have been destroyed. This means that
 the `data.relationships` must be an empty object and `data.attributes.listeners`
 must be an empty array in order for the service to qualify for destruction.
 
-Once a service is destroyed, any listeners associated with it will be freed
-after which the ports can be reused by other listeners.
-
 If there are open client connections that use the service when it is destroyed,
 they are allowed to gracefully close before the service is destroyed. This means
 that the destruction of a service can be acknowledged via the REST API before
@@ -466,6 +463,9 @@ DELETE /v1/services/:service/listeners/:name
 In the URI , the _:name_ must map to a listener and the _:service_ must map to a
 service. Both names must have all whitespace replaced with hyphens.
 
+When a listener is destroyed, the network port it listens on is available for
+reuse.
+
 #### Response
 
 Listener is destroyed:

Documentation/Reference/MaxAdmin.md

@@ -1547,8 +1547,8 @@ Example: create listener my-service my-new-listener 192.168.0.101 4006
 
 You can destroy created listeners with the `destroy listener` command. This will
 remove the persisted configuration and it will not be created on the next
-startup. The listener is stopped but it will remain a part of the runtime
-configuration until the next restart.
+startup. The listener is destroyed and the listening port is immediately
+available for reuse.
 
 ```
 destroy listener - Destroy a listener
@@ -1558,7 +1558,7 @@ Usage: destroy listener SERVICE NAME
 Parameters:
 NAME Listener to destroy
 
-The listener is stopped and it will be removed on the next restart of MaxScale
+The listener is deleted
 
 Example: destroy listener my-listener
 ```
@@ -1615,9 +1615,7 @@ Example: alter monitor my-monitor user=maxuser password=maxpwd
 ### Destroying Monitors
 
 To destroy a monitor, use the `destroy monitor` command. All servers need to be
-removed from the monitor before it can be destroyed. Only created monitors
-should be destroyed and they will remain a part of the runtime configuration
-until the next restart.
+removed from the monitor before it can be destroyed.
 
 ```
 destroy monitor - Destroy a monitor
@@ -1627,7 +1625,7 @@ Usage: destroy monitor NAME
 Parameters:
 NAME Monitor to destroy
 
-The monitor is stopped and it will be removed on the next restart of MaxScale
+The monitor is destroyed
 
 Example: destroy monitor my-monitor
 ```

Documentation/Reference/MaxCtrl.md

@@ -523,10 +523,7 @@ The monitor must be unlinked from all servers before it can be destroyed.
 
 `Usage: destroy listener <service> <name>`
 
-Destroying a monitor causes it to be removed on the next restart. Destroying a
-listener at runtime stops it from accepting new connections but it will still be
-bound to the listening socket. This means that new listeners cannot be created
-to replace destroyed listeners without restarting MaxScale.
+Destroying a listener closes the listening socket, opening it up for reuse.
 
 ### destroy service
 

maxctrl/lib/destroy.js

@@ -34,11 +34,7 @@ exports.builder = function(yargs) {
             })
         })
         .command('listener <service> <name>', 'Destroy an unused listener', function(yargs) {
-            return yargs.epilog('Destroying a monitor causes it to be removed on the next restart. ' +
-                                'Destroying a listener at runtime stops it from accepting new ' +
-                                'connections but it will still be bound to the listening socket. This ' +
-                                'means that new listeners cannot be created to replace destroyed listeners ' +
-                                'without restarting MaxScale.')
+            return yargs.epilog('Destroying a listener closes the listening socket, opening it up for reuse.')
                 .usage('Usage: destroy listener <service> <name>')
         }, function(argv) {
             maxctrl(argv, function(host) {

server/core/dcb.cc

@@ -1274,6 +1274,11 @@ void dcb_final_close(DCB* dcb)
                 }
             }
         }
+        else
+        {
+            // Only listeners are closed with a fd of -1
+            mxb_assert(dcb->dcb_role == DCB_ROLE_SERVICE_LISTENER);
+        }
 
         dcb->state = DCB_STATE_DISCONNECTED;
         dcb_remove_from_list(dcb);

server/core/service.cc

@@ -751,6 +751,11 @@ bool service_remove_listener(Service* service, const char* target)
             {
                 listener->listener->session->state = SESSION_STATE_LISTENER_STOPPED;
                 rval = true;
+
+                // TODO: This is not pretty but it works, revise when listeners are refactored. This is
+                // thread-safe as the listener is freed on the same thread that closes the socket.
+                close(listener->listener->fd);
+                listener->listener->fd = -1;
             }
             break;
         }