MXS-2710: Move client_count handling into client DCB

Due to the fact that both client connections and listeners use sessions in
2.3, the client_count tracking must be done inside the client DCB. In
addition to this, the max_connections check didn't take the current
pending connection into account which caused an off-by-one error.

This commit fixes the connection_limit test failure that was introduced by
commit 6306519e5e75575ba083ee2f0edfe7e624da5d26.

server/core/dcb.cc

@@ -56,6 +56,7 @@
 
 #include "internal/modules.h"
 #include "internal/session.h"
+#include "internal/service.hh"
 
 using maxscale::RoutingWorker;
 using maxbase::Worker;
@@ -218,6 +219,11 @@ DCB* dcb_alloc(dcb_role_t role, SERV_LISTENER* listener)
         newdcb->poll.owner = RoutingWorker::get_current();
     }
 
+    if (role == DCB_ROLE_CLIENT_HANDLER)
+    {
+        mxb::atomic::add(&listener->service->client_count, 1, mxb::atomic::RELAXED);
+    }
+
     return newdcb;
 }
 
@@ -282,6 +288,21 @@ void dcb_free_all_memory(DCB* dcb)
         this_thread.current_dcb = NULL;
     }
 
+    if (dcb->dcb_role == DCB_ROLE_CLIENT_HANDLER)
+    {
+        Service* service = static_cast<Service*>(dcb->service);
+        bool should_destroy = !mxb::atomic::load(&service->active);
+
+        if (mxb::atomic::add(&service->client_count, -1) == 1 && should_destroy)
+        {
+            // Destroy the service in the main routing worker thread
+            mxs::RoutingWorker* main_worker = mxs::RoutingWorker::get(mxs::RoutingWorker::MAIN);
+            main_worker->execute([service]() {
+                                     service_free(service);
+                                 }, Worker::EXECUTE_AUTO);
+        }
+    }
+
     DCB_CALLBACK* cb_dcb;
 
     if (dcb->protocol)
@@ -2518,7 +2539,7 @@ DCB* dcb_accept(DCB* dcb)
             }
 
             if (client_dcb->service->max_connections
-                && client_dcb->service->client_count >= client_dcb->service->max_connections)
+                && client_dcb->service->client_count > client_dcb->service->max_connections)
             {
                 // TODO: If connections can be queued, this is the place to put the
                 // TODO: connection on that queue.

server/core/session.cc

@@ -336,23 +336,6 @@ void session_close(MXS_SESSION* session)
     }
 }
 
-class ServiceDestroyTask : public Worker::DisposableTask
-{
-public:
-    ServiceDestroyTask(Service* service)
-        : m_service(service)
-    {
-    }
-
-    void execute(Worker& worker) override
-    {
-        service_free(m_service);
-    }
-
-private:
-    Service* m_service;
-};
-
 /**
  * Deallocate the specified session
  *
@@ -1253,7 +1236,6 @@ Session::Session(SERVICE* service)
 
     mxb::atomic::add(&service->stats.n_current, 1, mxb::atomic::RELAXED);
     mxb_assert(service->stats.n_current >= 0);
-    mxb::atomic::add(&service->client_count, 1, mxb::atomic::RELAXED);
 }
 
 Session::~Session()
@@ -1271,17 +1253,6 @@ Session::~Session()
 
     mxb::atomic::add(&service->stats.n_current, -1, mxb::atomic::RELAXED);
     mxb_assert(service->stats.n_current >= 0);
-
-    bool should_destroy = !mxb::atomic::load(&service->active);
-
-    if (mxb::atomic::add(&service->client_count, -1) == 1 && should_destroy)
-    {
-        // Destroy the service in the main routing worker thread
-        mxs::RoutingWorker* main_worker = mxs::RoutingWorker::get(mxs::RoutingWorker::MAIN);
-        main_worker->execute(
-            std::unique_ptr<ServiceDestroyTask>(new ServiceDestroyTask(static_cast<Service*>(service))),
-            Worker::EXECUTE_AUTO);
-    }
 }
 
 namespace

server/core/test/test_poll.cc

@@ -53,6 +53,7 @@ static int test1()
 
     SERVICE service;
     service.routerModule = (char*)"required by a check in dcb.cc";
+    dummy.service = &service;
 
     /* Poll tests */
     fprintf(stderr,