No, it's not a good idea to destroy client in the child nbd process

Should probably have been an obvious side effect :-)

Child process gets open file descriptors to parent's epoll/timerfd,
and it's totally OK to just close() all of them, but it's absolutely NOT
OK to run destructors - they modify the kernel state of epoll/timerfd
before destroying. So, basically, when we destroy the client in the child
process, we break it in the parent too. This also means that cluster_client_t
doesn't support fork(). :-)

src/client/nbd_proxy.cpp

@@ -709,21 +709,13 @@ help:
             ringloop->loop();
             ringloop->wait();
         }
-        destroy_client();
+        cli->flush();
-    }
+        delete cli;
-
+        delete epmgr;
-    void destroy_client()
+        delete ringloop;
-    {
+        cli = NULL;
-        if (cli)
+        epmgr = NULL;
-        {
+        ringloop = NULL;
-            cli->flush();
-            delete cli;
-            delete epmgr;
-            delete ringloop;
-            cli = NULL;
-            epmgr = NULL;
-            ringloop = NULL;
-        }
     }
 
     void load_module()
@@ -911,7 +903,6 @@ protected:
             // works correctly, and we don't accidentally break previous NBD devices while setting
             // up a new device. Forking to check every device is of course rather slow, so we also
             // do an additional check by calling list_mapped() before searching for a free NBD device.
-            destroy_client();
             if (bg)
             {
                 daemonize_fork();