CA-404370: enable NBD client only after completing handshake

Previously the client was being enabled as part of
`tapdisk_nbdserver_newclient_fd_new_fixed` after calling
`tapdisk_nbdserver_new_protocol_handshake`. This erroneously assumed
that protocol handshake was complete by the time that function
returned, which is not the case as it simply adds the fd to an event
for the scheduler. Protocol handshake is only complete when we reach
the end of the `tapdisk_nbdserver_handshake_cb` callback and enabling
the client before this means we have two events registered for the
same fd.

This was causing quicktest to fail after blktap2 was removed and NBD
is used for all control domain access to the I/O datapath.

Signed-off-by: Mark Syms <[email protected]>

drivers/tapdisk-nbdserver.c

@@ -1029,14 +1029,16 @@ void
 tapdisk_nbdserver_handshake_cb(event_id_t id, char mode, void *data)
 {
 	uint32_t cflags = 0;
+	int tmp_fd;
 
 	td_nbdserver_client_t *client = (td_nbdserver_client_t*)data;
 	td_nbdserver_t *server = client->server;
 
 	int rc = recv_fully_or_fail(server->handshake_fd, &cflags, sizeof(cflags));
 	if(rc < 0) {
 		ERR("Could not receive client flags");
-		return;
+		close(server->handshake_fd);
+		goto out;
 	}
 
 	cflags = be32toh (cflags);
@@ -1048,6 +1050,15 @@ tapdisk_nbdserver_handshake_cb(event_id_t id, char mode, void *data)
 		close(server->handshake_fd);
 	}
 
+	INFO("About to enable client on fd %d", client->client_fd);
+	if (tapdisk_nbdserver_enable_client(client) < 0) {
+		ERR("Error enabling client");
+		tmp_fd = client->client_fd;
+		tapdisk_nbdserver_free_client(client);
+		close(tmp_fd);
+	}
+
+out:
 	tapdisk_server_unregister_event(id);
 }
 
@@ -1157,13 +1168,6 @@ tapdisk_nbdserver_newclient_fd_new_fixed(td_nbdserver_t *server, int new_fd)
 		close(new_fd);
 		return;
 	}
-
-	INFO("About to enable client on fd %d", client->client_fd);
-	if (tapdisk_nbdserver_enable_client(client) < 0) {
-		ERR("Error enabling client");
-		tapdisk_nbdserver_free_client(client);
-		close(new_fd);
-	}
 }
 
 static void
@@ -1402,7 +1406,7 @@ tapdisk_nbdserver_newclient_unix(event_id_t id, char mode, void *data)
 		return;
 	}
 
-	INFO("server: got connection\n");
+	INFO("server: got connection fd = %d\n", new_fd);
 
 	tapdisk_nbdserver_newclient_fd_new_fixed(server, new_fd);
 }