dpif-linux: Fix a bug in creating port.

Based on the policy of 'OVS_VPORT_ATTR_UPCALL_PID', if upcall should
not be sent to userspace (i.e. the number of handler threads is zero),
the netlink message for creating vport should be an one-element array of
value 0.  However, dpif_linux_port_add__() fails to obey it and generates
zero-payload netlink message which causes the netlink transaction failing
with ERANGE error.

This is particularly bad when the 'flow-restore-wait' is set during upgrade,
since number of handler threads is not set in dpif-linux module and ovs is
not able to add port in datapath until the 'flow-restore-wait' is disabled.
Connection may lose due to this bug.

This bug was introduced by commit 1579cf6 (dpif-linux: Implement the
API functions to allow multiple handler threads read upcall.).

This commit fixes the bug by fixing the dpif_linux_port_add__() to generate
the correct netlink message when the number of handler threads is not set.

Bug #1239914.
Bug #1240598.
Bug #1240626.

Reported-by: Gurucharan Shetty <[email protected]>
Signed-off-by: Alex Wang <[email protected]>
Acked-by: Ben Pfaff <[email protected]>

lib/dpif-linux.c

@@ -676,7 +676,7 @@ dpif_linux_port_add__(struct dpif_linux *dpif, struct netdev *netdev,
 
     request.port_no = *port_nop;
     upcall_pids = vport_socksp_to_pids(socksp, dpif->n_handlers);
-    request.n_upcall_pids = dpif->n_handlers;
+    request.n_upcall_pids = socksp ? dpif->n_handlers : 1;
     request.upcall_pids = upcall_pids;
 
     error = dpif_linux_vport_transact(&request, &reply, &buf);