work around fd re-use bug in vishvananda/netlink

Closing a subscription can cause the associated fd to be re-used while
the subscription goroutine is also still trying to use it. Which can
have disastrous consequences.

So as a work around we don't close subscriptions. This means we leak
an fd, some buffer space, and goroutine. The latter will eventually
block in a channel send of a subscription event, since nothing is left
reading ont the other end. So at least it won't be consuming cycles.

net/if.go

@@ -19,9 +19,10 @@ func EnsureInterface(ifaceName string) (*net.Interface, error) {
 
 func ensureInterface(ifaceName string) (*net.Interface, error) {
 	ch := make(chan netlink.LinkUpdate)
-	done := make(chan struct{})
-	defer close(done)
-	if err := netlink.LinkSubscribe(ch, done); err != nil {
+	// NB: We do not supply (and eventually close) a 'done' channel
+	// here since that can cause incorrect file descriptor
+	// re-usage. See https://github.com/weaveworks/weave/issues/2120
+	if err := netlink.LinkSubscribe(ch, nil); err != nil {
 		return nil, err
 	}
 	// check for currently-existing interface after subscribing, to avoid race
@@ -46,9 +47,7 @@ func EnsureInterfaceAndMcastRoute(ifaceName string) (*net.Interface, error) {
 		return nil, err
 	}
 	ch := make(chan netlink.RouteUpdate)
-	done := make(chan struct{})
-	defer close(done)
-	if err := netlink.RouteSubscribe(ch, done); err != nil {
+	if err := netlink.RouteSubscribe(ch, nil); err != nil {
 		return nil, err
 	}
 	dest := net.IPv4(224, 0, 0, 0)