Add ReceiveBufferSize to LinkSubscribeOptions

Sometimes with `ListExisting` enabled and enough interfaces configured,
netlink will try to send more data than the buffer can hold and the
caller will get back ENOBUFS. Thus, it's useful to be able to configure
the netlink buffer size.

link_linux.go

@@ -2276,21 +2276,22 @@ type LinkUpdate struct {
 // LinkSubscribe takes a chan down which notifications will be sent
 // when links change.  Close the 'done' chan to stop subscription.
 func LinkSubscribe(ch chan<- LinkUpdate, done <-chan struct{}) error {
-	return linkSubscribeAt(netns.None(), netns.None(), ch, done, nil, false)
+	return linkSubscribeAt(netns.None(), netns.None(), ch, done, nil, false, 0)
 }
 
 // LinkSubscribeAt works like LinkSubscribe plus it allows the caller
 // to choose the network namespace in which to subscribe (ns).
 func LinkSubscribeAt(ns netns.NsHandle, ch chan<- LinkUpdate, done <-chan struct{}) error {
-	return linkSubscribeAt(ns, netns.None(), ch, done, nil, false)
+	return linkSubscribeAt(ns, netns.None(), ch, done, nil, false, 0)
 }
 
 // LinkSubscribeOptions contains a set of options to use with
 // LinkSubscribeWithOptions.
 type LinkSubscribeOptions struct {
-	Namespace     *netns.NsHandle
-	ErrorCallback func(error)
-	ListExisting  bool
+	Namespace         *netns.NsHandle
+	ErrorCallback     func(error)
+	ListExisting      bool
+	ReceiveBufferSize int
 }
 
 // LinkSubscribeWithOptions work like LinkSubscribe but enable to
@@ -2301,10 +2302,10 @@ func LinkSubscribeWithOptions(ch chan<- LinkUpdate, done <-chan struct{}, option
 		none := netns.None()
 		options.Namespace = &none
 	}
-	return linkSubscribeAt(*options.Namespace, netns.None(), ch, done, options.ErrorCallback, options.ListExisting)
+	return linkSubscribeAt(*options.Namespace, netns.None(), ch, done, options.ErrorCallback, options.ListExisting, options.ReceiveBufferSize)
 }
 
-func linkSubscribeAt(newNs, curNs netns.NsHandle, ch chan<- LinkUpdate, done <-chan struct{}, cberr func(error), listExisting bool) error {
+func linkSubscribeAt(newNs, curNs netns.NsHandle, ch chan<- LinkUpdate, done <-chan struct{}, cberr func(error), listExisting bool, rcvbuf int) error {
 	s, err := nl.SubscribeAt(newNs, curNs, unix.NETLINK_ROUTE, unix.RTNLGRP_LINK)
 	if err != nil {
 		return err
@@ -2324,6 +2325,12 @@ func linkSubscribeAt(newNs, curNs netns.NsHandle, ch chan<- LinkUpdate, done <-c
 			return err
 		}
 	}
+	if rcvbuf != 0 {
+		err = pkgHandle.SetSocketReceiveBufferSize(rcvbuf, false)
+		if err != nil {
+			return err
+		}
+	}
 	go func() {
 		defer close(ch)
 		for {