Monitor and throw error message in the logs if Weave bridge interface

is found to be in DOWN state

Fixes #3133 Do something more useful when the weave bridge is DOWN

On Weave restart there is already logic in place to create the bridge interface
and bring it up, this fix only monotors the weave bridge interface and logs error if
it is in DOWN state

net/bridge.go

@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"net"
 	"path/filepath"
+	"syscall"
 
 	"github.com/coreos/go-iptables/iptables"
 	"github.com/pkg/errors"
@@ -283,6 +284,10 @@ func EnsureBridge(procPath string, config *BridgeConfig, log *logrus.Logger, ips
 		return bridgeType, err
 	}
 
+	if err := monitorInterface(config.WeaveBridgeName, log); err != nil {
+		return bridgeType, err
+	}
+
 	if err := configureARPCache(procPath, config.WeaveBridgeName); err != nil {
 		return bridgeType, errors.Wrapf(err, "configuring ARP cache on bridge %q", config.WeaveBridgeName)
 	}
@@ -609,3 +614,27 @@ func reexpose(config *BridgeConfig, log *logrus.Logger) error {
 
 	return nil
 }
+
+func monitorInterface(ifaceName string, log *logrus.Logger) error {
+	_, err := netlink.LinkByName(ifaceName)
+	if err != nil {
+		return fmt.Errorf("Unable to find link %q: %s", ifaceName, err)
+	}
+
+	updatesChannel := make(chan netlink.LinkUpdate)
+	if err := netlink.LinkSubscribe(updatesChannel, nil); err != nil {
+		return errors.Wrapf(err, "error monitoring link %q for UP/DOWN notifications", ifaceName)
+	}
+
+	go func() {
+		for {
+			select {
+			case update := <-updatesChannel:
+				if update.Link.Attrs().Name == ifaceName && update.IfInfomsg.Flags&syscall.IFF_UP == 0 {
+					log.Errorf("Interface %q which needs to be in UP state for Weave functioning is found to be in DOWN state", ifaceName)
+				}
+			}
+		}
+	}()
+	return nil
+}