Avoid default gateway collisions

Default gateways truncate the endpoint name to 12 characters.  This can
make network endpoints ambiguous especially for load-balancing sandboxes
for networks with lenghty names (such as with our prefixes).  Address
this by detecting an overflow in the sanbox name length and instead
opting to name the gateway endpoint "gateway_<id>" which should never
collide.

Signed-off-by: Chris Telfer <ctelfer@docker.com>

libnetwork/default_gateway.go

@@ -49,9 +49,11 @@ func (sb *sandbox) setupDefaultGW() error {
 
 	createOptions := []EndpointOption{CreateOptionAnonymous()}
 
-	eplen := gwEPlen
-	if len(sb.containerID) < gwEPlen {
-		eplen = len(sb.containerID)
+	var gwName string
+	if len(sb.containerID) <= gwEPlen {
+		gwName = "gateway_" + sb.containerID
+	} else {
+		gwName = "gateway_" + sb.id[:gwEPlen]
 	}
 
 	sbLabels := sb.Labels()
@@ -69,7 +71,7 @@ func (sb *sandbox) setupDefaultGW() error {
 		createOptions = append(createOptions, epOption)
 	}
 
-	newEp, err := n.CreateEndpoint("gateway_"+sb.containerID[0:eplen], createOptions...)
+	newEp, err := n.CreateEndpoint(gwName, createOptions...)
 	if err != nil {
 		return fmt.Errorf("container %s: endpoint create on GW Network failed: %v", sb.containerID, err)
 	}