Node: Refuse to connect to unsupported Docker engine versions.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

cluster/node.go

@@ -106,6 +106,11 @@ func (n *Node) updateSpecs() error {
 	if err != nil {
 		return err
 	}
+	// Older versions of Docker don't expose the ID field and are not supported
+	// by Swarm.  Catch the error ASAP and refuse to connect.
+	if len(info.ID) == 0 {
+		return fmt.Errorf("Node %s is running an unsupported version of Docker Engine. Please upgrade.", n.Addr)
+	}
 	n.ID = info.ID
 	n.Name = info.Name
 	n.Cpus = info.NCPU
@@ -333,7 +338,7 @@ func (n *Node) Pull(image string) error {
 // Register an event handler.
 func (n *Node) Events(h EventHandler) error {
 	if n.eventHandler != nil {
-		return fmt.Errorf("event handler already set")
+		return errors.New("event handler already set")
 	}
 	n.eventHandler = h
 	return nil

cluster/node_test.go

@@ -12,6 +12,8 @@ import (
 
 var (
 	mockInfo = &dockerclient.Info{
+		ID:              "id",
+		Name:            "name",
 		NCPU:            10,
 		MemTotal:        20,
 		Driver:          "driver-test",
@@ -36,6 +38,17 @@ func TestNodeConnectionFailure(t *testing.T) {
 	client.Mock.AssertExpectations(t)
 }
 
+func TestOutdatedNode(t *testing.T) {
+	node := NewNode("test")
+	client := dockerclient.NewMockClient()
+	client.On("Info").Return(&dockerclient.Info{}, nil)
+
+	assert.Error(t, node.connectClient(client))
+	assert.False(t, node.IsConnected())
+
+	client.Mock.AssertExpectations(t)
+}
+
 func TestNodeSpecs(t *testing.T) {
 	node := NewNode("test")
 	assert.False(t, node.IsConnected())