[lxc] implement interface, add validation

builder/lxc/communicator.go

@@ -104,6 +104,10 @@ func (c *LxcAttachCommunicator) Download(src string, w io.Writer) error {
 	return nil
 }
 
+func (c *LxcAttachCommunicator) DownloadDir(src string, dst string, exclude []string) error {
+        return fmt.Errorf("DownloadDir is not implemented for lxc")
+}
+
 func (c *LxcAttachCommunicator) Execute(commandString string) (*exec.Cmd, error) {
 	log.Printf("Executing with lxc-attach in container: %s %s %s", c.ContainerName, c.RootFs, commandString)
 	command, err := c.CmdWrapper(

builder/lxc/config.go

@@ -8,6 +8,7 @@ import (
 	"github.com/mitchellh/packer/packer"
 	"github.com/mitchellh/packer/template/interpolate"
 	"time"
+	"os"
 )
 
 type Config struct {
@@ -62,6 +63,10 @@ func NewConfig(raws ...interface{}) (*Config, error) {
 		errs = packer.MultiErrorAppend(errs, fmt.Errorf("Failed parsing init_timeout: %s", err))
 	}
 
+	if _, err := os.Stat(c.ConfigFile); os.IsNotExist(err) {
+		errs = packer.MultiErrorAppend(errs, fmt.Errorf("LXC Config file appears to be missing: %s", c.ConfigFile))
+	}
+
 	if errs != nil && len(errs.Errors) > 0 {
 		return nil, errs
 	}