builder/amazon/chroot: verify we're on an EC2 instance

builder/amazon/chroot/step_check_ec2.go

 package chroot
 
 import (
+	"fmt"
+	"github.com/mitchellh/goamz/aws"
 	"github.com/mitchellh/multistep"
 	"github.com/mitchellh/packer/packer"
+	"log"
 )
 
+// StepCheckEC2 verifies that this builder is running on an EC2 instance.
 type StepCheckEC2 struct{}
 
 func (s *StepCheckEC2) Run(state map[string]interface{}) multistep.StepAction {
 	ui := state["ui"].(packer.Ui)
+
 	ui.Say("Verifying we're on an EC2 instance...")
+	id, err := aws.GetMetaData("instance-id")
+	if err != nil {
+		log.Printf("Error: %s", err)
+		err := fmt.Errorf(
+			"Error retrieving the ID of the instance Packer is running on.\n" +
+				"Please verify Packer is running on a proper AWS EC2 instance.")
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+	log.Printf("Instance ID: %s", string(id))
+
 	return multistep.ActionContinue
 }