builder/amazon/chroot: extract the source AMI info

builder/amazon/chroot/builder.go

@@ -21,6 +21,8 @@ const BuilderId = "mitchellh.amazon.chroot"
 type Config struct {
 	common.PackerConfig    `mapstructure:",squash"`
 	awscommon.AccessConfig `mapstructure:",squash"`
+
+	SourceAmi string `mapstructure:"source_ami"`
 }
 
 type Builder struct {
@@ -71,6 +73,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	// Build the steps
 	steps := []multistep.Step{
 		&StepCheckEC2{},
+		&StepSourceAMIInfo{},
 	}
 
 	// Run!

builder/amazon/chroot/step_source_ami_info.go

@@ -0,0 +1,52 @@
+package chroot
+
+import (
+	"fmt"
+	"github.com/mitchellh/goamz/ec2"
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
+)
+
+// StepSourceAMIInfo extracts critical information from the source AMI
+// that is used throughout the AMI creation process.
+//
+// Produces:
+//   source_image *ec2.Image - the source AMI info
+type StepSourceAMIInfo struct{}
+
+func (s *StepSourceAMIInfo) Run(state map[string]interface{}) multistep.StepAction {
+	config := state["config"].(*Config)
+	ec2conn := state["ec2"].(*ec2.EC2)
+	ui := state["ui"].(packer.Ui)
+
+	ui.Say("Inspecting the source AMI...")
+	imageResp, err := ec2conn.Images([]string{config.SourceAmi}, ec2.NewFilter())
+	if err != nil {
+		err := fmt.Errorf("Error querying AMI: %s", err)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	if len(imageResp.Images) == 0 {
+		err := fmt.Errorf("Source AMI '%s' was not found!", config.SourceAmi)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	image := imageResp.Images[0]
+
+	// It must be EBS-backed otherwise the build won't work
+	if image.RootDeviceType != "ebs" {
+		err := fmt.Errorf("The root device of the source AMI must be EBS-backed.")
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	state["source_image"] = image
+	return multistep.ActionContinue
+}
+
+func (s *StepSourceAMIInfo) Cleanup(map[string]interface{}) {}