builder/amazon/chroot: find available device

builder/amazon/chroot/builder.go

@@ -24,14 +24,13 @@ type Config struct {
 	common.PackerConfig    `mapstructure:",squash"`
 	awscommon.AccessConfig `mapstructure:",squash"`
 
-	ChrootMounts       [][]string `mapstructure:"chroot_mounts"`
-	CopyFiles          []string   `mapstructure:"copy_files"`
-	DevicePath         string     `mapstructure:"device_path"`
-	DevicePrefix       string     `mapstructure:"device_prefix"`
-	MountCommand       string     `mapstructure:"mount_command"`
-	MountPath          string     `mapstructure:"mount_path"`
-	SourceAmi          string     `mapstructure:"source_ami"`
-	UnmountCommand     string     `mapstructure:"unmount_command"`
+	ChrootMounts   [][]string `mapstructure:"chroot_mounts"`
+	CopyFiles      []string   `mapstructure:"copy_files"`
+	DevicePath     string     `mapstructure:"device_path"`
+	MountCommand   string     `mapstructure:"mount_command"`
+	MountPath      string     `mapstructure:"mount_path"`
+	SourceAmi      string     `mapstructure:"source_ami"`
+	UnmountCommand string     `mapstructure:"unmount_command"`
 }
 
 type Builder struct {
@@ -68,10 +67,6 @@ func (b *Builder) Prepare(raws ...interface{}) error {
 		b.config.CopyFiles = []string{"/etc/resolv.conf"}
 	}
 
-	if b.config.DevicePath == "" {
-		b.config.DevicePath = "/dev/sdh"
-	}
-
 	if b.config.MountCommand == "" {
 		b.config.MountCommand = "mount"
 	}
@@ -128,6 +123,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	steps := []multistep.Step{
 		&StepInstanceInfo{},
 		&StepSourceAMIInfo{},
+		&StepPrepareDevice{},
 		&StepCreateVolume{},
 		&StepAttachVolume{},
 		&StepMountDevice{},

builder/amazon/chroot/device.go

@@ -0,0 +1,61 @@
+package chroot
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// AvailableDevice finds an available device and returns it. Note that
+// you should externally hold a flock or something in order to guarantee
+// that this device is available across processes.
+func AvailableDevice() (string, error) {
+	prefix, err := devicePrefix()
+	if err != nil {
+		return "", err
+	}
+
+	letters := "fghijklmnop"
+	for _, letter := range letters {
+		for i := 1; i < 16; i++ {
+			device := fmt.Sprintf("/dev/%s%c%d", prefix, letter, i)
+			if _, err := os.Stat(device); err != nil {
+				return device, nil
+			}
+		}
+	}
+
+	return "", errors.New("available device could not be found")
+}
+
+// devicePrefix returns the prefix ("sd" or "xvd" or so on) of the devices
+// on the system.
+func devicePrefix() (string, error) {
+	available := []string{"sd", "xvd"}
+
+	f, err := os.Open("/sys/block")
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	dirs, err := f.Readdirnames(-1)
+	if dirs != nil && len(dirs) > 0 {
+		for _, dir := range dirs {
+			dirBase := filepath.Base(dir)
+			for _, prefix := range available {
+				if strings.HasPrefix(dirBase, prefix) {
+					return prefix, nil
+				}
+			}
+		}
+	}
+
+	if err != nil {
+		return "", err
+	}
+
+	return "", errors.New("device prefix could not be detected")
+}

builder/amazon/chroot/step_attach_volume.go

@@ -7,6 +7,7 @@ import (
 	"github.com/mitchellh/multistep"
 	awscommon "github.com/mitchellh/packer/builder/amazon/common"
 	"github.com/mitchellh/packer/packer"
+	"strings"
 )
 
 // StepAttachVolume attaches the previously created volume to an
@@ -21,16 +22,17 @@ type StepAttachVolume struct {
 }
 
 func (s *StepAttachVolume) Run(state map[string]interface{}) multistep.StepAction {
-	config := state["config"].(*Config)
 	ec2conn := state["ec2"].(*ec2.EC2)
+	device := state["device"].(string)
 	instance := state["instance"].(*ec2.Instance)
 	ui := state["ui"].(packer.Ui)
 	volumeId := state["volume_id"].(string)
 
-	device := config.DevicePath
+	// For the API call, it expects "sd" prefixed devices.
+	attachVolume := strings.Replace(device, "/xvd", "/sd", 1)
 
-	ui.Say("Attaching the root volume...")
-	_, err := ec2conn.AttachVolume(volumeId, instance.InstanceId, device)
+	ui.Say(fmt.Sprintf("Attaching the root volume to %s", attachVolume))
+	_, err := ec2conn.AttachVolume(volumeId, instance.InstanceId, attachVolume)
 	if err != nil {
 		err := fmt.Errorf("Error attaching volume: %s", err)
 		state["error"] = err
@@ -70,7 +72,6 @@ func (s *StepAttachVolume) Run(state map[string]interface{}) multistep.StepActio
 		return multistep.ActionHalt
 	}
 
-	state["device"] = device
 	state["attach_cleanup"] = s
 	return multistep.ActionContinue
 }

builder/amazon/chroot/step_prepare_device.go

@@ -0,0 +1,45 @@
+package chroot
+
+import (
+	"fmt"
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
+	"log"
+	"os"
+)
+
+// StepPrepareDevice finds an available device and sets it.
+type StepPrepareDevice struct {
+	mounts []string
+}
+
+func (s *StepPrepareDevice) Run(state map[string]interface{}) multistep.StepAction {
+	config := state["config"].(*Config)
+	ui := state["ui"].(packer.Ui)
+
+	device := config.DevicePath
+	if device == "" {
+		var err error
+		log.Println("Device path not specified, searching for available device...")
+		device, err = AvailableDevice()
+		if err != nil {
+			err := fmt.Errorf("Error finding available device: %s", err)
+			state["error"] = err
+			ui.Error(err.Error())
+			return multistep.ActionHalt
+		}
+	}
+
+	if _, err := os.Stat(device); err == nil {
+		err := fmt.Errorf("Device is in use: %s", device)
+		state["error"] = err
+		ui.Error(err.Error())
+		return multistep.ActionHalt
+	}
+
+	log.Printf("Device: %s", device)
+	state["device"] = device
+	return multistep.ActionContinue
+}
+
+func (s *StepPrepareDevice) Cleanup(state map[string]interface{}) {}