resource_aws_instance.go

package aws

import (
	"bytes"
	"crypto/sha1"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"log"
	"strings"
	"time"

	"github.com/hashicorp/aws-sdk-go/aws"
	"github.com/hashicorp/aws-sdk-go/gen/ec2"
	"github.com/hashicorp/terraform/helper/hashcode"
	"github.com/hashicorp/terraform/helper/resource"
	"github.com/hashicorp/terraform/helper/schema"
)

func resourceAwsInstance() *schema.Resource {
	return &schema.Resource{
		Create: resourceAwsInstanceCreate,
		Read:   resourceAwsInstanceRead,
		Update: resourceAwsInstanceUpdate,
		Delete: resourceAwsInstanceDelete,

		SchemaVersion: 1,
		MigrateState:  resourceAwsInstanceMigrateState,

		Schema: map[string]*schema.Schema{
			"ami": &schema.Schema{
				Type:     schema.TypeString,
				Required: true,
				ForceNew: true,
			},

			"associate_public_ip_address": &schema.Schema{
				Type:     schema.TypeBool,
				Optional: true,
				ForceNew: true,
			},

			"availability_zone": &schema.Schema{
				Type:     schema.TypeString,
				Optional: true,
				Computed: true,
				ForceNew: true,
			},

			"instance_type": &schema.Schema{
				Type:     schema.TypeString,
				Required: true,
				ForceNew: true,
			},

			"key_name": &schema.Schema{
				Type:     schema.TypeString,
				Optional: true,
				ForceNew: true,
				Computed: true,
			},

			"subnet_id": &schema.Schema{
				Type:     schema.TypeString,
				Optional: true,
				Computed: true,
				ForceNew: true,
			},

			"private_ip": &schema.Schema{
				Type:     schema.TypeString,
				Optional: true,
				ForceNew: true,
				Computed: true,
			},

			"source_dest_check": &schema.Schema{
				Type:     schema.TypeBool,
				Optional: true,
			},

			"user_data": &schema.Schema{
				Type:     schema.TypeString,
				Optional: true,
				ForceNew: true,
				StateFunc: func(v interface{}) string {
					switch v.(type) {
					case string:
						hash := sha1.Sum([]byte(v.(string)))
						return hex.EncodeToString(hash[:])
					default:
						return ""
					}
				},
			},

			"security_groups": &schema.Schema{
				Type:     schema.TypeSet,
				Optional: true,
				Computed: true,
				ForceNew: true,
				Elem:     &schema.Schema{Type: schema.TypeString},
				Set: func(v interface{}) int {
					return hashcode.String(v.(string))
				},
			},

			"public_dns": &schema.Schema{
				Type:     schema.TypeString,
				Computed: true,
			},

			"public_ip": &schema.Schema{
				Type:     schema.TypeString,
				Computed: true,
			},

			"private_dns": &schema.Schema{
				Type:     schema.TypeString,
				Computed: true,
			},

			"ebs_optimized": &schema.Schema{
				Type:     schema.TypeBool,
				Optional: true,
			},

			"iam_instance_profile": &schema.Schema{
				Type:     schema.TypeString,
				ForceNew: true,
				Optional: true,
			},

			"tenancy": &schema.Schema{
				Type:     schema.TypeString,
				Optional: true,
				Computed: true,
				ForceNew: true,
			},

			"tags": tagsSchema(),

			"block_device": &schema.Schema{
				Type:     schema.TypeMap,
				Optional: true,
				Removed:  "Split out into three sub-types; see Changelog and Docs",
			},

			"ebs_block_device": &schema.Schema{
				Type:     schema.TypeSet,
				Optional: true,
				Computed: true,
				Elem: &schema.Resource{
					Schema: map[string]*schema.Schema{
						"delete_on_termination": &schema.Schema{
							Type:     schema.TypeBool,
							Optional: true,
							Default:  true,
							ForceNew: true,
						},

						"device_name": &schema.Schema{
							Type:     schema.TypeString,
							Required: true,
							ForceNew: true,
						},

						"encrypted": &schema.Schema{
							Type:     schema.TypeBool,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},

						"iops": &schema.Schema{
							Type:     schema.TypeInt,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},

						"snapshot_id": &schema.Schema{
							Type:     schema.TypeString,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},

						"volume_size": &schema.Schema{
							Type:     schema.TypeInt,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},

						"volume_type": &schema.Schema{
							Type:     schema.TypeString,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},
					},
				},
				Set: func(v interface{}) int {
					var buf bytes.Buffer
					m := v.(map[string]interface{})
					buf.WriteString(fmt.Sprintf("%t-", m["delete_on_termination"].(bool)))
					buf.WriteString(fmt.Sprintf("%s-", m["device_name"].(string)))
					buf.WriteString(fmt.Sprintf("%t-", m["encrypted"].(bool)))
					// NOTE: Not considering IOPS in hash; when using gp2, IOPS can come
					// back set to something like "33", which throws off the set
					// calculation and generates an unresolvable diff.
					// buf.WriteString(fmt.Sprintf("%d-", m["iops"].(int)))
					buf.WriteString(fmt.Sprintf("%s-", m["snapshot_id"].(string)))
					buf.WriteString(fmt.Sprintf("%d-", m["volume_size"].(int)))
					buf.WriteString(fmt.Sprintf("%s-", m["volume_type"].(string)))
					return hashcode.String(buf.String())
				},
			},

			"ephemeral_block_device": &schema.Schema{
				Type:     schema.TypeSet,
				Optional: true,
				Computed: true,
				ForceNew: true,
				Elem: &schema.Resource{
					Schema: map[string]*schema.Schema{
						"device_name": &schema.Schema{
							Type:     schema.TypeString,
							Required: true,
						},

						"virtual_name": &schema.Schema{
							Type:     schema.TypeString,
							Required: true,
						},
					},
				},
				Set: func(v interface{}) int {
					var buf bytes.Buffer
					m := v.(map[string]interface{})
					buf.WriteString(fmt.Sprintf("%s-", m["device_name"].(string)))
					buf.WriteString(fmt.Sprintf("%s-", m["virtual_name"].(string)))
					return hashcode.String(buf.String())
				},
			},

			"root_block_device": &schema.Schema{
				// TODO: This is a set because we don't support singleton
				//       sub-resources today. We'll enforce that the set only ever has
				//       length zero or one below. When TF gains support for
				//       sub-resources this can be converted.
				Type:     schema.TypeSet,
				Optional: true,
				Computed: true,
				Elem: &schema.Resource{
					// "You can only modify the volume size, volume type, and Delete on
					// Termination flag on the block device mapping entry for the root
					// device volume." - bit.ly/ec2bdmap
					Schema: map[string]*schema.Schema{
						"delete_on_termination": &schema.Schema{
							Type:     schema.TypeBool,
							Optional: true,
							Default:  true,
							ForceNew: true,
						},

						"iops": &schema.Schema{
							Type:     schema.TypeInt,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},

						"volume_size": &schema.Schema{
							Type:     schema.TypeInt,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},

						"volume_type": &schema.Schema{
							Type:     schema.TypeString,
							Optional: true,
							Computed: true,
							ForceNew: true,
						},
					},
				},
				Set: func(v interface{}) int {
					var buf bytes.Buffer
					m := v.(map[string]interface{})
					buf.WriteString(fmt.Sprintf("%t-", m["delete_on_termination"].(bool)))
					// See the NOTE in "ebs_block_device" for why we skip iops here.
					// buf.WriteString(fmt.Sprintf("%d-", m["iops"].(int)))
					buf.WriteString(fmt.Sprintf("%d-", m["volume_size"].(int)))
					buf.WriteString(fmt.Sprintf("%s-", m["volume_type"].(string)))
					return hashcode.String(buf.String())
				},
			},
		},
	}
}

func resourceAwsInstanceCreate(d *schema.ResourceData, meta interface{}) error {
	ec2conn := meta.(*AWSClient).ec2conn

	// Figure out user data
	userData := ""
	if v := d.Get("user_data"); v != nil {
		userData = base64.StdEncoding.EncodeToString([]byte(v.(string)))
	}

	// check for non-default Subnet, and cast it to a String
	var hasSubnet bool
	subnet, hasSubnet := d.GetOk("subnet_id")
	subnetID := subnet.(string)

	placement := &ec2.Placement{
		AvailabilityZone: aws.String(d.Get("availability_zone").(string)),
	}

	if hasSubnet {
		// Tenancy is only valid inside a VPC
		// See http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_Placement.html
		if v := d.Get("tenancy").(string); v != "" {
			placement.Tenancy = aws.String(v)
		}
	}

	iam := &ec2.IAMInstanceProfileSpecification{
		Name: aws.String(d.Get("iam_instance_profile").(string)),
	}

	// Build the creation struct
	runOpts := &ec2.RunInstancesRequest{
		ImageID:            aws.String(d.Get("ami").(string)),
		Placement:          placement,
		InstanceType:       aws.String(d.Get("instance_type").(string)),
		MaxCount:           aws.Integer(1),
		MinCount:           aws.Integer(1),
		UserData:           aws.String(userData),
		EBSOptimized:       aws.Boolean(d.Get("ebs_optimized").(bool)),
		IAMInstanceProfile: iam,
	}

	associatePublicIPAddress := false
	if v := d.Get("associate_public_ip_address"); v != nil {
		associatePublicIPAddress = v.(bool)
	}

	var groups []string
	if v := d.Get("security_groups"); v != nil {
		// Security group names.
		// For a nondefault VPC, you must use security group IDs instead.
		// See http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_RunInstances.html
		for _, v := range v.(*schema.Set).List() {
			str := v.(string)
			groups = append(groups, str)
		}
	}

	if hasSubnet && associatePublicIPAddress {
		// If we have a non-default VPC / Subnet specified, we can flag
		// AssociatePublicIpAddress to get a Public IP assigned. By default these are not provided.
		// You cannot specify both SubnetId and the NetworkInterface.0.* parameters though, otherwise
		// you get: Network interfaces and an instance-level subnet ID may not be specified on the same request
		// You also need to attach Security Groups to the NetworkInterface instead of the instance,
		// to avoid: Network interfaces and an instance-level security groups may not be specified on
		// the same request
		ni := ec2.InstanceNetworkInterfaceSpecification{
			AssociatePublicIPAddress: aws.Boolean(associatePublicIPAddress),
			DeviceIndex:              aws.Integer(0),
			SubnetID:                 aws.String(subnetID),
		}

		if v, ok := d.GetOk("private_ip"); ok {
			ni.PrivateIPAddress = aws.String(v.(string))
		}

		if len(groups) > 0 {
			ni.Groups = groups
		}

		runOpts.NetworkInterfaces = []ec2.InstanceNetworkInterfaceSpecification{ni}
	} else {
		if subnetID != "" {
			runOpts.SubnetID = aws.String(subnetID)
		}

		if v, ok := d.GetOk("private_ip"); ok {
			runOpts.PrivateIPAddress = aws.String(v.(string))
		}
		if runOpts.SubnetID != nil &&
			*runOpts.SubnetID != "" {
			runOpts.SecurityGroupIDs = groups
		} else {
			runOpts.SecurityGroups = groups
		}
	}

	if v, ok := d.GetOk("key_name"); ok {
		runOpts.KeyName = aws.String(v.(string))
	}

	blockDevices := make([]ec2.BlockDeviceMapping, 0)

	if v, ok := d.GetOk("ebs_block_device"); ok {
		vL := v.(*schema.Set).List()
		for _, v := range vL {
			bd := v.(map[string]interface{})
			ebs := &ec2.EBSBlockDevice{
				DeleteOnTermination: aws.Boolean(bd["delete_on_termination"].(bool)),
			}

			if v, ok := bd["snapshot_id"].(string); ok && v != "" {
				ebs.SnapshotID = aws.String(v)
			}

			if v, ok := bd["volume_size"].(int); ok && v != 0 {
				ebs.VolumeSize = aws.Integer(v)
			}

			if v, ok := bd["volume_type"].(string); ok && v != "" {
				ebs.VolumeType = aws.String(v)
			}

			if v, ok := bd["iops"].(int); ok && v > 0 {
				ebs.IOPS = aws.Integer(v)
			}

			blockDevices = append(blockDevices, ec2.BlockDeviceMapping{
				DeviceName: aws.String(bd["device_name"].(string)),
				EBS:        ebs,
			})
		}
	}

	if v, ok := d.GetOk("ephemeral_block_device"); ok {
		vL := v.(*schema.Set).List()
		for _, v := range vL {
			bd := v.(map[string]interface{})
			blockDevices = append(blockDevices, ec2.BlockDeviceMapping{
				DeviceName:  aws.String(bd["device_name"].(string)),
				VirtualName: aws.String(bd["virtual_name"].(string)),
			})
		}
	}

	if v, ok := d.GetOk("root_block_device"); ok {
		vL := v.(*schema.Set).List()
		if len(vL) > 1 {
			return fmt.Errorf("Cannot specify more than one root_block_device.")
		}
		for _, v := range vL {
			bd := v.(map[string]interface{})
			ebs := &ec2.EBSBlockDevice{
				DeleteOnTermination: aws.Boolean(bd["delete_on_termination"].(bool)),
			}

			if v, ok := bd["volume_size"].(int); ok && v != 0 {
				ebs.VolumeSize = aws.Integer(v)
			}

			if v, ok := bd["volume_type"].(string); ok && v != "" {
				ebs.VolumeType = aws.String(v)
			}

			if v, ok := bd["iops"].(int); ok && v > 0 {
				ebs.IOPS = aws.Integer(v)
			}

			if dn, err := fetchRootDeviceName(d.Get("ami").(string), ec2conn); err == nil {
				blockDevices = append(blockDevices, ec2.BlockDeviceMapping{
					DeviceName: dn,
					EBS:        ebs,
				})
			} else {
				return err
			}
		}
	}

	if len(blockDevices) > 0 {
		runOpts.BlockDeviceMappings = blockDevices
	}

	// Create the instance
	log.Printf("[DEBUG] Run configuration: %#v", runOpts)
	runResp, err := ec2conn.RunInstances(runOpts)
	if err != nil {
		return fmt.Errorf("Error launching source instance: %s", err)
	}

	instance := &runResp.Instances[0]
	log.Printf("[INFO] Instance ID: %s", *instance.InstanceID)

	// Store the resulting ID so we can look this up later
	d.SetId(*instance.InstanceID)

	// Wait for the instance to become running so we can get some attributes
	// that aren't available until later.
	log.Printf(
		"[DEBUG] Waiting for instance (%s) to become running",
		*instance.InstanceID)

	stateConf := &resource.StateChangeConf{
		Pending:    []string{"pending"},
		Target:     "running",
		Refresh:    InstanceStateRefreshFunc(ec2conn, *instance.InstanceID),
		Timeout:    10 * time.Minute,
		Delay:      10 * time.Second,
		MinTimeout: 3 * time.Second,
	}

	instanceRaw, err := stateConf.WaitForState()
	if err != nil {
		return fmt.Errorf(
			"Error waiting for instance (%s) to become ready: %s",
			*instance.InstanceID, err)
	}

	instance = instanceRaw.(*ec2.Instance)

	// Initialize the connection info
	if instance.PublicIPAddress != nil {
		d.SetConnInfo(map[string]string{
			"type": "ssh",
			"host": *instance.PublicIPAddress,
		})
	}

	// Set our attributes
	if err := resourceAwsInstanceRead(d, meta); err != nil {
		return err
	}

	// Update if we need to
	return resourceAwsInstanceUpdate(d, meta)
}

func resourceAwsInstanceRead(d *schema.ResourceData, meta interface{}) error {
	ec2conn := meta.(*AWSClient).ec2conn

	resp, err := ec2conn.DescribeInstances(&ec2.DescribeInstancesRequest{
		InstanceIDs: []string{d.Id()},
	})
	if err != nil {
		// If the instance was not found, return nil so that we can show
		// that the instance is gone.
		if ec2err, ok := err.(aws.APIError); ok && ec2err.Code == "InvalidInstanceID.NotFound" {
			d.SetId("")
			return nil
		}

		// Some other error, report it
		return err
	}

	// If nothing was found, then return no state
	if len(resp.Reservations) == 0 {
		d.SetId("")
		return nil
	}

	instance := &resp.Reservations[0].Instances[0]

	// If the instance is terminated, then it is gone
	if *instance.State.Name == "terminated" {
		d.SetId("")
		return nil
	}

	if instance.Placement != nil {
		d.Set("availability_zone", instance.Placement.AvailabilityZone)
	}
	if instance.Placement.Tenancy != nil {
		d.Set("tenancy", instance.Placement.Tenancy)
	}

	d.Set("key_name", instance.KeyName)
	d.Set("public_dns", instance.PublicDNSName)
	d.Set("public_ip", instance.PublicIPAddress)
	d.Set("private_dns", instance.PrivateDNSName)
	d.Set("private_ip", instance.PrivateIPAddress)
	if len(instance.NetworkInterfaces) > 0 {
		d.Set("subnet_id", instance.NetworkInterfaces[0].SubnetID)
	} else {
		d.Set("subnet_id", instance.SubnetID)
	}
	d.Set("ebs_optimized", instance.EBSOptimized)
	d.Set("tags", tagsToMap(instance.Tags))

	// Determine whether we're referring to security groups with
	// IDs or names. We use a heuristic to figure this out. By default,
	// we use IDs if we're in a VPC. However, if we previously had an
	// all-name list of security groups, we use names. Or, if we had any
	// IDs, we use IDs.
	useID := instance.SubnetID != nil && *instance.SubnetID != ""
	if v := d.Get("security_groups"); v != nil {
		match := false
		for _, v := range v.(*schema.Set).List() {
			if strings.HasPrefix(v.(string), "sg-") {
				match = true
				break
			}
		}

		useID = match
	}

	// Build up the security groups
	sgs := make([]string, len(instance.SecurityGroups))
	for i, sg := range instance.SecurityGroups {
		if useID {
			sgs[i] = *sg.GroupID
		} else {
			sgs[i] = *sg.GroupName
		}
	}
	d.Set("security_groups", sgs)

	if err := readBlockDevices(d, instance, ec2conn); err != nil {
		return err
	}

	return nil
}

func resourceAwsInstanceUpdate(d *schema.ResourceData, meta interface{}) error {
	ec2conn := meta.(*AWSClient).ec2conn

	// SourceDestCheck can only be set on VPC instances
	if d.Get("subnet_id").(string) != "" {
		log.Printf("[INFO] Modifying instance %s", d.Id())
		err := ec2conn.ModifyInstanceAttribute(&ec2.ModifyInstanceAttributeRequest{
			InstanceID: aws.String(d.Id()),
			SourceDestCheck: &ec2.AttributeBooleanValue{
				Value: aws.Boolean(d.Get("source_dest_check").(bool)),
			},
		})
		if err != nil {
			return err
		}
	}

	// TODO(mitchellh): wait for the attributes we modified to
	// persist the change...

	if err := setTags(ec2conn, d); err != nil {
		return err
	} else {
		d.SetPartial("tags")
	}

	return nil
}

func resourceAwsInstanceDelete(d *schema.ResourceData, meta interface{}) error {
	ec2conn := meta.(*AWSClient).ec2conn

	log.Printf("[INFO] Terminating instance: %s", d.Id())
	req := &ec2.TerminateInstancesRequest{
		InstanceIDs: []string{d.Id()},
	}
	if _, err := ec2conn.TerminateInstances(req); err != nil {
		return fmt.Errorf("Error terminating instance: %s", err)
	}

	log.Printf(
		"[DEBUG] Waiting for instance (%s) to become terminated",
		d.Id())

	stateConf := &resource.StateChangeConf{
		Pending:    []string{"pending", "running", "shutting-down", "stopped", "stopping"},
		Target:     "terminated",
		Refresh:    InstanceStateRefreshFunc(ec2conn, d.Id()),
		Timeout:    10 * time.Minute,
		Delay:      10 * time.Second,
		MinTimeout: 3 * time.Second,
	}

	_, err := stateConf.WaitForState()
	if err != nil {
		return fmt.Errorf(
			"Error waiting for instance (%s) to terminate: %s",
			d.Id(), err)
	}

	d.SetId("")
	return nil
}

// InstanceStateRefreshFunc returns a resource.StateRefreshFunc that is used to watch
// an EC2 instance.
func InstanceStateRefreshFunc(conn *ec2.EC2, instanceID string) resource.StateRefreshFunc {
	return func() (interface{}, string, error) {
		resp, err := conn.DescribeInstances(&ec2.DescribeInstancesRequest{
			InstanceIDs: []string{instanceID},
		})
		if err != nil {
			if ec2err, ok := err.(aws.APIError); ok && ec2err.Code == "InvalidInstanceID.NotFound" {
				// Set this to nil as if we didn't find anything.
				resp = nil
			} else {
				log.Printf("Error on InstanceStateRefresh: %s", err)
				return nil, "", err
			}
		}

		if resp == nil || len(resp.Reservations) == 0 || len(resp.Reservations[0].Instances) == 0 {
			// Sometimes AWS just has consistency issues and doesn't see
			// our instance yet. Return an empty state.
			return nil, "", nil
		}

		i := &resp.Reservations[0].Instances[0]
		return i, *i.State.Name, nil
	}
}

func readBlockDevices(d *schema.ResourceData, instance *ec2.Instance, ec2conn *ec2.EC2) error {
	ibds, err := readBlockDevicesFromInstance(instance, ec2conn)
	if err != nil {
		return err
	}

	if err := d.Set("ebs_block_device", ibds["ebs"]); err != nil {
		return err
	}
	if ibds["root"] != nil {
		if err := d.Set("root_block_device", []interface{}{ibds["root"]}); err != nil {
			return err
		}
	}

	return nil
}

func readBlockDevicesFromInstance(instance *ec2.Instance, ec2conn *ec2.EC2) (map[string]interface{}, error) {
	blockDevices := make(map[string]interface{})
	blockDevices["ebs"] = make([]map[string]interface{}, 0)
	blockDevices["root"] = nil

	instanceBlockDevices := make(map[string]ec2.InstanceBlockDeviceMapping)
	for _, bd := range instance.BlockDeviceMappings {
		if bd.EBS != nil {
			instanceBlockDevices[*(bd.EBS.VolumeID)] = bd
		}
	}

	if len(instanceBlockDevices) == 0 {
		return nil, nil
	}

	volIDs := make([]string, 0, len(instanceBlockDevices))
	for volID := range instanceBlockDevices {
		volIDs = append(volIDs, volID)
	}

	// Need to call DescribeVolumes to get volume_size and volume_type for each
	// EBS block device
	volResp, err := ec2conn.DescribeVolumes(&ec2.DescribeVolumesRequest{
		VolumeIDs: volIDs,
	})
	if err != nil {
		return nil, err
	}

	for _, vol := range volResp.Volumes {
		instanceBd := instanceBlockDevices[*vol.VolumeID]
		bd := make(map[string]interface{})

		if instanceBd.EBS != nil && instanceBd.EBS.DeleteOnTermination != nil {
			bd["delete_on_termination"] = *instanceBd.EBS.DeleteOnTermination
		}
		if vol.Size != nil {
			bd["volume_size"] = *vol.Size
		}
		if vol.VolumeType != nil {
			bd["volume_type"] = *vol.VolumeType
		}
		if vol.IOPS != nil {
			bd["iops"] = *vol.IOPS
		}

		if blockDeviceIsRoot(instanceBd, instance) {
			blockDevices["root"] = bd
		} else {
			if instanceBd.DeviceName != nil {
				bd["device_name"] = *instanceBd.DeviceName
			}
			if vol.Encrypted != nil {
				bd["encrypted"] = *vol.Encrypted
			}
			if vol.SnapshotID != nil {
				bd["snapshot_id"] = *vol.SnapshotID
			}

			blockDevices["ebs"] = append(blockDevices["ebs"].([]map[string]interface{}), bd)
		}
	}

	return blockDevices, nil
}

func blockDeviceIsRoot(bd ec2.InstanceBlockDeviceMapping, instance *ec2.Instance) bool {
	return (bd.DeviceName != nil &&
		instance.RootDeviceName != nil &&
		*bd.DeviceName == *instance.RootDeviceName)
}

func fetchRootDeviceName(ami string, conn *ec2.EC2) (aws.StringValue, error) {
	if ami == "" {
		return nil, fmt.Errorf("Cannot fetch root device name for blank AMI ID.")
	}

	log.Printf("[DEBUG] Describing AMI %q to get root block device name", ami)
	req := &ec2.DescribeImagesRequest{ImageIDs: []string{ami}}
	if res, err := conn.DescribeImages(req); err == nil {
		if len(res.Images) == 1 {
			return res.Images[0].RootDeviceName, nil
		} else {
			return nil, fmt.Errorf("Expected 1 AMI for ID: %s, got: %#v", ami, res.Images)
		}
	} else {
		return nil, err
	}
}