aws_ami: Add root_snapshot_id attribute (#1572)

Current limitations in HCL prevent it from being possible to reference
an AMI's root snapshot ID via its ID. This is because the snapshot ID is
deeply nested in the `block_device_mappings` set. HCL is not
yet flexible enough to allow a lookup expression to extract the snapshot
ID embedded within that structure.

This makes it impossible to use resources like
`aws_snapshot_create_volume_permission` without manually doing the AMI
ID -> Snapshot ID translation outside of Terraform.

Here, we promote the Snapshot ID of the root volume to a top-level
`root_snapshot_id` attribute for AMI data sources and resources.

This field will be blank for instance store AMIs or for weird conditions
where there's no snapshot ID recorded (See commit message here:
bed5f62), but it enables usage of the
resource referenced above for the majority of scenarios.

aws/data_source_aws_ami.go

@@ -106,6 +106,10 @@ func dataSourceAwsAmi() *schema.Resource {
 				Type:     schema.TypeString,
 				Computed: true,
 			},
+			"root_snapshot_id": {
+				Type:     schema.TypeString,
+				Computed: true,
+			},
 			"sriov_net_support": {
 				Type:     schema.TypeString,
 				Computed: true,
@@ -284,6 +288,7 @@ func amiDescriptionAttributes(d *schema.ResourceData, image *ec2.Image) error {
 		d.Set("root_device_name", image.RootDeviceName)
 	}
 	d.Set("root_device_type", image.RootDeviceType)
+	d.Set("root_snapshot_id", amiRootSnapshotId(image))
 	if image.SriovNetSupport != nil {
 		d.Set("sriov_net_support", image.SriovNetSupport)
 	}
@@ -358,6 +363,22 @@ func amiProductCodes(m []*ec2.ProductCode) *schema.Set {
 	return s
 }
 
+// Returns the root snapshot ID for an image, if it has one
+func amiRootSnapshotId(image *ec2.Image) string {
+	if image.RootDeviceName == nil {
+		return ""
+	}
+	for _, bdm := range image.BlockDeviceMappings {
+		if bdm.DeviceName == nil || *bdm.DeviceName != *image.RootDeviceName {
+			continue
+		}
+		if bdm.Ebs != nil && bdm.Ebs.SnapshotId != nil {
+			return *bdm.Ebs.SnapshotId
+		}
+	}
+	return ""
+}
+
 // Returns the state reason.
 func amiStateReason(m *ec2.StateReason) map[string]interface{} {
 	s := make(map[string]interface{})

aws/data_source_aws_ami_test.go

@@ -41,6 +41,7 @@ func TestAccAWSAmiDataSource_natInstance(t *testing.T) {
 					resource.TestCheckResourceAttr("data.aws_ami.nat_ami", "product_codes.#", "0"),
 					resource.TestCheckResourceAttr("data.aws_ami.nat_ami", "root_device_name", "/dev/xvda"),
 					resource.TestCheckResourceAttr("data.aws_ami.nat_ami", "root_device_type", "ebs"),
+					resource.TestMatchResourceAttr("data.aws_ami.nat_ami", "root_snapshot_id", regexp.MustCompile("^snap-")),
 					resource.TestCheckResourceAttr("data.aws_ami.nat_ami", "sriov_net_support", "simple"),
 					resource.TestCheckResourceAttr("data.aws_ami.nat_ami", "state", "available"),
 					resource.TestCheckResourceAttr("data.aws_ami.nat_ami", "state_reason.code", "UNSET"),
@@ -78,6 +79,7 @@ func TestAccAWSAmiDataSource_windowsInstance(t *testing.T) {
 					resource.TestCheckResourceAttr("data.aws_ami.windows_ami", "product_codes.#", "0"),
 					resource.TestCheckResourceAttr("data.aws_ami.windows_ami", "root_device_name", "/dev/sda1"),
 					resource.TestCheckResourceAttr("data.aws_ami.windows_ami", "root_device_type", "ebs"),
+					resource.TestMatchResourceAttr("data.aws_ami.windows_ami", "root_snapshot_id", regexp.MustCompile("^snap-")),
 					resource.TestCheckResourceAttr("data.aws_ami.windows_ami", "sriov_net_support", "simple"),
 					resource.TestCheckResourceAttr("data.aws_ami.windows_ami", "state", "available"),
 					resource.TestCheckResourceAttr("data.aws_ami.windows_ami", "state_reason.code", "UNSET"),
@@ -112,6 +114,7 @@ func TestAccAWSAmiDataSource_instanceStore(t *testing.T) {
 					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "public", "true"),
 					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "product_codes.#", "0"),
 					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "root_device_type", "instance-store"),
+					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "root_snapshot_id", ""),
 					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "sriov_net_support", "simple"),
 					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "state", "available"),
 					resource.TestCheckResourceAttr("data.aws_ami.instance_store_ami", "state_reason.code", "UNSET"),

aws/resource_aws_ami.go

@@ -193,6 +193,7 @@ func resourceAwsAmiRead(d *schema.ResourceData, meta interface{}) error {
 	d.Set("kernel_id", image.KernelId)
 	d.Set("ramdisk_id", image.RamdiskId)
 	d.Set("root_device_name", image.RootDeviceName)
+	d.Set("root_snapshot_id", amiRootSnapshotId(image))
 	d.Set("sriov_net_support", image.SriovNetSupport)
 	d.Set("virtualization_type", image.VirtualizationType)
 
@@ -441,6 +442,10 @@ func resourceAwsAmiCommonSchema(computed bool) map[string]*schema.Schema {
 			Computed: computed,
 			ForceNew: !computed,
 		},
+		"root_snapshot_id": {
+			Type:     schema.TypeString,
+			Computed: true,
+		},
 		"sriov_net_support": {
 			Type:     schema.TypeString,
 			Optional: !computed,

aws/resource_aws_ami_test.go

@@ -3,6 +3,7 @@ package aws
 import (
 	"fmt"
 	"log"
+	"regexp"
 	"testing"
 	"time"
 
@@ -29,6 +30,8 @@ func TestAccAWSAMI_basic(t *testing.T) {
 					testAccCheckAmiExists("aws_ami.foo", &ami),
 					resource.TestCheckResourceAttr(
 						"aws_ami.foo", "name", fmt.Sprintf("tf-testing-%d", rInt)),
+					resource.TestMatchResourceAttr(
+						"aws_ami.foo", "root_snapshot_id", regexp.MustCompile("^snap-")),
 				),
 			},
 		},
@@ -202,12 +205,14 @@ func testAccCheckAmiEbsBlockDevice(bd *ec2.BlockDeviceMapping, ed *ec2.EbsBlockD
 
 func testAccAmiConfig_basic(rInt int) string {
 	return fmt.Sprintf(`
+data "aws_availability_zones" "available" {}
+
 resource "aws_ebs_volume" "foo" {
- 	availability_zone = "us-west-2a"
- 	size = 8
- 	tags {
- 	  Name = "testAccAmiConfig_basic"
- 	}
+  availability_zone = "${data.aws_availability_zones.available.names[0]}"
+  size = 8
+  tags {
+    Name = "testAccAmiConfig_basic"
+  }
 }
 
 resource "aws_ebs_snapshot" "foo" {
@@ -232,12 +237,14 @@ resource "aws_ami" "foo" {
 
 func testAccAmiConfig_snapshotSize(rInt int) string {
 	return fmt.Sprintf(`
+data "aws_availability_zones" "available" {}
+
 resource "aws_ebs_volume" "foo" {
- 	availability_zone = "us-west-2a"
- 	size = 20
- 	tags {
- 	  Name = "testAccAmiConfig_snapshotSize"
- 	}
+  availability_zone = "${data.aws_availability_zones.available.names[0]}"
+  size = 20
+  tags {
+    Name = "testAccAmiConfig_snapshotSize"
+  }
 }
 
 resource "aws_ebs_snapshot" "foo" {

website/docs/d/ami.html.markdown

@@ -108,6 +108,8 @@ interpolation.
   for machine images.
 * `root_device_name` - The device name of the root device.
 * `root_device_type` - The type of root device (ie: `ebs` or `instance-store`).
+* `root_snapshot_id` - The snapshot id associated with the root device, if any
+  (only applies to `ebs` root devices).
 * `sriov_net_support` - Specifies whether enhanced networking is enabled.
 * `state` - The current state of the AMI. If the state is `available`, the image
   is successfully registered and can be used to launch an instance.

website/docs/r/ami.html.markdown

@@ -42,6 +42,7 @@ The following arguments are supported:
 
 * `name` - (Required) A region-unique name for the AMI.
 * `description` - (Optional) A longer, human-readable description for the AMI.
+* `root_device_name` - (Optional) The name of the root device (for example, `/dev/sda1`, or `/dev/xvda`).
 * `virtualization_type` - (Optional) Keyword to choose what virtualization mode created instances
   will use. Can be either "paravirtual" (the default) or "hvm". The choice of virtualization type
   changes the set of further arguments that are required, as described below.
@@ -98,3 +99,4 @@ Nested `ephemeral_block_device` blocks have the following structure:
 The following attributes are exported:
 
 * `id` - The ID of the created AMI.
+* `root_snapshot_id` - The Snapshot ID for the root volume (for EBS-backed AMIs)