providers/aws: add name_prefix option to launch config

See #2911.

This adds a `name_prefix` option to `aws_launch_configuration` resources.

When specified, it is used instead of `terraform-` as the prefix for the
launch configuration.  It conflicts with `name`, so existing
functionality is unchanged.  `name` still sets the name explicitly.

Added an acceptance test, and updated the site documentation.

builtin/providers/aws/resource_aws_launch_configuration.go

@@ -30,6 +30,7 @@ func resourceAwsLaunchConfiguration() *schema.Resource {
 				Optional:      true,
 				Computed:      true,
 				ForceNew:      true,
+				ConflictsWith: []string{"name_prefix"},
 				ValidateFunc: func(v interface{}, k string) (ws []string, errors []error) {
 					// https://github.com/boto/botocore/blob/9f322b1/botocore/data/autoscaling/2011-01-01/service-2.json#L1932-L1939
 					value := v.(string)
@@ -41,6 +42,22 @@ func resourceAwsLaunchConfiguration() *schema.Resource {
 				},
 			},
 
+			"name_prefix": &schema.Schema{
+				Type:     schema.TypeString,
+				Optional: true,
+				ForceNew: true,
+				ValidateFunc: func(v interface{}, k string) (ws []string, errors []error) {
+					// https://github.com/boto/botocore/blob/9f322b1/botocore/data/autoscaling/2011-01-01/service-2.json#L1932-L1939
+					// uuid is 26 characters, limit the prefix to 229.
+					value := v.(string)
+					if len(value) > 229 {
+						errors = append(errors, fmt.Errorf(
+							"%q cannot be longer than 229 characters, name is limited to 255", k))
+					}
+					return
+				},
+			},
+
 			"image_id": &schema.Schema{
 				Type:     schema.TypeString,
 				Required: true,
@@ -386,6 +403,8 @@ func resourceAwsLaunchConfigurationCreate(d *schema.ResourceData, meta interface
 	var lcName string
 	if v, ok := d.GetOk("name"); ok {
 		lcName = v.(string)
+	} else if v, ok := d.GetOk("name_prefix"); ok {
+		lcName = resource.PrefixedUniqueId(v.(string))
 	} else {
 		lcName = resource.UniqueId()
 	}

builtin/providers/aws/resource_aws_launch_configuration_test.go

@@ -30,6 +30,14 @@ func TestAccAWSLaunchConfiguration_basic(t *testing.T) {
 						"aws_launch_configuration.bar", "terraform-"),
 				),
 			},
+			resource.TestStep{
+				Config: testAccAWSLaunchConfigurationPrefixNameConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSLaunchConfigurationExists("aws_launch_configuration.baz", &conf),
+					testAccCheckAWSLaunchConfigurationGeneratedNamePrefix(
+						"aws_launch_configuration.baz", "baz-"),
+				),
+			},
 		},
 	})
 }
@@ -255,3 +263,13 @@ resource "aws_launch_configuration" "bar" {
    associate_public_ip_address = false
 }
 `
+
+const testAccAWSLaunchConfigurationPrefixNameConfig = `
+resource "aws_launch_configuration" "baz" {
+   name_prefix = "baz-"
+   image_id = "ami-21f78e11"
+   instance_type = "t1.micro"
+   user_data = "foobar-user-data-change"
+   associate_public_ip_address = false
+}
+`

website/source/docs/providers/aws/r/launch_configuration.html.markdown

@@ -26,11 +26,13 @@ Launch Configurations cannot be updated after creation with the Amazon
 Web Service API. In order to update a Launch Configuration, Terraform will
 destroy the existing resource and create a replacement. In order to effectively
 use a Launch Configuration resource with an [AutoScaling Group resource][1],
-it's recommend to omit the Launch Configuration `name` attribute, and
-specify `create_before_destroy` in a [lifecycle][2] block, as shown:
+it's recommended to specify `create_before_destroy` in a [lifecycle][2] block.
+Either omit the Launch Configuration `name` attribute, or specify a partial name
+with `name_prefix`.  Example:
 
 ```
 resource "aws_launch_configuration" "as_conf" {
+    name_prefix = "terraform-lc-example-"
     image_id = "ami-1234"
     instance_type = "m1.small"
 
@@ -87,7 +89,9 @@ resource "aws_autoscaling_group" "bar" {
 The following arguments are supported:
 
 * `name` - (Optional) The name of the launch configuration. If you leave
-  this blank, Terraform will auto-generate it.
+  this blank, Terraform will auto-generate a unique name.
+* `name_prefix` - (Optional) Creates a unique name beginning with the specified
+  prefix. Conflicts with `name`.
 * `image_id` - (Required) The EC2 image ID to launch.
 * `instance_type` - (Required) The size of instance to launch.
 * `iam_instance_profile` - (Optional) The IAM instance profile to associate