provider/aws: ability to generate unique RDS identifier

Needed to truncate the identifier for SQL Server engines to keep it at
max 15 chars per the docs. Not a full UUID going into it, but should be
"unique enough" to not matter in practice.

Modified the basic test to use the generated value. Other tests are
still working w/ explicitly specified identifiers.

builtin/providers/aws/resource_aws_db_instance.go

@@ -85,7 +85,8 @@ func resourceAwsDbInstance() *schema.Resource {
 
 			"identifier": &schema.Schema{
 				Type:         schema.TypeString,
-				Required:     true,
+				Optional:     true,
+				Computed:     true,
 				ForceNew:     true,
 				ValidateFunc: validateRdsId,
 			},
@@ -291,12 +292,25 @@ func resourceAwsDbInstanceCreate(d *schema.ResourceData, meta interface{}) error
 	conn := meta.(*AWSClient).rdsconn
 	tags := tagsFromMapRDS(d.Get("tags").(map[string]interface{}))
 
+	identifier := d.Get("identifier").(string)
+	// Generate a unique ID for the user
+	if identifier == "" {
+		identifier = resource.PrefixedUniqueId("tf-")
+		// SQL Server identifier size is max 15 chars, so truncate
+		if engine := d.Get("engine").(string); engine != "" {
+			if strings.Contains(strings.ToLower(engine), "sqlserver") {
+				identifier = identifier[:15]
+			}
+		}
+		d.Set("identifier", identifier)
+	}
+
 	if v, ok := d.GetOk("replicate_source_db"); ok {
 		opts := rds.CreateDBInstanceReadReplicaInput{
 			SourceDBInstanceIdentifier: aws.String(v.(string)),
 			CopyTagsToSnapshot:         aws.Bool(d.Get("copy_tags_to_snapshot").(bool)),
 			DBInstanceClass:            aws.String(d.Get("instance_class").(string)),
-			DBInstanceIdentifier:       aws.String(d.Get("identifier").(string)),
+			DBInstanceIdentifier:       aws.String(identifier),
 			Tags:                       tags,
 		}
 		if attr, ok := d.GetOk("iops"); ok {

builtin/providers/aws/resource_aws_db_instance_test.go

@@ -362,10 +362,8 @@ func testAccCheckAWSDBInstanceExists(n string, v *rds.DBInstance) resource.TestC
 // Database names cannot collide, and deletion takes so long, that making the
 // name a bit random helps so able we can kill a test that's just waiting for a
 // delete and not be blocked on kicking off another one.
-var testAccAWSDBInstanceConfig = fmt.Sprintf(`
+var testAccAWSDBInstanceConfig = `
 resource "aws_db_instance" "bar" {
-	identifier = "foobarbaz-test-terraform-%d"
-
 	allocated_storage = 10
 	engine = "MySQL"
 	engine_version = "5.6.21"
@@ -383,7 +381,7 @@ resource "aws_db_instance" "bar" {
 	backup_retention_period = 0
 
 	parameter_group_name = "default.mysql5.6"
-}`, rand.New(rand.NewSource(time.Now().UnixNano())).Int())
+}`
 
 func testAccReplicaInstanceConfig(val int) string {
 	return fmt.Sprintf(`

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

@@ -28,16 +28,15 @@ for more information.
 
 ```
 resource "aws_db_instance" "default" {
-	identifier = "mydb-rds"
-	allocated_storage = 10
-	engine = "mysql"
-	engine_version = "5.6.17"
-	instance_class = "db.t1.micro"
-	name = "mydb"
-	username = "foo"
-	password = "bar"
-	db_subnet_group_name = "my_database_subnet_group"
-	parameter_group_name = "default.mysql5.6"
+  allocated_storage    = 10
+  engine               = "mysql"
+  engine_version       = "5.6.17"
+  instance_class       = "db.t1.micro"
+  name                 = "mydb"
+  username             = "foo"
+  password             = "bar"
+  db_subnet_group_name = "my_database_subnet_group"
+  parameter_group_name = "default.mysql5.6"
 }
 ```
 
@@ -51,7 +50,7 @@ The following arguments are supported:
 * `allocated_storage` - (Required unless a `snapshot_identifier` or `replicate_source_db` is provided) The allocated storage in gigabytes.
 * `engine` - (Required unless a `snapshot_identifier` or `replicate_source_db` is provided) The database engine to use.
 * `engine_version` - (Optional) The engine version to use.
-* `identifier` - (Required) The name of the RDS instance
+* `identifier` - (Optional) The name of the RDS instance, if omitted, Terraform will assign a random, unique name
 * `instance_class` - (Required) The instance type of the RDS instance.
 * `storage_type` - (Optional) One of "standard" (magnetic), "gp2" (general
 	purpose SSD), or "io1" (provisioned IOPS SSD). The default is "io1" if