providers/aws: fix bug w/ empty block dev mapping

fixes #1249

builtin/providers/aws/resource_aws_instance.go

@@ -744,6 +744,10 @@ func readBlockDevicesFromInstance(instance *ec2.Instance, ec2conn *ec2.EC2) (map
 		}
 	}
 
+	if len(instanceBlockDevices) == 0 {
+		return nil, nil
+	}
+
 	volIDs := make([]string, 0, len(instanceBlockDevices))
 	for volID := range instanceBlockDevices {
 		volIDs = append(volIDs, volID)

builtin/providers/aws/resource_aws_instance_test.go

@@ -14,6 +14,7 @@ import (
 
 func TestAccAWSInstance_normal(t *testing.T) {
 	var v ec2.Instance
+	var vol *ec2.Volume
 
 	testCheck := func(*terraform.State) error {
 		if *v.Placement.AvailabilityZone != "us-west-2a" {
@@ -35,6 +36,21 @@ func TestAccAWSInstance_normal(t *testing.T) {
 		Providers:    testAccProviders,
 		CheckDestroy: testAccCheckInstanceDestroy,
 		Steps: []resource.TestStep{
+			// Create a volume to cover #1249
+			resource.TestStep{
+				// Need a resource in this config so the provisioner will be available
+				Config: testAccInstanceConfig_pre,
+				Check: func(*terraform.State) error {
+					conn := testAccProvider.Meta().(*AWSClient).ec2conn
+					var err error
+					vol, err = conn.CreateVolume(&ec2.CreateVolumeRequest{
+						AvailabilityZone: aws.String("us-west-2a"),
+						Size:             aws.Integer(5),
+					})
+					return err
+				},
+			},
+
 			resource.TestStep{
 				Config: testAccInstanceConfig,
 				Check: resource.ComposeTestCheckFunc(
@@ -45,6 +61,8 @@ func TestAccAWSInstance_normal(t *testing.T) {
 						"aws_instance.foo",
 						"user_data",
 						"3dc39dda39be1205215e776bad998da361a5955d"),
+					resource.TestCheckResourceAttr(
+						"aws_instance.foo", "ebs_block_device.#", "0"),
 				),
 			},
 
@@ -61,8 +79,19 @@ func TestAccAWSInstance_normal(t *testing.T) {
 						"aws_instance.foo",
 						"user_data",
 						"3dc39dda39be1205215e776bad998da361a5955d"),
+					resource.TestCheckResourceAttr(
+						"aws_instance.foo", "ebs_block_device.#", "0"),
 				),
 			},
+
+			// Clean up volume created above
+			resource.TestStep{
+				Config: testAccInstanceConfig,
+				Check: func(*terraform.State) error {
+					conn := testAccProvider.Meta().(*AWSClient).ec2conn
+					return conn.DeleteVolume(&ec2.DeleteVolumeRequest{VolumeID: vol.VolumeID})
+				},
+			},
 		},
 	})
 }
@@ -393,6 +422,20 @@ func TestInstanceTenancySchema(t *testing.T) {
 	}
 }
 
+const testAccInstanceConfig_pre = `
+resource "aws_security_group" "tf_test_foo" {
+	name = "tf_test_foo"
+	description = "foo"
+
+	ingress {
+		protocol = "icmp"
+		from_port = -1
+		to_port = -1
+		cidr_blocks = ["0.0.0.0/0"]
+	}
+}
+`
+
 const testAccInstanceConfig = `
 resource "aws_security_group" "tf_test_foo" {
 	name = "tf_test_foo"