provider/aws: allow empty value for autoscaling schedule parameters

Specifically, this allows you to create a schedule which only changes
some of the group parameters without changing others, by setting the
parameters you wish to remain empty to -1.  This means you can adjust
min or max size without affecting the desired capacity, for example.

The ad hoc support for -1 isn't as nice as having real support for
leaving values out (see hashicorp#5694) but it solves a real use case.

Fixes hashicorp#5681.

builtin/providers/aws/resource_aws_autoscaling_schedule.go

@@ -97,9 +97,24 @@ func resourceAwsAutoscalingScheduleCreate(d *schema.ResourceData, meta interface
 		params.Recurrence = aws.String(attr.(string))
 	}
 
-	params.MinSize = aws.Int64(int64(d.Get("min_size").(int)))
-	params.MaxSize = aws.Int64(int64(d.Get("max_size").(int)))
-	params.DesiredCapacity = aws.Int64(int64(d.Get("desired_capacity").(int)))
+	// Scheduled actions don't need to set all three size parameters. For example,
+	// you may want to change the min or max without also forcing an immediate
+	// resize by changing a desired_capacity that may have changed due to other
+	// autoscaling rules. Since Terraform doesn't have a great pattern for
+	// differentiating between 0 and unset fields, we accept "-1" to mean "don't
+	// include this parameter in the action".
+	minSize := int64(d.Get("min_size").(int))
+	maxSize := int64(d.Get("max_size").(int))
+	desiredCapacity := int64(d.Get("desired_capacity").(int))
+	if minSize != -1 {
+		params.MinSize = aws.Int64(minSize)
+	}
+	if maxSize != -1 {
+		params.MaxSize = aws.Int64(maxSize)
+	}
+	if desiredCapacity != -1 {
+		params.DesiredCapacity = aws.Int64(desiredCapacity)
+	}
 
 	log.Printf("[INFO] Creating Autoscaling Scheduled Action: %s", d.Get("scheduled_action_name").(string))
 	_, err := autoscalingconn.PutScheduledUpdateGroupAction(params)
@@ -126,9 +141,23 @@ func resourceAwsAutoscalingScheduleRead(d *schema.ResourceData, meta interface{}
 
 	d.Set("autoscaling_group_name", sa.AutoScalingGroupName)
 	d.Set("arn", sa.ScheduledActionARN)
-	d.Set("desired_capacity", sa.DesiredCapacity)
-	d.Set("min_size", sa.MinSize)
-	d.Set("max_size", sa.MaxSize)
+
+	if sa.MinSize == nil {
+		d.Set("min_size", -1)
+	} else {
+		d.Set("min_size", sa.MinSize)
+	}
+	if sa.MaxSize == nil {
+		d.Set("max_size", -1)
+	} else {
+		d.Set("max_size", sa.MaxSize)
+	}
+	if sa.DesiredCapacity == nil {
+		d.Set("desired_capacity", -1)
+	} else {
+		d.Set("desired_capacity", sa.DesiredCapacity)
+	}
+
 	d.Set("recurrence", sa.Recurrence)
 
 	if sa.StartTime != nil {

builtin/providers/aws/resource_aws_autoscaling_schedule_test.go

@@ -104,6 +104,27 @@ func TestAccAWSAutoscalingSchedule_zeroValues(t *testing.T) {
 	})
 }
 
+func TestAccAWSAutoscalingSchedule_negativeOne(t *testing.T) {
+	var schedule autoscaling.ScheduledUpdateGroupAction
+
+	rName := fmt.Sprintf("tf-test-%d", acctest.RandInt())
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSAutoscalingScheduleDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccAWSAutoscalingScheduleConfig_negativeOne(rName),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckScalingScheduleExists("aws_autoscaling_schedule.foobar", &schedule),
+					testAccCheckScalingScheduleHasNoDesiredCapacity(&schedule),
+					resource.TestCheckResourceAttr("aws_autoscaling_schedule.foobar", "desired_capacity", "-1"),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckScalingScheduleExists(n string, policy *autoscaling.ScheduledUpdateGroupAction) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		rs, ok := s.RootModule().Resources[n]
@@ -159,6 +180,17 @@ func testAccCheckAWSAutoscalingScheduleDestroy(s *terraform.State) error {
 	return nil
 }
 
+func testAccCheckScalingScheduleHasNoDesiredCapacity(
+	schedule *autoscaling.ScheduledUpdateGroupAction) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		if schedule.DesiredCapacity == nil {
+			return nil
+		}
+		return fmt.Errorf("Expected not to set desired capacity, got %v",
+			aws.Int64Value(schedule.DesiredCapacity))
+	}
+}
+
 func testAccAWSAutoscalingScheduleConfig(r string) string {
 	return fmt.Sprintf(`
 resource "aws_launch_configuration" "foobar" {
@@ -265,3 +297,39 @@ resource "aws_autoscaling_schedule" "foobar" {
     autoscaling_group_name = "${aws_autoscaling_group.foobar.name}"
 }`, r, r)
 }
+
+func testAccAWSAutoscalingScheduleConfig_negativeOne(r string) string {
+	return fmt.Sprintf(`
+resource "aws_launch_configuration" "foobar" {
+    name = "%s"
+    image_id = "ami-21f78e11"
+    instance_type = "t1.micro"
+}
+
+resource "aws_autoscaling_group" "foobar" {
+    availability_zones = ["us-west-2a"]
+    name = "%s"
+    max_size = 1
+    min_size = 1
+    health_check_grace_period = 300
+    health_check_type = "ELB"
+    force_delete = true
+    termination_policies = ["OldestInstance"]
+    launch_configuration = "${aws_launch_configuration.foobar.name}"
+    tag {
+        key = "Foo"
+        value = "foo-bar"
+        propagate_at_launch = true
+    }
+}
+
+resource "aws_autoscaling_schedule" "foobar" {
+    scheduled_action_name = "foobar"
+    max_size = 3
+    min_size = 1
+    desired_capacity = -1
+    start_time = "2018-01-16T07:00:00Z"
+    end_time = "2018-01-16T13:00:00Z"
+    autoscaling_group_name = "${aws_autoscaling_group.foobar.name}"
+}`, r, r)
+}

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

@@ -46,10 +46,10 @@ The following arguments are supported:
                           If you try to schedule your action in the past, Auto Scaling returns an error message.
 * `recurrence` - (Optional) The time when recurring future actions will start. Start time is specified by the user following the Unix cron syntax format. 
 * `min_size` - (Optional) The minimum size for the Auto Scaling group. Default
-0.
+0. Set to -1 if you don't want to change the minimum size at the scheduled time.
 * `max_size` - (Optional) The maximum size for the Auto Scaling group. Default
-0.
-* `desired_capacity` - (Optional) The number of EC2 instances that should be running in the group. Default 0.
+0. Set to -1 if you don't want to change the maximum size at the scheduled time.
+* `desired_capacity` - (Optional) The number of EC2 instances that should be running in the group. Default 0.  Set to -1 if you don't want to change the desired capacity at the scheduled time.
 
 ~> **NOTE:** When `start_time` and `end_time` are specified with `recurrence` , they form the boundaries of when the recurring action will start and stop.