Default Target-Group of a listener cant be modified

[egarbi]

Terraform Version

Terraform v0.11.4

Terraform Configuration Files

module "ECS_ALB" {
  source  = "terraform-aws-modules/alb/aws"
  version = "3.1.0"

  load_balancer_name         = "${var.name}-${var.environment}"
  subnets                    = "${module.vpc.private_subnets}"
  load_balancer_is_internal  = "true"
  security_groups            = ["${module.security_groups.internal_elb}"]
  vpc_id                     = "${module.vpc.vpc_id}"
  enable_deletion_protection = true

  # Listener
  http_tcp_listeners_count = "1"
  http_tcp_listeners       = "${list(map("port", "8080", "protocol", "HTTP"))}"

  // Default Target-Group
  target_groups       = "${list(map("name", "first-${var.environment}", "backend_protocol", "HTTP", "backend_port", "8080"))}"
  target_groups_count = "1"

  #
  // Logging
  log_bucket_name = "${var.name}-${var.environment}-share-alb-logs"

  //Tagging
  tags = {
    Name = "${var.name}"

    Environment = "${var.environment}"
  }
}

Expected Behavior

The module should allow modify the default target-group (rule) associated with a listener.
For this particular example, let's suppose that I have already created a listener with the target-group first-testing as default and I want to change it to be default-testing.

Actual Behavior

Terraform apply fails with the following message:

module.stack.module.ECS_ALB.aws_lb_target_group.main: Destroying... (ID: arn:aws:elasticloadbalancing:us-west-4:...tgroup/first-testing/6f58a25341942b21)

Error: Error applying plan:

1 error(s) occurred:

* module.stack.module.ECS_ALB.aws_lb_target_group.main (destroy): 1 error(s) occurred:

* aws_lb_target_group.main: Error deleting Target Group: ResourceInUse: Target group 'arn:aws:elasticloadbalancing:us-west-4:12345678910:targetgroup/first-testing/6f58a25341942b21' is currently in use by a listener or a rule

Steps to Reproduce

1. create the resources using the configuration file above, init, plan and apply
2. modify the default target group as follow:

17c17
<   target_groups                 = "${list(map("name", "first-${var.environment}", "backend_protocol", "HTTP", "backend_port", "8080"))}"
---
>   target_groups                 = "${list(map("name", "default-${var.environment}", "backend_protocol", "HTTP", "backend_port", "8080"))}"

3. plan
4. apply

Maybe adding 'lifecycle {crate_before_destroy => true}` into aws_lb_target_group to solve it?
I will test later better

[brandonjbjelland]

Merged your PR. Thanks for tracking down the core of the problem.

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