Update lb_listener datasource schema to match lb_listener default_action forward schema

[rdelcampog]

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Relates #12574
Relates #10942

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Bug description

When the weighted target group functionality was added back in June 2020 in #12547, the lb_listener default_action block schema was not updated.

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Affected Resource(s)

aws_lb_listener (datasource)

Expected Behavior

Terraform should retrieve the specified AWS ELB listener configuration object for cases where there is a single target group configured, or weighted target groups are used on the resource.

Actual Behavior

If the aws_lb_listener datasource tries to retrieve a listener configured with a weighted target group forward default action:

data "aws_lb_listener" "example" {
  load_balancer_arn = aws_alb.example.arn
  port              = 443
}

locals {
  default_lb_listener_action = data.aws_lb_listener.example.default_action
}

output "default_lb_listener_action" {
  value = local.default_lb_listener_action
}

Terraform stops with an error:

Error: error setting default_action: Invalid address to set: []string{"default_action", "0", "forward"}

Steps to Reproduce

terraform apply

Terraform Configuration Files

This bug prevents gathering the current weight configured in the default action of an AWS ALB listener, needed for example to detect the current Target Group ARN that is currently receiving the 100% of the traffic (very useful for active side detection in blue-green deployments):

resource "aws_alb" "example" {
  name            = "example-alb"
  subnets         = module.vpc.public_subnets
  internal        = false
}

resource "aws_alb_listener" "example" {
  load_balancer_arn = aws_alb.example.arn
  port              = 443
  protocol          = "tcp"

  default_action {
    type = "forward"
    forward {
      target_group {
        arn    = "arn:aws:elasticloadbalancing:us-east-1:012345678901:targetgroup/tg-one/abcdefgh1234567"
        weight = 1
      }
      target_group {
        arn    = "arn:aws:elasticloadbalancing:us-east-1:012345678901:targetgroup/tg-two/abcdefgh1234567"
        weight = 0
      }
    }
  }
}

data "aws_lb_listener" "example" {
  load_balancer_arn = aws_alb.example.arn
  port              = 443
}

locals {
  default_lb_listener_action_targets = data.aws_lb_listener.example.default_action[0].forward[0].target_group
  lb_listener_forwarding_target_arn  = [for target_group in local.default_lb_listener_action_targets : target_group.arn if target_group.weight == 1][0]
}

output "lb_listener_forwarding_target_arn" {
  value = local.lb_listener_forwarding_target_arn
}

---

Release note for CHANGELOG:

* data-source/aws_lb_listener: Prevent error when retrieving a listener whose default action contains weighted target groups

Output from acceptance testing:

$ make testacc TEST=./aws TESTARGS='-run=TestAccDataSourceAWSLBListener'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -count 1 -parallel 20 -run=TestAccDataSourceAWSLBListener -timeout 120m
=== RUN   TestAccDataSourceAWSLBListener_basic
=== PAUSE TestAccDataSourceAWSLBListener_basic
=== RUN   TestAccDataSourceAWSLBListener_BackwardsCompatibility
=== PAUSE TestAccDataSourceAWSLBListener_BackwardsCompatibility
=== RUN   TestAccDataSourceAWSLBListener_https
=== PAUSE TestAccDataSourceAWSLBListener_https
=== CONT  TestAccDataSourceAWSLBListener_basic
=== CONT  TestAccDataSourceAWSLBListener_https
=== CONT  TestAccDataSourceAWSLBListener_BackwardsCompatibility
--- PASS: TestAccDataSourceAWSLBListener_basic (206.34s)
--- PASS: TestAccDataSourceAWSLBListener_https (216.49s)
--- PASS: TestAccDataSourceAWSLBListener_BackwardsCompatibility (217.41s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       219.182s

[rdelcampog]

@gdavison I would be very grateful if you could take a look to this bugfix when you have a while, it is a very simple change that I think can be approved quickly. Thank you! 😄

[bflad]

Added a covering acceptance test. Thank you, @rdelcampog for this fix. 🚀

[ghost]

This has been released in version 3.26.0 of the Terraform AWS provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template for triage. Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. Thanks!