resource/elasticache_subnet_group: appropriately ignore default tags configuration in default resource

[anGie44]

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Closes #19213

Notes

• Code change following a viable workaround which is to ignore_changes to tags_all (on pre-existing resources)
• Testing the "default" subnet group I believe requires importing, trying to create a new one in a test returns

"│ Error: error creating ElastiCache Subnet Group (default): InvalidParameterValue: The value for parameter CacheSubnetGroupName is not a valid identifier. The identifier default is reserved."

Using the following config with reference to the default ElastiCacheSubnetGroup in us-west-2:

provider "aws" {
  default_tags {
    tags = {
      Name = "terraform-default"
    }
  }
}

resource "aws_elasticache_subnet_group" "default" {
  name        = "default"
  description = "Default CacheSubnetGroup"
  subnet_ids  = data.aws_subnet_ids.test.ids
}

data "aws_subnet_ids" "test" {
  vpc_id = "XXXXXXX"
}

With v3.38.0 of the provider, Terraform plan after import returns:

aws_elasticache_subnet_group.default: Refreshing state... [id=default]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # aws_elasticache_subnet_group.default will be updated in-place
  ~ resource "aws_elasticache_subnet_group" "default" {
        id          = "default"
        name        = "default"
        tags        = {}
      ~ tags_all    = {
          + "Name" = "terraform-default"
        }
        # (3 unchanged attributes hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.

With provider built from this branch, Terraform plan after import returns:

aws_elasticache_subnet_group.default: Refreshing state... [id=default]

No changes. Infrastructure is up-to-date.

This means that Terraform did not detect any differences between your configuration and the remote system(s). As a result, there are no actions to take.

Output from acceptance testing:

ok  	github.com/terraform-providers/terraform-provider-aws/aws/internal/keyvaluetags	0.755s

--- PASS: TestAccAWSElasticacheSubnetGroup_basic (27.39s)
--- PASS: TestAccAWSElasticacheSubnetGroup_update (50.82s)
--- PASS: TestAccAWSElasticacheSubnetGroup_tags (64.58s)

[ewbankkit]

LGTM 🚀.

% terraform init

Initializing the backend...

Initializing provider plugins...
- Checking for available provider plugins...
- Downloading plugin for provider "aws" (hashicorp/aws) 3.40.0...

The following providers do not have any version constraints in configuration,
so the latest version was installed.

To prevent automatic upgrades to new major versions that may contain breaking
changes, it is recommended to add version = "..." constraints to the
corresponding provider blocks in configuration, with the constraint strings
suggested below.

* provider.aws: version = "~> 3.40"

Terraform has been successfully initialized!

% terraform import aws_elasticache_subnet_group.default default
aws_elasticache_subnet_group.default: Importing from ID "default"...
aws_elasticache_subnet_group.default: Import prepared!
  Prepared aws_elasticache_subnet_group for import
aws_elasticache_subnet_group.default: Refreshing state... [id=default]

Import successful!

The resources that were imported are shown above. These resources are now in
your Terraform state and will henceforth be managed by Terraform.

% terraform plan
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

data.aws_subnet_ids.test: Refreshing state...
aws_elasticache_subnet_group.default: Refreshing state... [id=default]

------------------------------------------------------------------------

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # aws_elasticache_subnet_group.default will be updated in-place
  ~ resource "aws_elasticache_subnet_group" "default" {
        arn         = "arn:aws:elasticache:us-west-2:123456789012:subnetgroup:default"
        description = "Default CacheSubnetGroup"
        id          = "default"
        name        = "default"
        subnet_ids  = [
            "subnet-xxxxxxxxxxxxxxxxx",
            "subnet-xxxxxxxxxxxxxxxxx",
            "subnet-xxxxxxxxxxxxxxxxx",
            "subnet-xxxxxxxxxxxxxxxxx",
        ]
        tags        = {}
      ~ tags_all    = {
          + "Name" = "terraform-default"
        }
    }

Plan: 0 to add, 1 to change, 0 to destroy.

% terraform apply

aws_elasticache_subnet_group.default: Modifying... [id=default]

Error: error updating tags: error tagging resource (arn:aws:elasticache:us-west-2:123456789012:subnetgroup:default): InvalidParameterValue: Tagging on default resources is not supported
	status code: 400, request id: 1ae50517-5045-46e5-8765-216055db901a

  on main.tf line 11, in resource "aws_elasticache_subnet_group" "default":
  11: resource "aws_elasticache_subnet_group" "default" {

% terraform init --plugin-dir=$HOME/go/bin

Initializing the backend...

Initializing provider plugins...

Terraform has been successfully initialized!

% terraform plan                          
Refreshing Terraform state in-memory prior to plan...
The refreshed state will be used to calculate this plan, but will not be
persisted to local or remote state storage.

data.aws_subnet_ids.test: Refreshing state...
aws_elasticache_subnet_group.default: Refreshing state... [id=default]

------------------------------------------------------------------------

No changes. Infrastructure is up-to-date.

This means that Terraform did not detect any differences between your
configuration and real physical resources that exist. As a result, no
actions need to be performed.

[ghost]

This has been released in version 3.42.0 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!

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