make aws_ec2_transit_gateway_vpc_attachment gracefully handle TGW without default route tables

[dmrzzz]

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Description

This is a feature request to implement what @ewbankkit suggested in #13512 (comment)

We probably should change the code so that transit_gateway_default_route_table_propagation and transit_gateway_default_route_table_association have no effect if the transit gateway has no equivalent default route table.

New or Affected Resource(s)

• aws_ec2_transit_gateway_vpc_attachment

Potential Terraform Configuration

provider "aws" {
  region = "us-west-2"
}

resource "aws_ec2_transit_gateway" "tgw" {
  auto_accept_shared_attachments  = "enable"
  default_route_table_association = "disable"
  default_route_table_propagation = "disable"
}

resource "aws_ec2_transit_gateway_vpc_attachment" "tgw_attach" {
  transit_gateway_id = aws_ec2_transit_gateway.tgw.id
  vpc_id             = aws_vpc.vpc.id
  subnet_ids         = [aws_subnet.subnet.id]

  # wish I didn't need these
  #transit_gateway_default_route_table_association = false
  #transit_gateway_default_route_table_propagation = false
}

resource "aws_vpc" "vpc" {
  cidr_block = "192.168.0.0/16"
}

resource "aws_subnet" "subnet" {
  vpc_id     = aws_vpc.vpc.id
  cidr_block = "192.168.0.0/24"
}

Status Quo

$ terraform -version
Terraform v0.15.1
on darwin_amd64
+ provider registry.terraform.io/hashicorp/aws v3.39.0

$ terraform apply -auto-approve
aws_ec2_transit_gateway.tgw: Creating...
aws_vpc.vpc: Creating...
aws_vpc.vpc: Creation complete after 3s [id=vpc-0929d0671d5f6092e]
aws_subnet.subnet: Creating...
aws_subnet.subnet: Creation complete after 2s [id=subnet-0109f5695d9c6c1d9]
aws_ec2_transit_gateway.tgw: Still creating... [10s elapsed]
aws_ec2_transit_gateway.tgw: Still creating... [20s elapsed]
aws_ec2_transit_gateway.tgw: Still creating... [30s elapsed]
aws_ec2_transit_gateway.tgw: Creation complete after 36s [id=tgw-0267f80c7549d3b01]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Creating...
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [10s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [20s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [30s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [40s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [50s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [1m0s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [1m10s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [1m20s elapsed]
aws_ec2_transit_gateway_vpc_attachment.tgw_attach: Still creating... [1m30s elapsed]
╷
│ Error: error updating EC2 Transit Gateway Attachment (tgw-attach-06566e808ef576afc) Route Table () association: error associating EC2 Transit Gateway Route Table () association (tgw-attach-06566e808ef576afc): MissingParameter: Missing required parameter in request: TransitGatewayRouteTableId.
│ 	status code: 400, request id: ad0505de-e1aa-42d7-843e-594485c69040
│ 
│   with aws_ec2_transit_gateway_vpc_attachment.tgw_attach,
│   on main.tf line 11, in resource "aws_ec2_transit_gateway_vpc_attachment" "tgw_attach":
│   11: resource "aws_ec2_transit_gateway_vpc_attachment" "tgw_attach" {
│ 
╵

It works fine (for an attachment in the same AWS account as the TGW) if I explicitly specify

  transit_gateway_default_route_table_association = false
  transit_gateway_default_route_table_propagation = false

but this is not DRY, and also contrasts poorly with the need to avoid specifying those same arguments when creating an attachment from a different AWS account (see #8383).

It would be much nicer for omitting the optional arguments to elicit reasonable default behavior in all cases.

References

• aws_ec2_transit_gateway_vpc_attachment does not set transit_gateway_default_route_table_association or transit_gateway_default_route_table_propagation correctly #8383
• error associating EC2 Transit Gateway Route Table () association MissingParameter: Missing required parameter in request: TransitGatewayRouteTableId. #13512

[github-actions]

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[dmrzzz]

Commenting to un-stale-ify.