terraform-provider-aws: Unable to use network_interface_id within aws_route_table without incurring a diff everytime

Terraform Version

$ terraform -v
Terraform v0.10.1

Affected Resource(s)

• aws_route_table

Terraform Configuration Files

provider "aws" {
  region = "ap-southeast-2"
  alias  = "local"
}

resource "aws_vpc" "default" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "subnet1" {
  vpc_id            = "${aws_vpc.default.id}"
  availability_zone = "ap-southeast-2a"
  cidr_block        = "10.0.1.0/24"
}

resource "aws_subnet" "subnet2" {
  vpc_id            = "${aws_vpc.default.id}"
  availability_zone = "ap-southeast-2a"
  cidr_block        = "10.0.2.0/24"
}

resource "aws_instance" "server" {
  instance_type = "t2.nano"
  ami           = "ami-ae6259cd"
  subnet_id     = "${aws_subnet.subnet1.id}"
}

resource "aws_network_interface" "test" {
  subnet_id   = "${aws_subnet.subnet2.id}"
  private_ips = ["10.0.2.100"]

  attachment {
    instance     = "${aws_instance.server.id}"
    device_index = 1
  }
}

resource "aws_route_table" "rt" {
  vpc_id = "${aws_vpc.default.id}"

  route {
    cidr_block           = "10.1.1.1/32"
    network_interface_id = "${aws_network_interface.test.id}"
  }
}

Expected Behavior

Running terraform apply for the second time there should be no modifications.

Actual Behavior

Running terraform apply for the second time there are modifications.

Steps to Reproduce

Please list the steps required to reproduce the issue, for example:

1. terraform apply
2. terraform apply again.

Important Factoids

network_interface_id or instance_id can be set for routes within aws_route_table though when setting network_interface_id it appears AWS sends back instance_id AND network_interface_id which triggers a diff.

$ terraform apply
...
Apply complete! Resources: 6 added, 0 changed, 0 destroyed.
$ terraform apply
...
aws_route_table.rt: Modifying... (ID: rtb-4b30d52c)
  route.1660468403.cidr_block:                "" => "10.1.1.1/32"
  route.1660468403.egress_only_gateway_id:    "" => ""
  route.1660468403.gateway_id:                "" => ""
  route.1660468403.instance_id:               "" => ""
  route.1660468403.ipv6_cidr_block:           "" => ""
  route.1660468403.nat_gateway_id:            "" => ""
  route.1660468403.network_interface_id:      "" => "eni-38498645"
  route.1660468403.vpc_peering_connection_id: "" => ""
  route.2141106289.cidr_block:                "10.1.1.1/32" => ""
  route.2141106289.egress_only_gateway_id:    "" => ""
  route.2141106289.gateway_id:                "" => ""
  route.2141106289.instance_id:               "i-06fbffdd7ceb7026f" => ""
  route.2141106289.ipv6_cidr_block:           "" => ""
  route.2141106289.nat_gateway_id:            "" => ""
  route.2141106289.network_interface_id:      "eni-38498645" => ""
  route.2141106289.vpc_peering_connection_id: "" => ""
aws_route_table.rt: Modifications complete (ID: rtb-4b30d52c)

Apply complete! Resources: 0 added, 1 changed, 0 destroyed.

This looks like it was fixed for standalone routes with aws_route but not in-line routes within aws_route_table.

Work around for now is to replace network_interface_id with instance_id.

resource "aws_route_table" "rt" {
  vpc_id = "${aws_vpc.default.id}"
    
 route {
    cidr_block           = "10.1.1.1/32"
    instance_id = "${aws_instance.server.id}"
  }
}

References

• https://github.com/hashicorp/terraform/issues/4311