Unnecessary plan change when using interpolation syntax with TypeSet

[kilokahn]

Terraform Version

v0.11.7

Terraform Configuration Files

variable "tenancy_ocid" {}

variable "user_ocid" {}
variable "fingerprint" {}
variable "private_key_path" {}
variable "compartment_ocid" {}
variable "region" {}

provider "oci" {
  region           = "${var.region}"
  tenancy_ocid     = "${var.tenancy_ocid}"
  user_ocid        = "${var.user_ocid}"
  fingerprint      = "${var.fingerprint}"
  private_key_path = "${var.private_key_path}"
}

resource "oci_core_vcn" "test_vcn" {
  cidr_block     = "10.0.0.0/16"
  compartment_id = "${var.compartment_ocid}"

  display_name = "mubaig-vcn"
}

resource "oci_core_route_table" "test_route_table" {
  #Required
  compartment_id = "${var.compartment_ocid}"
  count          = 1

  route_rules {
    network_entity_id = "${lookup(data.oci_core_private_ips.test_private_ips.private_ips[0], "id")}"
    destination = "0.0.0.0/0"
  }

  vcn_id = "${oci_core_vcn.test_vcn.id}"

  display_name = "mubaig-route-table"
}

data "oci_identity_availability_domains" "ad" {
  compartment_id = "${var.tenancy_ocid}"
}

resource "oci_core_subnet" "test_subnet" {
  availability_domain = "${lookup(data.oci_identity_availability_domains.ad.availability_domains[0],"name")}"
  cidr_block          = "10.0.1.0/24"
  display_name        = "mubaig-subnet"
  compartment_id      = "${var.compartment_ocid}"
  vcn_id              = "${oci_core_vcn.test_vcn.id}"
}

resource "oci_core_instance" "test_instance" {
  depends_on = ["oci_core_subnet.test_subnet"]
  count      = 2

  availability_domain = "${lookup(data.oci_identity_availability_domains.ad.availability_domains[0], "name")}"
  compartment_id      = "${var.compartment_ocid}"
  display_name        = "mubaig-test-instance"
  shape               = "VM.Standard2.1"

  create_vnic_details {
    subnet_id              = "${oci_core_subnet.test_subnet.id}"
    skip_source_dest_check = "true"
  }

  source_details {
    #Required
    source_id   = "ocid1.image.oc1.phx.aaaaaaaaozjbzisykoybkppaiwviyfzusjzokq7jzwxi7nvwdiopk7ligoia"
    source_type = "image"
  }
}

data "oci_core_private_ips" "test_private_ips" {
  depends_on = ["oci_core_instance.test_instance"]
  subnet_id  = "${element(oci_core_subnet.test_subnet.*.id, 0)}"
  ip_address = "${element(oci_core_instance.test_instance.*.private_ip, 0)}"
}

Debug Output

https://gist.github.com/kilokahn/ece94de49fdd37f749cbc00d434bef72

Crash Output

N/A

Expected Behavior

There should have been no change in the plan

Actual Behavior

TF plan shows a spurious diff. When apply is run, it shows that no changes are actually applied.

tf plan

 ~ update in-place
 <= read (data resources)

Terraform will perform the following actions:

 <= data.oci_core_private_ips.test_private_ips
      id:                                        <computed>
      ip_address:                                "10.0.1.2"
      private_ips.#:                             <computed>
      subnet_id:                                 "ocid1.subnet.oc1.phx.aaaaaaaa4emr5usnz5r7qa77yrjshxqtmjtkaysmd5hvgx5zfglak6pf543q"

  ~ oci_core_route_table.test_route_table
      route_rules.1367368352.network_entity_id:  "ocid1.privateip.oc1.phx.abyhqljs7kppu6jgqxwxovissfxzmnzvpbq7ubxd53tp74gfmmndcvyab3la" => ""
      route_rules.~2933718002.cidr_block:        "" => <computed>
      route_rules.~2933718002.destination:       "" => "0.0.0.0/0"
      route_rules.~2933718002.destination_type:  "" => <computed>
      route_rules.~2933718002.network_entity_id: "" => "${lookup(data.oci_core_private_ips.test_private_ips.private_ips[0], \"id\")}"

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

tf apply

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

Steps to Reproduce

1. terraform init
2. terraform plan
3. terraform apply
4. terraform plan # this should be empty, but it isn't
5. terraform apply # this shows that there is no change

Additional Context

The oci_route_table (https://github.com/terraform-providers/terraform-provider-oci/blob/master/oci/core_route_table_resource.go#L33) has an attribute for route_rules which is a TypeSet and it seems to the underlying reason for the problem.
The custom hash function that we define is getting the non-interpolated string in these cases (like ${lookup(data.oci_core_private_ips.test_private_ips.private_ips[0], "id")} rather than the actual value from the state). With such a literal string, it seems impossible for the provider code to correctly discern the correct hash value to be used. In general, passing a non-interpolated value to the hashing function (or any other provider overridden code) should be avoided by terraform, right?

References

There was a similar bug that was logged earlier - #2879 - which was mitigated by creating a new resource for the nested entry - which may not make sense in every scenario.

[apparentlymart]

Hi @kilokahn! Sorry for this confusing behavior.

It looks like you've found issue #18600 here. If you remove depends_on from the data "oci_core_private_ips" "test_private_ips" block the problem should go away. That depends_on argument is redundant anyway, since there's already a reference to oci_core_instance.test_instance in the ip_address argument.

Since this is the same as #18600, I'm going to close it out. Thanks for reporting this!

[kilokahn]

I can confirm that this works for me. Thanks for the quick response!

[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 have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.