Terraform crashes when trying to select an element from a concat containing a potentially empty list

[betabandido]

I am trying to build a module to configure a swagger endpoint that allows for either placing the endpoint under the root resource path or under a path named 'swagger'. I am following a similar approach to the one shown by Gruntwork (link to the article)

When I run terraform, it crashes with the following message: panic: runtime error: invalid memory address or nil pointer dereference (more details below).

I am including a minimal example that reproduces the error. In this example, the endpoint is represented just as an API gateway method (no integration, integration response or response resources are present).

Terraform Version

Terraform v0.9.5

Affected Resource(s)

Please list the resources as a list, for example:

• aws_api_gateway_method

Based on the nature of the problem, it might well be related to the core components, though.

Terraform Configuration Files

The module contains:

variable "api" {}
variable "parent" {}
variable "location" {}
variable "use_swagger_path" {
    default = true
}

resource "aws_api_gateway_resource" "swagger" {
  count       = "${var.use_swagger_path}"
  rest_api_id = "${var.api}"
  parent_id   = "${var.parent}"
  path_part   = "swagger"
}

resource "aws_api_gateway_method" "method" {
  rest_api_id   = "${var.api}"
  resource_id   = "${element(concat(aws_api_gateway_resource.swagger.*.id, list(var.parent)), 0)}"
  http_method   = "GET"
  authorization = "NONE"
}

The main file contains:

provider "aws" {
  profile = "${var.profile}"
  region  = "${var.region}"
}

variable "swagger_location" {
  default = "a_url"
}

resource "aws_api_gateway_rest_api" "api" {
  name = "api"
}

module "swagger_root" {
  source   = "modules/swagger"
  api      = "${aws_api_gateway_rest_api.api.id}"
  parent   = "${aws_api_gateway_rest_api.api.root_resource_id}"
  location = "${var.swagger_location}"
  use_swagger_path = false
}

Debug Output

https://gist.github.com/betabandido/2bcdcfbde244fdee016fc0f8f11d39ad

Panic Output

https://gist.github.com/betabandido/d516d992ef83f95e5d285daebcfdeb41

Expected Behavior

The API gateway resources should be created correctly.

Actual Behavior

Terraform crashed.

Steps to Reproduce

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

1. terraform get
2. terraform apply

[apparentlymart]

Hi @betabandido! Sorry for this crash, and thanks for reporting it.

I'm looking into this now, and will hopefully have an explanation (and ideally, a fix!) soon.

[apparentlymart]

Okay! This is a rather insidious edge-case.

The following is a minimal repro for this:

# root.tf

module "child" {
  source = "./child"
}

# child/child.tf

resource "null_resource" "a" {
  count = 0
}

resource "null_resource" "b" {
  triggers = {
      a_ids = "${join(" ", null_resource.a.*.id)}"
  }
}

The issue arises because of the shape of the intermediate state we're in when we resolve that null_resource.a.*.id variable in the child.

Normally by the time we resolve that null_resource.a.*.id expression there would be instances of null_resource.a in the module's state, but since we have count = 0 here no instances were created. A module's state isn't created until the first time a resource is written into it, so when we reach this point the module has no state at all, but this is not a situation this code was built to expect and so it crashes.

The extra-edgey part of this edge case is that it only applies to non-root modules, since Terraform creates an empty state for the root module very early on its processing, but non-root modules don't get a state until the first resource is created.

I'm working on a fix and will have a PR for it soon.

[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.

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