terraform test: Modules with no resources aren't saving their output values

[liamcervante]

Terraform Version

1.7.0-rc1
1.6.6

Terraform Configuration Files

# main.tf

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

# first/main.tf

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

output "id" {
    value = module.second.id
}

# second/main.tf

resource "tfcoremock_simple_resource" "resource" {}

output "id" {
  value = tfcoremock_simple_resource.resource.id
}

# main.tftest.hcl

run "run1" {
  command = apply

  assert {
    condition     = module.first.id != 0
    error_message = "Fail"
  }
}

Debug Output

N/A

Expected Behavior

test should pass

Actual Behavior

test fails

Steps to Reproduce

terraform init
terraform test

Additional Context

No response

References

• https://discuss.hashicorp.com/t/terraform-test-1-6-x-potential-bug-when-command-apply-unknown-condition-value/61363

[liamcervante]

Specifically, our definition of an empty module here: https://github.com/hashicorp/terraform/blob/main/internal/terraform/node_module_expand.go#L220 is causing the middle module in the configuration to be removed, and then the test assertion doesn't have access to the output from that module and so it returns unknown.

Interestingly, this is already fixed in main I think by this PR: #34343, which wasn't backported to v1.7.

[liamcervante]

This should be fixed in the next release of the v1.7 series.

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