Wrong parsing ot the destroy-time provisioner command, during a targeted destroy

[mvitale1989]

Terraform Version

Terraform v0.11.2                                  
+ provider.null v1.0.0    

Terraform Configuration Files

locals {
  msg_creation = "I should be shown at creation-time"
  msg_destruction = "I should be shown at destruction-time"
}

resource "null_resource" "myresource" {
  provisioner "local-exec" {
    command = "echo ${local.msg_creation}"
  }
  provisioner "local-exec" {
    command = "echo ${local.msg_destruction}"
    when = "destroy"
  }
}

Expected Behavior

Doing a targeted destroy of myresource should execute the destroy-time provisioner, and then remove the resource from the state.

Actual Behavior

Invoking terraform destroy --target null_resource.myresource --force prints out a possibly faulty error message:

null_resource.myresource: Refreshing state... (ID: 95776629882402507)
null_resource.myresource: Destroying... (ID: 95776629882402507)
null_resource.myresource: Provisioning with 'local-exec'...

Error: Error applying plan:

1 error(s) occurred:

* null_resource.myresource (destroy): 1 error(s) occurred:

* local-exec provisioner command must be a non-empty string

Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.

It's worth noting that this error is not printed out when doing a targetless destroy, e.g. terraform destroy --force. Also, a targeted apply does not incur in any problems.

Steps to Reproduce

Given the above config:

1. terraform init
2. terraform apply --auto-approve
3. terraform destroy --target null_resource.myresource --force

Found workaround

The workaround i used to overcome this problem might be useful in identifying the cause: to make the above steps work, I added the variable that appears in the destroy-time command into the creation-time provisioner block, as a comment.

Thus, the following configuration ensures that the terraform destroy --target null_resource.myresource --force works as well (notice the commented variable, in the creation-time provisioner command):

locals {
  msg_creation = "I should be shown at creation-time"
  msg_destruction = "I should be shown at destruction-time"
}

resource "null_resource" "myresource" {
  provisioner "local-exec" {
    command = "echo ${local.msg_creation} #${local.msg_destruction}"
  }
  provisioner "local-exec" {
    command = "echo ${local.msg_destruction}"
    when = "destroy"
  }
}

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