`ssh_private_key_file` used for both droplet creation and subsequent connection

Overview of the Issue

Previously I had an issue when providing a private SSH key to connect to my newly created droplet: #19

The fix provided in #20 and shipped with v1.0.1 fixed my issue and I've been using that version ever since.

Now I'm (finally) upgrading to a more recent version and my existing configuration results in a different error.

The private key specified in my config is being used as both the root key when creating the droplet and the ssh key when connecting as a user (which may not be root) to provision the droplet.

In my case, I have a user named deploy that already exists on the base image that I want to connect as. Giving root the same key as deploy is no bueno.

I noticed this because I have that key added to DO already separately, and when trying to create the root key it fails since there's a duplicate.

I believe this bug emerged in the following commit: https://github.com/digitalocean/packer-plugin-digitalocean/commit/0462cbc5d32f1013f2c067116bb836a2ef19030e

Versions of the plugin pre-1.2.0 work as expected.

My desired functionality is to be able to specify the root key and the communicator key separately. If I don't specify a root key then the plugin should generate a temporary one for me, but then connect using the supplied communicator user/key.

Reproduction Steps

Create a keypair on the host where you're running packer.
Add that key to Digital Ocean.
Create a packer buildfile that specifies the private key file but not the key ID from Digital Ocean
Run packer build
Observe the output (example below)

Plugin and Packer version

Packer: 1.9.4 Plugin: 1.2.0 and up

Simplified Packer Buildfile

variable "deploy_private_key_file" {
  type = string
  default = null
}

variable "digital_ocean_application_image_id" {
  type = string
  default = null
}

build {
  name = "application_server"
  source "source.digitalocean.default" {
    image = var.digital_ocean_application_image_id
    snapshot_name = "backend_server"
    snapshot_regions = ["tor1", "nyc3"]
    ssh_username = "deploy"
    ssh_private_key_file = var.deploy_private_key_file
  }
}

Operating system and Environment details

MacOS 13.6.3

Log Fragments and crash.log files

Full plugin output from my packer run:

Debug mode enabled. Builds will not be parallelized.
application_server.digitalocean.default: output will be in this color.

==> application_server.digitalocean.default: Using existing SSH private key
==> application_server.digitalocean.default: Pausing after run of step 'StepSSHKeyGen'. Press enter to continue. 
==> application_server.digitalocean.default: Pausing after run of step 'nullStep'. Press enter to continue. 
==> application_server.digitalocean.default: Importing SSH public key...
==> application_server.digitalocean.default: Error creating temporary SSH key: POST https://api.digitalocean.com/v2/account/keys: 422 (request "e362b52b-ac39-4c99-b14f-cb6bf0804dc0") SSH Key is already in use on your account
==> application_server.digitalocean.default: Pausing before cleanup of step 'nullStep'. Press enter to continue. 
==> application_server.digitalocean.default: Pausing before cleanup of step 'StepSSHKeyGen'. Press enter to continue. 
Build 'application_server.digitalocean.default' errored after 20 seconds 184 milliseconds: Error creating temporary SSH key: POST https://api.digitalocean.com/v2/account/keys: 422 (request "e362b52b-ac39-4c99-b14f-cb6bf0804dc0") SSH Key is already in use on your account

digitalocean / packer-plugin-digitalocean