This is the configuration I use. You can substitute `var.CONFIG` with your values. I am using git repositories as source for modules. Haven't tested with local modules.

cloud_function.tf

resource "google_cloudfunctions_function" "gcp_function" {
  depends_on  = [
    "null_resource.provision_ready",
  ]

  name                  = var.gcp_function_name
  runtime               = var.gcp_function_runtime
  available_memory_mb   = var.gcp_function_available_memory
  source_archive_bucket = var.gcp_function_archive_bucket
  source_archive_object = var.gcp_function_archive_object
  trigger_topic         = var.gcp_function_trigger_topic
  for_each              = var.gcp_function_environment_variables

  environment_variables = {
    key   = each.key
    value = each.value
  }
}

resource "null_resource" "provision_ready" {
  triggers = {
    provision_ready = var.provision_ready
  }
}

resource "null_resource" "function_complete" {
  depends_on  = [
    "null_resource.provision_ready",
    "google_cloudfunctions_function.gcp_function",
  ]
}

main.tf

module "grumman_pubsub" {
  source = ""
  
  provision_ready                       = var.provision_ready
  gcp_pubsub_topic_name                 = var.gcp_pubsub_topic_name
}

module "grumman_function" {
  source = ""
  
  provision_ready                     = module.grumman_pubsub.pubsub_complete
  gcp_function_name                   = var.gcp_function_name
  gcp_function_runtime                = var.gcp_function_runtime
  gcp_function_available_memory       = var.gcp_function_available_memory
  gcp_function_archive_bucket         = var.gcp_function_archive_bucket
  gcp_function_archive_object         = var.gcp_function_archive_object
  gcp_function_trigger_topic          = module.grumman_pubsub.pubsub_topic
  gcp_function_environment_variables  = var.gcp_function_environment_variables
}

pubsub_topic.tf

resource "google_pubsub_topic" "gcp_pubsub_topic" {
  depends_on  = [
    "null_resource.provision_ready",
  ]

  name  = var.gcp_pubsub_topic_name
}

resource "null_resource" "provision_ready" {
  triggers = {
    provision_ready = var.provision_ready
  }
}

resource "null_resource" "pubsub_complete" {
  depends_on  = [
    "null_resource.provision_ready",
    "google_pubsub_topic.gcp_pubsub_topic",
  ]
}

output "pubsub_topic" {
  value = google_pubsub_topic.gcp_pubsub_topic.name
}

output "pubsub_complete" {
  value = null_resource.pubsub_complete.id
}