added external_ip_address and use_external_ip_address variables

main.tf

@@ -85,7 +85,7 @@ module "nat-gateway" {
 
   access_config = [
     {
-      nat_ip = "${element(concat(google_compute_address.default.*.address, data.google_compute_address.default.*.address, list("")), 0)}"
+      nat_ip = "${element(concat(google_compute_address.default.*.address, data.google_compute_address.default.*.address, list(var.external_ip_address)), 0)}"
     },
   ]
 }
@@ -117,7 +117,7 @@ resource "google_compute_firewall" "nat-gateway" {
 }
 
 resource "google_compute_address" "default" {
-  count   = "${var.module_enabled && var.ip_address_name == "" ? 1 : 0}"
+  count   = "${var.module_enabled && var.ip_address_name == "" && var.use_external_ip_address != "true" ? 1 : 0}"
   name    = "${local.zonal_tag}"
   project = "${var.project}"
   region  = "${var.region}"

outputs.tf

@@ -31,7 +31,7 @@ output instance {
 
 output external_ip {
   description = "The external IP address of the NAT gateway instance."
-  value       = "${element(concat(google_compute_address.default.*.address, data.google_compute_address.default.*.address, list("")), 0)}"
+  value       = "${element(concat(google_compute_address.default.*.address, data.google_compute_address.default.*.address, list(var.external_ip_address)), 0)}"
 }
 
 output routing_tag_regional {

variables.tf

@@ -58,6 +58,16 @@ variable ip_address_name {
   default     = ""
 }
 
+variable external_ip_address {
+  description = "Address of existing IP reservation to use."
+  default     = ""
+}
+
+variable use_external_ip_address {
+  description = "Set to true when ip_address input variable is set."
+  default     = "false"
+}
+
 variable tags {
   description = "Additional compute instance network tags to apply route to."
   type        = "list"