validation checks for reservations

Author: PayalJakhar

modules/compute/gke-node-pool/README.md

@@ -305,6 +305,7 @@ limitations under the License.
 | [null_resource.enable_tcpxo_in_workload](https://registry.terraform.io/providers/hashicorp/null/latest/docs/resources/resource) | resource |
 | [null_resource.install_dependencies](https://registry.terraform.io/providers/hashicorp/null/latest/docs/resources/resource) | resource |
 | [google_compute_machine_types.machine_info](https://registry.terraform.io/providers/hashicorp/google/latest/docs/data-sources/compute_machine_types) | data source |
+| [google_compute_reservation.specific](https://registry.terraform.io/providers/hashicorp/google/latest/docs/data-sources/compute_reservation) | data source |
 | [google_compute_reservation.specific_reservations](https://registry.terraform.io/providers/hashicorp/google/latest/docs/data-sources/compute_reservation) | data source |
 | [google_container_cluster.gke_cluster](https://registry.terraform.io/providers/hashicorp/google/latest/docs/data-sources/container_cluster) | data source |
 

modules/compute/gke-node-pool/main.tf

@@ -67,6 +67,20 @@ data "google_container_cluster" "gke_cluster" {
   location = local.cluster_location
 }
 
+data "google_compute_reservation" "specific" {
+  count   = local.input_specific_reservations_count == 1 ? 1 : 0
+  project = var.project_id
+  zone    = var.zones[0]
+  name    = var.reservation_affinity.specific_reservations[0].name
+}
+
+locals {
+  reservation_available_count = (local.input_specific_reservations_count == 1) ? (
+    try(data.google_compute_reservation.specific[0].specific_reservation[0].count, 0) -
+    try(data.google_compute_reservation.specific[0].specific_reservation[0].in_use_count, 0)
+  ) : 0
+}
+
 resource "google_container_node_pool" "node_pool" {
   provider = google
 
@@ -379,6 +393,14 @@ resource "google_container_node_pool" "node_pool" {
       condition     = var.enable_flex_start == true ? (var.spot == false) : true
       error_message = "Both enable_flex_start and spot consumption option cannot be set to true at the same time."
     }
+    precondition {
+      condition     = var.reservation_affinity.consume_reservation_type != "SPECIFIC_RESERVATION" || (var.static_node_count != null && var.static_node_count <= local.reservation_available_count)
+      error_message = "Requested static_node_count (${coalesce(var.static_node_count, "not set")}) exceeds the available reservation capacity (${local.reservation_available_count})."
+    }
+    precondition {
+      condition     = local.input_specific_reservations_count == 1 || var.reservation_affinity.consume_reservation_type != "SPECIFIC_RESERVATION"
+      error_message = "Exactly one reservation must be specified when using SPECIFIC_RESERVATION."
+    }
   }
 }