fix: Enhance WI module usability with existing KSA (#557)

examples/workload_identity/main.tf

@@ -53,6 +53,7 @@ module "gke" {
   ]
 }
 
+# example without existing KSA
 module "workload_identity" {
   source              = "../../modules/workload-identity"
   project_id          = var.project_id
@@ -61,5 +62,27 @@ module "workload_identity" {
   use_existing_k8s_sa = false
 }
 
+
+# example with existing KSA
+resource "kubernetes_service_account" "test" {
+  metadata {
+    name = "foo-ksa"
+  }
+  secret {
+    name = "bar"
+  }
+}
+
+module "workload_identity_existing_ksa" {
+  source              = "../../modules/workload-identity"
+  project_id          = var.project_id
+  name                = "existing-${module.gke.name}"
+  cluster_name        = module.gke.name
+  location            = module.gke.location
+  namespace           = "default"
+  use_existing_k8s_sa = true
+  k8s_sa_name         = kubernetes_service_account.test.metadata.0.name
+}
+
 data "google_client_config" "default" {
 }

modules/workload-identity/README.md

@@ -51,9 +51,6 @@ resource "kubernetes_service_account" "preexisting" {
   metadata {
     name = "preexisting-sa"
     namespace = "prod"
-    annotations = {
-      "iam.gke.io/gcp-service-account" = "preexisting-sa@${var.project_id}.iam.gserviceaccount.com"
-    }
   }
 }
 
@@ -71,7 +68,9 @@ module "my-app-workload-identity" {
 
 | Name | Description | Type | Default | Required |
 |------|-------------|:----:|:-----:|:-----:|
+| cluster\_name | Cluster name. Required if using existing KSA. | string | `""` | no |
 | k8s\_sa\_name | Name for the existing Kubernetes service account | string | `"null"` | no |
+| location | Cluster location (region if regional cluster, zone if zonal cluster). Required if using existing KSA. | string | `""` | no |
 | name | Name for both service accounts | string | n/a | yes |
 | namespace | Namespace for k8s service account | string | `"default"` | no |
 | project\_id | GCP project ID | string | n/a | yes |

modules/workload-identity/main.tf

@@ -19,9 +19,23 @@ locals {
   gcp_sa_email            = google_service_account.cluster_service_account.email
 
   # This will cause terraform to block returning outputs until the service account is created
-  k8s_given_name       = var.k8s_sa_name != null ? var.k8s_sa_name : var.name
-  output_k8s_name      = var.use_existing_k8s_sa ? local.k8s_given_name : kubernetes_service_account.main[0].metadata[0].name
-  output_k8s_namespace = var.use_existing_k8s_sa ? var.namespace : kubernetes_service_account.main[0].metadata[0].namespace
+  k8s_given_name         = var.k8s_sa_name != null ? var.k8s_sa_name : var.name
+  output_k8s_name        = var.use_existing_k8s_sa ? local.k8s_given_name : kubernetes_service_account.main[0].metadata[0].name
+  output_k8s_namespace   = var.use_existing_k8s_sa ? var.namespace : kubernetes_service_account.main[0].metadata[0].namespace
+  token                  = var.use_existing_k8s_sa ? data.google_client_config.default.0.access_token : ""
+  cluster_ca_certificate = var.use_existing_k8s_sa ? data.google_container_cluster.primary.0.master_auth.0.cluster_ca_certificate : ""
+  cluster_endpoint       = var.use_existing_k8s_sa ? "https://${data.google_container_cluster.primary.0.endpoint}" : ""
+}
+
+data "google_container_cluster" "primary" {
+  count    = var.use_existing_k8s_sa ? 1 : 0
+  name     = var.cluster_name
+  project  = var.project_id
+  location = var.location
+}
+
+data "google_client_config" "default" {
+  count = var.use_existing_k8s_sa ? 1 : 0
 }
 
 resource "google_service_account" "cluster_service_account" {
@@ -51,11 +65,11 @@ module "annotate-sa" {
   enabled               = var.use_existing_k8s_sa
   skip_download         = true
 
-  create_cmd_entrypoint = "kubectl"
-  create_cmd_body       = "annotate sa -n ${local.output_k8s_namespace} ${local.k8s_given_name} iam.gke.io/gcp-service-account=${local.gcp_sa_email}"
+  create_cmd_entrypoint = "${path.module}/scripts/kubectl_wrapper.sh"
+  create_cmd_body       = "${local.cluster_endpoint} ${local.token} ${local.cluster_ca_certificate} kubectl annotate --overwrite sa -n ${local.output_k8s_namespace} ${local.k8s_given_name} iam.gke.io/gcp-service-account=${local.gcp_sa_email}"
 
-  destroy_cmd_entrypoint = "kubectl"
-  destroy_cmd_body       = "annotate sa -n ${local.output_k8s_namespace} ${local.k8s_given_name} iam.gke.io/gcp-service-account-"
+  destroy_cmd_entrypoint = "${path.module}/scripts/kubectl_wrapper.sh"
+  destroy_cmd_body       = "${local.cluster_endpoint} ${local.token} ${local.cluster_ca_certificate} kubectl annotate sa -n ${local.output_k8s_namespace} ${local.k8s_given_name} iam.gke.io/gcp-service-account-"
 }
 
 resource "google_service_account_iam_member" "main" {

modules/workload-identity/scripts/kubectl_wrapper.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+set -e
+
+if [ "$#" -lt 3 ]; then
+    >&2 echo "Not all expected arguments set."
+    exit 1
+fi
+
+HOST=$1
+TOKEN=$2
+CA_CERTIFICATE=$3
+
+shift 3
+
+RANDOM_ID="${RANDOM}_${RANDOM}"
+export TMPDIR="/tmp/kubectl_wrapper_${RANDOM_ID}"
+
+function cleanup {
+    rm -rf "${TMPDIR}"
+}
+trap cleanup EXIT
+
+mkdir "${TMPDIR}"
+
+export KUBECONFIG="${TMPDIR}/config"
+
+# shellcheck disable=SC1117
+base64 --help | grep "\--decode" && B64_ARG="--decode" || B64_ARG="-d"
+echo "${CA_CERTIFICATE}" | base64 ${B64_ARG} > "${TMPDIR}/ca_certificate"
+
+kubectl config set-cluster kubectl-wrapper --server="${HOST}" --certificate-authority="${TMPDIR}/ca_certificate" --embed-certs=true 1>/dev/null
+rm -f "${TMPDIR}/ca_certificate"
+kubectl config set-context kubectl-wrapper --cluster=kubectl-wrapper --user=kubectl-wrapper --namespace=default 1>/dev/null
+kubectl config set-credentials kubectl-wrapper --token="${TOKEN}" 1>/dev/null
+kubectl config use-context kubectl-wrapper 1>/dev/null
+kubectl version 1>/dev/null
+
+"$@"

modules/workload-identity/variables.tf

@@ -19,6 +19,18 @@ variable "name" {
   type        = string
 }
 
+variable "cluster_name" {
+  description = "Cluster name. Required if using existing KSA."
+  type        = string
+  default     = ""
+}
+
+variable "location" {
+  description = "Cluster location (region if regional cluster, zone if zonal cluster). Required if using existing KSA."
+  type        = string
+  default     = ""
+}
+
 variable "k8s_sa_name" {
   description = "Name for the existing Kubernetes service account"
   type        = string