provider/google: fix container instance group URLs

Google Container Engine's cluster API returned instance group manager
URLs when it meant to return instance group URLs. See #4336 for details
about the bug.

While this is undeniably an upstream problem, this PR:

* detects the error, meaning it will work as expected when the API is
  fixed.
* corrects the error by requesting the instance group manager, then
  retrieving its instance group URL, and using that instead.
* adds a test that exercises the error and the solution, to ensure it is
  functioning properly.

resource_container_cluster.go

@@ -13,6 +13,10 @@ import (
 	"google.golang.org/api/googleapi"
 )
 
+var (
+	instanceGroupManagerURL = regexp.MustCompile("^https://www.googleapis.com/compute/v1/projects/([a-z][a-z0-9-]{5}(?:[-a-z0-9]{0,23}[a-z0-9])?)/zones/([a-z0-9-]*)/instanceGroupManagers/([^/]*)")
+)
+
 func resourceContainerCluster() *schema.Resource {
 	return &schema.Resource{
 		Create: resourceContainerClusterCreate,
@@ -474,7 +478,28 @@ func resourceContainerClusterRead(d *schema.ResourceData, meta interface{}) erro
 	d.Set("network", d.Get("network").(string))
 	d.Set("subnetwork", cluster.Subnetwork)
 	d.Set("node_config", flattenClusterNodeConfig(cluster.NodeConfig))
-	d.Set("instance_group_urls", cluster.InstanceGroupUrls)
+
+	// container engine's API currently mistakenly returns the instance group manager's
+	// URL instead of the instance group's URL in its responses. This shim detects that
+	// error, and corrects it, by fetching the instance group manager URL and retrieving
+	// the instance group manager, then using that to look up the instance group URL, which
+	// is then substituted.
+	//
+	// This should be removed when the API response is fixed.
+	instanceGroupURLs := make([]string, 0, len(cluster.InstanceGroupUrls))
+	for _, u := range cluster.InstanceGroupUrls {
+		if !instanceGroupManagerURL.MatchString(u) {
+			instanceGroupURLs = append(instanceGroupURLs, u)
+			continue
+		}
+		matches := instanceGroupManagerURL.FindStringSubmatch(u)
+		instanceGroupManager, err := config.clientCompute.InstanceGroupManagers.Get(matches[1], matches[2], matches[3]).Do()
+		if err != nil {
+			return fmt.Errorf("Error reading instance group manager returned as an instance group URL: %s", err)
+		}
+		instanceGroupURLs = append(instanceGroupURLs, instanceGroupManager.InstanceGroup)
+	}
+	d.Set("instance_group_urls", instanceGroupURLs)
 
 	return nil
 }

resource_container_cluster_test.go

@@ -4,10 +4,11 @@ import (
 	"fmt"
 	"testing"
 
+	"strconv"
+
 	"github.com/hashicorp/terraform/helper/acctest"
 	"github.com/hashicorp/terraform/helper/resource"
 	"github.com/hashicorp/terraform/terraform"
-	"strconv"
 )
 
 func TestAccContainerCluster_basic(t *testing.T) {
@@ -116,6 +117,23 @@ func TestAccContainerCluster_network(t *testing.T) {
 	})
 }
 
+func TestAccContainerCluster_backend(t *testing.T) {
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckContainerClusterDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: testAccContainerCluster_backendRef,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckContainerClusterExists(
+						"google_container_cluster.primary"),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckContainerClusterDestroy(s *terraform.State) error {
 	config := testAccProvider.Meta().(*Config)
 
@@ -296,3 +314,49 @@ resource "google_container_cluster" "with_net_ref_by_name" {
 
 	network = "${google_compute_network.container_network.name}"
 }`, acctest.RandString(10), acctest.RandString(10), acctest.RandString(10))
+
+var testAccContainerCluster_backendRef = fmt.Sprintf(`
+resource "google_compute_backend_service" "my-backend-service" {
+  name      = "terraform-test-%s"
+  port_name = "http"
+  protocol  = "HTTP"
+
+  backend {
+    group = "${element(google_container_cluster.primary.instance_group_urls, 1)}"
+  }
+
+  health_checks = ["${google_compute_http_health_check.default.self_link}"]
+}
+
+resource "google_compute_http_health_check" "default" {
+  name               = "terraform-test-%s"
+  request_path       = "/"
+  check_interval_sec = 1
+  timeout_sec        = 1
+}
+
+resource "google_container_cluster" "primary" {
+  name               = "terraform-test-%s"
+  zone               = "us-central1-a"
+  initial_node_count = 3
+
+  additional_zones = [
+    "us-central1-b",
+    "us-central1-c",
+  ]
+
+  master_auth {
+    username = "mr.yoda"
+    password = "adoy.rm"
+  }
+
+  node_config {
+    oauth_scopes = [
+      "https://www.googleapis.com/auth/compute",
+      "https://www.googleapis.com/auth/devstorage.read_only",
+      "https://www.googleapis.com/auth/logging.write",
+      "https://www.googleapis.com/auth/monitoring",
+    ]
+  }
+}
+`, acctest.RandString(10), acctest.RandString(10), acctest.RandString(10))