Update google_compute_target_pool to no longer have a plan/apply loop with instance URLs (#666)

* Using instance URLs in `google_compute_target_pool` no longer has a plan/apply loop

* global regex; parts length check

google/resource_compute_target_pool.go

@@ -3,12 +3,15 @@ package google
 import (
 	"fmt"
 	"log"
+	"regexp"
 	"strings"
 
 	"github.com/hashicorp/terraform/helper/schema"
 	"google.golang.org/api/compute/v1"
 )
 
+var instancesSelfLinkPattern = regexp.MustCompile(fmt.Sprintf(zonalLinkBasePattern, "instances"))
+
 func resourceComputeTargetPool() *schema.Resource {
 	return &schema.Resource{
 		Create: resourceComputeTargetPoolCreate,
@@ -57,6 +60,25 @@ func resourceComputeTargetPool() *schema.Resource {
 				Computed: true,
 				ForceNew: false,
 				Elem:     &schema.Schema{Type: schema.TypeString},
+				DiffSuppressFunc: func(k, old, new string, d *schema.ResourceData) bool {
+					// instances are stored in state as "zone/name"
+					oldParts := strings.Split(old, "/")
+
+					// instances can also be specified in the config as a URL
+					if parts := instancesSelfLinkPattern.FindStringSubmatch(new); len(oldParts) == 2 && len(parts) == 4 {
+						// parts[0] = full match
+						// parts[1] = project
+						// parts[2] = zone
+						// parts[3] = instance name
+
+						oZone, oName := oldParts[0], oldParts[1]
+						nZone, nName := parts[2], parts[3]
+						if oZone == nZone && oName == nName {
+							return true
+						}
+					}
+					return false
+				},
 			},
 
 			"project": {

google/resource_compute_target_pool_test.go

@@ -79,12 +79,28 @@ resource "google_compute_http_health_check" "foobar" {
 	host = "example.com"
 }
 
+resource "google_compute_instance" "foobar" {
+	name         = "inst-tp-test-%s"
+	machine_type = "n1-standard-1"
+	zone         = "us-central1-a"
+
+	boot_disk {
+		initialize_params{
+			image = "debian-8-jessie-v20160803"
+		}
+	}
+
+	network_interface {
+		network = "default"
+	}
+}
+
 resource "google_compute_target_pool" "foobar" {
 	description = "Resource created for Terraform acceptance testing"
-	instances = ["us-central1-a/foo", "us-central1-b/bar"]
+	instances = ["${google_compute_instance.foobar.self_link}", "us-central1-b/bar"]
 	name = "tpool-test-%s"
 	session_affinity = "CLIENT_IP_PROTO"
 	health_checks = [
 		"${google_compute_http_health_check.foobar.name}"
 	]
-}`, acctest.RandString(10), acctest.RandString(10))
+}`, acctest.RandString(10), acctest.RandString(10), acctest.RandString(10))