Updating the resource to use a set instead of a list

By using a set for the availability zones, you can use things like
`availability_zones = ["${aws_instance.web.*.availability_zone}"]`
where is very likely multiple of the same zones will be added to the
set. If you use a list here, the list will say it’s changed (even if
you add the same zone) which will force a new resource.

builtin/providers/aws/resource_aws_elb.go

@@ -37,10 +37,13 @@ func resourceAwsElb() *schema.Resource {
 			},
 
 			"availability_zones": &schema.Schema{
-				Type:     schema.TypeList,
+				Type:     schema.TypeSet,
 				Elem:     &schema.Schema{Type: schema.TypeString},
 				Optional: true,
 				ForceNew: true,
+				Set: func(v interface{}) int {
+					return hashcode.String(v.(string))
+				},
 			},
 
 			"instances": &schema.Schema{
@@ -172,7 +175,7 @@ func resourceAwsElbCreate(d *schema.ResourceData, meta interface{}) error {
 	}
 
 	if v, ok := d.GetOk("availability_zones"); ok {
-		elbOpts.AvailZone = expandStringList(v.([]interface{}))
+		elbOpts.AvailZone = expandStringList(v.(*schema.Set).List())
 	}
 
 	if v, ok := d.GetOk("security_groups"); ok {
@@ -223,8 +226,7 @@ func resourceAwsElbCreate(d *schema.ResourceData, meta interface{}) error {
 			}
 		}
 	}
-	
-	
+
 	return resourceAwsElbUpdate(d, meta)
 }
 
@@ -255,6 +257,7 @@ func resourceAwsElbRead(d *schema.ResourceData, meta interface{}) error {
 	d.Set("name", lb.LoadBalancerName)
 	d.Set("dns_name", lb.DNSName)
 	d.Set("internal", lb.Scheme == "internal")
+	d.Set("availability_zones", flattenAvailabilityZones(lb.AvailabilityZones))
 	d.Set("instances", flattenInstances(lb.Instances))
 	d.Set("listener", flattenListeners(lb.Listeners))
 	d.Set("security_groups", lb.SecurityGroups)
@@ -306,25 +309,25 @@ func resourceAwsElbUpdate(d *schema.ResourceData, meta interface{}) error {
 				return fmt.Errorf("Failure deregistering instances: %s", err)
 			}
 		}
-		
+
 		d.SetPartial("instances")
 	}
 
 	log.Println("[INFO] outside modify attributes")
-		if d.HasChange("cross_zone_load_balancing") {
-			log.Println("[INFO] inside modify attributes")
-			attrs := elb.ModifyLoadBalancerAttributes{
-				LoadBalancerName: d.Get("name").(string),
-				LoadBalancerAttributes: elb.LoadBalancerAttributes{
-					CrossZoneLoadBalancingEnabled:  d.Get("cross_zone_load_balancing").(bool),
-				},
-			}
-			_, err := elbconn.ModifyLoadBalancerAttributes(&attrs)
-			if err != nil {
-				return fmt.Errorf("Failure configuring health check: %s", err)
-			}
-			d.SetPartial("cross_zone_load_balancing")
+	if d.HasChange("cross_zone_load_balancing") {
+		log.Println("[INFO] inside modify attributes")
+		attrs := elb.ModifyLoadBalancerAttributes{
+			LoadBalancerName: d.Get("name").(string),
+			LoadBalancerAttributes: elb.LoadBalancerAttributes{
+				CrossZoneLoadBalancingEnabled: d.Get("cross_zone_load_balancing").(bool),
+			},
 		}
+		_, err := elbconn.ModifyLoadBalancerAttributes(&attrs)
+		if err != nil {
+			return fmt.Errorf("Failure configuring health check: %s", err)
+		}
+		d.SetPartial("cross_zone_load_balancing")
+	}
 
 	d.Partial(false)
 	return resourceAwsElbRead(d, meta)

builtin/providers/aws/structure.go

@@ -172,6 +172,15 @@ func flattenInstances(list []elb.Instance) []string {
 	return result
 }
 
+// Flattens an array of AvailabilityZones into a []string
+func flattenAvailabilityZones(list []elb.AvailabilityZone) []string {
+	result := make([]string, 0, len(list))
+	for _, z := range list {
+		result = append(result, z.AvailabilityZone)
+	}
+	return result
+}
+
 // Flattens an array of Listeners into a []map[string]interface{}
 func flattenListeners(list []elb.Listener) []map[string]interface{} {
 	result := make([]map[string]interface{}, 0, len(list))