helper/schema: ensure ForceNew set when Update is not

If a given resource does not define an `Update` function, then all of
its attributes must be specified as `ForceNew`, lest Applys fail with
"doesn't support update" like #1367.

This is something we can detect automatically, so this adds a check for
it when we validate provider implementations.

builtin/providers/aws/resource_aws_launch_configuration.go

@@ -84,6 +84,7 @@ func resourceAwsLaunchConfiguration() *schema.Resource {
 			"associate_public_ip_address": &schema.Schema{
 				Type:     schema.TypeBool,
 				Optional: true,
+				ForceNew: true,
 				Default:  false,
 			},
 
@@ -96,6 +97,7 @@ func resourceAwsLaunchConfiguration() *schema.Resource {
 			"ebs_optimized": &schema.Schema{
 				Type:     schema.TypeBool,
 				Optional: true,
+				ForceNew: true,
 				Default:  false,
 			},
 

builtin/providers/google/resource_compute_route.go

@@ -75,6 +75,7 @@ func resourceComputeRoute() *schema.Resource {
 			"tags": &schema.Schema{
 				Type:     schema.TypeSet,
 				Optional: true,
+				ForceNew: true,
 				Elem:     &schema.Schema{Type: schema.TypeString},
 				Set: func(v interface{}) int {
 					return hashcode.String(v.(string))

helper/schema/resource.go

@@ -225,9 +225,31 @@ func (r *Resource) InternalValidate() error {
 		return errors.New("resource is nil")
 	}
 
+	if r.isTopLevel() {
+		// All non-Computed attributes must be ForceNew if Update is not defined
+		if r.Update == nil {
+			nonForceNewAttrs := make([]string, 0)
+			for k, v := range r.Schema {
+				if !v.ForceNew && !v.Computed {
+					nonForceNewAttrs = append(nonForceNewAttrs, k)
+				}
+			}
+			if len(nonForceNewAttrs) > 0 {
+				return fmt.Errorf(
+					"No Update defined, must set ForceNew on: %#v", nonForceNewAttrs)
+			}
+		}
+	}
+
 	return schemaMap(r.Schema).InternalValidate()
 }
 
+// Returns true if the resource is "top level" i.e. not a sub-resource.
+func (r *Resource) isTopLevel() bool {
+	// TODO: This is a heuristic; replace with a definitive attribute?
+	return r.Create != nil
+}
+
 // Determines if a given InstanceState needs to be migrated by checking the
 // stored version number with the current SchemaVersion
 func (r *Resource) checkSchemaVersion(is *terraform.InstanceState) (bool, int) {