helper/schema: nice error if update isn't supported

helper/schema/resource.go

@@ -2,6 +2,7 @@ package schema
 
 import (
 	"errors"
+	"fmt"
 
 	"github.com/hashicorp/terraform/terraform"
 )
@@ -73,6 +74,10 @@ func (r *Resource) Apply(
 		// We're creating, it is a new resource.
 		err = r.Create(data, meta)
 	} else {
+		if r.Update == nil {
+			return s, fmt.Errorf("%s doesn't support update", s.Type)
+		}
+
 		err = r.Update(data, meta)
 	}
 

helper/schema/resource_test.go

@@ -186,6 +186,50 @@ func TestResourceApply_update(t *testing.T) {
 	}
 }
 
+func TestResourceApply_updateNoCallback(t *testing.T) {
+	r := &Resource{
+		Schema: map[string]*Schema{
+			"foo": &Schema{
+				Type:     TypeInt,
+				Optional: true,
+			},
+		},
+	}
+
+	r.Update = nil
+
+	s := &terraform.ResourceState{
+		ID: "foo",
+		Attributes: map[string]string{
+			"foo": "12",
+		},
+	}
+
+	d := &terraform.ResourceDiff{
+		Attributes: map[string]*terraform.ResourceAttrDiff{
+			"foo": &terraform.ResourceAttrDiff{
+				New: "13",
+			},
+		},
+	}
+
+	actual, err := r.Apply(s, d, nil)
+	if err == nil {
+		t.Fatal("should error")
+	}
+
+	expected := &terraform.ResourceState{
+		ID: "foo",
+		Attributes: map[string]string{
+			"foo": "12",
+		},
+	}
+
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad: %#v", actual)
+	}
+}
+
 func TestResourceInternalValidate(t *testing.T) {
 	cases := []struct {
 		In  *Resource