helper/schema: if update sets the ID to blank, it deletes the resource

helper/schema/resource.go

@@ -117,7 +117,12 @@ func (r *Resource) Refresh(
 	}
 
 	err = r.Read(data, meta)
-	return data.State(), err
+	state := data.State()
+	if state.ID == "" {
+		state = nil
+	}
+
+	return state, err
 }
 
 // InternalValidate should be called to validate the structure

helper/schema/resource_test.go

@@ -304,3 +304,35 @@ func TestResourceRefresh(t *testing.T) {
 		t.Fatalf("bad: %#v", actual)
 	}
 }
+
+func TestResourceRefresh_delete(t *testing.T) {
+	r := &Resource{
+		Schema: map[string]*Schema{
+			"foo": &Schema{
+				Type:     TypeInt,
+				Optional: true,
+			},
+		},
+	}
+
+	r.Read = func(d *ResourceData, m interface{}) error {
+		d.SetId("")
+		return nil
+	}
+
+	s := &terraform.ResourceState{
+		ID: "bar",
+		Attributes: map[string]string{
+			"foo": "12",
+		},
+	}
+
+	actual, err := r.Refresh(s, 42)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if actual != nil {
+		t.Fatalf("bad: %#v", actual)
+	}
+}