helper/schema: use interface for equality check

/cc @svanharmelen

helper/schema/equal.go

@@ -0,0 +1,6 @@
+package schema
+
+// Equal is an interface that checks for deep equality between two objects.
+type Equal interface {
+	Equal(interface{}) bool
+}

helper/schema/resource_data.go

@@ -106,11 +106,11 @@ func (d *ResourceData) getRaw(key string, level getSource) getResult {
 func (d *ResourceData) HasChange(key string) bool {
 	o, n := d.GetChange(key)
 
-	// There is a special case needed for *schema.Set's as they contain
-	// a function and reflect.DeepEqual will only say two functions are
-	// equal when they are both nil (which in this case they are not).
-	if reflect.TypeOf(o).String() == "*schema.Set" {
-		return !reflect.DeepEqual(o.(*Set).m, n.(*Set).m)
+	// If the type implements the Equal interface, then call that
+	// instead of just doing a reflect.DeepEqual. An example where this is
+	// needed is *Set
+	if eq, ok := o.(Equal); ok {
+		return !eq.Equal(n)
 	}
 
 	return !reflect.DeepEqual(o, n)

helper/schema/set.go

@@ -2,6 +2,7 @@ package schema
 
 import (
 	"fmt"
+	"reflect"
 	"sort"
 	"sync"
 )
@@ -101,6 +102,15 @@ func (s *Set) Union(other *Set) *Set {
 	return result
 }
 
+func (s *Set) Equal(raw interface{}) bool {
+	other, ok := raw.(*Set)
+	if !ok {
+		return false
+	}
+
+	return reflect.DeepEqual(s.m, other.m)
+}
+
 func (s *Set) GoString() string {
 	return fmt.Sprintf("*Set(%#v)", s.m)
 }