GODRIVER-395: add zeroer implementation

Change-Id: I300499d79b01fa2e612a4c707d32757e01303534

Author: tychoish

bson/encode.go

@@ -96,6 +96,13 @@ type DocumentEncoder interface {
 	EncodeDocument(interface{}) (*Document, error)
 }
 
+// Zeroer allows custom struct types to implement a report of zero
+// state. All struct types that don't implement Zeroer or where IsZero
+// returns false are considered to be not zero.
+type Zeroer interface {
+	IsZero() bool
+}
+
 type encoder struct {
 	w io.Writer
 }
@@ -473,7 +480,10 @@ func (e *encoder) isZero(v reflect.Value) bool {
 	case reflect.Interface, reflect.Ptr:
 		return v.IsNil()
 	case reflect.Struct:
-		return reflect.Zero(v.Type()).Interface() == v.Interface()
+		if z, ok := v.Interface().(Zeroer); ok {
+			return z.IsZero()
+		}
+		return false
 	}
 
 	return false

bson/encode_test.go

@@ -9,11 +9,13 @@ package bson
 import (
 	"bytes"
 	"io"
+	"reflect"
 	"testing"
 	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/mongodb/mongo-go-driver/bson/objectid"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestEncoder(t *testing.T) {
@@ -911,6 +913,42 @@ func reflectionEncoderTest(t *testing.T) {
 	}
 }
 
+type zeroTest struct {
+	reportZero bool
+}
+
+func (z zeroTest) IsZero() bool { return z.reportZero }
+
+type nonZeroer struct {
+	value bool
+}
+
+func TestZeoerInterfaceUsedByDecoder(t *testing.T) {
+	enc := &encoder{}
+
+	// cases that are zero, because they are known types or pointers
+	var st *nonZeroer
+	assert.True(t, enc.isZero(reflect.ValueOf(st)))
+	assert.True(t, enc.isZero(reflect.ValueOf(0)))
+	assert.True(t, enc.isZero(reflect.ValueOf(false)))
+
+	// cases that shouldn't be zero
+	st = &nonZeroer{value: false}
+	assert.False(t, enc.isZero(reflect.ValueOf(struct{ val bool }{val: true})))
+	assert.False(t, enc.isZero(reflect.ValueOf(struct{ val bool }{val: false})))
+	assert.False(t, enc.isZero(reflect.ValueOf(st)))
+	st.value = true
+	assert.False(t, enc.isZero(reflect.ValueOf(st)))
+
+	// a test to see if the interface impacts the outcome
+	z := zeroTest{}
+	assert.False(t, enc.isZero(reflect.ValueOf(z)))
+
+	z.reportZero = true
+	assert.True(t, enc.isZero(reflect.ValueOf(z)))
+
+}
+
 func docToBytes(d *Document) []byte {
 	b, err := d.MarshalBSON()
 	if err != nil {