gob: don't invoke GobEncoder on zero values.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4801076

src/pkg/gob/encode.go

@@ -466,6 +466,27 @@ func (enc *Encoder) encodeInterface(b *bytes.Buffer, iv reflect.Value) {
 	enc.freeEncoderState(state)
 }
 
+// isZero returns whether the value is the zero of its type.
+func isZero(val reflect.Value) bool {
+	switch val.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
+		return val.Len() == 0
+	case reflect.Bool:
+		return !val.Bool()
+	case reflect.Complex64, reflect.Complex128:
+		return val.Complex() == 0
+	case reflect.Chan, reflect.Func, reflect.Ptr:
+		return val.IsNil()
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		return val.Int() == 0
+	case reflect.Float32, reflect.Float64:
+		return val.Float() == 0
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return val.Uint() == 0
+	}
+	panic("unknown type in isZero" + val.Type().String())
+}
+
 // encGobEncoder encodes a value that implements the GobEncoder interface.
 // The data is sent as a byte array.
 func (enc *Encoder) encodeGobEncoder(b *bytes.Buffer, v reflect.Value) {
@@ -614,6 +635,9 @@ func (enc *Encoder) gobEncodeOpFor(ut *userTypeInfo) (*encOp, int) {
 		} else {
 			v = reflect.ValueOf(unsafe.Unreflect(rt, p))
 		}
+		if !state.sendZero && isZero(v) {
+			return
+		}
 		state.update(i)
 		state.enc.encodeGobEncoder(state.b, v)
 	}

src/pkg/gob/gobencdec_test.go

@@ -466,3 +466,25 @@ func TestGobEncoderIgnoreNonStructField(t *testing.T) {
 		t.Errorf("expected 17 got %c", x.X)
 	}
 }
+
+func TestGobEncoderIgnoreNilEncoder(t *testing.T) {
+	b := new(bytes.Buffer)
+	// First a field that's a structure.
+	enc := NewEncoder(b)
+	err := enc.Encode(GobTest0{X: 18}) // G is nil
+	if err != nil {
+		t.Fatal("encode error:", err)
+	}
+	dec := NewDecoder(b)
+	x := new(GobTest0)
+	err = dec.Decode(x)
+	if err != nil {
+		t.Fatal("decode error:", err)
+	}
+	if x.X != 18 {
+		t.Errorf("expected x.X = 18, got %v", x.X)
+	}
+	if x.G != nil {
+		t.Errorf("expected x.G = nil, got %v", x.G)
+	}
+}