gob: isZero for struct values

Fixes #2577.

R=golang-dev, r, gri
CC=golang-dev
https://golang.org/cl/5492058

src/pkg/encoding/gob/encode.go

@@ -483,6 +483,13 @@ func isZero(val reflect.Value) bool {
 		return val.Float() == 0
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 		return val.Uint() == 0
+	case reflect.Struct:
+		for i := 0; i < val.NumField(); i++ {
+			if !isZero(val.Field(i)) {
+				return false
+			}
+		}
+		return true
 	}
 	panic("unknown type in isZero " + val.Type().String())
 }

src/pkg/encoding/gob/gobencdec_test.go

@@ -13,6 +13,7 @@ import (
 	"io"
 	"strings"
 	"testing"
+	"time"
 )
 
 // Types that implement the GobEncoder/Decoder interfaces.
@@ -526,3 +527,30 @@ func TestGobEncoderExtraIndirect(t *testing.T) {
 		t.Errorf("got = %q, want %q", got, gdb)
 	}
 }
+
+// Another bug: this caused a crash with the new Go1 Time type.
+
+type TimeBug struct {
+	T time.Time
+	S string
+	I int
+}
+
+func TestGobEncodeTime(t *testing.T) {
+	x := TimeBug{time.Now(), "hello", -55}
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	err := enc.Encode(x)
+	if err != nil {
+		t.Fatal("encode:", err)
+	}
+	var y TimeBug
+	dec := NewDecoder(b)
+	err = dec.Decode(&y)
+	if err != nil {
+		t.Fatal("decode:", err)
+	}
+	if x != y {
+		t.Fatal("%v != %v", x, y)
+	}
+}