reflect: make Value.Interface return immutable data

Fixes #3134.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5713049

src/pkg/reflect/all_test.go

@@ -1743,3 +1743,15 @@ func isValid(v Value) {
 		panic("zero Value")
 	}
 }
+
+func TestAlias(t *testing.T) {
+	x := string("hello")
+	v := ValueOf(&x).Elem()
+	oldvalue := v.Interface()
+	v.SetString("world")
+	newvalue := v.Interface()
+
+	if oldvalue != "hello" || newvalue != "world" {
+		t.Errorf("aliasing: old=%q new=%q, want hello, world", oldvalue, newvalue)
+	}
+}

src/pkg/reflect/value.go

@@ -842,6 +842,16 @@ func valueInterface(v Value, safe bool) interface{} {
 	var eface emptyInterface
 	eface.typ = v.typ.runtimeType()
 	eface.word = v.iword()
+
+	if v.flag&flagIndir != 0 && v.typ.size > ptrSize {
+		// eface.word is a pointer to the actual data,
+		// which might be changed.  We need to return
+		// a pointer to unchanging data, so make a copy.
+		ptr := unsafe_New(v.typ)
+		memmove(ptr, unsafe.Pointer(eface.word), v.typ.size)
+		eface.word = iword(ptr)
+	}
+
 	return *(*interface{})(unsafe.Pointer(&eface))
 }