From a72b87efa934957245449975a940763f49026a7c Mon Sep 17 00:00:00 2001
From: Russ Cox <rsc@golang.org>
Date: Thu, 1 Mar 2012 11:48:27 -0500
Subject: [PATCH] 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 | 12 ++++++++++++
 src/pkg/reflect/value.go    | 10 ++++++++++
 2 files changed, 22 insertions(+)

diff --git a/src/pkg/reflect/all_test.go b/src/pkg/reflect/all_test.go
index b21a99ec5c..6bb0613981 100644
--- a/src/pkg/reflect/all_test.go
+++ b/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)
+	}
+}
diff --git a/src/pkg/reflect/value.go b/src/pkg/reflect/value.go
index df65dcfa5b..39f8acdea4 100644
--- a/src/pkg/reflect/value.go
+++ b/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))
 }
 
-- 
2.48.1