reflect: keep RO flags unchanged in Value.Addr

Currently, Value.Addr collapses flagRO, which is a combination of
flagEmbedRO and flagStickyRO, to flagStickyRO. This causes exported
fields of unexported anonymous field from Value.Addr.Elem read only.

This commit fix this by keeping all bits of flagRO from origin
value in Value.Addr. This should be safe due to following reasons:
* Result of Value.Addr is not CanSet because of it is not CanAddr
   but not flagRO.
* Addr.Elem get same flagRO as origin, so it should behave same as
   origin in CanSet.

Fixes golang#32772.

Change-Id: I79e086628c0fb6569a50ce63f3b95916f997eda1
GitHub-Last-Rev: 78e280e
GitHub-Pull-Request: golang#32787
Reviewed-on: https://go-review.googlesource.com/c/go/+/183937
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/all_test.go

@@ -350,6 +350,7 @@ func TestCanSetField(t *testing.T) {
 	}
 
 	type testCase struct {
+		// -1 means Addr().Elem() of current value
 		index  []int
 		canSet bool
 	}
@@ -360,35 +361,65 @@ func TestCanSetField(t *testing.T) {
 		val: ValueOf(&S1{}),
 		cases: []testCase{
 			{[]int{0}, false},
+			{[]int{0, -1}, false},
 			{[]int{0, 0}, false},
+			{[]int{0, 0, -1}, false},
+			{[]int{0, -1, 0}, false},
+			{[]int{0, -1, 0, -1}, false},
 			{[]int{0, 1}, true},
+			{[]int{0, 1, -1}, true},
+			{[]int{0, -1, 1}, true},
+			{[]int{0, -1, 1, -1}, true},
 			{[]int{1}, false},
+			{[]int{1, -1}, false},
 			{[]int{2}, true},
+			{[]int{2, -1}, true},
 		},
 	}, {
 		val: ValueOf(&S2{embed: &embed{}}),
 		cases: []testCase{
 			{[]int{0}, false},
+			{[]int{0, -1}, false},
 			{[]int{0, 0}, false},
+			{[]int{0, 0, -1}, false},
+			{[]int{0, -1, 0}, false},
+			{[]int{0, -1, 0, -1}, false},
 			{[]int{0, 1}, true},
+			{[]int{0, 1, -1}, true},
+			{[]int{0, -1, 1}, true},
+			{[]int{0, -1, 1, -1}, true},
 			{[]int{1}, false},
 			{[]int{2}, true},
 		},
 	}, {
 		val: ValueOf(&S3{}),
 		cases: []testCase{
 			{[]int{0}, true},
+			{[]int{0, -1}, true},
 			{[]int{0, 0}, false},
+			{[]int{0, 0, -1}, false},
+			{[]int{0, -1, 0}, false},
+			{[]int{0, -1, 0, -1}, false},
 			{[]int{0, 1}, true},
+			{[]int{0, 1, -1}, true},
+			{[]int{0, -1, 1}, true},
+			{[]int{0, -1, 1, -1}, true},
 			{[]int{1}, false},
 			{[]int{2}, true},
 		},
 	}, {
 		val: ValueOf(&S4{Embed: &Embed{}}),
 		cases: []testCase{
 			{[]int{0}, true},
+			{[]int{0, -1}, true},
 			{[]int{0, 0}, false},
+			{[]int{0, 0, -1}, false},
+			{[]int{0, -1, 0}, false},
+			{[]int{0, -1, 0, -1}, false},
 			{[]int{0, 1}, true},
+			{[]int{0, 1, -1}, true},
+			{[]int{0, -1, 1}, true},
+			{[]int{0, -1, 1, -1}, true},
 			{[]int{1}, false},
 			{[]int{2}, true},
 		},
@@ -402,7 +433,11 @@ func TestCanSetField(t *testing.T) {
 					if f.Kind() == Ptr {
 						f = f.Elem()
 					}
-					f = f.Field(i)
+					if i == -1 {
+						f = f.Addr().Elem()
+					} else {
+						f = f.Field(i)
+					}
 				}
 				if got := f.CanSet(); got != tc.canSet {
 					t.Errorf("CanSet() = %v, want %v", got, tc.canSet)

src/reflect/value.go

@@ -269,7 +269,10 @@ func (v Value) Addr() Value {
 	if v.flag&flagAddr == 0 {
 		panic("reflect.Value.Addr of unaddressable value")
 	}
-	return Value{v.typ.ptrTo(), v.ptr, v.flag.ro() | flag(Ptr)}
+	// Preserve flagRO instead of using v.flag.ro() so that
+	// v.Addr().Elem() is equivalent to v (#32772)
+	fl := v.flag & flagRO
+	return Value{v.typ.ptrTo(), v.ptr, fl | flag(Ptr)}
 }
 
 // Bool returns v's underlying value.