reflect: allow Value.Bytes on addressable byte arrays

Modify Value.Bytes to be callable addressable byte arrays.
While related, the behavior of Value.SetBytes was not modified.

Fixes #47066

Change-Id: Ic3ba4432353b8da5f33b3188e20034a33b2f6ee8
Reviewed-on: https://go-review.googlesource.com/c/go/+/357331
Trust: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 47 insertions, 8 deletions

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index 866f38e687..5364166eab 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -3682,8 +3682,11 @@ func TestTagGet(t *testing.T) {
 }
 
 func TestBytes(t *testing.T) {
-	type B []byte
-	x := B{1, 2, 3, 4}
+	shouldPanic("on int Value", func() { ValueOf(0).Bytes() })
+	shouldPanic("of non-byte slice", func() { ValueOf([]string{}).Bytes() })
+
+	type S []byte
+	x := S{1, 2, 3, 4}
 	y := ValueOf(x).Bytes()
 	if !bytes.Equal(x, y) {
 		t.Fatalf("ValueOf(%v).Bytes() = %v", x, y)
@@ -3691,6 +3694,28 @@ func TestBytes(t *testing.T) {
 	if &x[0] != &y[0] {
 		t.Errorf("ValueOf(%p).Bytes() = %p", &x[0], &y[0])
 	}
+
+	type A [4]byte
+	a := A{1, 2, 3, 4}
+	shouldPanic("unaddressable", func() { ValueOf(a).Bytes() })
+	shouldPanic("on ptr Value", func() { ValueOf(&a).Bytes() })
+	b := ValueOf(&a).Elem().Bytes()
+	if !bytes.Equal(a[:], y) {
+		t.Fatalf("ValueOf(%v).Bytes() = %v", a, b)
+	}
+	if &a[0] != &b[0] {
+		t.Errorf("ValueOf(%p).Bytes() = %p", &a[0], &b[0])
+	}
+
+	// Per issue #24746, it was decided that Bytes can be called on byte slices
+	// that normally cannot be converted from per Go language semantics.
+	type B byte
+	type SB []B
+	type AB [4]B
+	ValueOf([]B{1, 2, 3, 4}).Bytes()  // should not panic
+	ValueOf(new([4]B)).Elem().Bytes() // should not panic
+	ValueOf(SB{1, 2, 3, 4}).Bytes()   // should not panic
+	ValueOf(new(AB)).Elem().Bytes()   // should not panic
 }
 
 func TestSetBytes(t *testing.T) {
diff --git a/src/reflect/value.go b/src/reflect/value.go
index dcc359dae4..89f0253570 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -286,14 +286,28 @@ func (v Value) Bool() bool {
 }
 
 // Bytes returns v's underlying value.
-// It panics if v's underlying value is not a slice of bytes.
+// It panics if v's underlying value is not a slice of bytes or
+// an addressable array of bytes.
 func (v Value) Bytes() []byte {
-	v.mustBe(Slice)
-	if v.typ.Elem().Kind() != Uint8 {
-		panic("reflect.Value.Bytes of non-byte slice")
+	switch v.kind() {
+	case Slice:
+		if v.typ.Elem().Kind() != Uint8 {
+			panic("reflect.Value.Bytes of non-byte slice")
+		}
+		// Slice is always bigger than a word; assume flagIndir.
+		return *(*[]byte)(v.ptr)
+	case Array:
+		if v.typ.Elem().Kind() != Uint8 {
+			panic("reflect.Value.Bytes of non-byte array")
+		}
+		if !v.CanAddr() {
+			panic("reflect.Value.Bytes of unaddressable byte array")
+		}
+		p := (*byte)(v.ptr)
+		n := int((*arrayType)(unsafe.Pointer(v.typ)).len)
+		return unsafe.Slice(p, n)
 	}
-	// Slice is always bigger than a word; assume flagIndir.
-	return *(*[]byte)(v.ptr)
+	panic(&ValueError{"reflect.Value.Bytes", v.kind()})
 }
 
 // runes returns v's underlying value.