simpler, more robust bypass

We make the bypass implementation a little simpler
by inferring the flag field position from available
reflect information and more robust by checking
that the flags that are set actually match the
semantics we expect.

We can restrict the use of unsafe to a single function: flagField.

spew/bypass.go

@@ -16,7 +16,9 @@
 // when the code is not running on Google App Engine, compiled by GopherJS, and
 // "-tags safe" is not added to the go build command line.  The "disableunsafe"
 // tag is deprecated and thus should not be used.
-// +build !js,!appengine,!safe,!disableunsafe
+// Go versions prior to 1.4 are disabled because they use a different layout
+// for interfaces which make the implementation of unsafeReflectValue more complex.
+// +build !js,!appengine,!safe,!disableunsafe,go1.4
 
 package spew
 
@@ -34,80 +36,49 @@ const (
 	ptrSize = unsafe.Sizeof((*byte)(nil))
 )
 
+type flag uintptr
+
 var (
-	// offsetPtr, offsetScalar, and offsetFlag are the offsets for the
-	// internal reflect.Value fields.  These values are valid before golang
-	// commit ecccf07e7f9d which changed the format.  The are also valid
-	// after commit 82f48826c6c7 which changed the format again to mirror
-	// the original format.  Code in the init function updates these offsets
-	// as necessary.
-	offsetPtr    = ptrSize
-	offsetScalar = uintptr(0)
-	offsetFlag   = ptrSize * 2
-
-	// flagKindWidth and flagKindShift indicate various bits that the
-	// reflect package uses internally to track kind information.
-	//
-	// flagRO indicates whether or not the value field of a reflect.Value is
-	// read-only.
-	//
-	// flagIndir indicates whether the value field of a reflect.Value is
-	// the actual data or a pointer to the data.
-	//
-	// These values are valid before golang commit 90a7c3c86944 which
-	// changed their positions.  Code in the init function updates these
-	// flags as necessary.
-	flagKindWidth = uintptr(5)
-	flagKindShift = flagKindWidth - 1
-	flagRO        = uintptr(1 << 0)
-	flagIndir     = uintptr(1 << 1)
+	// flagRO indicates whether the value field of a reflect.Value
+	// is read-only.
+	flagRO flag
+
+	// flagAddr indicates whether the address of the reflect.Value's
+	// value may be taken.
+	flagAddr flag
 )
 
-func init() {
-	// Older versions of reflect.Value stored small integers directly in the
-	// ptr field (which is named val in the older versions).  Versions
-	// between commits ecccf07e7f9d and 82f48826c6c7 added a new field named
-	// scalar for this purpose which unfortunately came before the flag
-	// field, so the offset of the flag field is different for those
-	// versions.
-	//
-	// This code constructs a new reflect.Value from a known small integer
-	// and checks if the size of the reflect.Value struct indicates it has
-	// the scalar field. When it does, the offsets are updated accordingly.
-	vv := reflect.ValueOf(0xf00)
-	if unsafe.Sizeof(vv) == (ptrSize * 4) {
-		offsetScalar = ptrSize * 2
-		offsetFlag = ptrSize * 3
-	}
+// flagKindMask holds the bits that make up the kind
+// part of the flags field. In all the supported versions,
+// it is in the lower 5 bits.
+const flagKindMask = flag(0x1f)
 
-	// Commit 90a7c3c86944 changed the flag positions such that the low
-	// order bits are the kind.  This code extracts the kind from the flags
-	// field and ensures it's the correct type.  When it's not, the flag
-	// order has been changed to the newer format, so the flags are updated
-	// accordingly.
-	upf := unsafe.Pointer(uintptr(unsafe.Pointer(&vv)) + offsetFlag)
-	upfv := *(*uintptr)(upf)
-	flagKindMask := uintptr((1<<flagKindWidth - 1) << flagKindShift)
-	if (upfv&flagKindMask)>>flagKindShift != uintptr(reflect.Int) {
-		flagKindShift = 0
-		flagRO = 1 << 5
-		flagIndir = 1 << 6
-
-		// Commit adf9b30e5594 modified the flags to separate the
-		// flagRO flag into two bits which specifies whether or not the
-		// field is embedded.  This causes flagIndir to move over a bit
-		// and means that flagRO is the combination of either of the
-		// original flagRO bit and the new bit.
-		//
-		// This code detects the change by extracting what used to be
-		// the indirect bit to ensure it's set.  When it's not, the flag
-		// order has been changed to the newer format, so the flags are
-		// updated accordingly.
-		if upfv&flagIndir == 0 {
-			flagRO = 3 << 5
-			flagIndir = 1 << 7
-		}
+// Different versions of Go have used different
+// bit layouts for the flags type. This table
+// records the known combinations.
+var okFlags = []struct {
+	ro, addr flag
+}{{
+	// From Go 1.4 to 1.5
+	ro:   1 << 5,
+	addr: 1 << 7,
+}, {
+	// Up to Go tip.
+	ro:   1<<5 | 1<<6,
+	addr: 1 << 8,
+}}
+
+var flagValOffset = func() uintptr {
+	field, ok := reflect.TypeOf(reflect.Value{}).FieldByName("flag")
+	if !ok {
+		panic("reflect.Value has no flag field")
 	}
+	return field.Offset
+}()
+
+// flagField returns a pointer to the flag field of a reflect.Value.
+func flagField(v *reflect.Value) *flag {
+	return (*flag)(unsafe.Pointer(uintptr(unsafe.Pointer(v)) + flagValOffset))
 }
 
 // unsafeReflectValue converts the passed reflect.Value into a one that bypasses
@@ -119,34 +90,56 @@ func init() {
 // This allows us to check for implementations of the Stringer and error
 // interfaces to be used for pretty printing ordinarily unaddressable and
 // inaccessible values such as unexported struct fields.
-func unsafeReflectValue(v reflect.Value) (rv reflect.Value) {
-	indirects := 1
-	vt := v.Type()
-	upv := unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + offsetPtr)
-	rvf := *(*uintptr)(unsafe.Pointer(uintptr(unsafe.Pointer(&v)) + offsetFlag))
-	if rvf&flagIndir != 0 {
-		vt = reflect.PtrTo(v.Type())
-		indirects++
-	} else if offsetScalar != 0 {
-		// The value is in the scalar field when it's not one of the
-		// reference types.
-		switch vt.Kind() {
-		case reflect.Uintptr:
-		case reflect.Chan:
-		case reflect.Func:
-		case reflect.Map:
-		case reflect.Ptr:
-		case reflect.UnsafePointer:
-		default:
-			upv = unsafe.Pointer(uintptr(unsafe.Pointer(&v)) +
-				offsetScalar)
-		}
+func unsafeReflectValue(v reflect.Value) reflect.Value {
+	if !v.IsValid() || (v.CanInterface() && v.CanAddr()) {
+		return v
 	}
+	flagFieldPtr := flagField(&v)
+	*flagFieldPtr &^= flagRO
+	*flagFieldPtr |= flagAddr
+	return v
+}
 
-	pv := reflect.NewAt(vt, upv)
-	rv = pv
-	for i := 0; i < indirects; i++ {
-		rv = rv.Elem()
+// Sanity checks against future reflect package changes
+// to the type or semantics of the Value.flag field.
+func init() {
+	field, ok := reflect.TypeOf(reflect.Value{}).FieldByName("flag")
+	if !ok {
+		panic("reflect.Value has no flag field")
+	}
+	if field.Type.Kind() != reflect.TypeOf(flag(0)).Kind() {
+		panic("reflect.Value flag field has changed kind")
+	}
+	type t0 int
+	var t struct {
+		A t0
+		// t0 will have flagEmbedRO set.
+		t0
+		// a will have flagStickyRO set
+		a t0
+	}
+	vA := reflect.ValueOf(t).FieldByName("A")
+	va := reflect.ValueOf(t).FieldByName("a")
+	vt0 := reflect.ValueOf(t).FieldByName("t0")
+
+	// Infer flagRO from the difference between the flags
+	// for the (otherwise identical) fields in t.
+	flagPublic := *flagField(&vA)
+	flagWithRO := *flagField(&va) | *flagField(&vt0)
+	flagRO = flagPublic ^ flagWithRO
+
+	// Infer flagAddr from the difference between a value
+	// taken from a pointer and not.
+	vPtrA := reflect.ValueOf(&t).Elem().FieldByName("A")
+	flagNoPtr := *flagField(&vA)
+	flagPtr := *flagField(&vPtrA)
+	flagAddr = flagNoPtr ^ flagPtr
+
+	// Check that the inferred flags tally with one of the known versions.
+	for _, f := range okFlags {
+		if flagRO == f.ro && flagAddr == f.addr {
+			return
+		}
 	}
-	return rv
+	panic("reflect.Value read-only flag has changed semantics")
 }

spew/bypasssafe.go

@@ -16,7 +16,7 @@
 // when the code is running on Google App Engine, compiled by GopherJS, or
 // "-tags safe" is added to the go build command line.  The "disableunsafe"
 // tag is deprecated and thus should not be used.
-// +build js appengine safe disableunsafe
+// +build js appengine safe disableunsafe !go1.4
 
 package spew
 

spew/internalunsafe_test.go

@@ -16,7 +16,7 @@
 // when the code is not running on Google App Engine, compiled by GopherJS, and
 // "-tags safe" is not added to the go build command line.  The "disableunsafe"
 // tag is deprecated and thus should not be used.
-// +build !js,!appengine,!safe,!disableunsafe
+// +build !js,!appengine,!safe,!disableunsafe,go1.4
 
 /*
 This test file is part of the spew package rather than than the spew_test
@@ -30,7 +30,6 @@ import (
 	"bytes"
 	"reflect"
 	"testing"
-	"unsafe"
 )
 
 // changeKind uses unsafe to intentionally change the kind of a reflect.Value to
@@ -38,13 +37,13 @@ import (
 // fallback code which punts to the standard fmt library for new types that
 // might get added to the language.
 func changeKind(v *reflect.Value, readOnly bool) {
-	rvf := (*uintptr)(unsafe.Pointer(uintptr(unsafe.Pointer(v)) + offsetFlag))
-	*rvf = *rvf | ((1<<flagKindWidth - 1) << flagKindShift)
+	flags := flagField(v)
 	if readOnly {
-		*rvf |= flagRO
+		*flags |= flagRO
 	} else {
-		*rvf &= ^uintptr(flagRO)
+		*flags &^= flagRO
 	}
+	*flags |= flagKindMask
 }
 
 // TestAddedReflectValue tests functionaly of the dump and formatter code which