Add support for limited mode without unsafe pkg.

This commit adds support for compiling spew without the unsafe package.
When compiled without the unsafe package, some of the more advanced
features such as invoking stringers on pointers from non-pointer
variables and unexported struct fields are not available.

By default, spew will be compiled in the limited mode for Google App
Engine since the unsafe package is not available there.  Additionally,
spew can be compiled without the unsafe package manually by specifying
the "disableunsafe" build tag.

Finally, a new package-level constant named "UnsafeDisabled" has been
exposed which can be used to programmatically determine if spew was
compiled with access to the unsafe package.

1 files changed, 13 insertions, 122 deletions

diff --git a/spew/common.go b/spew/common.go
index 8252cd3..14f02dc 100644
--- a/spew/common.go
+++ b/spew/common.go
@@ -23,116 +23,8 @@ import (
 	"reflect"
 	"sort"
 	"strconv"
-	"unsafe"
 )
 
-const (
-	// ptrSize is the size of a pointer on the current arch.
-	ptrSize = unsafe.Sizeof((*byte)(nil))
-)
-
-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    = uintptr(ptrSize)
-	offsetScalar = uintptr(0)
-	offsetFlag   = uintptr(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 = uintptr(flagKindWidth - 1)
-	flagRO        = uintptr(1 << 0)
-	flagIndir     = uintptr(1 << 1)
-)
-
-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
-	}
-
-	// 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
-	}
-}
-
-// unsafeReflectValue converts the passed reflect.Value into a one that bypasses
-// the typical safety restrictions preventing access to unaddressable and
-// unexported data.  It works by digging the raw pointer to the underlying
-// value out of the protected value and generating a new unprotected (unsafe)
-// reflect.Value to it.
-//
-// 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)
-		}
-	}
-
-	pv := reflect.NewAt(vt, upv)
-	rv = pv
-	for i := 0; i < indirects; i++ {
-		rv = rv.Elem()
-	}
-	return rv
-}
-
 // Some constants in the form of bytes to avoid string overhead.  This mirrors
 // the technique used in the fmt package.
 var (
@@ -194,9 +86,14 @@ func handleMethods(cs *ConfigState, w io.Writer, v reflect.Value) (handled bool)
 	// We need an interface to check if the type implements the error or
 	// Stringer interface.  However, the reflect package won't give us an
 	// interface on certain things like unexported struct fields in order
-	// to enforce visibility rules.  We use unsafe to bypass these restrictions
-	// since this package does not mutate the values.
+	// to enforce visibility rules.  We use unsafe, when it's available,
+	// to bypass these restrictions since this package does not mutate the
+	// values.
 	if !v.CanInterface() {
+		if UnsafeDisabled {
+			return false
+		}
+
 		v = unsafeReflectValue(v)
 	}
 
@@ -206,21 +103,15 @@ func handleMethods(cs *ConfigState, w io.Writer, v reflect.Value) (handled bool)
 	// mutate the value, however, types which choose to satisify an error or
 	// Stringer interface with a pointer receiver should not be mutating their
 	// state inside these interface methods.
-	var viface interface{}
-	if !cs.DisablePointerMethods {
-		if !v.CanAddr() {
-			v = unsafeReflectValue(v)
-		}
-		viface = v.Addr().Interface()
-	} else {
-		if v.CanAddr() {
-			v = v.Addr()
-		}
-		viface = v.Interface()
+	if !cs.DisablePointerMethods && !UnsafeDisabled && !v.CanAddr() {
+		v = unsafeReflectValue(v)
+	}
+	if v.CanAddr() {
+		v = v.Addr()
 	}
 
 	// Is it an error or Stringer?
-	switch iface := viface.(type) {
+	switch iface := v.Interface().(type) {
 	case error:
 		defer catchPanic(w, v)
 		if cs.ContinueOnMethod {