os: use filepathlite.IsAbs

It is better to have a single implementation of IsAbs, which is quite
tricky to get right on Windows.

Change-Id: I45933b0ceff2920d9eddb61e62aacb2602c3dc8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/582498
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/os/file_windows.go

@@ -6,6 +6,7 @@ package os
 
 import (
 	"errors"
+	"internal/filepathlite"
 	"internal/godebug"
 	"internal/poll"
 	"internal/syscall/windows"
@@ -313,7 +314,7 @@ func Symlink(oldname, newname string) error {
 		return &LinkError{"symlink", oldname, newname, err}
 	}
 	var o *uint16
-	if isAbs(oldname) {
+	if filepathlite.IsAbs(oldname) {
 		o, err = syscall.UTF16PtrFromString(fixLongPath(oldname))
 	} else {
 		// Do not use fixLongPath on oldname for relative symlinks,

src/os/path_windows.go

@@ -5,6 +5,7 @@
 package os
 
 import (
+	"internal/filepathlite"
 	"internal/syscall/windows"
 	"syscall"
 )
@@ -44,18 +45,6 @@ func basename(name string) string {
 	return name
 }
 
-func isAbs(path string) (b bool) {
-	v := volumeName(path)
-	if v == "" {
-		return false
-	}
-	path = path[len(v):]
-	if path == "" {
-		return false
-	}
-	return IsPathSeparator(path[0])
-}
-
 func volumeName(path string) (v string) {
 	if len(path) < 2 {
 		return ""
@@ -174,7 +163,7 @@ func addExtendedPrefix(path string) string {
 	// The MSDN docs appear to say that a normal path that is 248 bytes long
 	// will work; empirically the path must be less then 248 bytes long.
 	pathLength := len(path)
-	if !isAbs(path) {
+	if !filepathlite.IsAbs(path) {
 		// If the path is relative, we need to prepend the working directory
 		// plus a separator to the path before we can determine if it's too long.
 		// We don't want to call syscall.Getwd here, as that call is expensive to do

src/os/types_windows.go

@@ -5,6 +5,7 @@
 package os
 
 import (
+	"internal/filepathlite"
 	"internal/godebug"
 	"internal/syscall/windows"
 	"sync"
@@ -338,7 +339,7 @@ func (fs *fileStat) loadFileId() error {
 // and set name from path.
 func (fs *fileStat) saveInfoFromPath(path string) error {
 	fs.path = path
-	if !isAbs(fs.path) {
+	if !filepathlite.IsAbs(fs.path) {
 		var err error
 		fs.path, err = syscall.FullPath(fs.path)
 		if err != nil {