os: use filepathlite.VolumeName

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

Change-Id: Ibba82dd71fe10b594cb6f782582430aa422e7078
Reviewed-on: https://go-review.googlesource.com/c/go/+/582499
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/os/file_windows.go

@@ -292,10 +292,10 @@ func Symlink(oldname, newname string) error {
 
 	// need the exact location of the oldname when it's relative to determine if it's a directory
 	destpath := oldname
-	if v := volumeName(oldname); v == "" {
+	if v := filepathlite.VolumeName(oldname); v == "" {
 		if len(oldname) > 0 && IsPathSeparator(oldname[0]) {
 			// oldname is relative to the volume containing newname.
-			if v = volumeName(newname); v != "" {
+			if v = filepathlite.VolumeName(newname); v != "" {
 				// Prepend the volume explicitly, because it may be different from the
 				// volume of the current working directory.
 				destpath = v + oldname

src/os/path.go

@@ -5,6 +5,7 @@
 package os
 
 import (
+	"internal/filepathlite"
 	"syscall"
 )
 
@@ -43,7 +44,7 @@ func MkdirAll(path string, perm FileMode) error {
 
 	// If there is a parent directory, and it is not the volume name,
 	// recurse to ensure parent directory exists.
-	if parent := path[:i]; len(parent) > len(volumeName(path)) {
+	if parent := path[:i]; len(parent) > len(filepathlite.VolumeName(path)) {
 		err = MkdirAll(parent, perm)
 		if err != nil {
 			return err

src/os/path_plan9.go

@@ -13,7 +13,3 @@ const (
 func IsPathSeparator(c uint8) bool {
 	return PathSeparator == c
 }
-
-func volumeName(p string) string {
-	return ""
-}

src/os/path_unix.go

@@ -69,7 +69,3 @@ func splitPath(path string) (string, string) {
 
 	return dirname, basename
 }
-
-func volumeName(p string) string {
-	return ""
-}

src/os/path_windows.go

@@ -45,44 +45,6 @@ func basename(name string) string {
 	return name
 }
 
-func volumeName(path string) (v string) {
-	if len(path) < 2 {
-		return ""
-	}
-	// with drive letter
-	c := path[0]
-	if path[1] == ':' &&
-		('0' <= c && c <= '9' || 'a' <= c && c <= 'z' ||
-			'A' <= c && c <= 'Z') {
-		return path[:2]
-	}
-	// is it UNC
-	if l := len(path); l >= 5 && IsPathSeparator(path[0]) && IsPathSeparator(path[1]) &&
-		!IsPathSeparator(path[2]) && path[2] != '.' {
-		// first, leading `\\` and next shouldn't be `\`. its server name.
-		for n := 3; n < l-1; n++ {
-			// second, next '\' shouldn't be repeated.
-			if IsPathSeparator(path[n]) {
-				n++
-				// third, following something characters. its share name.
-				if !IsPathSeparator(path[n]) {
-					if path[n] == '.' {
-						break
-					}
-					for ; n < l; n++ {
-						if IsPathSeparator(path[n]) {
-							break
-						}
-					}
-					return path[:n]
-				}
-				break
-			}
-		}
-	}
-	return ""
-}
-
 func fromSlash(path string) string {
 	// Replace each '/' with '\\' if present
 	var pathbuf []byte
@@ -106,7 +68,7 @@ func fromSlash(path string) string {
 }
 
 func dirname(path string) string {
-	vol := volumeName(path)
+	vol := filepathlite.VolumeName(path)
 	i := len(path) - 1
 	for i >= len(vol) && !IsPathSeparator(path[i]) {
 		i--