lib/osutil: Fix TraversesSymlink with symlinked fs root on windows (fixes #4875) (#4886)

lib/osutil/traversessymlink.go

@@ -32,29 +32,22 @@ func (e NotADirectoryError) Error() string {
 	return fmt.Sprintf("not a directory: %s", e.path)
 }
 
-// TraversesSymlink returns an error if base and any path component of name up to and
-// including filepath.Join(base, name) traverses a symlink.
-// Base and name must both be clean and name must be relative to base.
+// TraversesSymlink returns an error if any path component of name (including name
+// itself) traverses a symlink.
 func TraversesSymlink(filesystem fs.Filesystem, name string) error {
-	base := "."
-	path := base
-	info, err := filesystem.Lstat(path)
+	var err error
+	name, err = fs.Canonicalize(name)
 	if err != nil {
 		return err
 	}
-	if !info.IsDir() {
-		return &NotADirectoryError{
-			path: base,
-		}
-	}
 
 	if name == "." {
-		// The result of calling TraversesSymlink("some/where", filepath.Dir("foo"))
+		// The result of calling TraversesSymlink(filesystem, filepath.Dir("foo"))
 		return nil
 	}
 
-	parts := strings.Split(name, string(fs.PathSeparator))
-	for _, part := range parts {
+	var path string
+	for _, part := range strings.Split(name, string(fs.PathSeparator)) {
 		path = filepath.Join(path, part)
 		info, err := filesystem.Lstat(path)
 		if err != nil {
@@ -65,12 +58,12 @@ func TraversesSymlink(filesystem fs.Filesystem, name string) error {
 		}
 		if info.IsSymlink() {
 			return &TraversesSymlinkError{
-				path: strings.TrimPrefix(path, base),
+				path: path,
 			}
 		}
 		if !info.IsDir() {
 			return &NotADirectoryError{
-				path: strings.TrimPrefix(path, base),
+				path: path,
 			}
 		}
 	}