godoc/vfs: add io/fs adapter

Change-Id: Iaeea108ce2803d3ee1ece5d2d67fbe353576fbd7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/291669
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

godoc/vfs/fs.go

@@ -0,0 +1,79 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.16
+
+package vfs
+
+import (
+	"io/fs"
+	"os"
+	"path"
+	"strings"
+)
+
+// FromFS converts an fs.FS to the FileSystem interface.
+func FromFS(fsys fs.FS) FileSystem {
+	return &fsysToFileSystem{fsys}
+}
+
+type fsysToFileSystem struct {
+	fsys fs.FS
+}
+
+func (f *fsysToFileSystem) fsPath(name string) string {
+	name = path.Clean(name)
+	if name == "/" {
+		return "."
+	}
+	return strings.TrimPrefix(name, "/")
+}
+
+func (f *fsysToFileSystem) Open(name string) (ReadSeekCloser, error) {
+	file, err := f.fsys.Open(f.fsPath(name))
+	if err != nil {
+		return nil, err
+	}
+	if rsc, ok := file.(ReadSeekCloser); ok {
+		return rsc, nil
+	}
+	return &noSeekFile{f.fsPath(name), file}, nil
+}
+
+func (f *fsysToFileSystem) Lstat(name string) (os.FileInfo, error) {
+	return fs.Stat(f.fsys, f.fsPath(name))
+}
+
+func (f *fsysToFileSystem) Stat(name string) (os.FileInfo, error) {
+	return fs.Stat(f.fsys, f.fsPath(name))
+}
+
+func (f *fsysToFileSystem) RootType(name string) RootType { return "" }
+
+func (f *fsysToFileSystem) ReadDir(name string) ([]os.FileInfo, error) {
+	dirs, err := fs.ReadDir(f.fsys, f.fsPath(name))
+	var infos []os.FileInfo
+	for _, d := range dirs {
+		info, err1 := d.Info()
+		if err1 != nil {
+			if err == nil {
+				err = err1
+			}
+			continue
+		}
+		infos = append(infos, info)
+	}
+	return infos, err
+}
+
+func (f *fsysToFileSystem) String() string { return "io/fs" }
+
+type noSeekFile struct {
+	path string
+	fs.File
+}
+
+func (f *noSeekFile) Seek(offset int64, whence int) (int64, error) {
+	return 0, &fs.PathError{Op: "seek", Path: f.path, Err: fs.ErrInvalid}
+}