tailscale: update tailfs file and package names (#11590)

This change updates the tailfs file and package names to their new
naming convention.

Updates #tailscale/corp#16827

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

drive/driveimpl/shared/readonlydir.go

@@ -0,0 +1,111 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package shared contains types and functions shared by different tailfs
+// packages.
+package shared
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"sync"
+)
+
+// DirFile implements webdav.File for a virtual directory.
+// It mimics the behavior of an os.File that is pointing at a real directory.
+type DirFile struct {
+	// Info provides the fs.FileInfo for this directory
+	Info fs.FileInfo
+	// LoadChildren is used to load the fs.FileInfos for this directory's
+	// children. It is called at most once in order to support listing
+	// children.
+	LoadChildren func() ([]fs.FileInfo, error)
+
+	// loadChildrenMu guards children and loadedChildren.
+	loadChildrenMu sync.Mutex
+	children       []fs.FileInfo
+	loadedChildren bool
+}
+
+// Readdir implements interface webdav.File. It lazily loads information about
+// children when it is called.
+func (d *DirFile) Readdir(count int) ([]fs.FileInfo, error) {
+	err := d.loadChildrenIfNecessary()
+	if err != nil {
+		return nil, err
+	}
+
+	if count <= 0 {
+		result := d.children
+		d.children = nil
+		return result, nil
+	}
+
+	n := len(d.children)
+	if count < n {
+		n = count
+	}
+	result := d.children[:n]
+	d.children = d.children[n:]
+	if len(d.children) == 0 {
+		err = io.EOF
+	}
+	return result, err
+}
+
+func (d *DirFile) loadChildrenIfNecessary() error {
+	d.loadChildrenMu.Lock()
+	defer d.loadChildrenMu.Unlock()
+
+	if !d.loadedChildren {
+		var err error
+		d.children, err = d.LoadChildren()
+		if err != nil {
+			return err
+		}
+		d.loadedChildren = true
+	}
+	return nil
+}
+
+// Stat implements interface webdav.File.
+func (d *DirFile) Stat() (fs.FileInfo, error) {
+	return d.Info, nil
+}
+
+// Close implements interface webdav.File. It does nothing and never returns an
+// error.
+func (d *DirFile) Close() error {
+	return nil
+}
+
+// Read implements interface webdav.File. As this is a directory, it always
+// fails with an fs.PathError.
+func (d *DirFile) Read(b []byte) (int, error) {
+	return 0, &fs.PathError{
+		Op:   "read",
+		Path: d.Info.Name(),
+		Err:  errors.New("is a directory"),
+	}
+}
+
+// Write implements interface webdav.File. As this is a directory, it always
+// fails with an fs.PathError.
+func (d *DirFile) Write(b []byte) (int, error) {
+	return 0, &fs.PathError{
+		Op:   "write",
+		Path: d.Info.Name(),
+		Err:  errors.New("bad file descriptor"),
+	}
+}
+
+// Seek implements interface webdav.File. As this is a directory, it always
+// fails with an fs.PathError.
+func (d *DirFile) Seek(offset int64, whence int) (int64, error) {
+	return 0, &fs.PathError{
+		Op:   "seek",
+		Path: d.Info.Name(),
+		Err:  errors.New("invalid argument"),
+	}
+}