tailfs: initial implementation

Add a WebDAV-based folder sharing mechanism that is exposed to local clients at
100.100.100.100:8080 and to remote peers via a new peerapi endpoint at
/v0/tailfs.

Add the ability to manage folder sharing via the new 'share' CLI sub-command.

Updates tailscale/corp#16827

Signed-off-by: Percy Wegmann <percy@tailscale.com>

tailfs/webdavfs/writeonly_file.go

@@ -0,0 +1,89 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package webdavfs
+
+import (
+	"context"
+	"errors"
+	"io"
+	"io/fs"
+	"os"
+
+	"tailscale.com/tailfs/shared"
+)
+
+type writeOnlyFile struct {
+	io.WriteCloser
+	name       string
+	perm       os.FileMode
+	fs         *webdavFS
+	finalError chan error
+}
+
+// Readdir implements webdav.File. As this is a file, this always fails with an
+// os.PathError.
+func (f *writeOnlyFile) Readdir(count int) ([]fs.FileInfo, error) {
+	return nil, &os.PathError{
+		Op:   "readdir",
+		Path: f.name,
+		Err:  errors.New("is a file"), // TODO(oxtoacart): make sure this and below errors match what a regular os.File does
+	}
+}
+
+// Seek implements webdav.File. This always fails with an os.PathError.
+func (f *writeOnlyFile) Seek(offset int64, whence int) (int64, error) {
+	return 0, &os.PathError{
+		Op:   "seek",
+		Path: f.name,
+		Err:  errors.New("seek not supported"),
+	}
+}
+
+// Stat implements webdav.File.
+func (f *writeOnlyFile) Stat() (fs.FileInfo, error) {
+	fi, err := f.fs.Stat(context.Background(), f.name)
+	if err != nil {
+		// use static info for newly created file
+		now := f.fs.now()
+		fi = &shared.StaticFileInfo{
+			Named:       f.name,
+			Sized:       0,
+			Moded:       f.perm,
+			BirthedTime: now,
+			ModdedTime:  now,
+			Dir:         false,
+		}
+	}
+	return fi, nil
+}
+
+// Read implements webdav.File. As this is a write-only file, it always fails
+// with an os.PathError.
+func (f *writeOnlyFile) Read(p []byte) (int, error) {
+	return 0, &os.PathError{
+		Op:   "write",
+		Path: f.name,
+		Err:  errors.New("write-only"),
+	}
+}
+
+// Write implements webdav.File.
+func (f *writeOnlyFile) Write(p []byte) (int, error) {
+	select {
+	case err := <-f.finalError:
+		return 0, err
+	default:
+		return f.WriteCloser.Write(p)
+	}
+}
+
+// Close implements webdav.File.
+func (f *writeOnlyFile) Close() error {
+	err := f.WriteCloser.Close()
+	writeErr := <-f.finalError
+	if writeErr != nil {
+		return writeErr
+	}
+	return err
+}