feature/drive: start factoring out Taildrive, add ts_omit_drive build tag

As of this commit (per the issue), the Taildrive code remains where it
was, but in new files that are protected by the new ts_omit_drive
build tag. Future commits will move it.

Updates #17058

Change-Id: Idf0a51db59e41ae8da6ea2b11d238aefc48b219e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled_drive.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_drive
+
+package main
+
+import (
+	"errors"
+	"fmt"
+
+	"tailscale.com/drive/driveimpl"
+	"tailscale.com/tsd"
+	"tailscale.com/types/logger"
+	"tailscale.com/wgengine"
+)
+
+func init() {
+	subCommands["serve-taildrive"] = &serveDriveFunc
+
+	hookSetSysDrive.Set(func(sys *tsd.System, logf logger.Logf) {
+		sys.Set(driveimpl.NewFileSystemForRemote(logf))
+	})
+	hookSetWgEnginConfigDrive.Set(func(conf *wgengine.Config, logf logger.Logf) {
+		conf.DriveForLocal = driveimpl.NewFileSystemForLocal(logf)
+	})
+}
+
+var serveDriveFunc = serveDrive
+
+// serveDrive serves one or more Taildrives on localhost using the WebDAV
+// protocol. On UNIX and MacOS tailscaled environment, Taildrive spawns child
+// tailscaled processes in serve-taildrive mode in order to access the fliesystem
+// as specific (usually unprivileged) users.
+//
+// serveDrive prints the address on which it's listening to stdout so that the
+// parent process knows where to connect to.
+func serveDrive(args []string) error {
+	if len(args) == 0 {
+		return errors.New("missing shares")
+	}
+	if len(args)%2 != 0 {
+		return errors.New("need <sharename> <path> pairs")
+	}
+	s, err := driveimpl.NewFileServer()
+	if err != nil {
+		return fmt.Errorf("unable to start Taildrive file server: %v", err)
+	}
+	shares := make(map[string]string)
+	for i := 0; i < len(args); i += 2 {
+		shares[args[i]] = args[i+1]
+	}
+	s.SetShares(shares)
+	fmt.Printf("%v\n", s.Addr())
+	return s.Serve()
+}