cmd/tailscale, cmd/tailscaled, paths: add paths package for default paths

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

paths/paths_unix.go

@@ -0,0 +1,37 @@
+// Copyright (c) 2020 Tailscale Inc & 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 !windows
+
+package paths
+
+import (
+	"fmt"
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+func init() {
+	stateFileFunc = stateFileUnix
+}
+
+func stateFileUnix() string {
+	// TODO: use other default paths on other GOOSes probably. This works for Linux.
+	const varLib = "/var/lib/tailscale/tailscaled.state"
+	try := varLib
+	for i := 0; i < 3; i++ { // check writability of the file, /var/lib/tailscale, and /var/lib
+		err := unix.Access(try, unix.O_RDWR)
+		println(fmt.Sprintf("Access(%q) = %v", try, err))
+		if err == nil {
+			return varLib
+		}
+		try = filepath.Dir(try)
+	}
+
+	// TODO: try some $HOME/.tailscale or XDG path? But will it
+	// even work usefully enough as non-root? Probably not. Maybe
+	// best to require it be explicit in that case.
+	return ""
+}