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

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

paths/paths.go

@@ -0,0 +1,40 @@
+// 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.
+
+// Package paths returns platform and user-specific default paths to
+// Tailscale files and directories.
+package paths
+
+import (
+	"os"
+	"runtime"
+)
+
+// LegacyConfigPath is the path used by the pre-tailscaled "relaynode"
+// daemon's config file.
+const LegacyConfigPath = "/var/lib/tailscale/relay.conf"
+
+// DefaultTailscaledSocket returns the path to the tailscaled Unix socket
+// or the empty string if there's no reasonable default.
+func DefaultTailscaledSocket() string {
+	if runtime.GOOS == "windows" {
+		return ""
+	}
+	if fi, err := os.Stat("/run"); err == nil && fi.IsDir() {
+		return "/run/tailscale/tailscaled.sock"
+	}
+	return "tailscaled.sock"
+}
+
+var stateFileFunc func() string
+
+// DefaultTailscaledStateFile returns the default path to the
+// tailscaled state file, or the empty string if there's no reasonable
+// default value.
+func DefaultTailscaledStateFile() string {
+	if f := stateFileFunc; f != nil {
+		return f()
+	}
+	return ""
+}