ipn/ipnserver paths: add paths.LegacyStateFilePath

Moving this information into a centralized place so that it is accessible to
code in subsequent commits.

Updates #3011

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

ipn/ipnserver/server.go

@@ -20,7 +20,6 @@ import (
 	"os/exec"
 	"os/signal"
 	"os/user"
-	"path/filepath"
 	"runtime"
 	"strconv"
 	"strings"
@@ -602,7 +601,7 @@ func tryWindowsAppDataMigration(logf logger.Logf, path string) string {
 		// what they are doing.
 		return path
 	}
-	oldFile := filepath.Join(os.Getenv("LocalAppData"), "Tailscale", "server-state.conf")
+	oldFile := paths.LegacyStateFilePath()
 	return paths.TryConfigFileMigration(logf, oldFile, path)
 }
 

paths/paths_unix.go

@@ -81,3 +81,8 @@ func ensureStateDirPerms(dir string) error {
 	}
 	return os.Chmod(dir, perm)
 }
+
+// LegacyStateFilePath is not applicable to UNIX; it is just stubbed out.
+func LegacyStateFilePath() string {
+	return ""
+}

paths/paths_windows.go

@@ -148,3 +148,9 @@ func ensureStateDirPerms(dirPath string) error {
 	return windows.SetNamedSecurityInfo(dirPath, windows.SE_FILE_OBJECT, flags,
 		sids.User, sids.PrimaryGroup, dacl, nil)
 }
+
+// LegacyStateFilePath returns the legacy path to the state file when it was stored under the
+// current user's %LocalAppData%.
+func LegacyStateFilePath() string {
+	return filepath.Join(os.Getenv("LocalAppData"), "Tailscale", "server-state.conf")
+}