ipn/desktop: add a new package for managing desktop sessions on Windows

This PR adds a new package, ipn/desktop, which provides a platform-agnostic
interface for enumerating desktop sessions and registering session callbacks.
Currently, it is implemented only for Windows.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/desktop/session.go

@@ -0,0 +1,58 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package desktop
+
+import (
+	"fmt"
+
+	"tailscale.com/ipn/ipnauth"
+)
+
+// SessionID is a unique identifier of a desktop session.
+type SessionID uint
+
+// SessionStatus is the status of a desktop session.
+type SessionStatus int
+
+const (
+	// ClosedSession is a session that does not exist, is not yet initialized by the OS,
+	// or has been terminated.
+	ClosedSession SessionStatus = iota
+	// ForegroundSession is a session that a user can interact with,
+	// such as when attached to a physical console or an active,
+	// unlocked RDP connection.
+	ForegroundSession
+	// BackgroundSession indicates that the session is locked, disconnected,
+	// or otherwise running without user presence or interaction.
+	BackgroundSession
+)
+
+// String implements [fmt.Stringer].
+func (s SessionStatus) String() string {
+	switch s {
+	case ClosedSession:
+		return "Closed"
+	case ForegroundSession:
+		return "Foreground"
+	case BackgroundSession:
+		return "Background"
+	default:
+		panic("unreachable")
+	}
+}
+
+// Session is a state of a desktop session at a given point in time.
+type Session struct {
+	ID     SessionID     // Identifier of the session; can be reused after the session is closed.
+	Status SessionStatus // The status of the session, such as foreground or background.
+	User   ipnauth.Actor // User logged into the session.
+}
+
+// Description returns a human-readable description of the session.
+func (s *Session) Description() string {
+	if maybeUsername, _ := s.User.Username(); maybeUsername != "" { // best effort
+		return fmt.Sprintf("Session %d - %q (%s)", s.ID, maybeUsername, s.Status)
+	}
+	return fmt.Sprintf("Session %d (%s)", s.ID, s.Status)
+}