ipn/ipnauth: implement an Actor representing tailscaled itself

Updates #14823

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

ipn/ipnauth/self.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipnauth
+
+import (
+	"tailscale.com/ipn"
+)
+
+// Self is a caller identity that represents the tailscaled itself and therefore
+// has unlimited access.
+var Self Actor = unrestricted{}
+
+// unrestricted is an [Actor] that has unlimited access to the currently running
+// tailscaled instance. It's typically used for operations performed by tailscaled
+// on its own, or upon a request from the control plane, rather on behalf of a user.
+type unrestricted struct{}
+
+// UserID implements [Actor].
+func (u unrestricted) UserID() ipn.WindowsUserID { return "" }
+
+// Username implements [Actor].
+func (u unrestricted) Username() (string, error) { return "", nil }
+
+// ClientID implements [Actor].
+// It always returns (NoClientID, false) because the tailscaled itself
+// is not a connected LocalAPI client.
+func (u unrestricted) ClientID() (_ ClientID, ok bool) { return NoClientID, false }
+
+// CheckProfileAccess implements [Actor].
+func (u unrestricted) CheckProfileAccess(_ ipn.LoginProfileView, _ ProfileAccess) error {
+	// Unrestricted access to all profiles.
+	return nil
+}
+
+// IsLocalSystem implements [Actor].
+//
+// Deprecated: this method exists for compatibility with the current (as of 2025-01-28)
+// permission model and will be removed as we progress on tailscale/corp#18342.
+func (u unrestricted) IsLocalSystem() bool { return false }
+
+// IsLocalAdmin implements [Actor].
+//
+// Deprecated: this method exists for compatibility with the current (as of 2025-01-28)
+// permission model and will be removed as we progress on tailscale/corp#18342.
+func (u unrestricted) IsLocalAdmin(operatorUID string) bool { return false }