Constant lightning::chain::channelmonitor::CLOSED_CHANNEL_UPDATE_ID

pub const CLOSED_CHANNEL_UPDATE_ID: u64 = std::u64::MAX; // 0xf_fff_fff_fff_fff_fffu64

If: (1) a channel has been force closed and (2) we receive a preimage from a forward link that allows us to spend an HTLC output on this channel’s (the backward link’s) broadcasted commitment transaction then we allow the ChannelManager to send a ChannelMonitorUpdate with this update ID, with the update providing said payment preimage. No other update types are allowed after force-close.