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Correctly handle new ChannelMonitorUpdates to old post-FC chans #4136

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Merged
merged 1 commit into from
Oct 7, 2025
+10 −3
Merged

Correctly handle new ChannelMonitorUpdates to old post-FC chans #4136

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13 changes: 10 additions & 3 deletions lightning/src/ln/channelmanager.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1601,6 +1601,8 @@ where
/// the highest `update_id` of all the pending in-flight updates (note that any pending updates
/// not yet applied sitting in [`ChannelManager::pending_background_events`] will also be
/// considered as they are also in [`Self::in_flight_monitor_updates`]).
///
/// Note that channels which were closed prior to LDK 0.1 may have a value here of `u64::MAX`.
closed_channel_monitor_update_ids: BTreeMap<ChannelId, u64>,
/// The peer is currently connected (i.e. we've seen a
/// [`BaseMessageHandler::peer_connected`] and no corresponding
Expand Down Expand Up @@ -13262,7 +13264,8 @@ where
.closed_channel_monitor_update_ids
.get_mut(&channel_id)
.expect("Channels originating a payment resolution must have a monitor");
*update_id += 1;
// Note that for channels closed pre-0.1, the latest update_id is `u64::MAX`.
*update_id = update_id.saturating_add(1);
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Given the current and future caveats described in the commit msg, does this need a log on overflow?

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Hmm, not sure if its really needed. Maybe the commit message isn't clear, but I don't see any issues with this with the current code. I do call out that its possible that other code changes in the future which introduces an issue, but I don't think we really need to log a warning on the assumption that future code might break really ancient channel closes.

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No, I think the commit message is clear.

For someone looking at this statement though, it must be hard to understand why this is safe. I thought a warning log would both help debug any future issue with this, and at the same time add a bit more documentation. Or maybe move some of the commit message to a code comment.

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I added comments.

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It's still pretty thin and doesn't explain why it is okay to not increment when at the max already.


let update = ChannelMonitorUpdate {
update_id: *update_id,
Expand Down Expand Up @@ -16523,7 +16526,9 @@ where
should_queue_fc_update = !monitor.no_further_updates_allowed();
let mut latest_update_id = monitor.get_latest_update_id();
if should_queue_fc_update {
latest_update_id += 1;
// Note that for channels closed pre-0.1, the latest update_id is
// `u64::MAX`.
latest_update_id = latest_update_id.saturating_add(1);
}
per_peer_state
.entry(counterparty_node_id)
Expand Down Expand Up @@ -17170,7 +17175,9 @@ where
.closed_channel_monitor_update_ids
.get_mut(channel_id)
.expect("Channels originating a preimage must have a monitor");
*update_id += 1;
// Note that for channels closed pre-0.1, the latest
// update_id is `u64::MAX`.
*update_id = update_id.saturating_add(1);

pending_background_events.push(BackgroundEvent::MonitorUpdateRegeneratedOnStartup {
counterparty_node_id: monitor.get_counterparty_node_id(),
Expand Down