f add comments suggested by val

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -813,8 +813,18 @@ macro_rules! handle_chan_restoration_locked {
 
 			let chanmon_update: Option<ChannelMonitorUpdate> = $chanmon_update; // Force type-checking to resolve
 			if chanmon_update.is_some() {
-				assert!($commitment_update.is_some());
+				// On reconnect (or monitor restoration), we, by definition, only resend a
+				// funding_locked if there have been no commitment updates, so the only channel
+				// monitor update which could also be associated with a funding_locked would be
+				// the funding_created/funding_signed monitor update. That monitor update failing
+				// implies that we won't send funding_locked until its been updated, so we can't
+				// have a funding_locked and a monitor update here (so we don't bother to handle it
+				// correctly below).
 				assert!($funding_locked.is_none());
+				// A channel monitor update makes no sense without either a funding_locked or a
+				// commitment update to process after it. Since we can't have a funding_locked, we
+				// only bother to handle the monitor-update + commitment_update case below.
+				assert!($commitment_update.is_some());
 			}
 
 			if let Some(msg) = $funding_locked {