f new comment

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -3981,21 +3981,10 @@ where
 		};
 		debug_assert!(!sources.is_empty());
 
-		// If we are claiming an MPP payment, we check that all channels which contain a claimable
-		// HTLC still exist. While this isn't guaranteed to remain true if a channel closes while
-		// we're claiming (or even after we claim, before the commitment update dance completes),
-		// it should be a relatively rare race, and we'd rather not claim HTLCs that require us to
-		// go on-chain (and lose the on-chain fee to do so) than just reject the payment.
-		//
-		// Note that we'll still always get our funds - as long as the generated
-		// `ChannelMonitorUpdate` makes it out to the relevant monitor we can claim on-chain.
-		//
-		// If we find an HTLC which we would need to claim but for which we do not have a
-		// channel, we will fail all parts of the MPP payment. While we could wait and see if
-		// the sender retries the already-failed path(s), it should be a pretty rare case where
-		// we got all the HTLCs and then a channel closed while we were waiting for the user to
-		// provide the preimage, so worrying too much about the optimal handling isn't worth
-		// it.
+		// Just in case one HTLC has been failed between when we generated the `PaymentClaimable`
+		// and when we got here we need to check that the amount we're about to claim matches the
+		// amount we told the user in the last `PaymentClaimable`. We also do a sanity-check that
+		// the MPP parts all have the same `total_msat`.
 		let mut claimable_amt_msat = 0;
 		let mut prev_total_msat = None;
 		let mut expected_amt_msat = None;