Include inbound-claimed-HTLCs in reported channel balances

Given the balance is reported as "total balance if we went to chain
ignoring fees", it seems reasonable to include claimed HTLCs - if
we went to chain we'd get those funds, less on-chain fees. Further,
if we do not include them, its possible to have pending outbound
holding-cell HTLCs underflow the balance calculation, causing a
panic in debug mode, and bogus values in release.

This resolves a subtraction underflow bug found by the
`chanmon_consistency` fuzz target.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -2155,8 +2155,15 @@ impl<Signer: Sign> Channel<Signer> {
 	/// This is the amount that would go to us if we close the channel, ignoring any on-chain fees.
 	/// See also [`Channel::get_inbound_outbound_available_balance_msat`]
 	pub fn get_balance_msat(&self) -> u64 {
-		self.value_to_self_msat
-			- self.get_outbound_pending_htlc_stats(None).pending_htlcs_value_msat
+		// Include our local balance, plus any inbound HTLCs we know the preimage for, minus any
+		// HTLCs sent or which will be sent after commitment signed's are exchanged.
+		let mut balance_msat = self.value_to_self_msat;
+		for ref htlc in self.pending_inbound_htlcs.iter() {
+			if let InboundHTLCState::LocalRemoved(InboundHTLCRemovalReason::Fulfill(_)) = htlc.state {
+				balance_msat += htlc.amount_msat;
+			}
+		}
+		balance_msat - self.get_outbound_pending_htlc_stats(None).pending_htlcs_value_msat
 	}
 
 	pub fn get_holder_counterparty_selected_channel_reserve_satoshis(&self) -> (u64, Option<u64>) {

lightning/src/ln/functional_tests.rs

@@ -8101,6 +8101,43 @@ fn test_bump_txn_sanitize_tracking_maps() {
 	}
 }
 
+#[test]
+fn test_pending_claimed_htlc_no_balance_underflow() {
+	// Tests that if we have a pending outbound HTLC as well as a claimed-but-not-fully-removed
+	// HTLC we will not underflow when we call `Channel::get_balance_msat()`.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100_000, 0, InitFeatures::known(), InitFeatures::known());
+
+	let payment_preimage = route_payment(&nodes[0], &[&nodes[1]], 1_010_000).0;
+	nodes[1].node.claim_funds(payment_preimage);
+	check_added_monitors!(nodes[1], 1);
+	let fulfill_ev = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
+
+	nodes[0].node.handle_update_fulfill_htlc(&nodes[1].node.get_our_node_id(), &fulfill_ev.update_fulfill_htlcs[0]);
+	expect_payment_sent_without_paths!(nodes[0], payment_preimage);
+	nodes[0].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &fulfill_ev.commitment_signed);
+	check_added_monitors!(nodes[0], 1);
+	let (_raa, _cs) = get_revoke_commit_msgs!(nodes[0], nodes[1].node.get_our_node_id());
+
+	// At this point nodes[1] has received 1,010k msat (10k msat more than their reserve) and can
+	// send an HTLC back (though it will go in the holding cell). Send an HTLC back and check we
+	// can get our balance.
+
+	// Get a route from nodes[1] to nodes[0] by getting a route going the other way and then flip
+	// the public key of the only hop. This works around ChannelDetails not showing the
+	// almost-claimed HTLC as available balance.
+	let (mut route, _, _, _) = get_route_and_payment_hash!(nodes[0], nodes[1], 10_000);
+	route.payment_params = None; // This is all wrong, but unnecessary
+	route.paths[0][0].pubkey = nodes[0].node.get_our_node_id();
+	let (_, payment_hash_2, payment_secret_2) = get_payment_preimage_hash!(nodes[0]);
+	nodes[1].node.send_payment(&route, payment_hash_2, &Some(payment_secret_2)).unwrap();
+
+	assert_eq!(nodes[1].node.list_channels()[0].balance_msat, 1_000_000);
+}
+
 #[test]
 fn test_channel_conf_timeout() {
 	// Tests that, for inbound channels, we give up on them if the funding transaction does not