Splice: Better balance checking

* Regression test added for Issue ElementsProject#6572 (issuecomment-1730808863) w/stuck HTLC
* `check_balance` adjusted to calculate pending HTLCs explicitly
* Test confirmed to fail prior to PR ElementsProject#6713

ChangeLog-Fixed: Issue splicing with pending / stuck HTLCs fixed.

Author: ddustin

channeld/channeld.c

@@ -2961,13 +2961,35 @@ static struct amount_sat check_balances(struct peer *peer,
 			  funding_amount_res, min_multiplied;
 	struct amount_msat funding_amount,
 			   initiator_fee, accepter_fee;
-	struct amount_msat in[NUM_TX_ROLES], out[NUM_TX_ROLES];
+	struct amount_msat in[NUM_TX_ROLES], out[NUM_TX_ROLES],
+			   pending_htlcs[NUM_TX_ROLES];
+	struct htlc_map_iter it;
+	const struct htlc *htlc;
 	bool opener = our_role == TX_INITIATOR;
 	u8 *msg;
 
+	/* The channel funds less any pending htlcs */
 	in[TX_INITIATOR] = peer->channel->view->owed[opener ? LOCAL : REMOTE];
 	in[TX_ACCEPTER] = peer->channel->view->owed[opener ? REMOTE : LOCAL];
 
+	/* pending_htlcs holds the value of all pending htlcs for each side */
+	pending_htlcs[TX_INITIATOR] = AMOUNT_MSAT(0);
+	pending_htlcs[TX_ACCEPTER] = AMOUNT_MSAT(0);
+	for (htlc = htlc_map_first(peer->channel->htlcs, &it);
+	     htlc;
+	     htlc = htlc_map_next(peer->channel->htlcs, &it)) {
+		struct amount_msat *itr;
+
+		if (htlc_owner(htlc) == opener ? LOCAL : REMOTE)
+			itr = &pending_htlcs[TX_INITIATOR];
+		else
+			itr = &pending_htlcs[TX_ACCEPTER];
+
+		if (!amount_msat_add(itr, *itr, htlc->amount))
+			peer_failed_warn(peer->pps, &peer->channel_id,
+					 "Unable to add HTLC balance");
+	}
+
 	for (size_t i = 0; i < psbt->num_inputs; i++)
 		if (i != chan_input_index)
 			add_amount_to_side(peer, in,
@@ -2985,12 +3007,22 @@ static struct amount_sat check_balances(struct peer *peer,
 					   psbt_output_get_amount(psbt, i),
 					   &psbt->outputs[i].unknowns);
 
-	/* Calculate total channel output amount */
+	/* Calculate original channel output amount */
 	if (!amount_msat_add(&funding_amount,
 			     peer->channel->view->owed[LOCAL],
 			     peer->channel->view->owed[REMOTE]))
 		peer_failed_warn(peer->pps, &peer->channel_id,
 				 "Unable to calculate starting channel amount");
+	if (!amount_msat_add(&funding_amount,
+			     funding_amount,
+			     pending_htlcs[TX_INITIATOR]))
+		peer_failed_warn(peer->pps, &peer->channel_id,
+				 "Unable to calculate starting channel amount");
+	if (!amount_msat_add(&funding_amount,
+			     funding_amount,
+			     pending_htlcs[TX_ACCEPTER]))
+		peer_failed_warn(peer->pps, &peer->channel_id,
+				 "Unable to calculate starting channel amount");
 
 	/* Tasks:
 	 *   Add up total funding_amount
@@ -3033,9 +3065,13 @@ static struct amount_sat check_balances(struct peer *peer,
 		peer_failed_warn(peer->pps, &peer->channel_id,
 				 "Initiator funding is less than commited"
 				 " amount. Initiator contributing %s but they"
-				 " committed to %s.",
+				 " committed to %s. Pending offered HTLC"
+				 " balance of %s is not available for this"
+				 " operation.",
 				 fmt_amount_msat(tmpctx, in[TX_INITIATOR]),
-				 fmt_amount_msat(tmpctx, out[TX_INITIATOR]));
+				 fmt_amount_msat(tmpctx, out[TX_INITIATOR]),
+				 fmt_amount_msat(tmpctx,
+				 		 pending_htlcs[TX_INITIATOR]));
 	}
 
 	if (!amount_msat_sub(&initiator_fee, in[TX_INITIATOR], out[TX_INITIATOR]))
@@ -3051,9 +3087,13 @@ static struct amount_sat check_balances(struct peer *peer,
 		peer_failed_warn(peer->pps, &peer->channel_id,
 				 "Accepter funding is less than commited"
 				 " amount. Accepter contributing %s but they"
-				 " committed to %s.",
+				 " committed to %s. Pending offered HTLC"
+				 " balance of %s is not available for this"
+				 " operation.",
 				 fmt_amount_msat(tmpctx, in[TX_INITIATOR]),
-				 fmt_amount_msat(tmpctx, out[TX_INITIATOR]));
+				 fmt_amount_msat(tmpctx, out[TX_INITIATOR]),
+				 fmt_amount_msat(tmpctx,
+				 		 pending_htlcs[TX_INITIATOR]));
 	}
 
 	if (!amount_msat_sub(&accepter_fee, in[TX_ACCEPTER], out[TX_ACCEPTER]))

tests/test_splicing.py

@@ -276,3 +276,44 @@ def test_commit_crash_splice(node_factory, bitcoind):
     # Check that the splice doesn't generate a unilateral close transaction
     time.sleep(5)
     assert l1.db_query("SELECT count(*) as c FROM channeltxs;")[0]['c'] == 0
+
+
+@pytest.mark.openchannel('v1')
+@pytest.mark.openchannel('v2')
+@unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd doesnt yet support PSBT features we need')
+def test_splice_stuck_htlc(node_factory, bitcoind, executor):
+    l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True, opts={'experimental-splicing': None})
+
+    l3.rpc.dev_ignore_htlcs(id=l2.info['id'], ignore=True)
+
+    inv = l3.rpc.invoice(10000000, '1', 'no_1')
+    executor.submit(l1.rpc.pay, inv['bolt11'])
+    l3.daemon.wait_for_log('their htlc 0 dev_ignore_htlcs')
+
+    # Now we should have a stuck invoice between l1 -> l2
+
+    chan_id = l1.get_channel_id(l2)
+
+    # add extra sats to pay fee
+    funds_result = l1.rpc.fundpsbt("109000sat", "slow", 166, excess_as_change=True)
+
+    result = l1.rpc.splice_init(chan_id, 100000, funds_result['psbt'])
+    result = l1.rpc.splice_update(chan_id, result['psbt'])
+    result = l1.rpc.signpsbt(result['psbt'])
+    result = l1.rpc.splice_signed(chan_id, result['signed_psbt'])
+
+    l2.daemon.wait_for_log(r'CHANNELD_NORMAL to CHANNELD_AWAITING_SPLICE')
+    l1.daemon.wait_for_log(r'CHANNELD_NORMAL to CHANNELD_AWAITING_SPLICE')
+
+    mempool = bitcoind.rpc.getrawmempool(True)
+    assert len(list(mempool.keys())) == 1
+    assert result['txid'] in list(mempool.keys())
+
+    bitcoind.generate_block(6, wait_for_mempool=1)
+
+    l2.daemon.wait_for_log(r'CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')
+    l1.daemon.wait_for_log(r'CHANNELD_AWAITING_SPLICE to CHANNELD_NORMAL')
+
+    # Check that the splice doesn't generate a unilateral close transaction
+    time.sleep(5)
+    assert l1.db_query("SELECT count(*) as c FROM channeltxs;")[0]['c'] == 0