closingd: retransmit 'funding_locked' if we reconnect without any update

As per BOLT02 #message-retransmission :
if `next_commitment_number` is 1 in both the `channel_reestablish` it sent and received:
    - MUST retransmit `funding_locked`

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - bolt11: support for parsing feature bits (field `9`).
 
+- Protocol: we now retransmit `funding_locked` upon reconnection while closing if there was no update
 ### Changed
 
 - JSON API: `txprepare` now uses `outputs` as parameter other than `destination` and `satoshi`

channeld/channeld.c

@@ -2315,6 +2315,8 @@ static void peer_reconnect(struct peer *peer,
 	    && next_commitment_number == 1) {
 		u8 *msg;
 
+		status_debug("Retransmitting funding_locked for channel %s",
+		             type_to_string(tmpctx, struct channel_id, &peer->channel_id));
 		/* Contains per commit point #1, for first post-opening commit */
 		msg = towire_funding_locked(NULL,
 					    &peer->channel_id,

closingd/closingd.c

@@ -108,25 +108,17 @@ static u8 *closing_read_peer_msg(const tal_t *ctx,
 	}
 }
 
-static void do_reconnect(struct per_peer_state *pps,
-			 const struct channel_id *channel_id,
-			 const u64 next_index[NUM_SIDES],
-			 u64 revocations_received,
-			 const u8 *channel_reestablish,
-			 const u8 *final_scriptpubkey,
-			 const struct secret *last_remote_per_commit_secret)
+static struct pubkey get_per_commitment_point(u64 commitment_number)
 {
 	u8 *msg;
-	struct channel_id their_channel_id;
-	u64 next_local_commitment_number, next_remote_revocation_number;
-	struct pubkey my_current_per_commitment_point;
+	struct pubkey commitment_point;
 	struct secret *s;
 
 	/* Our current per-commitment point is the commitment point in the last
 	 * received signed commitment; HSM gives us that and the previous
 	 * secret (which we don't need). */
 	msg = towire_hsm_get_per_commitment_point(NULL,
-						  next_index[LOCAL]-1);
+	                                          commitment_number);
 	if (!wire_sync_write(HSM_FD, take(msg)))
 		status_failed(STATUS_FAIL_HSM_IO,
 			      "Writing get_per_commitment_point to HSM: %s",
@@ -138,11 +130,29 @@ static void do_reconnect(struct per_peer_state *pps,
 			      "Reading resp get_per_commitment_point reply: %s",
 			      strerror(errno));
 	if (!fromwire_hsm_get_per_commitment_point_reply(tmpctx, msg,
-					&my_current_per_commitment_point,
-					&s))
+	                                                 &commitment_point,
+	                                                 &s))
 		status_failed(STATUS_FAIL_HSM_IO,
-			      "Bad per_commitment_point reply %s",
-			      tal_hex(tmpctx, msg));
+		              "Bad per_commitment_point reply %s",
+		              tal_hex(tmpctx, msg));
+
+	return commitment_point;
+}
+
+static void do_reconnect(struct per_peer_state *pps,
+			 const struct channel_id *channel_id,
+			 const u64 next_index[NUM_SIDES],
+			 u64 revocations_received,
+			 const u8 *channel_reestablish,
+			 const u8 *final_scriptpubkey,
+			 const struct secret *last_remote_per_commit_secret)
+{
+	u8 *msg;
+	struct channel_id their_channel_id;
+	u64 next_local_commitment_number, next_remote_revocation_number;
+	struct pubkey my_current_per_commitment_point, next_commitment_point;
+
+	my_current_per_commitment_point = get_per_commitment_point(next_index[LOCAL]-1);
 
 	/* BOLT #2:
 	 *
@@ -207,8 +217,20 @@ static void do_reconnect(struct per_peer_state *pps,
 	msg = towire_shutdown(NULL, channel_id, final_scriptpubkey);
 	sync_crypto_write(pps, take(msg));
 
-	/* FIXME: Spec says to re-xmit funding_locked here if we haven't
-	 * done any updates. */
+	/* BOLT #2:
+	 *
+	 * A node:
+	 *...
+	 *   - if `next_commitment_number` is 1 in both the `channel_reestablish` it sent and received:
+	 *     - MUST retransmit `funding_locked`.
+	 */
+	if (next_index[REMOTE] == 1 && next_index[LOCAL] == 1) {
+		status_debug("Retransmitting funding_locked for channel %s",
+		             type_to_string(tmpctx, struct channel_id, channel_id));
+		next_commitment_point = get_per_commitment_point(next_index[LOCAL]);
+		msg = towire_funding_locked(NULL, channel_id, &next_commitment_point);
+		sync_crypto_write(pps, take(msg));
+	}
 }
 
 static void send_offer(struct per_peer_state *pps,

tests/test_connection.py

@@ -391,6 +391,33 @@ def test_reconnect_gossiping(node_factory):
     l2.daemon.wait_for_log('processing now old peer gone')
 
 
+@unittest.skipIf(not DEVELOPER, "needs dev-disconnect")
+def test_reconnect_no_update(node_factory, executor):
+    """
+    This tests if the `funding_locked` is sent if we receive a
+    `channel_reestablish` message with `next_commitment_number` == 1 and
+    our `next_commitment_number` == 1.
+    """
+    disconnects = ["@WIRE_FUNDING_LOCKED", "@WIRE_SHUTDOWN"]
+    # Allow bad gossip because it might receive WIRE_CHANNEL_UPDATE before
+    # announcement before of the disconnection
+    l1 = node_factory.get_node(may_reconnect=True, allow_bad_gossip=True)
+    l2 = node_factory.get_node(disconnect=disconnects, may_reconnect=True)
+
+    # For channeld reconnection
+    l1.rpc.connect(l2.info["id"], "localhost", l2.port)
+    fundchannel_exec = executor.submit(l1.fund_channel, l2, 10**6, False)
+    l1.daemon.wait_for_log(r"channeld.* Retransmitting funding_locked for channel")
+    l1.stop()
+
+    # For closingd reconnection
+    scid = fundchannel_exec.result()
+    l1.daemon.start()
+    executor.submit(l1.rpc.close, scid, 0)
+    l2.daemon.wait_for_log(r"closingd.* Retransmitting funding_locked for channel")
+    l1.daemon.wait_for_log(r"CLOSINGD_COMPLETE")
+
+
 def test_connect_stresstest(node_factory, executor):
     # This test is unreliable, but it's better than nothing.
     l1 = node_factory.get_node(may_reconnect=True)