lightningd: message for channeld to tell us that channel risks penalty.

For option_data_loss_protect, the peer can prove to us that it's ahead;
it gives us the (hopefully honest!) per_commitment_point it will use,
and we make sure we don't broadcast the commitment transaction we have.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

channeld/channel.c

@@ -2309,6 +2309,7 @@ static void req_in(struct peer *peer, const u8 *msg)
 	case WIRE_CHANNEL_GOT_SHUTDOWN:
 	case WIRE_CHANNEL_SHUTDOWN_COMPLETE:
 	case WIRE_CHANNEL_DEV_REENABLE_COMMIT_REPLY:
+	case WIRE_CHANNEL_FAIL_FALLEN_BEHIND:
 		break;
 	}
 	master_badmsg(-1, msg);

channeld/channel_wire.csv

@@ -165,3 +165,8 @@ channel_feerates,1027
 channel_feerates,,feerate,u32
 channel_feerates,,min_feerate,u32
 channel_feerates,,max_feerate,u32
+
+# Peer presented proof it was from the future.
+channel_fail_fallen_behind,1028
+channel_fail_fallen_behind,,remote_per_commitment_point,struct pubkey
+

lightningd/channel.c

@@ -231,6 +231,7 @@ struct channel *new_channel(struct peer *peer, u64 dbid,
 	channel->connected = connected;
 	channel->local_basepoints = *local_basepoints;
 	channel->local_funding_pubkey = *local_funding_pubkey;
+	channel->future_per_commitment_point = NULL;
 
 	list_add_tail(&peer->channels, &channel->list);
 	tal_add_destructor(channel, destroy_channel);

lightningd/channel.h

@@ -105,6 +105,10 @@ struct channel {
 
 	/* Does gossipd need to know if the owner dies? (ie. not onchaind) */
 	bool connected;
+
+	/* Do we have an "impossible" future per_commitment_point from
+	 * peer via option_data_loss_protect? */
+	struct pubkey *future_per_commitment_point;
 };
 
 struct channel *new_channel(struct peer *peer, u64 dbid,

lightningd/channel_control.c

@@ -4,6 +4,7 @@
 #include <common/memleak.h>
 #include <common/timeout.h>
 #include <common/utils.h>
+#include <common/wire_error.h>
 #include <errno.h>
 #include <gossipd/gossip_constants.h>
 #include <hsmd/gen_hsm_client_wire.h>
@@ -101,6 +102,32 @@ static void peer_got_shutdown(struct channel *channel, const u8 *msg)
 	wallet_channel_save(ld->wallet, channel);
 }
 
+static void channel_fail_fallen_behind(struct channel *channel, const u8 *msg)
+{
+	struct pubkey per_commitment_point;
+	struct channel_id cid;
+
+	if (!fromwire_channel_fail_fallen_behind(msg, &per_commitment_point)) {
+		channel_internal_error(channel,
+				       "bad channel_fail_fallen_behind %s",
+				       tal_hex(tmpctx, msg));
+		return;
+	}
+
+	/* FIXME: Save in db! */
+	channel->future_per_commitment_point
+		= tal_dup(channel, struct pubkey, &per_commitment_point);
+
+	/* We don't fail yet, since we want daemon to send them an error
+	 * to trigger rebroadcasting.  But make sure we set error now in
+	 * case something else goes wrong! */
+	derive_channel_id(&cid,
+			  &channel->funding_txid,
+			  channel->funding_outnum);
+	channel->error = towire_errorfmt(channel, &cid,
+					 "Catastrophic failure: please close channel");
+}
+
 static void peer_start_closingd_after_shutdown(struct channel *channel,
 					       const u8 *msg,
 					       const int *fds)
@@ -147,6 +174,9 @@ static unsigned channel_msg(struct subd *sd, const u8 *msg, const int *fds)
 			return 2;
 		peer_start_closingd_after_shutdown(sd->channel, msg, fds);
 		break;
+	case WIRE_CHANNEL_FAIL_FALLEN_BEHIND:
+		channel_fail_fallen_behind(sd->channel, msg);
+		break;
 
 	/* And we never get these from channeld. */
 	case WIRE_CHANNEL_INIT:

lightningd/peer_control.c

@@ -370,15 +370,28 @@ register_close_command(struct lightningd *ld,
 void drop_to_chain(struct lightningd *ld, struct channel *channel,
 		   bool cooperative)
 {
+	/* BOLT #2:
+	 *
+	 * - if `next_remote_revocation_number` is greater than expected
+	 *   above, AND `your_last_per_commitment_secret` is correct for that
+	 *   `next_remote_revocation_number` minus 1:
+	 *      - MUST NOT broadcast its commitment transaction.
+	 */
+	if (channel->future_per_commitment_point && !cooperative) {
+		log_broken(channel->log,
+			   "Cannot broadcast our commitment tx:"
+			   " they have a future one");
+	} else {
 		sign_last_tx(channel);
 
 		/* Keep broadcasting until we say stop (can fail due to dup,
 		 * if they beat us to the broadcast). */
 		broadcast_tx(ld->topology, channel, channel->last_tx, NULL);
 
-	resolve_close_command(ld, channel, cooperative);
-
 		remove_sig(channel->last_tx);
+	}
+
+	resolve_close_command(ld, channel, cooperative);
 }
 
 void channel_errmsg(struct channel *channel,