close: Do not publish unilateral when witnessing a close onchain

Changelog-Changed: close: We no longer attempt to publish a unilateral close that'd fail anyway when we witness a close onchain.

Author: cdecker

lightningd/channel.c

@@ -902,22 +902,35 @@ void channel_fail_permanent(struct channel *channel,
 	struct lightningd *ld = channel->peer->ld;
 	va_list ap;
 	char *why;
+	/* Do we want to rebroadcast close transactions? If we're
+	 * witnessing the close on-chain there is no point in doing
+	 * this. */
+	bool rebroadcast;
 
 	va_start(ap, fmt);
 	why = tal_vfmt(tmpctx, fmt, ap);
 	va_end(ap);
 
-	log_unusual(channel->log, "Peer permanent failure in %s: %s",
-		    channel_state_name(channel), why);
+	log_unusual(channel->log,
+		    "Peer permanent failure in %s: %s (reason=%s)",
+		    channel_state_name(channel), why,
+		    channel_change_state_reason_str(reason));
 
 	/* We can have multiple errors, eg. onchaind failures. */
 	if (!channel->error)
 		channel->error = towire_errorfmt(channel,
 						 &channel->cid, "%s", why);
 
 	channel_set_owner(channel, NULL);
-	/* Drop non-cooperatively (unilateral) to chain. */
-	drop_to_chain(ld, channel, false);
+
+	/* Drop non-cooperatively (unilateral) to chain. If we detect
+	 * the close from the blockchain (i.e., reason is
+	 * REASON_ONCHAIN, or FUNDING_SPEND_SEEN) then we can observe
+	 * passively, and not broadcast our own unilateral close, as
+	 * it doesn't stand a chance anyway. */
+	rebroadcast = !(channel->state == ONCHAIN ||
+			channel->state == FUNDING_SPEND_SEEN);
+	drop_to_chain(ld, channel, false, rebroadcast);
 
 	if (channel_state_wants_onchain_fail(channel->state))
 		channel_set_state(channel,

lightningd/closing_control.c

@@ -303,7 +303,7 @@ static void peer_closing_complete(struct channel *channel, const u8 *msg)
 		return;
 
 	/* Channel gets dropped to chain cooperatively. */
-	drop_to_chain(channel->peer->ld, channel, true);
+	drop_to_chain(channel->peer->ld, channel, true, true /* rebroadcast */);
 	channel_set_state(channel,
 			  CLOSINGD_SIGEXCHANGE,
 			  CLOSINGD_COMPLETE,

lightningd/peer_control.c

@@ -348,7 +348,7 @@ static enum watch_result closed_inflight_depth_cb(struct lightningd *ld,
 }
 
 void drop_to_chain(struct lightningd *ld, struct channel *channel,
-		   bool cooperative)
+		   bool cooperative, bool rebroadcast)
 {
 	struct channel_inflight *inflight;
 	const char *cmd_id;
@@ -387,6 +387,10 @@ void drop_to_chain(struct lightningd *ld, struct channel *channel,
 		log_broken(channel->log,
 			   "Cannot broadcast our commitment tx:"
 			   " it's invalid! (ancient channel?)");
+	} else if (!rebroadcast && !cooperative) {
+		log_unusual(channel->log,
+			    "Not dropping our unilateral close onchain since "
+			    "we already saw theirs confirm.");
 	} else {
 		struct bitcoin_tx *tx COMPILER_WANTS_INIT("gcc 12.3.0");
 
@@ -448,10 +452,10 @@ void resend_closing_transactions(struct lightningd *ld)
 			case CLOSED:
 				continue;
 			case CLOSINGD_COMPLETE:
-				drop_to_chain(ld, channel, true);
+				drop_to_chain(ld, channel, true, true);
 				continue;
 			case AWAITING_UNILATERAL:
-				drop_to_chain(ld, channel, false);
+				drop_to_chain(ld, channel, false, true);
 				continue;
 			}
 			abort();

lightningd/peer_control.h

@@ -109,7 +109,16 @@ void peer_set_dbid(struct peer *peer, u64 dbid);
 /* At startup, re-send any transactions we want bitcoind to have */
 void resend_closing_transactions(struct lightningd *ld);
 
-void drop_to_chain(struct lightningd *ld, struct channel *channel, bool cooperative);
+/**
+ * Initiate the close of a channel.
+ *
+ * @param rebroadcast: Whether we should be broadcasting our
+ *   commitment transaction in order to close the channel, or not.
+ */
+void drop_to_chain(struct lightningd *ld,
+		   struct channel *channel,
+		   bool cooperative,
+		   bool rebroadcast);
 
 void update_channel_from_inflight(struct lightningd *ld,
 				  struct channel *channel,