lightningd: don't disconnect when sending error for unknown channel_reestablish

When a peer sends WIRE_CHANNEL_REESTABLISH for a channel we don't know
about (e.g. dual-funding where we deleted the unsaved channel on
disconnect but the peer saved it in DUALOPEND_OPEN_COMMIT_READY), we
send an error and disconnect.  The disconnect races with the error
delivery: the peer's dualopend may not receive the error before the
connection drops, so the channel survives as a transient failure and
the peer retries on every reconnect, creating an infinite loop.

Fix this by sending the error without disconnecting for the
WIRE_CHANNEL_REESTABLISH case, so the peer's dualopend reliably
receives the error and cleans up the stale channel.

Changelog-Fixed: dual-funding reconnect loop when peer doesn't know about a saved channel
Fixes: #8822
Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: vincenzopalazzo

lightningd/peer_control.c

@@ -2137,6 +2137,21 @@ void handle_peer_spoke(struct lightningd *ld, const u8 *msg)
 						"Channel is closed and forgotten");
 			goto send_error;
 		}
+		/* Unknown channel: send error but don't disconnect.
+		 * If we hang up, the peer's dualopend may not
+		 * receive the error (it races with the disconnect),
+		 * leaving the peer with a saved channel that we
+		 * don't know about, causing an infinite reconnect
+		 * loop. */
+		log_peer_unusual(ld->log, &peer->id,
+				 "Unknown channel %s for %s",
+				 fmt_channel_id(tmpctx,
+						&channel_id),
+				 peer_wire_name(msgtype));
+		error = towire_errorfmt(tmpctx, &channel_id,
+					"Unknown channel for %s",
+					peer_wire_name(msgtype));
+		goto send_error_nohangup;
 	}
 
 	/* Weird message?  Log and reply with error. */
@@ -2162,6 +2177,15 @@ void handle_peer_spoke(struct lightningd *ld, const u8 *msg)
 							peer->connectd_counter)));
 	return;
 
+send_error_nohangup:
+	log_peer_debug(ld->log, &peer->id, "Telling connectd to send error %s",
+		       tal_hex(tmpctx, error));
+	subd_send_msg(ld->connectd,
+		      take(towire_connectd_peer_send_msg(NULL, &peer->id,
+							 peer->connectd_counter,
+							 error)));
+	return;
+
 tell_connectd:
 	subd_send_msg(ld->connectd,
 		      take(towire_connectd_peer_connect_subd(NULL, &id,