channeld: detect splice on reestablish via funding_tx_index, not txid

is_splice_active compared the peer's my_current_funding_locked txid
against peer->channel->funding.txid, which is wrong: once a splice locks,
funding.txid becomes the splice txid, both sides agree on it, the check
falls through, and channel_ready is incorrectly retransmitted on reconnect.

Resolve the txid to its funding_tx_index (the current channel funding or a
pending splice inflight) and treat > 0 as a splice.

Changelog-Fixed: channeld: correctly detect splice transactions when deciding whether to retransmit `channel_ready` on reconnect.

Author: nGoline

channeld/channeld.c

@@ -5645,6 +5645,23 @@ static bool capture_premature_msg(const u8 ***shit_lnd_says, const u8 *msg)
 	return true;
 }
 
+/* Returns the funding_tx_index of the funding tx with this txid: the current
+ * channel funding, or a pending splice inflight.  Returns 0 (the original
+ * funding) if unknown.
+ */
+static u32 funding_tx_index_for_txid(const struct peer *peer,
+				     const struct bitcoin_txid *txid)
+{
+	if (bitcoin_txid_eq(txid, &peer->channel->funding.txid))
+		return peer->channel->funding_tx_index;
+	for (size_t i = 0; i < tal_count(peer->splice_state->inflights); i++) {
+		const struct inflight *inf = peer->splice_state->inflights[i];
+		if (bitcoin_txid_eq(txid, &inf->outpoint.txid))
+			return inf->funding_tx_index;
+	}
+	return 0;
+}
+
 static void peer_reconnect(struct peer *peer,
 			   const struct secret *last_remote_per_commit_secret)
 {
@@ -5967,17 +5984,6 @@ static void peer_reconnect(struct peer *peer,
 				     "next_funding_txid not recognized.");
 	}
 
-	/* "none of those channel_reestablish messages contain
-	 * my_current_funding_locked or next_funding for a splice transaction" */
-	bool is_splice_active = local_next_funding
-		|| peer->splice_state->locked_ready[LOCAL]
-		|| remote_next_funding
-		|| (recv_tlvs
-		    && recv_tlvs->my_current_funding_locked
-		    && !bitcoin_txid_eq(
-			    &recv_tlvs->my_current_funding_locked->my_current_funding_locked_txid,
-			    &peer->channel->funding.txid));
-
 	/* BOLT #2:
 	 *
 	 * A node:
@@ -5996,7 +6002,17 @@ static void peer_reconnect(struct peer *peer,
 	if (peer->channel_ready[LOCAL]
 	    && peer->next_index[LOCAL] == 1
 	    && next_commitment_number == 1
-	    && !is_splice_active) {
+	    /* "none of those channel_reestablish messages contain
+	     * my_current_funding_locked or next_funding for a splice
+	     * transaction": a funding_tx_index of 1 or more means the txid came
+	     * from a splice (0 is the original funding). */
+	    && !local_next_funding
+	    && !peer->splice_state->locked_ready[LOCAL]
+	    && !remote_next_funding
+	    && !(recv_tlvs
+		 && recv_tlvs->my_current_funding_locked
+		 && funding_tx_index_for_txid(peer,
+			&recv_tlvs->my_current_funding_locked->my_current_funding_locked_txid) > 0)) {
 		struct tlv_channel_ready_tlvs *tlvs = tlv_channel_ready_tlvs_new(tmpctx);
 
 		tlvs->short_channel_id = &peer->local_alias;

tests/test_splicing_disconnect.py

@@ -69,7 +69,6 @@ def test_splice_disconnect_sig(node_factory, bitcoind):
 @pytest.mark.openchannel('v1')
 @pytest.mark.openchannel('v2')
 @unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd doesnt yet support PSBT features we need')
-@pytest.mark.xfail(strict=True, reason="channel_ready wrongly retransmitted after splice until funding_tx_index detection")
 def test_splice_reconnect_after_lock_no_channel_ready(node_factory, bitcoind):
     # Once a splice locks, channel funding txid is updated to the splice txid.
     # On reconnect we must still recognise the peer's `my_current_funding_locked`