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)
 {
@@ -5974,9 +5991,8 @@ static void peer_reconnect(struct peer *peer,
 		|| 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));
+		    && funding_tx_index_for_txid(peer,
+			    &recv_tlvs->my_current_funding_locked->my_current_funding_locked_txid) > 0);
 
 	/* BOLT #2:
 	 *

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`
@@ -104,6 +103,9 @@ def test_splice_reconnect_after_lock_no_channel_ready(node_factory, bitcoind):
     # the new funding_tx_index columns) and reconnects/reestablishes.
     l1.restart()
 
+    # Force the reconnect rather than waiting on auto-reconnect backoff.
+    l1.rpc.connect(l2.info['id'], 'localhost', l2.port)
+
     l1.daemon.wait_for_log(r'peer_in WIRE_CHANNEL_REESTABLISH')
     l2.daemon.wait_for_log(r'peer_in WIRE_CHANNEL_REESTABLISH')
     l1.daemon.wait_for_log(r'billboard: Channel ready for use.')