bitcoin/tx.c: use 330 sat dust limit for P2TR/P2WPKH change outputs Fixes #8395

Fix by checking is_elements: Elements keeps 546 sat, Bitcoin uses 330 sat.
Changelog-Fixed: Transactions now correctly create change outputs >= 330 sat for P2TR/P2WPKH instead of absorbing them as fees

Author: Andezion

bitcoin/tx.c

@@ -985,9 +985,13 @@ struct amount_sat change_amount(struct amount_sat excess, u32 feerate_perkw,
 	if (!amount_sat_sub(&excess, excess, fee))
 		return AMOUNT_SAT(0);
 
-	/* Must be non-dust */
-	if (!amount_sat_greater_eq(excess, chainparams->dust_limit))
-		return AMOUNT_SAT(0);
+	if (chainparams->is_elements) {
+		if (!amount_sat_greater_eq(excess, AMOUNT_SAT(546)))
+			return AMOUNT_SAT(0);
+	} else {
+		if (!amount_sat_greater_eq(excess, AMOUNT_SAT(330)))
+			return AMOUNT_SAT(0);
+	}
 
 	return excess;
 }

tests/test_p2tr_change_dust.py

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+"""Test P2TR change outputs with dust limit 330 sat (issue #8395)."""
+import unittest
+
+from fixtures import *  # noqa: F401,F403
+from fixtures import TEST_NETWORK
+from utils import wait_for
+
+
+@unittest.skipIf(TEST_NETWORK == 'liquid-regtest', "P2TR not yet supported on Elements")
+def test_p2tr_change_dust_limit(node_factory, bitcoind):
+
+    l1 = node_factory.get_node(feerates=(253, 253, 253, 253))
+
+    addr = l1.rpc.newaddr('p2tr')['p2tr']
+    bitcoind.rpc.sendtoaddress(addr, 1.0)
+    bitcoind.generate_block(1)
+    wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == 1)
+
+    outputs = l1.rpc.listfunds()['outputs']
+    assert len(outputs) == 1
+    utxo = outputs[0]
+
+    utxo_amount = int(utxo['amount_msat'] / 1000)
+
+    target_amount = utxo_amount - 450
+
+    result = l1.rpc.fundpsbt(
+        satoshi=f"{target_amount}sat",
+        feerate="253perkw",
+        startweight=0,
+        excess_as_change=True
+    )
+
+    assert 'change_outnum' in result, "Expected change output to be created"
+
+    psbt = bitcoind.rpc.decodepsbt(result['psbt'])
+
+    change_outnum = result['change_outnum']
+    if 'tx' in psbt:
+        change_output = psbt['tx']['vout'][change_outnum]
+        change_amount_btc = float(change_output['value'])
+    else:
+        change_output = psbt['outputs'][change_outnum]
+        change_amount_btc = float(change_output['amount'])
+
+    change_amount_sat = int(change_amount_btc * 100_000_000)
+
+    print(f"Change amount: {change_amount_sat} sat")
+
+    assert change_amount_sat >= 330, f"Change {change_amount_sat} sat should be >= 330 sat"
+    assert change_amount_sat <= 546, f"Change {change_amount_sat} sat should be <= 546 sat (for this test)"
+
+    print(f"SUCCESS: P2TR change output of {change_amount_sat} sat created (between 330 and 546 sat)")