elements: Add removal of fee output if empty

Needed for zero-fee anchor outputs

Author: cdecker

bitcoin/test/run-tx-bitcoin_tx_2of2_input_witness_weight.c

@@ -27,6 +27,9 @@ struct amount_sat amount_asset_to_sat(struct amount_asset *asset UNNEEDED)
 				       struct amount_sat a UNNEEDED,
 				       struct amount_sat b UNNEEDED)
 { fprintf(stderr, "amount_sat_add called!\n"); abort(); }
+/* Generated stub for amount_sat_eq */
+bool amount_sat_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_eq called!\n"); abort(); }
 /* Generated stub for amount_sat_greater_eq */
 bool amount_sat_greater_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
 { fprintf(stderr, "amount_sat_greater_eq called!\n"); abort(); }

bitcoin/tx.c

@@ -542,9 +542,34 @@ struct bitcoin_tx *bitcoin_tx(const tal_t *ctx,
 	return tx;
 }
 
+static void elements_maybe_remove_fee_output(struct bitcoin_tx *tx)
+{
+	struct amount_sat fee = bitcoin_tx_compute_fee(tx);
+	int pos;
+
+	/* If we aren't using elements, we don't add explicit fee outputs */
+	if (!chainparams->is_elements)
+		return;
+
+	/* If we have a fee we must keep the fee output. */
+	if (!amount_sat_eq(fee, AMOUNT_SAT(0)))
+		return;
+
+	/* Try to find any existing fee output */
+	for (pos = 0; pos < tx->wtx->num_outputs; pos++) {
+		if (elements_tx_output_is_fee(tx, pos))
+			break;
+	}
+
+	if (pos != tx->wtx->num_outputs) {
+		wally_tx_remove_output(tx->wtx, pos);
+	}
+}
+
 void bitcoin_tx_finalize(struct bitcoin_tx *tx)
 {
 	elements_tx_add_fee_output(tx);
+	elements_maybe_remove_fee_output(tx);
 	assert(bitcoin_tx_check(tx));
 }