chore: Add fee output on elements for anchor spends and resends

The fee output is mandatory in `elements`-based chains, for the
homomorphic amount commitment to work out. These transactions were
missing that output.

Author: cdecker

lightningd/anchorspend.c

@@ -288,6 +288,13 @@ static struct wally_psbt *anchor_psbt(const tal_t *ctx,
 	psbt_append_output(psbt,
 			   scriptpubkey_p2tr(tmpctx, &final_key),
 			   change);
+
+	/* And finally, if we're running on an elements chain we also need to
+	 * add an explicit fee output. */
+	if (is_elements(chainparams)) {
+		psbt_elements_normalize_fees(psbt);
+	}
+
 	return psbt;
 }
 

lightningd/onchain_control.c

@@ -1141,6 +1141,10 @@ static bool consider_onchain_htlc_tx_rebroadcast(struct channel *channel,
 		return true;
 	}
 
+	if (chainparams->is_elements) {
+		psbt_elements_normalize_fees(psbt);
+	}
+
 	/* Now, get HSM to sign off. */
 	hsm_utxos = utxos_to_hsm_utxos(tmpctx, utxos);
 	msg = towire_hsmd_sign_htlc_tx_mingle(NULL,