f Make sure transaction is relayable

.. rather than undercutting the fee, we now choose a method that might
slightly overestimate fees, ensuring its relayability. We also make sure
the net fee is always larger than the fee rate floor.

src/wallet.rs

@@ -4,7 +4,7 @@ use crate::config::BDK_WALLET_SYNC_TIMEOUT_SECS;
 use crate::fee_estimator::{ConfirmationTarget, FeeEstimator};
 use crate::Error;
 
-use lightning::chain::chaininterface::BroadcasterInterface;
+use lightning::chain::chaininterface::{BroadcasterInterface, FEERATE_FLOOR_SATS_PER_KW};
 
 use lightning::events::bump_transaction::{Utxo, WalletSource};
 use lightning::ln::msgs::{DecodeError, UnsignedGossipMessage};
@@ -258,13 +258,9 @@ where
 				OnchainSendType::SendAllRetainingReserve { cur_anchor_reserve_sats } => {
 					let spendable_amount_sats =
 						self.get_spendable_amount_sats(cur_anchor_reserve_sats).unwrap_or(0);
-					// TODO: can we make this closer resemble the actual transaction?
-					// As draining the wallet always will only add one output, this method likely
-					// under-estimates the fee rate a bit.
 					let mut tmp_tx_builder = locked_wallet.build_tx();
 					tmp_tx_builder
-						.drain_wallet()
-						.drain_to(address.script_pubkey())
+						.add_recipient(address.script_pubkey(), spendable_amount_sats)
 						.fee_rate(fee_rate)
 						.enable_rbf();
 					let tmp_tx_details = match tmp_tx_builder.finish() {
@@ -279,7 +275,8 @@ where
 						},
 					};
 
-					let estimated_tx_fee_sats = tmp_tx_details.fee.unwrap_or(0);
+					let estimated_tx_fee_sats =
+						tmp_tx_details.fee.unwrap_or(0).max(FEERATE_FLOOR_SATS_PER_KW as u64);
 					let estimated_spendable_amount_sats =
 						spendable_amount_sats.saturating_sub(estimated_tx_fee_sats);