Rewrite test_update_fee_that_funder_cannot_afford to avoid magic

Instead of magic hard-coded constants, its better for tests to
derive the values used so that they change if constants are changed
and so that it is easier to re-derive constants in the future as
needed.

Author: TheBlueMatt

lightning/src/ln/functional_tests.rs

@@ -584,11 +584,14 @@ fn test_update_fee_that_funder_cannot_afford() {
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
 	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
-	let channel_value = 1888;
-	let chan = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, channel_value, 700000, InitFeatures::known(), InitFeatures::known());
+	let channel_value = 5000;
+	let push_sats = 700;
+	let chan = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, channel_value, push_sats * 1000, InitFeatures::known(), InitFeatures::known());
 	let channel_id = chan.2;
+	let bs_channel_reserve_sats = Channel::<EnforcingSigner>::get_holder_selected_channel_reserve_satoshis(channel_value);
 
-	let feerate = 260;
+	// Calculate the maximum feerate that A can afford:
+	let feerate = ((channel_value - bs_channel_reserve_sats - push_sats) * 1000 / COMMITMENT_TX_BASE_WEIGHT) as u32;
 	{
 		let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
 		*feerate_lock = feerate;
@@ -601,24 +604,22 @@ fn test_update_fee_that_funder_cannot_afford() {
 
 	commitment_signed_dance!(nodes[1], nodes[0], update_msg.commitment_signed, false);
 
-	//Confirm that the new fee based on the last local commitment txn is what we expected based on the feerate of 260 set above.
-	//This value results in a fee that is exactly what the funder can afford (277 sat + 1000 sat channel reserve)
+	// Confirm that the new fee based on the last local commitment txn is what we expected based on the feerate set above.
 	{
 		let commitment_tx = get_local_commitment_txn!(nodes[1], channel_id)[0].clone();
 
-		//We made sure neither party's funds are below the dust limit so -2 non-HTLC txns from number of outputs
-		let num_htlcs = commitment_tx.output.len() - 2;
-		let total_fee: u64 = feerate as u64 * (COMMITMENT_TX_BASE_WEIGHT + (num_htlcs as u64) * COMMITMENT_TX_WEIGHT_PER_HTLC) / 1000;
+		//We made sure neither party's funds are below the dust limit and there are no HTLCs here
+		assert_eq!(commitment_tx.output.len(), 2);
+		let total_fee: u64 = feerate as u64 * COMMITMENT_TX_BASE_WEIGHT / 1000;
 		let mut actual_fee = commitment_tx.output.iter().fold(0, |acc, output| acc + output.value);
 		actual_fee = channel_value - actual_fee;
 		assert_eq!(total_fee, actual_fee);
 	}
 
-	//Add 2 to the previous fee rate to the final fee increases by 1 (with no HTLCs the fee is essentially
-	//fee_rate*(724/1000) so the increment of 1*0.724 is rounded back down)
 	{
+		// Increment the feerate by a small constant, accouting for rounding errors
 		let mut feerate_lock = chanmon_cfgs[0].fee_estimator.sat_per_kw.lock().unwrap();
-		*feerate_lock = feerate + 2;
+		*feerate_lock += 4;
 	}
 	nodes[0].node.timer_tick_occurred();
 	check_added_monitors!(nodes[0], 1);