Test that txn pay at least a minimum relay fee in functional tests

This also pays a fee on the transactions we generate in response to
SpendableOutputDescriptors in tests.

This fixes the known issues in #630, though we should test for
standardness in other ways as well.

Author: TheBlueMatt

lightning/src/ln/functional_test_utils.rs

@@ -481,14 +481,26 @@ pub fn create_announced_chan_between_nodes_with_value<'a, 'b, 'c, 'd>(nodes: &'a
 macro_rules! check_spends {
 	($tx: expr, $($spends_txn: expr),*) => {
 		{
-			$tx.verify(|out_point| {
+			let get_output = |out_point: &bitcoin::blockdata::transaction::OutPoint| {
 				$(
 					if out_point.txid == $spends_txn.txid() {
 						return $spends_txn.output.get(out_point.vout as usize).cloned()
 					}
 				)*
 				None
-			}).unwrap();
+			};
+			let mut total_value_in = 0;
+			for input in $tx.input.iter() {
+				total_value_in += get_output(&input.previous_output).unwrap().value;
+			}
+			let mut total_value_out = 0;
+			for output in $tx.output.iter() {
+				total_value_out += output.value;
+			}
+			let min_fee = ($tx.get_weight() as u64 + 3) / 4; // One sat per vbyte (ie per weight/4, rounded up)
+			// Input amount must be no larger than output amount + minimum relay fee
+			assert!(total_value_out + min_fee <= total_value_in);
+			$tx.verify(get_output).unwrap();
 		}
 	}
 }

lightning/src/ln/functional_tests.rs

@@ -4686,6 +4686,7 @@ macro_rules! check_spendable_outputs {
 										input: vec![input],
 										output: vec![outp],
 									};
+									spend_tx.output[0].value -= (spend_tx.get_weight() + 2 + 1 + 73 + 35 + 3) as u64 / 4; // (Max weight + 3 (to round up)) / 4
 									let secp_ctx = Secp256k1::new();
 									let keys = $keysinterface.derive_channel_keys($chan_value, key_derivation_params.0, key_derivation_params.1);
 									let remotepubkey = keys.pubkeys().payment_point;
@@ -4720,6 +4721,7 @@ macro_rules! check_spendable_outputs {
 
 										let delayed_payment_pubkey = PublicKey::from_secret_key(&secp_ctx, &delayed_payment_key);
 										let witness_script = chan_utils::get_revokeable_redeemscript(revocation_pubkey, *to_self_delay, &delayed_payment_pubkey);
+										spend_tx.output[0].value -= (spend_tx.get_weight() + 2 + 1 + 73 + 1 + witness_script.len() + 1 + 3) as u64 / 4; // (Max weight + 3 (to round up)) / 4
 										let sighash = Message::from_slice(&bip143::SigHashCache::new(&spend_tx).signature_hash(0, &witness_script, output.value, SigHashType::All)[..]).unwrap();
 										let local_delayedsig = secp_ctx.sign(&sighash, &delayed_payment_key);
 										spend_tx.input[0].witness.push(local_delayedsig.serialize_der().to_vec());
@@ -4747,6 +4749,7 @@ macro_rules! check_spendable_outputs {
 										input: vec![input],
 										output: vec![outp.clone()],
 									};
+									spend_tx.output[0].value -= (spend_tx.get_weight() + 2 + 1 + 73 + 35 + 3) as u64 / 4; // (Max weight + 3 (to round up)) / 4
 									let secret = {
 										match ExtendedPrivKey::new_master(Network::Testnet, &$node.node_seed) {
 											Ok(master_key) => {