Follow-up nits from #1199 (phantom node support)

Author: valentinewallace

fuzz/src/full_stack.rs

@@ -390,6 +390,9 @@ pub fn do_test(data: &[u8], logger: &Arc<dyn Logger>) {
 		best_block: BestBlock::from_genesis(network),
 	};
 	let channelmanager = Arc::new(ChannelManager::new(fee_est.clone(), monitor.clone(), broadcast.clone(), Arc::clone(&logger), keys_manager.clone(), config, params));
+	// Adding new calls to `KeysInterface::get_secure_random_bytes` during startup can change all the
+	// keys subsequently generated in this test. Rather than regenerating all the messages manually,
+	// it's easier to just increment the counter here so the keys don't change.
 	keys_manager.counter.fetch_sub(1, Ordering::AcqRel);
 	let our_id = PublicKey::from_secret_key(&Secp256k1::signing_only(), &keys_manager.get_node_secret(Recipient::Node).unwrap());
 	let network_graph = Arc::new(NetworkGraph::new(genesis_block(network).block_hash()));

lightning/src/util/scid_utils.rs

@@ -104,18 +104,15 @@ pub(crate) mod fake_scid {
 			let rand_bytes = keys_manager.get_secure_random_bytes();
 
 			let segwit_activation_height = segwit_activation_height(genesis_hash);
-			let mut valid_block_range = if highest_seen_blockheight > segwit_activation_height {
-				highest_seen_blockheight - segwit_activation_height
-			} else {
-				1
-			};
+			let mut blocks_since_segwit_activation = highest_seen_blockheight.saturating_sub(segwit_activation_height);
+
 			// We want to ensure that this fake channel won't conflict with any transactions we haven't
 			// seen yet, in case `highest_seen_blockheight` is updated before we get full information
 			// about transactions confirmed in the given block.
-			if valid_block_range > BLOCKS_PER_MONTH { valid_block_range -= BLOCKS_PER_MONTH; }
+			blocks_since_segwit_activation = blocks_since_segwit_activation.saturating_sub(BLOCKS_PER_MONTH);
 
 			let rand_for_height = u32::from_be_bytes(rand_bytes[..4].try_into().unwrap());
-			let fake_scid_height = segwit_activation_height + rand_for_height % valid_block_range;
+			let fake_scid_height = segwit_activation_height + rand_for_height % (blocks_since_segwit_activation + 1);
 
 			let rand_for_tx_index = u32::from_be_bytes(rand_bytes[4..8].try_into().unwrap());
 			let fake_scid_tx_index = rand_for_tx_index % MAX_TX_INDEX;