Disallow empty fee estimates on Mainnet

We generally want to properly be able to detect whenever a fee estimation would
fail, as we need to fail startup if we can't retrieve the latest fee
rates.

However, currently the `convert_fee_estimates` function would fallback
to a default of 1sat/vbyte if the retrieved estimate map is empty. This
is fine/potentially needed for, e.g., Signet where Esplora's
`fee-estimates` endpoint would return an empty dictionary.

However, we need to detect the empty map and fail our fee estimation if
we encounter such a case, rather than using the 1 sat/vbyte fallback on
mainnet, where differences in fee estimation could lead to costly
force-closures. Here we do this and just return a
`FeerateEstimationFailed` if we hit such a case.

src/builder.rs

@@ -508,8 +508,11 @@ fn build_with_store_internal<K: KVStore + Sync + Send + 'static>(
 				tx_sync.client().clone(),
 				Arc::clone(&logger),
 			));
-			let fee_estimator =
-				Arc::new(OnchainFeeEstimator::new(tx_sync.client().clone(), Arc::clone(&logger)));
+			let fee_estimator = Arc::new(OnchainFeeEstimator::new(
+				tx_sync.client().clone(),
+				Arc::clone(&config),
+				Arc::clone(&logger),
+			));
 			(blockchain, tx_sync, tx_broadcaster, fee_estimator)
 		}
 		None => {
@@ -523,8 +526,11 @@ fn build_with_store_internal<K: KVStore + Sync + Send + 'static>(
 				tx_sync.client().clone(),
 				Arc::clone(&logger),
 			));
-			let fee_estimator =
-				Arc::new(OnchainFeeEstimator::new(tx_sync.client().clone(), Arc::clone(&logger)));
+			let fee_estimator = Arc::new(OnchainFeeEstimator::new(
+				tx_sync.client().clone(),
+				Arc::clone(&config),
+				Arc::clone(&logger),
+			));
 			(blockchain, tx_sync, tx_broadcaster, fee_estimator)
 		}
 	};

src/fee_estimator.rs

@@ -1,5 +1,5 @@
 use crate::logger::{log_error, log_trace, Logger};
-use crate::Error;
+use crate::{Config, Error};
 
 use lightning::chain::chaininterface::{
 	ConfirmationTarget, FeeEstimator, FEERATE_FLOOR_SATS_PER_KW,
@@ -8,28 +8,30 @@ use lightning::chain::chaininterface::{
 use bdk::FeeRate;
 use esplora_client::AsyncClient as EsploraClient;
 
+use bitcoin::Network;
 use bitcoin::blockdata::weight::Weight;
 
 use std::collections::HashMap;
 use std::ops::Deref;
-use std::sync::RwLock;
+use std::sync::{Arc, RwLock};
 
 pub(crate) struct OnchainFeeEstimator<L: Deref>
 where
 	L::Target: Logger,
 {
 	fee_rate_cache: RwLock<HashMap<ConfirmationTarget, FeeRate>>,
 	esplora_client: EsploraClient,
+	config: Arc<Config>,
 	logger: L,
 }
 
 impl<L: Deref> OnchainFeeEstimator<L>
 where
 	L::Target: Logger,
 {
-	pub(crate) fn new(esplora_client: EsploraClient, logger: L) -> Self {
+	pub(crate) fn new(esplora_client: EsploraClient, config: Arc<Config>, logger: L) -> Self {
 		let fee_rate_cache = RwLock::new(HashMap::new());
-		Self { fee_rate_cache, esplora_client, logger }
+		Self { fee_rate_cache, esplora_client, config, logger }
 	}
 
 	pub(crate) async fn update_fee_estimates(&self) -> Result<(), Error> {
@@ -61,6 +63,16 @@ where
 				Error::FeerateEstimationUpdateFailed
 			})?;
 
+			if estimates.is_empty() && self.config.network == Network::Bitcoin {
+				// Ensure we fail if we didn't receive any estimates.
+				log_error!(
+					self.logger,
+					"Failed to retrieve fee rate estimates for {:?}: empty fee estimates are dissallowed on Mainnet.",
+					target,
+				);
+				return Err(Error::FeerateEstimationUpdateFailed);
+			}
+
 			let converted_estimates = esplora_client::convert_fee_rate(num_blocks, estimates)
 				.map_err(|e| {
 					log_error!(