Add exponential backoff for sync failures

Previously, chain synchronization failures would retry immediately
without any delay, which could lead to tight retry loops and high
CPU usage during failures.

This change introduces exponential backoff for transient
errors, starting at 2 seconds and doubling each time up to a maximum
of 300 seconds. Persistent errors also now delay retries by the
maximum backoff duration to prevent rapid loops while maintaining
eventual recovery.

Fixes #587

Author: tosynthegeek

src/chain/mod.rs

@@ -36,7 +36,7 @@ use lightning_transaction_sync::EsploraSyncClient;
 use lightning_block_sync::gossip::UtxoSource;
 use lightning_block_sync::init::{synchronize_listeners, validate_best_block_header};
 use lightning_block_sync::poll::{ChainPoller, ChainTip, ValidatedBlockHeader};
-use lightning_block_sync::SpvClient;
+use lightning_block_sync::{BlockSourceErrorKind, SpvClient};
 
 use bdk_esplora::EsploraAsyncExt;
 use bdk_wallet::Update as BdkUpdate;
@@ -425,6 +425,9 @@ impl ChainSource {
 					"Starting initial synchronization of chain listeners. This might take a while..",
 				);
 
+				let mut backoff = CHAIN_POLLING_INTERVAL_SECS;
+				const MAX_BACKOFF_SECS: u64 = 300;
+
 				loop {
 					let channel_manager_best_block_hash =
 						channel_manager.current_best_block().block_hash;
@@ -504,8 +507,24 @@ impl ChainSource {
 
 						Err(e) => {
 							log_error!(logger, "Failed to synchronize chain listeners: {:?}", e);
-							tokio::time::sleep(Duration::from_secs(CHAIN_POLLING_INTERVAL_SECS))
-								.await;
+							if e.kind() == BlockSourceErrorKind::Transient {
+								log_info!(
+									logger,
+									"Transient error syncing chain listeners: {:?}. Retrying in {} seconds.",
+									e,
+									backoff
+								);
+								tokio::time::sleep(Duration::from_secs(backoff)).await;
+								backoff = std::cmp::min(backoff * 2, MAX_BACKOFF_SECS);
+							} else {
+								log_error!(
+									logger,
+									"Persistent error syncing chain listeners: {:?}. Retrying in {} seconds.",
+									e,
+									MAX_BACKOFF_SECS
+								);
+								tokio::time::sleep(Duration::from_secs(MAX_BACKOFF_SECS)).await;
+							}
 						},
 					}
 				}