Require chain::Listen impls in block sync be Send + Sync

Users who want to use `lightning-block-sync`'s `init` module would
be reasonable in wanting to use it in a multithreaded environment,
however because it takes a list of listeners as `dyn chain::Listen`
without any `Send` or `Sync` bound they fail in doing so.

Here we add a `Send + Sync` bound, requiring any listeners be both.
This could be less generic for users with their own `chain::Listen`
listener that is *not* `Send` or `Sync`, but given multi-threading
support is important and the LDK-included `chain::Listen`
implementations are `Send + Sync`, this seems like an acceptable
tradeoff.

Author: TheBlueMatt

lightning-block-sync/src/init.rs

@@ -101,8 +101,8 @@ BlockSourceResult<ValidatedBlockHeader> {
 /// 	let mut cache = UnboundedCache::new();
 /// 	let mut monitor_listener = (monitor, &*tx_broadcaster, &*fee_estimator, &*logger);
 /// 	let listeners = vec![
-/// 		(monitor_block_hash, &monitor_listener as &dyn chain::Listen),
-/// 		(manager_block_hash, &manager as &dyn chain::Listen),
+/// 		(monitor_block_hash, &monitor_listener as &(dyn chain::Listen + Send + Sync)),
+/// 		(manager_block_hash, &manager as &(dyn chain::Listen + Send + Sync)),
 /// 	];
 /// 	let chain_tip = init::synchronize_listeners(
 /// 		block_source, Network::Bitcoin, &mut cache, listeners).await.unwrap();
@@ -125,7 +125,7 @@ pub async fn synchronize_listeners<B: BlockSource, C: Cache>(
 	block_source: &mut B,
 	network: Network,
 	header_cache: &mut C,
-	mut chain_listeners: Vec<(BlockHash, &dyn chain::Listen)>,
+	mut chain_listeners: Vec<(BlockHash, &(dyn chain::Listen + Send + Sync))>,
 ) -> BlockSourceResult<ValidatedBlockHeader> {
 	let best_header = validate_best_block_header(block_source).await?;
 
@@ -198,7 +198,7 @@ impl<'a, C: Cache> Cache for ReadOnlyCache<'a, C> {
 }
 
 /// Wrapper for supporting dynamically sized chain listeners.
-struct DynamicChainListener<'a>(&'a dyn chain::Listen);
+struct DynamicChainListener<'a>(&'a (dyn chain::Listen + Send + Sync));
 
 impl<'a> chain::Listen for DynamicChainListener<'a> {
 	fn block_connected(&self, _block: &Block, _height: u32) {
@@ -211,7 +211,7 @@ impl<'a> chain::Listen for DynamicChainListener<'a> {
 }
 
 /// A set of dynamically sized chain listeners, each paired with a starting block height.
-struct ChainListenerSet<'a>(Vec<(u32, &'a dyn chain::Listen)>);
+struct ChainListenerSet<'a>(Vec<(u32, &'a (dyn chain::Listen + Send + Sync))>);
 
 impl<'a> chain::Listen for ChainListenerSet<'a> {
 	fn block_connected(&self, block: &Block, height: u32) {
@@ -249,9 +249,9 @@ mod tests {
 			.expect_block_connected(*chain.at_height(4));
 
 		let listeners = vec![
-			(chain.at_height(1).block_hash, &listener_1 as &dyn chain::Listen),
-			(chain.at_height(2).block_hash, &listener_2 as &dyn chain::Listen),
-			(chain.at_height(3).block_hash, &listener_3 as &dyn chain::Listen),
+			(chain.at_height(1).block_hash, &listener_1 as &(dyn chain::Listen + Send + Sync)),
+			(chain.at_height(2).block_hash, &listener_2 as &(dyn chain::Listen + Send + Sync)),
+			(chain.at_height(3).block_hash, &listener_3 as &(dyn chain::Listen + Send + Sync)),
 		];
 		let mut cache = chain.header_cache(0..=4);
 		match synchronize_listeners(&mut chain, Network::Bitcoin, &mut cache, listeners).await {
@@ -284,9 +284,9 @@ mod tests {
 			.expect_block_connected(*main_chain.at_height(4));
 
 		let listeners = vec![
-			(fork_chain_1.tip().block_hash, &listener_1 as &dyn chain::Listen),
-			(fork_chain_2.tip().block_hash, &listener_2 as &dyn chain::Listen),
-			(fork_chain_3.tip().block_hash, &listener_3 as &dyn chain::Listen),
+			(fork_chain_1.tip().block_hash, &listener_1 as &(dyn chain::Listen + Send + Sync)),
+			(fork_chain_2.tip().block_hash, &listener_2 as &(dyn chain::Listen + Send + Sync)),
+			(fork_chain_3.tip().block_hash, &listener_3 as &(dyn chain::Listen + Send + Sync)),
 		];
 		let mut cache = fork_chain_1.header_cache(2..=4);
 		cache.extend(fork_chain_2.header_cache(3..=4));
@@ -327,9 +327,9 @@ mod tests {
 			.expect_block_connected(*main_chain.at_height(4));
 
 		let listeners = vec![
-			(fork_chain_1.tip().block_hash, &listener_1 as &dyn chain::Listen),
-			(fork_chain_2.tip().block_hash, &listener_2 as &dyn chain::Listen),
-			(fork_chain_3.tip().block_hash, &listener_3 as &dyn chain::Listen),
+			(fork_chain_1.tip().block_hash, &listener_1 as &(dyn chain::Listen + Send + Sync)),
+			(fork_chain_2.tip().block_hash, &listener_2 as &(dyn chain::Listen + Send + Sync)),
+			(fork_chain_3.tip().block_hash, &listener_3 as &(dyn chain::Listen + Send + Sync)),
 		];
 		let mut cache = fork_chain_1.header_cache(2..=4);
 		cache.extend(fork_chain_2.header_cache(3..=4));
@@ -351,7 +351,7 @@ mod tests {
 			.expect_block_disconnected(*old_tip)
 			.expect_block_connected(*new_tip);
 
-		let listeners = vec![(old_tip.block_hash, &listener as &dyn chain::Listen)];
+		let listeners = vec![(old_tip.block_hash, &listener as &(dyn chain::Listen + Send + Sync))];
 		let mut cache = fork_chain.header_cache(2..=2);
 		match synchronize_listeners(&mut main_chain, Network::Bitcoin, &mut cache, listeners).await {
 			Ok(_) => {