Expose an option to substantially reduce sleep time in futures BP

Some users have suggested that waking every 100ms can be
CPU-intensive in deployments with hundreds or thousands of nodes
all running on the same machine. Thus, we add an option to the
futures-based `background-processor` to avoid waking every 100ms to
check for iOS having backgrounded our app and cut our TCP sockets.

This cuts the normal sleep time down from 100ms to 10s, for those
who turn it on.

Author: TheBlueMatt

lightning-background-processor/src/lib.rs

@@ -116,6 +116,13 @@ const FIRST_NETWORK_PRUNE_TIMER: u64 = 60;
 #[cfg(test)]
 const FIRST_NETWORK_PRUNE_TIMER: u64 = 1;
 
+#[cfg(feature = "futures")]
+/// core::cmp::min is not currently const, so we define a trivial (and equivalent) replacement
+const fn min_u64(a: u64, b: u64) -> u64 { if a < b { a } else { b } }
+#[cfg(feature = "futures")]
+const FASTEST_TIMER: u64 = min_u64(min_u64(FRESHNESS_TIMER, PING_TIMER),
+	min_u64(SCORER_PERSIST_TIMER, FIRST_NETWORK_PRUNE_TIMER));
+
 /// Either [`P2PGossipSync`] or [`RapidGossipSync`].
 pub enum GossipSync<
 	P: Deref<Target = P2PGossipSync<G, U, L>>,
@@ -258,7 +265,8 @@ macro_rules! define_run_body {
 	($persister: ident, $chain_monitor: ident, $process_chain_monitor_events: expr,
 	 $channel_manager: ident, $process_channel_manager_events: expr,
 	 $gossip_sync: ident, $peer_manager: ident, $logger: ident, $scorer: ident,
-	 $loop_exit_check: expr, $await: expr, $get_timer: expr, $timer_elapsed: expr)
+	 $loop_exit_check: expr, $await: expr, $get_timer: expr, $timer_elapsed: expr,
+	 $check_slow_await: expr)
 	=> { {
 		log_trace!($logger, "Calling ChannelManager's timer_tick_occurred on startup");
 		$channel_manager.timer_tick_occurred();
@@ -288,9 +296,10 @@ macro_rules! define_run_body {
 
 			// We wait up to 100ms, but track how long it takes to detect being put to sleep,
 			// see `await_start`'s use below.
-			let mut await_start = $get_timer(1);
+			let mut await_start = None;
+			if $check_slow_await { await_start = Some($get_timer(1)); }
 			let updates_available = $await;
-			let await_slow = $timer_elapsed(&mut await_start, 1);
+			let await_slow = if $check_slow_await { $timer_elapsed(&mut await_start.unwrap(), 1) } else { false };
 
 			if updates_available {
 				log_trace!($logger, "Persisting ChannelManager...");
@@ -449,6 +458,11 @@ use core::task;
 /// feature, doing so will skip calling [`NetworkGraph::remove_stale_channels_and_tracking`],
 /// you should call [`NetworkGraph::remove_stale_channels_and_tracking_with_time`] regularly
 /// manually instead.
+///
+/// The `mobile_interruptable_platform` flag should be set if we're currently running on a
+/// mobile device, where we may need to check for interruption of the application regularly. If you
+/// are unsure, you should set the flag, as the performance impact of it is minimal unless there
+/// are hundreds or thousands of simultaneous process calls running.
 #[cfg(feature = "futures")]
 pub async fn process_events_async<
 	'a,
@@ -484,7 +498,7 @@ pub async fn process_events_async<
 >(
 	persister: PS, event_handler: EventHandler, chain_monitor: M, channel_manager: CM,
 	gossip_sync: GossipSync<PGS, RGS, G, UL, L>, peer_manager: PM, logger: L, scorer: Option<S>,
-	sleeper: Sleeper,
+	sleeper: Sleeper, mobile_interruptable_platform: bool,
 ) -> Result<(), lightning::io::Error>
 where
 	UL::Target: 'static + UtxoLookup,
@@ -526,7 +540,7 @@ where
 			let fut = Selector {
 				a: channel_manager.get_persistable_update_future(),
 				b: chain_monitor.get_update_future(),
-				c: sleeper(Duration::from_millis(100)),
+				c: sleeper(if mobile_interruptable_platform { Duration::from_millis(100) } else { Duration::from_secs(FASTEST_TIMER) }),
 			};
 			match fut.await {
 				SelectorOutput::A => true,
@@ -541,7 +555,7 @@ where
 			let mut waker = dummy_waker();
 			let mut ctx = task::Context::from_waker(&mut waker);
 			core::pin::Pin::new(fut).poll(&mut ctx).is_ready()
-		})
+		}, mobile_interruptable_platform)
 }
 
 #[cfg(feature = "std")]
@@ -660,7 +674,7 @@ impl BackgroundProcessor {
 					channel_manager.get_persistable_update_future(),
 					chain_monitor.get_update_future()
 				).wait_timeout(Duration::from_millis(100)),
-				|_| Instant::now(), |time: &Instant, dur| time.elapsed().as_secs() > dur)
+				|_| Instant::now(), |time: &Instant, dur| time.elapsed().as_secs() > dur, false)
 		});
 		Self { stop_thread: stop_thread_clone, thread_handle: Some(handle) }
 	}