Avoid persisting a ChannelManager update after each timer tick

Currently, when a user calls `ChannelManager::timer_tick_occurred`
we always set the persister's update flag to true. This results in
a ChannelManager persistence after each timer tick, even when
nothing happened.

Instead, we add a new flag to `PersistenceNotifierGuard` to
indicate if we should skip setting the update flag.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -530,6 +530,7 @@ impl BestBlock {
 /// updates are ready for persistence).
 struct PersistenceNotifierGuard<'a> {
 	persistence_notifier: &'a PersistenceNotifier,
+	persistence_update: bool,
 	// We hold onto this result so the lock doesn't get released immediately.
 	_read_guard: RwLockReadGuard<'a, ()>,
 }
@@ -540,14 +541,17 @@ impl<'a> PersistenceNotifierGuard<'a> {
 
 		Self {
 			persistence_notifier: notifier,
+			persistence_update: true,
 			_read_guard: read_guard,
 		}
 	}
 }
 
 impl<'a> Drop for PersistenceNotifierGuard<'a> {
 	fn drop(&mut self) {
-		self.persistence_notifier.notify();
+		if self.persistence_update {
+			self.persistence_notifier.notify();
+		}
 	}
 }
 
@@ -2094,9 +2098,13 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	/// BroadcastChannelUpdate events in timer_tick_occurred.
 	///
 	/// Expects the caller to have a total_consistency_lock read lock.
-	fn process_background_events(&self) {
+	fn process_background_events(&self) -> bool {
 		let mut background_events = Vec::new();
 		mem::swap(&mut *self.pending_background_events.lock().unwrap(), &mut background_events);
+		if background_events.is_empty() {
+			return false;
+		}
+
 		for event in background_events.drain(..) {
 			match event {
 				BackgroundEvent::ClosingMonitorUpdate((funding_txo, update)) => {
@@ -2106,6 +2114,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 				},
 			}
 		}
+		true
 	}
 
 	#[cfg(any(test, feature = "_test_utils"))]
@@ -2121,8 +2130,8 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	///
 	/// Note that in some rare cases this may generate a `chain::Watch::update_channel` call.
 	pub fn timer_tick_occurred(&self) {
-		let _persistence_guard = PersistenceNotifierGuard::new(&self.total_consistency_lock, &self.persistence_notifier);
-		self.process_background_events();
+		let mut persistence_guard = PersistenceNotifierGuard::new(&self.total_consistency_lock, &self.persistence_notifier);
+		let mut did_work = self.process_background_events();
 
 		let mut channel_state_lock = self.channel_state.lock().unwrap();
 		let channel_state = &mut *channel_state_lock;
@@ -2138,6 +2147,7 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 							msg: update
 						});
 					}
+					did_work = true;
 					chan.set_update_status(UpdateStatus::Disabled);
 				},
 				UpdateStatus::EnabledStaged if chan.is_live() => {
@@ -2146,11 +2156,16 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 							msg: update
 						});
 					}
+					did_work = true;
 					chan.set_update_status(UpdateStatus::Enabled);
 				},
 				_ => {},
 			}
 		}
+
+		if !did_work {
+			persistence_guard.persistence_update = false;
+		}
 	}
 
 	/// Indicates that the preimage for payment_hash is unknown or the received amount is incorrect