Simplify event processing loop exit condition somewhat

Instead of having potentially two event processors running at once,
but only at different parts of the loop, we simplify the event
processing loop a bit by ensuring only one event processor can run
at once until the first is ready to exit.

Note that we still have to ensure we unset the mutual exclusion
flag inside the `pending_events` lock as otherwise we can race and
end up with two event processors called and both return without
every pending event having been processed.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -1718,14 +1718,11 @@ macro_rules! handle_new_monitor_update {
 
 macro_rules! process_events_body {
 	($self: expr, $event_to_handle: expr, $handle_event: expr) => {
-		let mut processed_all_events = false;
-		while !processed_all_events {
-			if $self.pending_events_processor.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed).is_err() {
-				return;
-			}
-
-			let mut result = NotifyOption::SkipPersist;
-
+		if $self.pending_events_processor.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed).is_err() {
+			return;
+		}
+		let mut result = NotifyOption::SkipPersist;
+		loop {
 			{
 				// We'll acquire our total consistency lock so that we can be sure no other
 				// persists happen while processing monitor events.
@@ -1754,23 +1751,26 @@ macro_rules! process_events_body {
 				}
 			}
 
-			{
-				let mut pending_events = $self.pending_events.lock().unwrap();
-				pending_events.drain(..num_events);
-				processed_all_events = pending_events.is_empty();
-				$self.pending_events_processor.store(false, Ordering::Release);
-			}
-
+			let mut processed_all_events = true;
 			if !post_event_actions.is_empty() {
 				$self.handle_post_event_actions(post_event_actions);
 				// If we had some actions, go around again as we may have more events now
 				processed_all_events = false;
 			}
 
-			if result == NotifyOption::DoPersist {
-				$self.persistence_notifier.notify();
+			{
+				let mut pending_events = $self.pending_events.lock().unwrap();
+				pending_events.drain(..num_events);
+				processed_all_events = pending_events.is_empty();
+				if processed_all_events {
+					$self.pending_events_processor.store(false, Ordering::Release);
+					break;
+				}
 			}
 		}
+		if result == NotifyOption::DoPersist {
+			$self.persistence_notifier.notify();
+		}
 	}
 }