Add reenatrancy guard for process_pending_htlc_forwards

We add a reenatrancy guard to disallow entering
`process_pending_htlc_forwards` multiple times. This makes sure that
we'd skip any additional processing calls if a prior round/batch of
processing is still underway.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -6337,6 +6337,11 @@ where
 	///
 	/// Will regularly be called by the background processor.
 	pub fn process_pending_htlc_forwards(&self) {
+		static REENTRANCY_GUARD: AtomicBool = AtomicBool::new(false);
+		if REENTRANCY_GUARD.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed).is_err() {
+			return;
+		}
+
 		let _persistence_guard = PersistenceNotifierGuard::optionally_notify(self, || {
 			let mut should_persist = NotifyOption::SkipPersistNoEvents;
 			self.process_pending_update_add_htlcs();
@@ -7175,6 +7180,7 @@ where
 			}
 			should_persist
 		});
+		REENTRANCY_GUARD.store(false, Ordering::Release);
 	}
 
 	/// Free the background events, generally called from [`PersistenceNotifierGuard`] constructors.