f - Add debug_assertions of channel_state in fail_htlc_backwards_internal

Author: ViktorTigerstrom

lightning/src/ln/channelmanager.rs

@@ -3881,6 +3881,15 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	/// Note that while this function pushes events as well as FailHTLC's to fail htlcs for
 	/// designated channels, no assumptions are made that the channels are still available.
 	fn fail_htlc_backwards_internal(&self, source: HTLCSource, payment_hash: &PaymentHash, onion_error: HTLCFailReason,destination: HTLCDestination) {
+		#[cfg(debug_assertions)]
+		{
+			// Ensure that the `channel_state` lock is not held when calling this function.
+			// This ensures that future code doesn't introduce a lock_order requirement for
+			// `forward_htlcs` to be locked after the `channel_state` lock, which calling this
+			// function with the `channel_state` locked would.
+			assert!(self.channel_state.try_lock().is_ok());
+		}
+
 		//TODO: There is a timing attack here where if a node fails an HTLC back to us they can
 		//identify whether we sent it or not based on the (I presume) very different runtime
 		//between the branches here. We should make this async and move it into the forward HTLCs