Don't hold channel_state lock for entire duration of claim_funds

When `claim_funds` has to claim multiple HTLCs as a part of a
single MPP payment, it currently does so holding the
`channel_state` lock for the entire duration of the claim loop.
Here we swap that for taking the lock once for each HTLC. This
allows us to be more flexible with locks going forward, and
ultimately isn't a huge change - if our counterparty intends to
force-close a channel, us choosing to ignore it by holding the
`channel_state` lock for the duration of the claim isn't going to
result in a commitment update, it will just result in the preimage
already being in the `ChannelMonitor`.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4294,8 +4294,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		let mut expected_amt_msat = None;
 		let mut valid_mpp = true;
 		let mut errs = Vec::new();
-		let mut channel_state_lock = self.channel_state.lock().unwrap();
-		let channel_state = &mut *channel_state_lock;
+		let mut channel_state = Some(self.channel_state.lock().unwrap());
 		for htlc in sources.iter() {
 			let chan_id = match self.short_to_chan_info.read().unwrap().get(&htlc.prev_hop.short_channel_id) {
 				Some((_cp_id, chan_id)) => chan_id.clone(),
@@ -4305,7 +4304,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 				}
 			};
 
-			if let None = channel_state.by_id.get(&chan_id) {
+			if let None = channel_state.as_ref().unwrap().by_id.get(&chan_id) {
 				valid_mpp = false;
 				break;
 			}
@@ -4343,7 +4342,9 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		}
 		if valid_mpp {
 			for htlc in sources.drain(..) {
-				match self.claim_funds_from_hop(&mut channel_state_lock, htlc.prev_hop, payment_preimage,
+				if channel_state.is_none() { channel_state = Some(self.channel_state.lock().unwrap()); }
+				match self.claim_funds_from_hop(channel_state.take().unwrap(), htlc.prev_hop,
+					payment_preimage,
 					|_| Some(MonitorUpdateCompletionAction::PaymentClaimed { payment_hash }))
 				{
 					ClaimFundsFromHop::MonitorUpdateFail(pk, err, _) => {
@@ -4353,7 +4354,12 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 							log_error!(self.logger, "Temporary failure claiming HTLC, treating as success: {}", err.err.err);
 						} else { errs.push((pk, err)); }
 					},
-					ClaimFundsFromHop::PrevHopForceClosed => unreachable!("We already checked for channel existence, we can't fail here!"),
+					ClaimFundsFromHop::PrevHopForceClosed => {
+						// This should be incredibly rare - we checked that all the channels were
+						// open above, though as we release the lock at each loop iteration it's
+						// still possible. We should still claim the HTLC on-chain through the
+						// closed-channel-update generated in claim_funds_from_hop.
+					},
 					ClaimFundsFromHop::DuplicateClaim => {
 						// While we should never get here in most cases, if we do, it likely
 						// indicates that the HTLC was timed out some time ago and is no longer
@@ -4364,7 +4370,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 				}
 			}
 		}
-		mem::drop(channel_state_lock);
+		mem::drop(channel_state);
 		if !valid_mpp {
 			for htlc in sources.drain(..) {
 				let mut htlc_msat_height_data = byte_utils::be64_to_array(htlc.value).to_vec();
@@ -4385,13 +4391,13 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 	}
 
 	fn claim_funds_from_hop<ComplFunc: FnOnce(Option<u64>) -> Option<MonitorUpdateCompletionAction>>(&self,
-		channel_state_lock: &mut MutexGuard<ChannelHolder<<K::Target as KeysInterface>::Signer>>,
+		mut channel_state_lock: MutexGuard<ChannelHolder<<K::Target as KeysInterface>::Signer>>,
 		prev_hop: HTLCPreviousHopData, payment_preimage: PaymentPreimage, completion_action: ComplFunc)
 	-> ClaimFundsFromHop {
 		//TODO: Delay the claimed_funds relaying just like we do outbound relay!
 
 		let chan_id = prev_hop.outpoint.to_channel_id();
-		let channel_state = &mut **channel_state_lock;
+		let channel_state = &mut *channel_state_lock;
 		if let hash_map::Entry::Occupied(mut chan) = channel_state.by_id.entry(chan_id) {
 			match chan.get_mut().get_update_fulfill_htlc_and_commit(prev_hop.htlc_id, payment_preimage, &self.logger) {
 				Ok(msgs_monitor_option) => {
@@ -4501,7 +4507,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		}
 	}
 
-	fn claim_funds_internal(&self, mut channel_state_lock: MutexGuard<ChannelHolder<<K::Target as KeysInterface>::Signer>>, source: HTLCSource, payment_preimage: PaymentPreimage, forwarded_htlc_value_msat: Option<u64>, from_onchain: bool, next_channel_id: [u8; 32]) {
+	fn claim_funds_internal(&self, channel_state_lock: MutexGuard<ChannelHolder<<K::Target as KeysInterface>::Signer>>, source: HTLCSource, payment_preimage: PaymentPreimage, forwarded_htlc_value_msat: Option<u64>, from_onchain: bool, next_channel_id: [u8; 32]) {
 		match source {
 			HTLCSource::OutboundRoute { session_priv, payment_id, path, .. } => {
 				mem::drop(channel_state_lock);
@@ -4548,7 +4554,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 			},
 			HTLCSource::PreviousHopData(hop_data) => {
 				let prev_outpoint = hop_data.outpoint;
-				let res = self.claim_funds_from_hop(&mut channel_state_lock, hop_data, payment_preimage,
+				let res = self.claim_funds_from_hop(channel_state_lock, hop_data, payment_preimage,
 					|htlc_claim_value_msat| {
 						if let Some(forwarded_htlc_value) = forwarded_htlc_value_msat {
 							let fee_earned_msat = if let Some(claimed_htlc_value) = htlc_claim_value_msat {
@@ -4566,7 +4572,6 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 							}})
 						} else { None }
 					});
-				mem::drop(channel_state_lock);
 				if let ClaimFundsFromHop::MonitorUpdateFail(pk, err, _) = res {
 					let result: Result<(), _> = Err(err);
 					let _ = handle_error!(self, result, pk);