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

@@ -4281,10 +4281,14 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		};
 		debug_assert!(!sources.is_empty());
 
-		// If we are claiming an MPP payment, we have to take special care to ensure that each
-		// channel exists before claiming all of the payments (inside one lock).
-		// Note that channel existance is sufficient as we should always get a monitor update
-		// which will take care of the real HTLC claim enforcement.
+		// If we are claiming an MPP payment, we check that all channels which contain a claimable
+		// HTLC still exist. While this isn't guaranteed to remain true if a channel closes while
+		// we're claiming (or even after we claim, before the commitment update dance completes),
+		// it should be a relatively rare race, and we'd rather not claim HTLCs that require us to
+		// go on-chain (and lose the on-chain fee to do so) than just reject the payment.
+		//
+		// Note that we'll still always get our funds - as long as the generated
+		// `ChannelMonitorUpdate` makes it out to the relevant monitor we can claim on-chain.
 		//
 		// If we find an HTLC which we would need to claim but for which we do not have a
 		// channel, we will fail all parts of the MPP payment. While we could wait and see if
@@ -4297,8 +4301,7 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		let mut valid_mpp = true;
 		let mut errs = Vec::new();
 		let mut claimed_any_htlcs = false;
-		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(),
@@ -4308,7 +4311,7 @@ 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;
 			}
@@ -4346,7 +4349,8 @@ 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) {
 					ClaimFundsFromHop::MonitorUpdateFail(pk, err, _) => {
 						if let msgs::ErrorAction::IgnoreError = err.err.action {
 							// We got a temporary failure updating monitor, but will claim the
@@ -4355,7 +4359,12 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 							claimed_any_htlcs = true;
 						} 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
@@ -4366,7 +4375,7 @@ 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 = htlc.value.to_be_bytes().to_vec();
@@ -4393,11 +4402,11 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 		}
 	}
 
-	fn claim_funds_from_hop(&self, channel_state_lock: &mut MutexGuard<ChannelHolder<<K::Target as KeysInterface>::Signer>>, prev_hop: HTLCPreviousHopData, payment_preimage: PaymentPreimage) -> ClaimFundsFromHop {
+	fn claim_funds_from_hop(&self, mut channel_state_lock: MutexGuard<ChannelHolder<<K::Target as KeysInterface>::Signer>>, prev_hop: HTLCPreviousHopData, payment_preimage: PaymentPreimage) -> 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) => {
@@ -4497,7 +4506,7 @@ 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);
@@ -4544,7 +4553,7 @@ 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);
 				let claimed_htlc = if let ClaimFundsFromHop::DuplicateClaim = res { false } else { true };
 				let htlc_claim_value_msat = match res {
 					ClaimFundsFromHop::MonitorUpdateFail(_, _, amt_opt) => amt_opt,
@@ -4558,7 +4567,6 @@ impl<M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelManager<M, T, K, F
 					// update to. Instead, we simply document in `PaymentForwarded` that this
 					// can happen.
 				}
-				mem::drop(channel_state_lock);
 				if let ClaimFundsFromHop::MonitorUpdateFail(pk, err, _) = res {
 					let result: Result<(), _> = Err(err);
 					let _ = handle_error!(self, result, pk);