Check the PK of the source of an error before closing chans from it

When we receive an error message from a peer, it can indicate a
channel which we should close. However, we previously did not
check that the counterparty who sends us such a message is the
counterparty with whom we have the channel, allowing any
connected peer to make us force-close any channel we have as long
as they know the channel id.

This commit simply changes the force-close logic to check that the
sender matches the channel's counterparty node_id, though as noted
in lightningdevkit#105, we eventually need to change the indexing anyway to allow
absurdly terrible peers to open channels with us.

Found during review of lightningdevkit#777.

lightning/src/ln/channelmanager.rs

@@ -916,19 +916,22 @@ impl<ChanSigner: ChannelKeys, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref>
 		}
 	}
 
-	/// Force closes a channel, immediately broadcasting the latest local commitment transaction to
-	/// the chain and rejecting new HTLCs on the given channel. Fails if channel_id is unknown to the manager.
-	pub fn force_close_channel(&self, channel_id: &[u8; 32]) -> Result<(), APIError>{
-		let _consistency_lock = self.total_consistency_lock.read().unwrap();
-
+	fn force_close_channel_with_peer(&self, channel_id: &[u8; 32], peer_node_id: Option<&PublicKey>) -> Result<(), APIError> {
 		let mut chan = {
 			let mut channel_state_lock = self.channel_state.lock().unwrap();
 			let channel_state = &mut *channel_state_lock;
-			if let Some(chan) = channel_state.by_id.remove(channel_id) {
-				if let Some(short_id) = chan.get_short_channel_id() {
+			if let hash_map::Entry::Occupied(chan) = channel_state.by_id.entry(channel_id.clone()) {
+				if let Some(node_id) = peer_node_id {
+					if chan.get().get_counterparty_node_id() != *node_id {
+						// Error or Ok here doesn't matter - the result is only exposed publicly
+						// when peer_node_id is None anyway.
+						return Ok(());
+					}
+				}
+				if let Some(short_id) = chan.get().get_short_channel_id() {
 					channel_state.short_to_id.remove(&short_id);
 				}
-				chan
+				chan.remove_entry().1
 			} else {
 				return Err(APIError::ChannelUnavailable{err: "No such channel".to_owned()});
 			}
@@ -945,6 +948,13 @@ impl<ChanSigner: ChannelKeys, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref>
 		Ok(())
 	}
 
+	/// Force closes a channel, immediately broadcasting the latest local commitment transaction to
+	/// the chain and rejecting new HTLCs on the given channel. Fails if channel_id is unknown to the manager.
+	pub fn force_close_channel(&self, channel_id: &[u8; 32]) -> Result<(), APIError> {
+		let _consistency_lock = self.total_consistency_lock.read().unwrap();
+		self.force_close_channel_with_peer(channel_id, None)
+	}
+
 	/// Force close all channels, immediately broadcasting the latest local commitment transaction
 	/// for each to the chain and rejecting new HTLCs on each.
 	pub fn force_close_all_channels(&self) {
@@ -3474,12 +3484,12 @@ impl<ChanSigner: ChannelKeys, M: Deref + Sync + Send, T: Deref + Sync + Send, K:
 			for chan in self.list_channels() {
 				if chan.remote_network_id == *counterparty_node_id {
 					// Untrusted messages from peer, we throw away the error if id points to a non-existent channel
-					let _ = self.force_close_channel(&msg.channel_id);
+					let _ = self.force_close_channel_with_peer(&chan.channel_id, Some(counterparty_node_id));
 				}
 			}
 		} else {
 			// Untrusted messages from peer, we throw away the error if id points to a non-existent channel
-			let _ = self.force_close_channel(&msg.channel_id);
+			let _ = self.force_close_channel_with_peer(&msg.channel_id, Some(counterparty_node_id));
 		}
 	}
 }