Add DecodeError::DangerousValue for decoding invalid channel managers

This would help distinguish different types of errors when deserialzing
a channel manager. InvalidValue was used previously but this could be
because it is an old serialization format, whereas DangerousValue is a
lot more clear on why the deserialization failed.

fuzz/src/router.rs

@@ -157,6 +157,7 @@ pub fn do_test<Out: test_logger::Output>(data: &[u8], out: Out) {
 					msgs::DecodeError::ShortRead => panic!("We picked the length..."),
 					msgs::DecodeError::Io(e) => panic!("{:?}", e),
 					msgs::DecodeError::UnsupportedCompression => return,
+					msgs::DecodeError::DangerousValue => return,
 				}
 			}
 		}}

lightning/src/ln/channelmanager.rs

@@ -10918,7 +10918,7 @@ where
 						}
 					}
 					if chan.get_latest_unblocked_monitor_update_id() > max_in_flight_update_id {
-						// If the channel is ahead of the monitor, return InvalidValue:
+						// If the channel is ahead of the monitor, return DangerousValue:
 						log_error!(logger, "A ChannelMonitor is stale compared to the current ChannelManager! This indicates a potentially-critical violation of the chain::Watch API!");
 						log_error!(logger, " The ChannelMonitor for channel {} is at update_id {} with update_id through {} in-flight",
 							chan.context.channel_id(), monitor.get_latest_update_id(), max_in_flight_update_id);
@@ -10927,7 +10927,7 @@ where
 						log_error!(logger, " client applications must ensure that ChannelMonitor data is always available and the latest to avoid funds loss!");
 						log_error!(logger, " Without the latest ChannelMonitor we cannot continue without risking funds.");
 						log_error!(logger, " Please ensure the chain::Watch API requirements are met and file a bug report at https://github.com/lightningdevkit/rust-lightning");
-						return Err(DecodeError::InvalidValue);
+						return Err(DecodeError::DangerousValue);
 					}
 				} else {
 					// We shouldn't have persisted (or read) any unfunded channel types so none should have been

lightning/src/ln/msgs.rs

@@ -91,6 +91,16 @@ pub enum DecodeError {
 	Io(io::ErrorKind),
 	/// The message included zlib-compressed values, which we don't support.
 	UnsupportedCompression,
+	/// Value is validly encoded but is dangerous to use.
+	///
+	/// This is used for things like [`ChannelManager`] deserialization where we want to ensure
+	/// that we don't use a [`ChannelManager`] which is in out of sync with the [`ChannelMonitor`].
+	/// This indicates that there is a critical implementation flaw in the storage implementation
+	/// and it's unsafe to continue.
+	///
+	/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+	/// [`ChannelMonitor`]: crate::chain::channelmonitor::ChannelMonitor
+	DangerousValue,
 }
 
 /// An [`init`] message to be sent to or received from a peer.
@@ -1796,6 +1806,7 @@ impl fmt::Display for DecodeError {
 			DecodeError::BadLengthDescriptor => f.write_str("A length descriptor in the packet didn't describe the later data correctly"),
 			DecodeError::Io(ref e) => fmt::Debug::fmt(e, f),
 			DecodeError::UnsupportedCompression => f.write_str("We don't support receiving messages with zlib-compressed fields"),
+			DecodeError::DangerousValue => f.write_str("Value would be dangerous to continue execution with"),
 		}
 	}
 }

lightning/src/ln/peer_handler.rs

@@ -1551,6 +1551,7 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 												}
 												(msgs::DecodeError::BadLengthDescriptor, _) => return Err(PeerHandleError { }),
 												(msgs::DecodeError::Io(_), _) => return Err(PeerHandleError { }),
+												(msgs::DecodeError::DangerousValue, _) => return Err(PeerHandleError { }),
 											}
 										}
 									};

lightning/src/ln/reload_tests.rs

@@ -412,7 +412,7 @@ fn test_manager_serialize_deserialize_inconsistent_monitor() {
 	}
 
 	let mut nodes_0_read = &nodes_0_serialized[..];
-	if let Err(msgs::DecodeError::InvalidValue) =
+	if let Err(msgs::DecodeError::DangerousValue) =
 		<(BlockHash, ChannelManager<&test_utils::TestChainMonitor, &test_utils::TestBroadcaster, &test_utils::TestKeysInterface, &test_utils::TestKeysInterface, &test_utils::TestKeysInterface, &test_utils::TestFeeEstimator, &test_utils::TestRouter, &test_utils::TestLogger>)>::read(&mut nodes_0_read, ChannelManagerReadArgs {
 		default_config: UserConfig::default(),
 		entropy_source: keys_manager,