[rust-lightning] / lightning / src / chain / chainmonitor.rs

// This file is Copyright its original authors, visible in version control
// history.
//
// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
// You may not use this file except in accordance with one or both of these
// licenses.

//! Logic to connect off-chain channel management with on-chain transaction monitoring.
//!
//! [`ChainMonitor`] is an implementation of [`chain::Watch`] used both to process blocks and to
//! update [`ChannelMonitor`]s accordingly. If any on-chain events need further processing, it will
//! make those available as [`MonitorEvent`]s to be consumed.
//!
//! [`ChainMonitor`] is parameterized by an optional chain source, which must implement the
//! [`chain::Filter`] trait. This provides a mechanism to signal new relevant outputs back to light
//! clients, such that transactions spending those outputs are included in block data.
//!
//! [`ChainMonitor`] may be used directly to monitor channels locally or as a part of a distributed
//! setup to monitor channels remotely. In the latter case, a custom [`chain::Watch`] implementation
//! would be responsible for routing each update to a remote server and for retrieving monitor
//! events. The remote server would make use of [`ChainMonitor`] for block processing and for
//! servicing [`ChannelMonitor`] updates from the client.

use bitcoin::blockdata::block::BlockHeader;
use bitcoin::hash_types::Txid;

use chain;
use chain::{ChannelMonitorUpdateStatus, Filter, WatchedOutput};
use chain::chaininterface::{BroadcasterInterface, FeeEstimator};
use chain::channelmonitor::{ChannelMonitor, ChannelMonitorUpdate, Balance, MonitorEvent, TransactionOutputs, LATENCY_GRACE_PERIOD_BLOCKS};
use chain::transaction::{OutPoint, TransactionData};
use chain::keysinterface::Sign;
use util::atomic_counter::AtomicCounter;
use util::logger::Logger;
use util::errors::APIError;
use util::events;
use util::events::EventHandler;
use ln::channelmanager::ChannelDetails;

use prelude::*;
use sync::{RwLock, RwLockReadGuard, Mutex, MutexGuard};
use core::ops::Deref;
use core::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use bitcoin::secp256k1::PublicKey;

#[derive(Clone, Copy, Hash, PartialEq, Eq)]
/// A specific update's ID stored in a `MonitorUpdateId`, separated out to make the contents
/// entirely opaque.
enum UpdateOrigin {
        /// An update that was generated by the `ChannelManager` (via our `chain::Watch`
        /// implementation). This corresponds to an actual [`ChannelMonitorUpdate::update_id`] field
        /// and [`ChannelMonitor::get_latest_update_id`].
        OffChain(u64),
        /// An update that was generated during blockchain processing. The ID here is specific to the
        /// generating [`ChainMonitor`] and does *not* correspond to any on-disk IDs.
        ChainSync(u64),
}

/// An opaque identifier describing a specific [`Persist`] method call.
#[derive(Clone, Copy, Hash, PartialEq, Eq)]
pub struct MonitorUpdateId {
        contents: UpdateOrigin,
}

impl MonitorUpdateId {
        pub(crate) fn from_monitor_update(update: &ChannelMonitorUpdate) -> Self {
                Self { contents: UpdateOrigin::OffChain(update.update_id) }
        }
        pub(crate) fn from_new_monitor<ChannelSigner: Sign>(monitor: &ChannelMonitor<ChannelSigner>) -> Self {
                Self { contents: UpdateOrigin::OffChain(monitor.get_latest_update_id()) }
        }
}

/// `Persist` defines behavior for persisting channel monitors: this could mean
/// writing once to disk, and/or uploading to one or more backup services.
///
/// Each method can return three possible values:
///  * If persistence (including any relevant `fsync()` calls) happens immediately, the
///    implementation should return [`ChannelMonitorUpdateStatus::Completed`], indicating normal
///    channel operation should continue.
///  * If persistence happens asynchronously, implementations should first ensure the
///    [`ChannelMonitor`] or [`ChannelMonitorUpdate`] are written durably to disk, and then return
///    [`ChannelMonitorUpdateStatus::InProgress`] while the update continues in the background.
///    Once the update completes, [`ChainMonitor::channel_monitor_updated`] should be called with
///    the corresponding [`MonitorUpdateId`].
///
///    Note that unlike the direct [`chain::Watch`] interface,
///    [`ChainMonitor::channel_monitor_updated`] must be called once for *each* update which occurs.
///
///  * If persistence fails for some reason, implementations should return
///    [`ChannelMonitorUpdateStatus::PermanentFailure`], in which case the channel will likely be
///    closed without broadcasting the latest state. See
///    [`ChannelMonitorUpdateStatus::PermanentFailure`] for more details.
pub trait Persist<ChannelSigner: Sign> {
        /// Persist a new channel's data in response to a [`chain::Watch::watch_channel`] call. This is
        /// called by [`ChannelManager`] for new channels, or may be called directly, e.g. on startup.
        ///
        /// The data can be stored any way you want, but the identifier provided by LDK is the
        /// channel's outpoint (and it is up to you to maintain a correct mapping between the outpoint
        /// and the stored channel data). Note that you **must** persist every new monitor to disk.
        ///
        /// The `update_id` is used to identify this call to [`ChainMonitor::channel_monitor_updated`],
        /// if you return [`ChannelMonitorUpdateStatus::InProgress`].
        ///
        /// See [`Writeable::write`] on [`ChannelMonitor`] for writing out a `ChannelMonitor`
        /// and [`ChannelMonitorUpdateStatus`] for requirements when returning errors.
        ///
        /// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
        /// [`Writeable::write`]: crate::util::ser::Writeable::write
        fn persist_new_channel(&self, channel_id: OutPoint, data: &ChannelMonitor<ChannelSigner>, update_id: MonitorUpdateId) -> ChannelMonitorUpdateStatus;

        /// Update one channel's data. The provided [`ChannelMonitor`] has already applied the given
        /// update.
        ///
        /// Note that on every update, you **must** persist either the [`ChannelMonitorUpdate`] or the
        /// updated monitor itself to disk/backups. See the [`Persist`] trait documentation for more
        /// details.
        ///
        /// During blockchain synchronization operations, this may be called with no
        /// [`ChannelMonitorUpdate`], in which case the full [`ChannelMonitor`] needs to be persisted.
        /// Note that after the full [`ChannelMonitor`] is persisted any previous
        /// [`ChannelMonitorUpdate`]s which were persisted should be discarded - they can no longer be
        /// applied to the persisted [`ChannelMonitor`] as they were already applied.
        ///
        /// If an implementer chooses to persist the updates only, they need to make
        /// sure that all the updates are applied to the `ChannelMonitors` *before*
        /// the set of channel monitors is given to the `ChannelManager`
        /// deserialization routine. See [`ChannelMonitor::update_monitor`] for
        /// applying a monitor update to a monitor. If full `ChannelMonitors` are
        /// persisted, then there is no need to persist individual updates.
        ///
        /// Note that there could be a performance tradeoff between persisting complete
        /// channel monitors on every update vs. persisting only updates and applying
        /// them in batches. The size of each monitor grows `O(number of state updates)`
        /// whereas updates are small and `O(1)`.
        ///
        /// The `update_id` is used to identify this call to [`ChainMonitor::channel_monitor_updated`],
        /// if you return [`ChannelMonitorUpdateStatus::InProgress`].
        ///
        /// See [`Writeable::write`] on [`ChannelMonitor`] for writing out a `ChannelMonitor`,
        /// [`Writeable::write`] on [`ChannelMonitorUpdate`] for writing out an update, and
        /// [`ChannelMonitorUpdateStatus`] for requirements when returning errors.
        ///
        /// [`Writeable::write`]: crate::util::ser::Writeable::write
        fn update_persisted_channel(&self, channel_id: OutPoint, update: &Option<ChannelMonitorUpdate>, data: &ChannelMonitor<ChannelSigner>, update_id: MonitorUpdateId) -> ChannelMonitorUpdateStatus;
}

struct MonitorHolder<ChannelSigner: Sign> {
        monitor: ChannelMonitor<ChannelSigner>,
        /// The full set of pending monitor updates for this Channel.
        ///
        /// Note that this lock must be held during updates to prevent a race where we call
        /// update_persisted_channel, the user returns a
        /// [`ChannelMonitorUpdateStatus::InProgress`], and then calls channel_monitor_updated
        /// immediately, racing our insertion of the pending update into the contained Vec.
        ///
        /// Beyond the synchronization of updates themselves, we cannot handle user events until after
        /// any chain updates have been stored on disk. Thus, we scan this list when returning updates
        /// to the ChannelManager, refusing to return any updates for a ChannelMonitor which is still
        /// being persisted fully to disk after a chain update.
        ///
        /// This avoids the possibility of handling, e.g. an on-chain claim, generating a claim monitor
        /// event, resulting in the relevant ChannelManager generating a PaymentSent event and dropping
        /// the pending payment entry, and then reloading before the monitor is persisted, resulting in
        /// the ChannelManager re-adding the same payment entry, before the same block is replayed,
        /// resulting in a duplicate PaymentSent event.
        pending_monitor_updates: Mutex<Vec<MonitorUpdateId>>,
        /// When the user returns a PermanentFailure error from an update_persisted_channel call during
        /// block processing, we inform the ChannelManager that the channel should be closed
        /// asynchronously. In order to ensure no further changes happen before the ChannelManager has
        /// processed the closure event, we set this to true and return PermanentFailure for any other
        /// chain::Watch events.
        channel_perm_failed: AtomicBool,
        /// The last block height at which no [`UpdateOrigin::ChainSync`] monitor updates were present
        /// in `pending_monitor_updates`.
        /// If it's been more than [`LATENCY_GRACE_PERIOD_BLOCKS`] since we started waiting on a chain
        /// sync event, we let monitor events return to `ChannelManager` because we cannot hold them up
        /// forever or we'll end up with HTLC preimages waiting to feed back into an upstream channel
        /// forever, risking funds loss.
        last_chain_persist_height: AtomicUsize,
}

impl<ChannelSigner: Sign> MonitorHolder<ChannelSigner> {
        fn has_pending_offchain_updates(&self, pending_monitor_updates_lock: &MutexGuard<Vec<MonitorUpdateId>>) -> bool {
                pending_monitor_updates_lock.iter().any(|update_id|
                        if let UpdateOrigin::OffChain(_) = update_id.contents { true } else { false })
        }
        fn has_pending_chainsync_updates(&self, pending_monitor_updates_lock: &MutexGuard<Vec<MonitorUpdateId>>) -> bool {
                pending_monitor_updates_lock.iter().any(|update_id|
                        if let UpdateOrigin::ChainSync(_) = update_id.contents { true } else { false })
        }
}

/// A read-only reference to a current ChannelMonitor.
///
/// Note that this holds a mutex in [`ChainMonitor`] and may block other events until it is
/// released.
pub struct LockedChannelMonitor<'a, ChannelSigner: Sign> {
        lock: RwLockReadGuard<'a, HashMap<OutPoint, MonitorHolder<ChannelSigner>>>,
        funding_txo: OutPoint,
}

impl<ChannelSigner: Sign> Deref for LockedChannelMonitor<'_, ChannelSigner> {
        type Target = ChannelMonitor<ChannelSigner>;
        fn deref(&self) -> &ChannelMonitor<ChannelSigner> {
                &self.lock.get(&self.funding_txo).expect("Checked at construction").monitor
        }
}

/// An implementation of [`chain::Watch`] for monitoring channels.
///
/// Connected and disconnected blocks must be provided to `ChainMonitor` as documented by
/// [`chain::Watch`]. May be used in conjunction with [`ChannelManager`] to monitor channels locally
/// or used independently to monitor channels remotely. See the [module-level documentation] for
/// details.
///
/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
/// [module-level documentation]: crate::chain::chainmonitor
pub struct ChainMonitor<ChannelSigner: Sign, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref>
        where C::Target: chain::Filter,
        T::Target: BroadcasterInterface,
        F::Target: FeeEstimator,
        L::Target: Logger,
        P::Target: Persist<ChannelSigner>,
{
        monitors: RwLock<HashMap<OutPoint, MonitorHolder<ChannelSigner>>>,
        /// When we generate a [`MonitorUpdateId`] for a chain-event monitor persistence, we need a
        /// unique ID, which we calculate by simply getting the next value from this counter. Note that
        /// the ID is never persisted so it's ok that they reset on restart.
        sync_persistence_id: AtomicCounter,
        chain_source: Option<C>,
        broadcaster: T,
        logger: L,
        fee_estimator: F,
        persister: P,
        /// "User-provided" (ie persistence-completion/-failed) [`MonitorEvent`]s. These came directly
        /// from the user and not from a [`ChannelMonitor`].
        pending_monitor_events: Mutex<Vec<(OutPoint, Vec<MonitorEvent>, Option<PublicKey>)>>,
        /// The best block height seen, used as a proxy for the passage of time.
        highest_chain_height: AtomicUsize,
}

impl<ChannelSigner: Sign, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref> ChainMonitor<ChannelSigner, C, T, F, L, P>
where C::Target: chain::Filter,
            T::Target: BroadcasterInterface,
            F::Target: FeeEstimator,
            L::Target: Logger,
            P::Target: Persist<ChannelSigner>,
{
        /// Dispatches to per-channel monitors, which are responsible for updating their on-chain view
        /// of a channel and reacting accordingly based on transactions in the given chain data. See
        /// [`ChannelMonitor::block_connected`] for details. Any HTLCs that were resolved on chain will
        /// be returned by [`chain::Watch::release_pending_monitor_events`].
        ///
        /// Calls back to [`chain::Filter`] if any monitor indicated new outputs to watch. Subsequent
        /// calls must not exclude any transactions matching the new outputs nor any in-block
        /// descendants of such transactions. It is not necessary to re-fetch the block to obtain
        /// updated `txdata`.
        ///
        /// Calls which represent a new blockchain tip height should set `best_height`.
        fn process_chain_data<FN>(&self, header: &BlockHeader, best_height: Option<u32>, txdata: &TransactionData, process: FN)
        where
                FN: Fn(&ChannelMonitor<ChannelSigner>, &TransactionData) -> Vec<TransactionOutputs>
        {
                let monitor_states = self.monitors.write().unwrap();
                if let Some(height) = best_height {
                        // If the best block height is being updated, update highest_chain_height under the
                        // monitors write lock.
                        let old_height = self.highest_chain_height.load(Ordering::Acquire);
                        let new_height = height as usize;
                        if new_height > old_height {
                                self.highest_chain_height.store(new_height, Ordering::Release);
                        }
                }

                for (funding_outpoint, monitor_state) in monitor_states.iter() {
                        let monitor = &monitor_state.monitor;
                        let mut txn_outputs;
                        {
                                txn_outputs = process(monitor, txdata);
                                let update_id = MonitorUpdateId {
                                        contents: UpdateOrigin::ChainSync(self.sync_persistence_id.get_increment()),
                                };
                                let mut pending_monitor_updates = monitor_state.pending_monitor_updates.lock().unwrap();
                                if let Some(height) = best_height {
                                        if !monitor_state.has_pending_chainsync_updates(&pending_monitor_updates) {
                                                // If there are not ChainSync persists awaiting completion, go ahead and
                                                // set last_chain_persist_height here - we wouldn't want the first
                                                // InProgress to always immediately be considered "overly delayed".
                                                monitor_state.last_chain_persist_height.store(height as usize, Ordering::Release);
                                        }
                                }

                                log_trace!(self.logger, "Syncing Channel Monitor for channel {}", log_funding_info!(monitor));
                                match self.persister.update_persisted_channel(*funding_outpoint, &None, monitor, update_id) {
                                        ChannelMonitorUpdateStatus::Completed =>
                                                log_trace!(self.logger, "Finished syncing Channel Monitor for channel {}", log_funding_info!(monitor)),
                                        ChannelMonitorUpdateStatus::PermanentFailure => {
                                                monitor_state.channel_perm_failed.store(true, Ordering::Release);
                                                self.pending_monitor_events.lock().unwrap().push((*funding_outpoint, vec![MonitorEvent::UpdateFailed(*funding_outpoint)], monitor.get_counterparty_node_id()));
                                        },
                                        ChannelMonitorUpdateStatus::InProgress => {
                                                log_debug!(self.logger, "Channel Monitor sync for channel {} in progress, holding events until completion!", log_funding_info!(monitor));
                                                pending_monitor_updates.push(update_id);
                                        },
                                }
                        }

                        // Register any new outputs with the chain source for filtering, storing any dependent
                        // transactions from within the block that previously had not been included in txdata.
                        if let Some(ref chain_source) = self.chain_source {
                                let block_hash = header.block_hash();
                                for (txid, mut outputs) in txn_outputs.drain(..) {
                                        for (idx, output) in outputs.drain(..) {
                                                // Register any new outputs with the chain source for filtering
                                                let output = WatchedOutput {
                                                        block_hash: Some(block_hash),
                                                        outpoint: OutPoint { txid, index: idx as u16 },
                                                        script_pubkey: output.script_pubkey,
                                                };
                                                chain_source.register_output(output)
                                        }
                                }
                        }
                }
        }

        /// Creates a new `ChainMonitor` used to watch on-chain activity pertaining to channels.
        ///
        /// When an optional chain source implementing [`chain::Filter`] is provided, the chain monitor
        /// will call back to it indicating transactions and outputs of interest. This allows clients to
        /// pre-filter blocks or only fetch blocks matching a compact filter. Otherwise, clients may
        /// always need to fetch full blocks absent another means for determining which blocks contain
        /// transactions relevant to the watched channels.
        pub fn new(chain_source: Option<C>, broadcaster: T, logger: L, feeest: F, persister: P) -> Self {
                Self {
                        monitors: RwLock::new(HashMap::new()),
                        sync_persistence_id: AtomicCounter::new(),
                        chain_source,
                        broadcaster,
                        logger,
                        fee_estimator: feeest,
                        persister,
                        pending_monitor_events: Mutex::new(Vec::new()),
                        highest_chain_height: AtomicUsize::new(0),
                }
        }

        /// Gets the balances in the contained [`ChannelMonitor`]s which are claimable on-chain or
        /// claims which are awaiting confirmation.
        ///
        /// Includes the balances from each [`ChannelMonitor`] *except* those included in
        /// `ignored_channels`, allowing you to filter out balances from channels which are still open
        /// (and whose balance should likely be pulled from the [`ChannelDetails`]).
        ///
        /// See [`ChannelMonitor::get_claimable_balances`] for more details on the exact criteria for
        /// inclusion in the return value.
        pub fn get_claimable_balances(&self, ignored_channels: &[&ChannelDetails]) -> Vec<Balance> {
                let mut ret = Vec::new();
                let monitor_states = self.monitors.read().unwrap();
                for (_, monitor_state) in monitor_states.iter().filter(|(funding_outpoint, _)| {
                        for chan in ignored_channels {
                                if chan.funding_txo.as_ref() == Some(funding_outpoint) {
                                        return false;
                                }
                        }
                        true
                }) {
                        ret.append(&mut monitor_state.monitor.get_claimable_balances());
                }
                ret
        }

        /// Gets the [`LockedChannelMonitor`] for a given funding outpoint, returning an `Err` if no
        /// such [`ChannelMonitor`] is currently being monitored for.
        ///
        /// Note that the result holds a mutex over our monitor set, and should not be held
        /// indefinitely.
        pub fn get_monitor(&self, funding_txo: OutPoint) -> Result<LockedChannelMonitor<'_, ChannelSigner>, ()> {
                let lock = self.monitors.read().unwrap();
                if lock.get(&funding_txo).is_some() {
                        Ok(LockedChannelMonitor { lock, funding_txo })
                } else {
                        Err(())
                }
        }

        /// Lists the funding outpoint of each [`ChannelMonitor`] being monitored.
        ///
        /// Note that [`ChannelMonitor`]s are not removed when a channel is closed as they are always
        /// monitoring for on-chain state resolutions.
        pub fn list_monitors(&self) -> Vec<OutPoint> {
                self.monitors.read().unwrap().keys().map(|outpoint| *outpoint).collect()
        }

        #[cfg(test)]
        pub fn remove_monitor(&self, funding_txo: &OutPoint) -> ChannelMonitor<ChannelSigner> {
                self.monitors.write().unwrap().remove(funding_txo).unwrap().monitor
        }

        /// Indicates the persistence of a [`ChannelMonitor`] has completed after
        /// [`ChannelMonitorUpdateStatus::InProgress`] was returned from an update operation.
        ///
        /// Thus, the anticipated use is, at a high level:
        ///  1) This [`ChainMonitor`] calls [`Persist::update_persisted_channel`] which stores the
        ///     update to disk and begins updating any remote (e.g. watchtower/backup) copies,
        ///     returning [`ChannelMonitorUpdateStatus::InProgress`],
        ///  2) once all remote copies are updated, you call this function with the
        ///     `completed_update_id` that completed, and once all pending updates have completed the
        ///     channel will be re-enabled.
        //      Note that we re-enable only after `UpdateOrigin::OffChain` updates complete, we don't
        //      care about `UpdateOrigin::ChainSync` updates for the channel state being updated. We
        //      only care about `UpdateOrigin::ChainSync` for returning `MonitorEvent`s.
        ///
        /// Returns an [`APIError::APIMisuseError`] if `funding_txo` does not match any currently
        /// registered [`ChannelMonitor`]s.
        pub fn channel_monitor_updated(&self, funding_txo: OutPoint, completed_update_id: MonitorUpdateId) -> Result<(), APIError> {
                let monitors = self.monitors.read().unwrap();
                let monitor_data = if let Some(mon) = monitors.get(&funding_txo) { mon } else {
                        return Err(APIError::APIMisuseError { err: format!("No ChannelMonitor matching funding outpoint {:?} found", funding_txo) });
                };
                let mut pending_monitor_updates = monitor_data.pending_monitor_updates.lock().unwrap();
                pending_monitor_updates.retain(|update_id| *update_id != completed_update_id);

                match completed_update_id {
                        MonitorUpdateId { contents: UpdateOrigin::OffChain(_) } => {
                                // Note that we only check for `UpdateOrigin::OffChain` failures here - if
                                // we're being told that a `UpdateOrigin::OffChain` monitor update completed,
                                // we only care about ensuring we don't tell the `ChannelManager` to restore
                                // the channel to normal operation until all `UpdateOrigin::OffChain` updates
                                // complete.
                                // If there's some `UpdateOrigin::ChainSync` update still pending that's okay
                                // - we can still update our channel state, just as long as we don't return
                                // `MonitorEvent`s from the monitor back to the `ChannelManager` until they
                                // complete.
                                let monitor_is_pending_updates = monitor_data.has_pending_offchain_updates(&pending_monitor_updates);
                                if monitor_is_pending_updates || monitor_data.channel_perm_failed.load(Ordering::Acquire) {
                                        // If there are still monitor updates pending (or an old monitor update
                                        // finished after a later one perm-failed), we cannot yet construct an
                                        // Completed event.
                                        return Ok(());
                                }
                                self.pending_monitor_events.lock().unwrap().push((funding_txo, vec![MonitorEvent::Completed {
                                        funding_txo,
                                        monitor_update_id: monitor_data.monitor.get_latest_update_id(),
                                }], monitor_data.monitor.get_counterparty_node_id()));
                        },
                        MonitorUpdateId { contents: UpdateOrigin::ChainSync(_) } => {
                                if !monitor_data.has_pending_chainsync_updates(&pending_monitor_updates) {
                                        monitor_data.last_chain_persist_height.store(self.highest_chain_height.load(Ordering::Acquire), Ordering::Release);
                                        // The next time release_pending_monitor_events is called, any events for this
                                        // ChannelMonitor will be returned.
                                }
                        },
                }
                Ok(())
        }

        /// This wrapper avoids having to update some of our tests for now as they assume the direct
        /// chain::Watch API wherein we mark a monitor fully-updated by just calling
        /// channel_monitor_updated once with the highest ID.
        #[cfg(any(test, fuzzing))]
        pub fn force_channel_monitor_updated(&self, funding_txo: OutPoint, monitor_update_id: u64) {
                let monitors = self.monitors.read().unwrap();
                let counterparty_node_id = monitors.get(&funding_txo).and_then(|m| m.monitor.get_counterparty_node_id());
                self.pending_monitor_events.lock().unwrap().push((funding_txo, vec![MonitorEvent::Completed {
                        funding_txo,
                        monitor_update_id,
                }], counterparty_node_id));
        }

        #[cfg(any(test, fuzzing, feature = "_test_utils"))]
        pub fn get_and_clear_pending_events(&self) -> Vec<events::Event> {
                use util::events::EventsProvider;
                let events = core::cell::RefCell::new(Vec::new());
                let event_handler = |event: &events::Event| events.borrow_mut().push(event.clone());
                self.process_pending_events(&event_handler);
                events.into_inner()
        }
}

impl<ChannelSigner: Sign, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref>
chain::Listen for ChainMonitor<ChannelSigner, C, T, F, L, P>
where
        C::Target: chain::Filter,
        T::Target: BroadcasterInterface,
        F::Target: FeeEstimator,
        L::Target: Logger,
        P::Target: Persist<ChannelSigner>,
{
        fn filtered_block_connected(&self, header: &BlockHeader, txdata: &TransactionData, height: u32) {
                log_debug!(self.logger, "New best block {} at height {} provided via block_connected", header.block_hash(), height);
                self.process_chain_data(header, Some(height), &txdata, |monitor, txdata| {
                        monitor.block_connected(
                                header, txdata, height, &*self.broadcaster, &*self.fee_estimator, &*self.logger)
                });
        }

        fn block_disconnected(&self, header: &BlockHeader, height: u32) {
                let monitor_states = self.monitors.read().unwrap();
                log_debug!(self.logger, "Latest block {} at height {} removed via block_disconnected", header.block_hash(), height);
                for monitor_state in monitor_states.values() {
                        monitor_state.monitor.block_disconnected(
                                header, height, &*self.broadcaster, &*self.fee_estimator, &*self.logger);
                }
        }
}

impl<ChannelSigner: Sign, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref>
chain::Confirm for ChainMonitor<ChannelSigner, C, T, F, L, P>
where
        C::Target: chain::Filter,
        T::Target: BroadcasterInterface,
        F::Target: FeeEstimator,
        L::Target: Logger,
        P::Target: Persist<ChannelSigner>,
{
        fn transactions_confirmed(&self, header: &BlockHeader, txdata: &TransactionData, height: u32) {
                log_debug!(self.logger, "{} provided transactions confirmed at height {} in block {}", txdata.len(), height, header.block_hash());
                self.process_chain_data(header, None, txdata, |monitor, txdata| {
                        monitor.transactions_confirmed(
                                header, txdata, height, &*self.broadcaster, &*self.fee_estimator, &*self.logger)
                });
        }

        fn transaction_unconfirmed(&self, txid: &Txid) {
                log_debug!(self.logger, "Transaction {} reorganized out of chain", txid);
                let monitor_states = self.monitors.read().unwrap();
                for monitor_state in monitor_states.values() {
                        monitor_state.monitor.transaction_unconfirmed(txid, &*self.broadcaster, &*self.fee_estimator, &*self.logger);
                }
        }

        fn best_block_updated(&self, header: &BlockHeader, height: u32) {
                log_debug!(self.logger, "New best block {} at height {} provided via best_block_updated", header.block_hash(), height);
                self.process_chain_data(header, Some(height), &[], |monitor, txdata| {
                        // While in practice there shouldn't be any recursive calls when given empty txdata,
                        // it's still possible if a chain::Filter implementation returns a transaction.
                        debug_assert!(txdata.is_empty());
                        monitor.best_block_updated(
                                header, height, &*self.broadcaster, &*self.fee_estimator, &*self.logger)
                });
        }

        fn get_relevant_txids(&self) -> Vec<Txid> {
                let mut txids = Vec::new();
                let monitor_states = self.monitors.read().unwrap();
                for monitor_state in monitor_states.values() {
                        txids.append(&mut monitor_state.monitor.get_relevant_txids());
                }

                txids.sort_unstable();
                txids.dedup();
                txids
        }
}

impl<ChannelSigner: Sign, C: Deref , T: Deref , F: Deref , L: Deref , P: Deref >
chain::Watch<ChannelSigner> for ChainMonitor<ChannelSigner, C, T, F, L, P>
where C::Target: chain::Filter,
            T::Target: BroadcasterInterface,
            F::Target: FeeEstimator,
            L::Target: Logger,
            P::Target: Persist<ChannelSigner>,
{
        /// Adds the monitor that watches the channel referred to by the given outpoint.
        ///
        /// Calls back to [`chain::Filter`] with the funding transaction and outputs to watch.
        ///
        /// Note that we persist the given `ChannelMonitor` while holding the `ChainMonitor`
        /// monitors lock.
        fn watch_channel(&self, funding_outpoint: OutPoint, monitor: ChannelMonitor<ChannelSigner>) -> ChannelMonitorUpdateStatus {
                let mut monitors = self.monitors.write().unwrap();
                let entry = match monitors.entry(funding_outpoint) {
                        hash_map::Entry::Occupied(_) => {
                                log_error!(self.logger, "Failed to add new channel data: channel monitor for given outpoint is already present");
                                return ChannelMonitorUpdateStatus::PermanentFailure
                        },
                        hash_map::Entry::Vacant(e) => e,
                };
                log_trace!(self.logger, "Got new ChannelMonitor for channel {}", log_funding_info!(monitor));
                let update_id = MonitorUpdateId::from_new_monitor(&monitor);
                let mut pending_monitor_updates = Vec::new();
                let persist_res = self.persister.persist_new_channel(funding_outpoint, &monitor, update_id);
                match persist_res {
                        ChannelMonitorUpdateStatus::InProgress => {
                                log_info!(self.logger, "Persistence of new ChannelMonitor for channel {} in progress", log_funding_info!(monitor));
                                pending_monitor_updates.push(update_id);
                        },
                        ChannelMonitorUpdateStatus::PermanentFailure => {
                                log_error!(self.logger, "Persistence of new ChannelMonitor for channel {} failed", log_funding_info!(monitor));
                                return persist_res;
                        },
                        ChannelMonitorUpdateStatus::Completed => {
                                log_info!(self.logger, "Persistence of new ChannelMonitor for channel {} completed", log_funding_info!(monitor));
                        }
                }
                if let Some(ref chain_source) = self.chain_source {
                        monitor.load_outputs_to_watch(chain_source);
                }
                entry.insert(MonitorHolder {
                        monitor,
                        pending_monitor_updates: Mutex::new(pending_monitor_updates),
                        channel_perm_failed: AtomicBool::new(false),
                        last_chain_persist_height: AtomicUsize::new(self.highest_chain_height.load(Ordering::Acquire)),
                });
                persist_res
        }

        /// Note that we persist the given `ChannelMonitor` update while holding the
        /// `ChainMonitor` monitors lock.
        fn update_channel(&self, funding_txo: OutPoint, update: ChannelMonitorUpdate) -> ChannelMonitorUpdateStatus {
                // Update the monitor that watches the channel referred to by the given outpoint.
                let monitors = self.monitors.read().unwrap();
                match monitors.get(&funding_txo) {
                        None => {
                                log_error!(self.logger, "Failed to update channel monitor: no such monitor registered");

                                // We should never ever trigger this from within ChannelManager. Technically a
                                // user could use this object with some proxying in between which makes this
                                // possible, but in tests and fuzzing, this should be a panic.
                                #[cfg(any(test, fuzzing))]
                                panic!("ChannelManager generated a channel update for a channel that was not yet registered!");
                                #[cfg(not(any(test, fuzzing)))]
                                ChannelMonitorUpdateStatus::PermanentFailure
                        },
                        Some(monitor_state) => {
                                let monitor = &monitor_state.monitor;
                                log_trace!(self.logger, "Updating ChannelMonitor for channel {}", log_funding_info!(monitor));
                                let update_res = monitor.update_monitor(&update, &self.broadcaster, &*self.fee_estimator, &self.logger);
                                if update_res.is_err() {
                                        log_error!(self.logger, "Failed to update ChannelMonitor for channel {}.", log_funding_info!(monitor));
                                }
                                // Even if updating the monitor returns an error, the monitor's state will
                                // still be changed. So, persist the updated monitor despite the error.
                                let update_id = MonitorUpdateId::from_monitor_update(&update);
                                let mut pending_monitor_updates = monitor_state.pending_monitor_updates.lock().unwrap();
                                let persist_res = self.persister.update_persisted_channel(funding_txo, &Some(update), monitor, update_id);
                                match persist_res {
                                        ChannelMonitorUpdateStatus::InProgress => {
                                                pending_monitor_updates.push(update_id);
                                                log_debug!(self.logger, "Persistence of ChannelMonitorUpdate for channel {} in progress", log_funding_info!(monitor));
                                        },
                                        ChannelMonitorUpdateStatus::PermanentFailure => {
                                                monitor_state.channel_perm_failed.store(true, Ordering::Release);
                                                log_error!(self.logger, "Persistence of ChannelMonitorUpdate for channel {} failed", log_funding_info!(monitor));
                                        },
                                        ChannelMonitorUpdateStatus::Completed => {
                                                log_debug!(self.logger, "Persistence of ChannelMonitorUpdate for channel {} completed", log_funding_info!(monitor));
                                        },
                                }
                                if update_res.is_err() {
                                        ChannelMonitorUpdateStatus::PermanentFailure
                                } else if monitor_state.channel_perm_failed.load(Ordering::Acquire) {
                                        ChannelMonitorUpdateStatus::PermanentFailure
                                } else {
                                        persist_res
                                }
                        }
                }
        }

        fn release_pending_monitor_events(&self) -> Vec<(OutPoint, Vec<MonitorEvent>, Option<PublicKey>)> {
                let mut pending_monitor_events = self.pending_monitor_events.lock().unwrap().split_off(0);
                for monitor_state in self.monitors.read().unwrap().values() {
                        let is_pending_monitor_update = monitor_state.has_pending_chainsync_updates(&monitor_state.pending_monitor_updates.lock().unwrap());
                        if is_pending_monitor_update &&
                                        monitor_state.last_chain_persist_height.load(Ordering::Acquire) + LATENCY_GRACE_PERIOD_BLOCKS as usize
                                                > self.highest_chain_height.load(Ordering::Acquire)
                        {
                                log_info!(self.logger, "A Channel Monitor sync is still in progress, refusing to provide monitor events!");
                        } else {
                                if monitor_state.channel_perm_failed.load(Ordering::Acquire) {
                                        // If a `UpdateOrigin::ChainSync` persistence failed with `PermanantFailure`,
                                        // we don't really know if the latest `ChannelMonitor` state is on disk or not.
                                        // We're supposed to hold monitor updates until the latest state is on disk to
                                        // avoid duplicate events, but the user told us persistence is screw-y and may
                                        // not complete. We can't hold events forever because we may learn some payment
                                        // preimage, so instead we just log and hope the user complied with the
                                        // `PermanentFailure` requirements of having at least the local-disk copy
                                        // updated.
                                        log_info!(self.logger, "A Channel Monitor sync returned PermanentFailure. Returning monitor events but duplicate events may appear after reload!");
                                }
                                if is_pending_monitor_update {
                                        log_error!(self.logger, "A ChannelMonitor sync took longer than {} blocks to complete.", LATENCY_GRACE_PERIOD_BLOCKS);
                                        log_error!(self.logger, "   To avoid funds-loss, we are allowing monitor updates to be released.");
                                        log_error!(self.logger, "   This may cause duplicate payment events to be generated.");
                                }
                                let monitor_events = monitor_state.monitor.get_and_clear_pending_monitor_events();
                                if monitor_events.len() > 0 {
                                        let monitor_outpoint = monitor_state.monitor.get_funding_txo().0;
                                        let counterparty_node_id = monitor_state.monitor.get_counterparty_node_id();
                                        pending_monitor_events.push((monitor_outpoint, monitor_events, counterparty_node_id));
                                }
                        }
                }
                pending_monitor_events
        }
}

impl<ChannelSigner: Sign, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref> events::EventsProvider for ChainMonitor<ChannelSigner, C, T, F, L, P>
        where C::Target: chain::Filter,
              T::Target: BroadcasterInterface,
              F::Target: FeeEstimator,
              L::Target: Logger,
              P::Target: Persist<ChannelSigner>,
{
        /// Processes [`SpendableOutputs`] events produced from each [`ChannelMonitor`] upon maturity.
        ///
        /// An [`EventHandler`] may safely call back to the provider, though this shouldn't be needed in
        /// order to handle these events.
        ///
        /// [`SpendableOutputs`]: events::Event::SpendableOutputs
        fn process_pending_events<H: Deref>(&self, handler: H) where H::Target: EventHandler {
                let mut pending_events = Vec::new();
                for monitor_state in self.monitors.read().unwrap().values() {
                        pending_events.append(&mut monitor_state.monitor.get_and_clear_pending_events());
                }
                for event in pending_events.drain(..) {
                        handler.handle_event(&event);
                }
        }
}

#[cfg(test)]
mod tests {
        use bitcoin::{BlockHeader, TxMerkleNode};
        use bitcoin::hashes::Hash;
        use ::{check_added_monitors, check_closed_broadcast, check_closed_event};
        use ::{expect_payment_sent, expect_payment_claimed, expect_payment_sent_without_paths, expect_payment_path_successful, get_event_msg};
        use ::{get_htlc_update_msgs, get_local_commitment_txn, get_revoke_commit_msgs, get_route_and_payment_hash, unwrap_send_err};
        use chain::{ChannelMonitorUpdateStatus, Confirm, Watch};
        use chain::channelmonitor::LATENCY_GRACE_PERIOD_BLOCKS;
        use ln::channelmanager::{self, PaymentSendFailure};
        use ln::functional_test_utils::*;
        use ln::msgs::ChannelMessageHandler;
        use util::errors::APIError;
        use util::events::{ClosureReason, MessageSendEvent, MessageSendEventsProvider};

        #[test]
        fn test_async_ooo_offchain_updates() {
                // Test that if we have multiple offchain updates being persisted and they complete
                // out-of-order, the ChainMonitor waits until all have completed before informing the
                // ChannelManager.
                let chanmon_cfgs = create_chanmon_cfgs(2);
                let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
                let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
                let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
                create_announced_chan_between_nodes(&nodes, 0, 1, channelmanager::provided_init_features(), channelmanager::provided_init_features());

                // Route two payments to be claimed at the same time.
                let (payment_preimage_1, payment_hash_1, _) = route_payment(&nodes[0], &[&nodes[1]], 1_000_000);
                let (payment_preimage_2, payment_hash_2, _) = route_payment(&nodes[0], &[&nodes[1]], 1_000_000);

                chanmon_cfgs[1].persister.offchain_monitor_updates.lock().unwrap().clear();
                chanmon_cfgs[1].persister.set_update_ret(ChannelMonitorUpdateStatus::InProgress);

                nodes[1].node.claim_funds(payment_preimage_1);
                check_added_monitors!(nodes[1], 1);
                expect_payment_claimed!(nodes[1], payment_hash_1, 1_000_000);
                nodes[1].node.claim_funds(payment_preimage_2);
                check_added_monitors!(nodes[1], 1);
                expect_payment_claimed!(nodes[1], payment_hash_2, 1_000_000);

                chanmon_cfgs[1].persister.set_update_ret(ChannelMonitorUpdateStatus::Completed);

                let persistences = chanmon_cfgs[1].persister.offchain_monitor_updates.lock().unwrap().clone();
                assert_eq!(persistences.len(), 1);
                let (funding_txo, updates) = persistences.iter().next().unwrap();
                assert_eq!(updates.len(), 2);

                // Note that updates is a HashMap so the ordering here is actually random. This shouldn't
                // fail either way but if it fails intermittently it's depending on the ordering of updates.
                let mut update_iter = updates.iter();
                nodes[1].chain_monitor.chain_monitor.channel_monitor_updated(*funding_txo, update_iter.next().unwrap().clone()).unwrap();
                assert!(nodes[1].chain_monitor.release_pending_monitor_events().is_empty());
                assert!(nodes[1].node.get_and_clear_pending_msg_events().is_empty());
                nodes[1].chain_monitor.chain_monitor.channel_monitor_updated(*funding_txo, update_iter.next().unwrap().clone()).unwrap();

                // Now manually walk the commitment signed dance - because we claimed two payments
                // back-to-back it doesn't fit into the neat walk commitment_signed_dance does.

                let updates = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
                nodes[0].node.handle_update_fulfill_htlc(&nodes[1].node.get_our_node_id(), &updates.update_fulfill_htlcs[0]);
                expect_payment_sent_without_paths!(nodes[0], payment_preimage_1);
                nodes[0].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &updates.commitment_signed);
                check_added_monitors!(nodes[0], 1);
                let (as_first_raa, as_first_update) = get_revoke_commit_msgs!(nodes[0], nodes[1].node.get_our_node_id());

                nodes[1].node.handle_revoke_and_ack(&nodes[0].node.get_our_node_id(), &as_first_raa);
                check_added_monitors!(nodes[1], 1);
                let bs_second_updates = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
                nodes[1].node.handle_commitment_signed(&nodes[0].node.get_our_node_id(), &as_first_update);
                check_added_monitors!(nodes[1], 1);
                let bs_first_raa = get_event_msg!(nodes[1], MessageSendEvent::SendRevokeAndACK, nodes[0].node.get_our_node_id());

                nodes[0].node.handle_update_fulfill_htlc(&nodes[1].node.get_our_node_id(), &bs_second_updates.update_fulfill_htlcs[0]);
                expect_payment_sent_without_paths!(nodes[0], payment_preimage_2);
                nodes[0].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &bs_second_updates.commitment_signed);
                check_added_monitors!(nodes[0], 1);
                nodes[0].node.handle_revoke_and_ack(&nodes[1].node.get_our_node_id(), &bs_first_raa);
                expect_payment_path_successful!(nodes[0]);
                check_added_monitors!(nodes[0], 1);
                let (as_second_raa, as_second_update) = get_revoke_commit_msgs!(nodes[0], nodes[1].node.get_our_node_id());

                nodes[1].node.handle_revoke_and_ack(&nodes[0].node.get_our_node_id(), &as_second_raa);
                check_added_monitors!(nodes[1], 1);
                nodes[1].node.handle_commitment_signed(&nodes[0].node.get_our_node_id(), &as_second_update);
                check_added_monitors!(nodes[1], 1);
                let bs_second_raa = get_event_msg!(nodes[1], MessageSendEvent::SendRevokeAndACK, nodes[0].node.get_our_node_id());

                nodes[0].node.handle_revoke_and_ack(&nodes[1].node.get_our_node_id(), &bs_second_raa);
                expect_payment_path_successful!(nodes[0]);
                check_added_monitors!(nodes[0], 1);
        }

        fn do_chainsync_pauses_events(block_timeout: bool) {
                // When a chainsync monitor update occurs, any MonitorUpdates should be held before being
                // passed upstream to a `ChannelManager` via `Watch::release_pending_monitor_events`. This
                // tests that behavior, as well as some ways it might go wrong.
                let chanmon_cfgs = create_chanmon_cfgs(2);
                let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
                let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
                let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
                let channel = create_announced_chan_between_nodes(
                        &nodes, 0, 1, channelmanager::provided_init_features(), channelmanager::provided_init_features());

                // Get a route for later and rebalance the channel somewhat
                send_payment(&nodes[0], &[&nodes[1]], 10_000_000);
                let (route, second_payment_hash, _, second_payment_secret) = get_route_and_payment_hash!(nodes[0], nodes[1], 100_000);

                // First route a payment that we will claim on chain and give the recipient the preimage.
                let (payment_preimage, payment_hash, _) = route_payment(&nodes[0], &[&nodes[1]], 1_000_000);
                nodes[1].node.claim_funds(payment_preimage);
                expect_payment_claimed!(nodes[1], payment_hash, 1_000_000);
                nodes[1].node.get_and_clear_pending_msg_events();
                check_added_monitors!(nodes[1], 1);
                let remote_txn = get_local_commitment_txn!(nodes[1], channel.2);
                assert_eq!(remote_txn.len(), 2);

                // Temp-fail the block connection which will hold the channel-closed event
                chanmon_cfgs[0].persister.chain_sync_monitor_persistences.lock().unwrap().clear();
                chanmon_cfgs[0].persister.set_update_ret(ChannelMonitorUpdateStatus::InProgress);

                // Connect B's commitment transaction, but only to the ChainMonitor/ChannelMonitor. The
                // channel is now closed, but the ChannelManager doesn't know that yet.
                let new_header = BlockHeader {
                        version: 2, time: 0, bits: 0, nonce: 0,
                        prev_blockhash: nodes[0].best_block_info().0,
                        merkle_root: TxMerkleNode::all_zeros() };
                nodes[0].chain_monitor.chain_monitor.transactions_confirmed(&new_header,
                        &[(0, &remote_txn[0]), (1, &remote_txn[1])], nodes[0].best_block_info().1 + 1);
                assert!(nodes[0].chain_monitor.release_pending_monitor_events().is_empty());
                nodes[0].chain_monitor.chain_monitor.best_block_updated(&new_header, nodes[0].best_block_info().1 + 1);
                assert!(nodes[0].chain_monitor.release_pending_monitor_events().is_empty());

                // If the ChannelManager tries to update the channel, however, the ChainMonitor will pass
                // the update through to the ChannelMonitor which will refuse it (as the channel is closed).
                chanmon_cfgs[0].persister.set_update_ret(ChannelMonitorUpdateStatus::Completed);
                unwrap_send_err!(nodes[0].node.send_payment(&route, second_payment_hash, &Some(second_payment_secret)),
                        true, APIError::ChannelUnavailable { ref err },
                        assert!(err.contains("ChannelMonitor storage failure")));
                check_added_monitors!(nodes[0], 2); // After the failure we generate a close-channel monitor update
                check_closed_broadcast!(nodes[0], true);
                check_closed_event!(nodes[0], 1, ClosureReason::ProcessingError { err: "ChannelMonitor storage failure".to_string() });

                // However, as the ChainMonitor is still waiting for the original persistence to complete,
                // it won't yet release the MonitorEvents.
                assert!(nodes[0].chain_monitor.release_pending_monitor_events().is_empty());

                if block_timeout {
                        // After three blocks, pending MontiorEvents should be released either way.
                        let latest_header = BlockHeader {
                                version: 2, time: 0, bits: 0, nonce: 0,
                                prev_blockhash: nodes[0].best_block_info().0,
                                merkle_root: TxMerkleNode::all_zeros() };
                        nodes[0].chain_monitor.chain_monitor.best_block_updated(&latest_header, nodes[0].best_block_info().1 + LATENCY_GRACE_PERIOD_BLOCKS);
                } else {
                        let persistences = chanmon_cfgs[0].persister.chain_sync_monitor_persistences.lock().unwrap().clone();
                        for (funding_outpoint, update_ids) in persistences {
                                for update_id in update_ids {
                                        nodes[0].chain_monitor.chain_monitor.channel_monitor_updated(funding_outpoint, update_id).unwrap();
                                }
                        }
                }

                expect_payment_sent!(nodes[0], payment_preimage);
        }

        #[test]
        fn chainsync_pauses_events() {
                do_chainsync_pauses_events(false);
                do_chainsync_pauses_events(true);
        }

        #[test]
        fn update_during_chainsync_fails_channel() {
                let chanmon_cfgs = create_chanmon_cfgs(2);
                let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
                let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
                let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
                create_announced_chan_between_nodes(&nodes, 0, 1, channelmanager::provided_init_features(), channelmanager::provided_init_features());

                chanmon_cfgs[0].persister.chain_sync_monitor_persistences.lock().unwrap().clear();
                chanmon_cfgs[0].persister.set_update_ret(ChannelMonitorUpdateStatus::PermanentFailure);

                connect_blocks(&nodes[0], 1);
                // Before processing events, the ChannelManager will still think the Channel is open and
                // there won't be any ChannelMonitorUpdates
                assert_eq!(nodes[0].node.list_channels().len(), 1);
                check_added_monitors!(nodes[0], 0);
                // ... however once we get events once, the channel will close, creating a channel-closed
                // ChannelMonitorUpdate.
                check_closed_broadcast!(nodes[0], true);
                check_closed_event!(nodes[0], 1, ClosureReason::ProcessingError { err: "Failed to persist ChannelMonitor update during chain sync".to_string() });
                check_added_monitors!(nodes[0], 1);
        }
}