[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::{Block, BlockHeader};
use bitcoin::hash_types::Txid;

use chain;
use chain::{ChannelMonitorUpdateErr, Filter, WatchedOutput};
use chain::chaininterface::{BroadcasterInterface, FeeEstimator};
use chain::channelmonitor::{ChannelMonitor, ChannelMonitorUpdate, Balance, MonitorEvent, TransactionOutputs};
use chain::transaction::{OutPoint, TransactionData};
use chain::keysinterface::Sign;
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;

#[derive(Clone, Copy, Hash, PartialEq, Eq)]
enum UpdateOrigin {
        OffChain(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 `Ok(())`, 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
///    `Err(ChannelMonitorUpdateErr::TemporaryFailure)` 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
///    `Err(ChannelMonitorUpdateErr::PermanentFailure)`, in which case the channel will likely be
///    closed without broadcasting the latest state. See
///    [`ChannelMonitorUpdateErr::PermanentFailure`] for more details.
pub trait Persist<ChannelSigner: Sign> {
        /// Persist a new channel's data. 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 [`ChannelMonitorUpdateErr::TemporaryFailure`].
        ///
        /// See [`Writeable::write`] on [`ChannelMonitor`] for writing out a `ChannelMonitor`
        /// and [`ChannelMonitorUpdateErr`] for requirements when returning errors.
        ///
        /// [`Writeable::write`]: crate::util::ser::Writeable::write
        fn persist_new_channel(&self, channel_id: OutPoint, data: &ChannelMonitor<ChannelSigner>, update_id: MonitorUpdateId) -> Result<(), ChannelMonitorUpdateErr>;

        /// 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.
        ///
        /// 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 [`ChannelMonitorUpdateErr::TemporaryFailure`].
        ///
        /// See [`Writeable::write`] on [`ChannelMonitor`] for writing out a `ChannelMonitor`,
        /// [`Writeable::write`] on [`ChannelMonitorUpdate`] for writing out an update, and
        /// [`ChannelMonitorUpdateErr`] for requirements when returning errors.
        ///
        /// [`Writeable::write`]: crate::util::ser::Writeable::write
        fn update_persisted_channel(&self, channel_id: OutPoint, update: &ChannelMonitorUpdate, data: &ChannelMonitor<ChannelSigner>, update_id: MonitorUpdateId) -> Result<(), ChannelMonitorUpdateErr>;
}

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 TemporaryFailure, and then calls
        /// channel_monitor_updated immediately, racing our insertion of the pending update into the
        /// contained Vec.
        pending_monitor_updates: Mutex<Vec<MonitorUpdateId>>,
}

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 })
        }
}

/// 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>>>,
        chain_source: Option<C>,
        broadcaster: T,
        logger: L,
        fee_estimator: F,
        persister: P,
        pending_monitor_events: Mutex<Vec<MonitorEvent>>,
}

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`.
        fn process_chain_data<FN>(&self, header: &BlockHeader, txdata: &TransactionData, process: FN)
        where
                FN: Fn(&ChannelMonitor<ChannelSigner>, &TransactionData) -> Vec<TransactionOutputs>
        {
                let mut dependent_txdata = Vec::new();
                let monitor_states = self.monitors.read().unwrap();
                for monitor_state in monitor_states.values() {
                        let mut txn_outputs = process(&monitor_state.monitor, txdata);

                        // 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 and recurse
                                                // if it indicates that there are dependent transactions within the block
                                                // that had not been previously included in txdata.
                                                let output = WatchedOutput {
                                                        block_hash: Some(block_hash),
                                                        outpoint: OutPoint { txid, index: idx as u16 },
                                                        script_pubkey: output.script_pubkey,
                                                };
                                                if let Some(tx) = chain_source.register_output(output) {
                                                        dependent_txdata.push(tx);
                                                }
                                        }
                                }
                        }
                }

                // Recursively call for any dependent transactions that were identified by the chain source.
                if !dependent_txdata.is_empty() {
                        dependent_txdata.sort_unstable_by_key(|(index, _tx)| *index);
                        dependent_txdata.dedup_by_key(|(index, _tx)| *index);
                        let txdata: Vec<_> = dependent_txdata.iter().map(|(index, tx)| (*index, tx)).collect();
                        self.process_chain_data(header, &txdata, process);
                }
        }

        /// 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()),
                        chain_source,
                        broadcaster,
                        logger,
                        fee_estimator: feeest,
                        persister,
                        pending_monitor_events: Mutex::new(Vec::new()),
                }
        }

        /// 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
        /// [`ChannelMonitorUpdateErr::TemporaryFailure`] 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 [`ChannelMonitorUpdateErr::TemporaryFailure`],
        ///  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 { .. } => {
                                // 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 {
                                        // If there are still monitor updates pending, we cannot yet construct an
                                        // UpdateCompleted event.
                                        return Ok(());
                                }
                                self.pending_monitor_events.lock().unwrap().push(MonitorEvent::UpdateCompleted {
                                        funding_txo,
                                        monitor_update_id: monitor_data.monitor.get_latest_update_id(),
                                });
                        }
                }
                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, feature = "fuzztarget"))]
        pub fn force_channel_monitor_updated(&self, funding_txo: OutPoint, monitor_update_id: u64) {
                self.pending_monitor_events.lock().unwrap().push(MonitorEvent::UpdateCompleted {
                        funding_txo,
                        monitor_update_id,
                });
        }

        #[cfg(any(test, feature = "fuzztarget", 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 block_connected(&self, block: &Block, height: u32) {
                let header = &block.header;
                let txdata: Vec<_> = block.txdata.iter().enumerate().collect();
                log_debug!(self.logger, "New best block {} at height {} provided via block_connected", header.block_hash(), height);
                self.process_chain_data(header, &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, 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, &[], |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>) -> Result<(), ChannelMonitorUpdateErr> {
                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 Err(ChannelMonitorUpdateErr::PermanentFailure)},
                        hash_map::Entry::Vacant(e) => e,
                };
                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);
                if persist_res.is_err() {
                        log_error!(self.logger, "Failed to persist new channel data: {:?}", persist_res);
                }
                if persist_res == Err(ChannelMonitorUpdateErr::PermanentFailure) {
                        return persist_res;
                } else if persist_res.is_err() {
                        pending_monitor_updates.push(update_id);
                }
                {
                        let funding_txo = monitor.get_funding_txo();
                        log_trace!(self.logger, "Got new Channel Monitor for channel {}", log_bytes!(funding_txo.0.to_channel_id()[..]));

                        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) });
                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) -> Result<(), ChannelMonitorUpdateErr> {
                // 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, feature = "fuzztarget"))]
                                panic!("ChannelManager generated a channel update for a channel that was not yet registered!");
                                #[cfg(not(any(test, feature = "fuzztarget")))]
                                Err(ChannelMonitorUpdateErr::PermanentFailure)
                        },
                        Some(monitor_state) => {
                                let monitor = &monitor_state.monitor;
                                log_trace!(self.logger, "Updating Channel Monitor for channel {}", log_funding_info!(monitor));
                                let update_res = monitor.update_monitor(&update, &self.broadcaster, &self.fee_estimator, &self.logger);
                                if let Err(e) = &update_res {
                                        log_error!(self.logger, "Failed to update channel monitor: {:?}", e);
                                }
                                // 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, &update, monitor, update_id);
                                if let Err(e) = persist_res {
                                        if e == ChannelMonitorUpdateErr::TemporaryFailure {
                                                pending_monitor_updates.push(update_id);
                                        }
                                        log_error!(self.logger, "Failed to persist channel monitor update: {:?}", e);
                                }
                                if update_res.is_err() {
                                        Err(ChannelMonitorUpdateErr::PermanentFailure)
                                } else {
                                        persist_res
                                }
                        }
                }
        }

        fn release_pending_monitor_events(&self) -> Vec<MonitorEvent> {
                let mut pending_monitor_events = self.pending_monitor_events.lock().unwrap().split_off(0);
                for monitor_state in self.monitors.read().unwrap().values() {
                        pending_monitor_events.append(&mut monitor_state.monitor.get_and_clear_pending_monitor_events());
                }
                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 ::{check_added_monitors, get_local_commitment_txn};
        use ln::features::InitFeatures;
        use ln::functional_test_utils::*;
        use util::events::MessageSendEventsProvider;
        use util::test_utils::{OnRegisterOutput, TxOutReference};

        /// Tests that in-block dependent transactions are processed by `block_connected` when not
        /// included in `txdata` but returned by [`chain::Filter::register_output`]. For instance,
        /// a (non-anchor) commitment transaction's HTLC output may be spent in the same block as the
        /// commitment transaction itself. An Electrum client may filter the commitment transaction but
        /// needs to return the HTLC transaction so it can be processed.
        #[test]
        fn connect_block_checks_dependent_transactions() {
                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, InitFeatures::known(), InitFeatures::known());

                // Send a payment, saving nodes[0]'s revoked commitment and HTLC-Timeout transactions.
                let (commitment_tx, htlc_tx) = {
                        let payment_preimage = route_payment(&nodes[0], &vec!(&nodes[1])[..], 5_000_000).0;
                        let mut txn = get_local_commitment_txn!(nodes[0], channel.2);
                        claim_payment(&nodes[0], &vec!(&nodes[1])[..], payment_preimage);

                        assert_eq!(txn.len(), 2);
                        (txn.remove(0), txn.remove(0))
                };

                // Set expectations on nodes[1]'s chain source to return dependent transactions.
                let htlc_output = TxOutReference(commitment_tx.clone(), 0);
                let to_local_output = TxOutReference(commitment_tx.clone(), 1);
                let htlc_timeout_output = TxOutReference(htlc_tx.clone(), 0);
                nodes[1].chain_source
                        .expect(OnRegisterOutput { with: htlc_output, returns: Some((1, htlc_tx)) })
                        .expect(OnRegisterOutput { with: to_local_output, returns: None })
                        .expect(OnRegisterOutput { with: htlc_timeout_output, returns: None });

                // Notify nodes[1] that nodes[0]'s revoked commitment transaction was mined. The chain
                // source should return the dependent HTLC transaction when the HTLC output is registered.
                mine_transaction(&nodes[1], &commitment_tx);

                // Clean up so uninteresting assertions don't fail.
                check_added_monitors!(nodes[1], 1);
                nodes[1].node.get_and_clear_pending_msg_events();
                nodes[1].node.get_and_clear_pending_events();
        }
}