Add Event::ChannelClosed generation at channel shutdown

[rust-lightning] / lightning / src / util / events.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.

//! Events are returned from various bits in the library which indicate some action must be taken
//! by the client.
//!
//! Because we don't have a built-in runtime, it's up to the client to call events at a time in the
//! future, as well as generate and broadcast funding transactions handle payment preimages and a
//! few other things.

use chain::keysinterface::SpendableOutputDescriptor;
use ln::msgs;
use ln::{PaymentPreimage, PaymentHash, PaymentSecret};
use routing::network_graph::NetworkUpdate;
use util::ser::{Writeable, Writer, MaybeReadable, Readable, VecReadWrapper, VecWriteWrapper};

use bitcoin::blockdata::script::Script;

use bitcoin::secp256k1::key::PublicKey;

use io;
use prelude::*;
use core::cmp;
use core::time::Duration;
use core::ops::Deref;

/// Some information provided on receipt of payment depends on whether the payment received is a
/// spontaneous payment or a "conventional" lightning payment that's paying an invoice.
#[derive(Clone, Debug)]
pub enum PaymentPurpose {
        /// Information for receiving a payment that we generated an invoice for.
        InvoicePayment {
                /// The preimage to the payment_hash, if the payment hash (and secret) were fetched via
                /// [`ChannelManager::create_inbound_payment`]. If provided, this can be handed directly to
                /// [`ChannelManager::claim_funds`].
                ///
                /// [`ChannelManager::create_inbound_payment`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment
                /// [`ChannelManager::claim_funds`]: crate::ln::channelmanager::ChannelManager::claim_funds
                payment_preimage: Option<PaymentPreimage>,
                /// The "payment secret". This authenticates the sender to the recipient, preventing a
                /// number of deanonymization attacks during the routing process.
                /// It is provided here for your reference, however its accuracy is enforced directly by
                /// [`ChannelManager`] using the values you previously provided to
                /// [`ChannelManager::create_inbound_payment`] or
                /// [`ChannelManager::create_inbound_payment_for_hash`].
                ///
                /// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
                /// [`ChannelManager::create_inbound_payment`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment
                /// [`ChannelManager::create_inbound_payment_for_hash`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment_for_hash
                payment_secret: PaymentSecret,
                /// This is the `user_payment_id` which was provided to
                /// [`ChannelManager::create_inbound_payment_for_hash`] or
                /// [`ChannelManager::create_inbound_payment`]. It has no meaning inside of LDK and is
                /// simply copied here. It may be used to correlate PaymentReceived events with invoice
                /// metadata stored elsewhere.
                ///
                /// [`ChannelManager::create_inbound_payment`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment
                /// [`ChannelManager::create_inbound_payment_for_hash`]: crate::ln::channelmanager::ChannelManager::create_inbound_payment_for_hash
                user_payment_id: u64,
        },
        /// Because this is a spontaneous payment, the payer generated their own preimage rather than us
        /// (the payee) providing a preimage.
        SpontaneousPayment(PaymentPreimage),
}

/// An Event which you should probably take some action in response to.
///
/// Note that while Writeable and Readable are implemented for Event, you probably shouldn't use
/// them directly as they don't round-trip exactly (for example FundingGenerationReady is never
/// written as it makes no sense to respond to it after reconnecting to peers).
#[derive(Clone, Debug)]
pub enum Event {
        /// Used to indicate that the client should generate a funding transaction with the given
        /// parameters and then call ChannelManager::funding_transaction_generated.
        /// Generated in ChannelManager message handling.
        /// Note that *all inputs* in the funding transaction must spend SegWit outputs or your
        /// counterparty can steal your funds!
        FundingGenerationReady {
                /// The random channel_id we picked which you'll need to pass into
                /// ChannelManager::funding_transaction_generated.
                temporary_channel_id: [u8; 32],
                /// The value, in satoshis, that the output should have.
                channel_value_satoshis: u64,
                /// The script which should be used in the transaction output.
                output_script: Script,
                /// The value passed in to ChannelManager::create_channel
                user_channel_id: u64,
        },
        /// Indicates we've received money! Just gotta dig out that payment preimage and feed it to
        /// ChannelManager::claim_funds to get it....
        /// Note that if the preimage is not known or the amount paid is incorrect, you should call
        /// ChannelManager::fail_htlc_backwards to free up resources for this HTLC and avoid
        /// network congestion.
        /// The amount paid should be considered 'incorrect' when it is less than or more than twice
        /// the amount expected.
        /// If you fail to call either ChannelManager::claim_funds or
        /// ChannelManager::fail_htlc_backwards within the HTLC's timeout, the HTLC will be
        /// automatically failed.
        PaymentReceived {
                /// The hash for which the preimage should be handed to the ChannelManager.
                payment_hash: PaymentHash,
                /// The value, in thousandths of a satoshi, that this payment is for. Note that you must
                /// compare this to the expected value before accepting the payment (as otherwise you are
                /// providing proof-of-payment for less than the value you expected!).
                amt: u64,
                /// Information for claiming this received payment, based on whether the purpose of the
                /// payment is to pay an invoice or to send a spontaneous payment.
                purpose: PaymentPurpose,
        },
        /// Indicates an outbound payment we made succeeded (i.e. it made it all the way to its target
        /// and we got back the payment preimage for it).
        ///
        /// Note for MPP payments: in rare cases, this event may be preceded by a `PaymentFailed` event.
        /// In this situation, you SHOULD treat this payment as having succeeded.
        PaymentSent {
                /// The preimage to the hash given to ChannelManager::send_payment.
                /// Note that this serves as a payment receipt, if you wish to have such a thing, you must
                /// store it somehow!
                payment_preimage: PaymentPreimage,
        },
        /// Indicates an outbound payment we made failed. Probably some intermediary node dropped
        /// something. You may wish to retry with a different route.
        PaymentFailed {
                /// The hash which was given to ChannelManager::send_payment.
                payment_hash: PaymentHash,
                /// Indicates the payment was rejected for some reason by the recipient. This implies that
                /// the payment has failed, not just the route in question. If this is not set, you may
                /// retry the payment via a different route.
                rejected_by_dest: bool,
                /// Any failure information conveyed via the Onion return packet by a node along the failed
                /// payment route.
                ///
                /// Should be applied to the [`NetworkGraph`] so that routing decisions can take into
                /// account the update. [`NetGraphMsgHandler`] is capable of doing this.
                ///
                /// [`NetworkGraph`]: crate::routing::network_graph::NetworkGraph
                /// [`NetGraphMsgHandler`]: crate::routing::network_graph::NetGraphMsgHandler
                network_update: Option<NetworkUpdate>,
                /// For both single-path and multi-path payments, this is set if all paths of the payment have
                /// failed. This will be set to false if (1) this is an MPP payment and (2) other parts of the
                /// larger MPP payment were still in flight when this event was generated.
                all_paths_failed: bool,
#[cfg(test)]
                error_code: Option<u16>,
#[cfg(test)]
                error_data: Option<Vec<u8>>,
        },
        /// Used to indicate that ChannelManager::process_pending_htlc_forwards should be called at a
        /// time in the future.
        PendingHTLCsForwardable {
                /// The minimum amount of time that should be waited prior to calling
                /// process_pending_htlc_forwards. To increase the effort required to correlate payments,
                /// you should wait a random amount of time in roughly the range (now + time_forwardable,
                /// now + 5*time_forwardable).
                time_forwardable: Duration,
        },
        /// Used to indicate that an output which you should know how to spend was confirmed on chain
        /// and is now spendable.
        /// Such an output will *not* ever be spent by rust-lightning, and are not at risk of your
        /// counterparty spending them due to some kind of timeout. Thus, you need to store them
        /// somewhere and spend them when you create on-chain transactions.
        SpendableOutputs {
                /// The outputs which you should store as spendable by you.
                outputs: Vec<SpendableOutputDescriptor>,
        },
        /// This event is generated when a payment has been successfully forwarded through us and a
        /// forwarding fee earned.
        PaymentForwarded {
                /// The fee, in milli-satoshis, which was earned as a result of the payment.
                ///
                /// Note that if we force-closed the channel over which we forwarded an HTLC while the HTLC
                /// was pending, the amount the next hop claimed will have been rounded down to the nearest
                /// whole satoshi. Thus, the fee calculated here may be higher than expected as we still
                /// claimed the full value in millisatoshis from the source. In this case,
                /// `claim_from_onchain_tx` will be set.
                ///
                /// If the channel which sent us the payment has been force-closed, we will claim the funds
                /// via an on-chain transaction. In that case we do not yet know the on-chain transaction
                /// fees which we will spend and will instead set this to `None`. It is possible duplicate
                /// `PaymentForwarded` events are generated for the same payment iff `fee_earned_msat` is
                /// `None`.
                fee_earned_msat: Option<u64>,
                /// If this is `true`, the forwarded HTLC was claimed by our counterparty via an on-chain
                /// transaction.
                claim_from_onchain_tx: bool,
        },
        /// Used to indicate that a channel was closed at the given timestamp.
        ChannelClosed  {
                /// The channel_id which has been barren from further off-chain updates but
                /// funding output might still be not resolved yet.
                channel_id: [u8; 32],
                /// A human-readable error message
                err: String
        }
}

impl Writeable for Event {
        fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
                match self {
                        &Event::FundingGenerationReady { .. } => {
                                0u8.write(writer)?;
                                // We never write out FundingGenerationReady events as, upon disconnection, peers
                                // drop any channels which have not yet exchanged funding_signed.
                        },
                        &Event::PaymentReceived { ref payment_hash, ref amt, ref purpose } => {
                                1u8.write(writer)?;
                                let mut payment_secret = None;
                                let mut user_payment_id = None;
                                let payment_preimage;
                                match &purpose {
                                        PaymentPurpose::InvoicePayment { payment_preimage: preimage, payment_secret: secret, user_payment_id: id } => {
                                                payment_secret = Some(secret);
                                                payment_preimage = *preimage;
                                                user_payment_id = Some(id);
                                        },
                                        PaymentPurpose::SpontaneousPayment(preimage) => {
                                                payment_preimage = Some(*preimage);
                                        }
                                }
                                write_tlv_fields!(writer, {
                                        (0, payment_hash, required),
                                        (2, payment_secret, option),
                                        (4, amt, required),
                                        (6, user_payment_id, option),
                                        (8, payment_preimage, option),
                                });
                        },
                        &Event::PaymentSent { ref payment_preimage } => {
                                2u8.write(writer)?;
                                write_tlv_fields!(writer, {
                                        (0, payment_preimage, required),
                                });
                        },
                        &Event::PaymentFailed { ref payment_hash, ref rejected_by_dest, ref network_update, ref all_paths_failed,
                                #[cfg(test)]
                                ref error_code,
                                #[cfg(test)]
                                ref error_data,
                        } => {
                                3u8.write(writer)?;
                                #[cfg(test)]
                                error_code.write(writer)?;
                                #[cfg(test)]
                                error_data.write(writer)?;
                                write_tlv_fields!(writer, {
                                        (0, payment_hash, required),
                                        (1, network_update, option),
                                        (2, rejected_by_dest, required),
                                        (3, all_paths_failed, required),
                                });
                        },
                        &Event::PendingHTLCsForwardable { time_forwardable: _ } => {
                                4u8.write(writer)?;
                                write_tlv_fields!(writer, {});
                                // We don't write the time_fordwardable out at all, as we presume when the user
                                // deserializes us at least that much time has elapsed.
                        },
                        &Event::SpendableOutputs { ref outputs } => {
                                5u8.write(writer)?;
                                write_tlv_fields!(writer, {
                                        (0, VecWriteWrapper(outputs), required),
                                });
                        },
                        &Event::PaymentForwarded { fee_earned_msat, claim_from_onchain_tx } => {
                                7u8.write(writer)?;
                                write_tlv_fields!(writer, {
                                        (0, fee_earned_msat, option),
                                        (2, claim_from_onchain_tx, required),
                                });
                        },
                        &Event::ChannelClosed { ref channel_id, ref err } => {
                                6u8.write(writer)?;
                                channel_id.write(writer)?;
                                (err.len() as u16).write(writer)?;
                                writer.write_all(err.as_bytes())?;
                                write_tlv_fields!(writer, {});
                        },
                }
                Ok(())
        }
}
impl MaybeReadable for Event {
        fn read<R: io::Read>(reader: &mut R) -> Result<Option<Self>, msgs::DecodeError> {
                match Readable::read(reader)? {
                        0u8 => Ok(None),
                        1u8 => {
                                let f = || {
                                        let mut payment_hash = PaymentHash([0; 32]);
                                        let mut payment_preimage = None;
                                        let mut payment_secret = None;
                                        let mut amt = 0;
                                        let mut user_payment_id = None;
                                        read_tlv_fields!(reader, {
                                                (0, payment_hash, required),
                                                (2, payment_secret, option),
                                                (4, amt, required),
                                                (6, user_payment_id, option),
                                                (8, payment_preimage, option),
                                        });
                                        let purpose = match payment_secret {
                                                Some(secret) => PaymentPurpose::InvoicePayment {
                                                        payment_preimage,
                                                        payment_secret: secret,
                                                        user_payment_id: if let Some(id) = user_payment_id {
                                                                id
                                                        } else { return Err(msgs::DecodeError::InvalidValue) }
                                                },
                                                None if payment_preimage.is_some() => PaymentPurpose::SpontaneousPayment(payment_preimage.unwrap()),
                                                None => return Err(msgs::DecodeError::InvalidValue),
                                        };
                                        Ok(Some(Event::PaymentReceived {
                                                payment_hash,
                                                amt,
                                                purpose,
                                        }))
                                };
                                f()
                        },
                        2u8 => {
                                let f = || {
                                        let mut payment_preimage = PaymentPreimage([0; 32]);
                                        read_tlv_fields!(reader, {
                                                (0, payment_preimage, required),
                                        });
                                        Ok(Some(Event::PaymentSent {
                                                payment_preimage,
                                        }))
                                };
                                f()
                        },
                        3u8 => {
                                let f = || {
                                        #[cfg(test)]
                                        let error_code = Readable::read(reader)?;
                                        #[cfg(test)]
                                        let error_data = Readable::read(reader)?;
                                        let mut payment_hash = PaymentHash([0; 32]);
                                        let mut rejected_by_dest = false;
                                        let mut network_update = None;
                                        let mut all_paths_failed = Some(true);
                                        read_tlv_fields!(reader, {
                                                (0, payment_hash, required),
                                                (1, network_update, ignorable),
                                                (2, rejected_by_dest, required),
                                                (3, all_paths_failed, option),
                                        });
                                        Ok(Some(Event::PaymentFailed {
                                                payment_hash,
                                                rejected_by_dest,
                                                network_update,
                                                all_paths_failed: all_paths_failed.unwrap(),
                                                #[cfg(test)]
                                                error_code,
                                                #[cfg(test)]
                                                error_data,
                                        }))
                                };
                                f()
                        },
                        4u8 => {
                                let f = || {
                                        read_tlv_fields!(reader, {});
                                        Ok(Some(Event::PendingHTLCsForwardable {
                                                time_forwardable: Duration::from_secs(0)
                                        }))
                                };
                                f()
                        },
                        5u8 => {
                                let f = || {
                                        let mut outputs = VecReadWrapper(Vec::new());
                                        read_tlv_fields!(reader, {
                                                (0, outputs, required),
                                        });
                                        Ok(Some(Event::SpendableOutputs { outputs: outputs.0 }))
                                };
                                f()
                        },
                        7u8 => {
                                let f = || {
                                        let mut fee_earned_msat = None;
                                        let mut claim_from_onchain_tx = false;
                                        read_tlv_fields!(reader, {
                                                (0, fee_earned_msat, option),
                                                (2, claim_from_onchain_tx, required),
                                        });
                                        Ok(Some(Event::PaymentForwarded { fee_earned_msat, claim_from_onchain_tx }))
                                };
                                f()
                        },
                        // Versions prior to 0.0.100 did not ignore odd types, instead returning InvalidValue.
                        x if x % 2 == 1 => Ok(None),
                        6u8 => {
                                let f = || {
                                        let channel_id = Readable::read(reader)?;
                                        let err = {
                                                let mut size: usize = <u16 as Readable>::read(reader)? as usize;
                                                let mut data = vec![];
                                                let data_len = reader.read_to_end(&mut data)?;
                                                size = cmp::min(data_len, size);
                                                match String::from_utf8(data[..size as usize].to_vec()) {
                                                        Ok(s) => s,
                                                        Err(_) => return Err(msgs::DecodeError::InvalidValue),
                                                }
                                        };
                                        read_tlv_fields!(reader, {});
                                        Ok(Some(Event::ChannelClosed { channel_id, err: err }))
                                };
                                f()
                        },
                        _ => Err(msgs::DecodeError::InvalidValue)
                }
        }
}

/// An event generated by ChannelManager which indicates a message should be sent to a peer (or
/// broadcast to most peers).
/// These events are handled by PeerManager::process_events if you are using a PeerManager.
#[derive(Clone, Debug)]
pub enum MessageSendEvent {
        /// Used to indicate that we've accepted a channel open and should send the accept_channel
        /// message provided to the given peer.
        SendAcceptChannel {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::AcceptChannel,
        },
        /// Used to indicate that we've initiated a channel open and should send the open_channel
        /// message provided to the given peer.
        SendOpenChannel {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::OpenChannel,
        },
        /// Used to indicate that a funding_created message should be sent to the peer with the given node_id.
        SendFundingCreated {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::FundingCreated,
        },
        /// Used to indicate that a funding_signed message should be sent to the peer with the given node_id.
        SendFundingSigned {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::FundingSigned,
        },
        /// Used to indicate that a funding_locked message should be sent to the peer with the given node_id.
        SendFundingLocked {
                /// The node_id of the node which should receive these message(s)
                node_id: PublicKey,
                /// The funding_locked message which should be sent.
                msg: msgs::FundingLocked,
        },
        /// Used to indicate that an announcement_signatures message should be sent to the peer with the given node_id.
        SendAnnouncementSignatures {
                /// The node_id of the node which should receive these message(s)
                node_id: PublicKey,
                /// The announcement_signatures message which should be sent.
                msg: msgs::AnnouncementSignatures,
        },
        /// Used to indicate that a series of HTLC update messages, as well as a commitment_signed
        /// message should be sent to the peer with the given node_id.
        UpdateHTLCs {
                /// The node_id of the node which should receive these message(s)
                node_id: PublicKey,
                /// The update messages which should be sent. ALL messages in the struct should be sent!
                updates: msgs::CommitmentUpdate,
        },
        /// Used to indicate that a revoke_and_ack message should be sent to the peer with the given node_id.
        SendRevokeAndACK {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::RevokeAndACK,
        },
        /// Used to indicate that a closing_signed message should be sent to the peer with the given node_id.
        SendClosingSigned {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::ClosingSigned,
        },
        /// Used to indicate that a shutdown message should be sent to the peer with the given node_id.
        SendShutdown {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::Shutdown,
        },
        /// Used to indicate that a channel_reestablish message should be sent to the peer with the given node_id.
        SendChannelReestablish {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The message which should be sent.
                msg: msgs::ChannelReestablish,
        },
        /// Used to indicate that a channel_announcement and channel_update should be broadcast to all
        /// peers (except the peer with node_id either msg.contents.node_id_1 or msg.contents.node_id_2).
        ///
        /// Note that after doing so, you very likely (unless you did so very recently) want to call
        /// ChannelManager::broadcast_node_announcement to trigger a BroadcastNodeAnnouncement event.
        /// This ensures that any nodes which see our channel_announcement also have a relevant
        /// node_announcement, including relevant feature flags which may be important for routing
        /// through or to us.
        BroadcastChannelAnnouncement {
                /// The channel_announcement which should be sent.
                msg: msgs::ChannelAnnouncement,
                /// The followup channel_update which should be sent.
                update_msg: msgs::ChannelUpdate,
        },
        /// Used to indicate that a node_announcement should be broadcast to all peers.
        BroadcastNodeAnnouncement {
                /// The node_announcement which should be sent.
                msg: msgs::NodeAnnouncement,
        },
        /// Used to indicate that a channel_update should be broadcast to all peers.
        BroadcastChannelUpdate {
                /// The channel_update which should be sent.
                msg: msgs::ChannelUpdate,
        },
        /// Used to indicate that a channel_update should be sent to a single peer.
        /// In contrast to [`Self::BroadcastChannelUpdate`], this is used when the channel is a
        /// private channel and we shouldn't be informing all of our peers of channel parameters.
        SendChannelUpdate {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The channel_update which should be sent.
                msg: msgs::ChannelUpdate,
        },
        /// Broadcast an error downstream to be handled
        HandleError {
                /// The node_id of the node which should receive this message
                node_id: PublicKey,
                /// The action which should be taken.
                action: msgs::ErrorAction
        },
        /// Query a peer for channels with funding transaction UTXOs in a block range.
        SendChannelRangeQuery {
                /// The node_id of this message recipient
                node_id: PublicKey,
                /// The query_channel_range which should be sent.
                msg: msgs::QueryChannelRange,
        },
        /// Request routing gossip messages from a peer for a list of channels identified by
        /// their short_channel_ids.
        SendShortIdsQuery {
                /// The node_id of this message recipient
                node_id: PublicKey,
                /// The query_short_channel_ids which should be sent.
                msg: msgs::QueryShortChannelIds,
        },
        /// Sends a reply to a channel range query. This may be one of several SendReplyChannelRange events
        /// emitted during processing of the query.
        SendReplyChannelRange {
                /// The node_id of this message recipient
                node_id: PublicKey,
                /// The reply_channel_range which should be sent.
                msg: msgs::ReplyChannelRange,
        }
}

/// A trait indicating an object may generate message send events
pub trait MessageSendEventsProvider {
        /// Gets the list of pending events which were generated by previous actions, clearing the list
        /// in the process.
        fn get_and_clear_pending_msg_events(&self) -> Vec<MessageSendEvent>;
}

/// A trait indicating an object may generate events.
///
/// Events are processed by passing an [`EventHandler`] to [`process_pending_events`].
///
/// # Requirements
///
/// See [`process_pending_events`] for requirements around event processing.
///
/// When using this trait, [`process_pending_events`] will call [`handle_event`] for each pending
/// event since the last invocation. The handler must either act upon the event immediately
/// or preserve it for later handling.
///
/// Note, handlers may call back into the provider and thus deadlocking must be avoided. Be sure to
/// consult the provider's documentation on the implication of processing events and how a handler
/// may safely use the provider (e.g., see [`ChannelManager::process_pending_events`] and
/// [`ChainMonitor::process_pending_events`]).
///
/// (C-not implementable) As there is likely no reason for a user to implement this trait on their
/// own type(s).
///
/// [`process_pending_events`]: Self::process_pending_events
/// [`handle_event`]: EventHandler::handle_event
/// [`ChannelManager::process_pending_events`]: crate::ln::channelmanager::ChannelManager#method.process_pending_events
/// [`ChainMonitor::process_pending_events`]: crate::chain::chainmonitor::ChainMonitor#method.process_pending_events
pub trait EventsProvider {
        /// Processes any events generated since the last call using the given event handler.
        ///
        /// Subsequent calls must only process new events. However, handlers must be capable of handling
        /// duplicate events across process restarts. This may occur if the provider was recovered from
        /// an old state (i.e., it hadn't been successfully persisted after processing pending events).
        fn process_pending_events<H: Deref>(&self, handler: H) where H::Target: EventHandler;
}

/// A trait implemented for objects handling events from [`EventsProvider`].
pub trait EventHandler {
        /// Handles the given [`Event`].
        ///
        /// See [`EventsProvider`] for details that must be considered when implementing this method.
        fn handle_event(&self, event: &Event);
}

impl<F> EventHandler for F where F: Fn(&Event) {
        fn handle_event(&self, event: &Event) {
                self(event)
        }
}