[rust-lightning] / lightning-invoice / src / payment.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.

//! A module for paying Lightning invoices and sending spontaneous payments.
//!
//! Defines an [`InvoicePayer`] utility for sending payments, parameterized by [`Payer`] and
//! [`Router`] traits. Implementations of [`Payer`] provide the payer's node id, channels, and means
//! to send a payment over a [`Route`]. Implementations of [`Router`] find a [`Route`] between payer
//! and payee using information provided by the payer and from the payee's [`Invoice`], when
//! applicable.
//!
//! [`InvoicePayer`] uses its [`Router`] parameterization for optionally notifying scorers upon
//! receiving the [`Event::PaymentPathFailed`] and [`Event::PaymentPathSuccessful`] events.
//! It also does the same for payment probe failure and success events using [`Event::ProbeFailed`]
//! and [`Event::ProbeSuccessful`].
//!
//! [`InvoicePayer`] is capable of retrying failed payments. It accomplishes this by implementing
//! [`EventHandler`] which decorates a user-provided handler. It will intercept any
//! [`Event::PaymentPathFailed`] events and retry the failed paths for a fixed number of total
//! attempts or until retry is no longer possible. In such a situation, [`InvoicePayer`] will pass
//! along the events to the user-provided handler.
//!
//! # Example
//!
//! ```
//! # extern crate lightning;
//! # extern crate lightning_invoice;
//! # extern crate secp256k1;
//! #
//! # use lightning::io;
//! # use lightning::ln::{PaymentHash, PaymentPreimage, PaymentSecret};
//! # use lightning::ln::channelmanager::{ChannelDetails, PaymentId, PaymentSendFailure};
//! # use lightning::ln::msgs::LightningError;
//! # use lightning::routing::gossip::NodeId;
//! # use lightning::routing::router::{Route, RouteHop, RouteParameters};
//! # use lightning::routing::scoring::{ChannelUsage, Score};
//! # use lightning::util::events::{Event, EventHandler, EventsProvider};
//! # use lightning::util::logger::{Logger, Record};
//! # use lightning::util::ser::{Writeable, Writer};
//! # use lightning_invoice::Invoice;
//! # use lightning_invoice::payment::{InFlightHtlcs, InvoicePayer, Payer, Retry, Router};
//! # use secp256k1::PublicKey;
//! # use std::cell::RefCell;
//! # use std::ops::Deref;
//! #
//! # struct FakeEventProvider {}
//! # impl EventsProvider for FakeEventProvider {
//! #     fn process_pending_events<H: Deref>(&self, handler: H) where H::Target: EventHandler {}
//! # }
//! #
//! # struct FakePayer {}
//! # impl Payer for FakePayer {
//! #     fn node_id(&self) -> PublicKey { unimplemented!() }
//! #     fn first_hops(&self) -> Vec<ChannelDetails> { unimplemented!() }
//! #     fn send_payment(
//! #         &self, route: &Route, payment_hash: PaymentHash, payment_secret: &Option<PaymentSecret>,
//! #         payment_id: PaymentId
//! #     ) -> Result<(), PaymentSendFailure> { unimplemented!() }
//! #     fn send_spontaneous_payment(
//! #         &self, route: &Route, payment_preimage: PaymentPreimage, payment_id: PaymentId,
//! #     ) -> Result<(), PaymentSendFailure> { unimplemented!() }
//! #     fn retry_payment(
//! #         &self, route: &Route, payment_id: PaymentId
//! #     ) -> Result<(), PaymentSendFailure> { unimplemented!() }
//! #     fn abandon_payment(&self, payment_id: PaymentId) { unimplemented!() }
//! # }
//! #
//! # struct FakeRouter {}
//! # impl Router for FakeRouter {
//! #     fn find_route(
//! #         &self, payer: &PublicKey, params: &RouteParameters, payment_hash: &PaymentHash,
//! #         first_hops: Option<&[&ChannelDetails]>, _inflight_htlcs: InFlightHtlcs
//! #     ) -> Result<Route, LightningError> { unimplemented!() }
//! #
//! #     fn notify_payment_path_failed(&self, path: &[&RouteHop], short_channel_id: u64) {  unimplemented!() }
//! #     fn notify_payment_path_successful(&self, path: &[&RouteHop]) {  unimplemented!() }
//! #     fn notify_payment_probe_successful(&self, path: &[&RouteHop]) {  unimplemented!() }
//! #     fn notify_payment_probe_failed(&self, path: &[&RouteHop], short_channel_id: u64) { unimplemented!() }
//! # }
//! #
//! # struct FakeScorer {}
//! # impl Writeable for FakeScorer {
//! #     fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> { unimplemented!(); }
//! # }
//! # impl Score for FakeScorer {
//! #     fn channel_penalty_msat(
//! #         &self, _short_channel_id: u64, _source: &NodeId, _target: &NodeId, _usage: ChannelUsage
//! #     ) -> u64 { 0 }
//! #     fn payment_path_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
//! #     fn payment_path_successful(&mut self, _path: &[&RouteHop]) {}
//! #     fn probe_failed(&mut self, _path: &[&RouteHop], _short_channel_id: u64) {}
//! #     fn probe_successful(&mut self, _path: &[&RouteHop]) {}
//! # }
//! #
//! # struct FakeLogger {}
//! # impl Logger for FakeLogger {
//! #     fn log(&self, record: &Record) { unimplemented!() }
//! # }
//! #
//! # fn main() {
//! let event_handler = |event: &Event| {
//!     match event {
//!         Event::PaymentPathFailed { .. } => println!("payment failed after retries"),
//!         Event::PaymentSent { .. } => println!("payment successful"),
//!         _ => {},
//!     }
//! };
//! # let payer = FakePayer {};
//! # let router = FakeRouter {};
//! # let scorer = RefCell::new(FakeScorer {});
//! # let logger = FakeLogger {};
//! let invoice_payer = InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));
//!
//! let invoice = "...";
//! if let Ok(invoice) = invoice.parse::<Invoice>() {
//!     invoice_payer.pay_invoice(&invoice).unwrap();
//!
//! # let event_provider = FakeEventProvider {};
//!     loop {
//!         event_provider.process_pending_events(&invoice_payer);
//!     }
//! }
//! # }
//! ```
//!
//! # Note
//!
//! The [`Route`] is computed before each payment attempt. Any updates affecting path finding such
//! as updates to the network graph or changes to channel scores should be applied prior to
//! retries, typically by way of composing [`EventHandler`]s accordingly.

use crate::Invoice;

use bitcoin_hashes::Hash;
use bitcoin_hashes::sha256::Hash as Sha256;

use crate::prelude::*;
use lightning::io;
use lightning::ln::{PaymentHash, PaymentPreimage, PaymentSecret};
use lightning::ln::channelmanager::{ChannelDetails, PaymentId, PaymentSendFailure};
use lightning::ln::msgs::LightningError;
use lightning::routing::gossip::NodeId;
use lightning::routing::router::{PaymentParameters, Route, RouteHop, RouteParameters};
use lightning::util::errors::APIError;
use lightning::util::events::{Event, EventHandler};
use lightning::util::logger::Logger;
use lightning::util::ser::Writeable;
use crate::time_utils::Time;
use crate::sync::Mutex;

use secp256k1::PublicKey;

use core::fmt;
use core::fmt::{Debug, Display, Formatter};
use core::ops::Deref;
use core::time::Duration;
#[cfg(feature = "std")]
use std::time::SystemTime;

/// A utility for paying [`Invoice`]s and sending spontaneous payments.
///
/// See [module-level documentation] for details.
///
/// [module-level documentation]: crate::payment
pub type InvoicePayer<P, R, L, E> = InvoicePayerUsingTime::<P, R, L, E, ConfiguredTime>;

#[cfg(not(feature = "no-std"))]
type ConfiguredTime = std::time::Instant;
#[cfg(feature = "no-std")]
use crate::time_utils;
#[cfg(feature = "no-std")]
type ConfiguredTime = time_utils::Eternity;

/// (C-not exported) generally all users should use the [`InvoicePayer`] type alias.
pub struct InvoicePayerUsingTime<P: Deref, R: Router, L: Deref, E: EventHandler, T: Time>
where
        P::Target: Payer,
        L::Target: Logger,
{
        payer: P,
        router: R,
        logger: L,
        event_handler: E,
        /// Caches the overall attempts at making a payment, which is updated prior to retrying.
        payment_cache: Mutex<HashMap<PaymentHash, PaymentInfo<T>>>,
        retry: Retry,
}

/// Used by [`InvoicePayerUsingTime::payment_cache`] to track the payments that are either
/// currently being made, or have outstanding paths that need retrying.
struct PaymentInfo<T: Time> {
        attempts: PaymentAttempts<T>,
        paths: Vec<Vec<RouteHop>>,
}

impl<T: Time> PaymentInfo<T> {
        fn new() -> Self {
                PaymentInfo {
                        attempts: PaymentAttempts::new(),
                        paths: vec![],
                }
        }
}

/// Storing minimal payment attempts information required for determining if a outbound payment can
/// be retried.
#[derive(Clone, Copy)]
struct PaymentAttempts<T: Time> {
        /// This count will be incremented only after the result of the attempt is known. When it's 0,
        /// it means the result of the first attempt is now known yet.
        count: usize,
        /// This field is only used when retry is [`Retry::Timeout`] which is only build with feature std
        first_attempted_at: T
}

impl<T: Time> PaymentAttempts<T> {
        fn new() -> Self {
                PaymentAttempts {
                        count: 0,
                        first_attempted_at: T::now()
                }
        }
}

impl<T: Time> Display for PaymentAttempts<T> {
        fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
                #[cfg(feature = "no-std")]
                return write!( f, "attempts: {}", self.count);
                #[cfg(not(feature = "no-std"))]
                return write!(
                        f,
                        "attempts: {}, duration: {}s",
                        self.count,
                        T::now().duration_since(self.first_attempted_at).as_secs()
                );
        }
}

/// A trait defining behavior of an [`Invoice`] payer.
///
/// While the behavior of [`InvoicePayer`] provides idempotency of duplicate `send_*payment` calls
/// with the same [`PaymentHash`], it is up to the `Payer` to provide idempotency across restarts.
///
/// [`ChannelManager`] provides idempotency for duplicate payments with the same [`PaymentId`].
///
/// In order to trivially ensure idempotency for payments, the default `Payer` implementation
/// reuses the [`PaymentHash`] bytes as the [`PaymentId`]. Custom implementations wishing to
/// provide payment idempotency with a different idempotency key (i.e. [`PaymentId`]) should map
/// the [`Invoice`] or spontaneous payment target pubkey to their own idempotency key.
///
/// [`ChannelManager`]: lightning::ln::channelmanager::ChannelManager
pub trait Payer {
        /// Returns the payer's node id.
        fn node_id(&self) -> PublicKey;

        /// Returns the payer's channels.
        fn first_hops(&self) -> Vec<ChannelDetails>;

        /// Sends a payment over the Lightning Network using the given [`Route`].
        fn send_payment(
                &self, route: &Route, payment_hash: PaymentHash, payment_secret: &Option<PaymentSecret>,
                payment_id: PaymentId
        ) -> Result<(), PaymentSendFailure>;

        /// Sends a spontaneous payment over the Lightning Network using the given [`Route`].
        fn send_spontaneous_payment(
                &self, route: &Route, payment_preimage: PaymentPreimage, payment_id: PaymentId
        ) -> Result<(), PaymentSendFailure>;

        /// Retries a failed payment path for the [`PaymentId`] using the given [`Route`].
        fn retry_payment(&self, route: &Route, payment_id: PaymentId) -> Result<(), PaymentSendFailure>;

        /// Signals that no further retries for the given payment will occur.
        fn abandon_payment(&self, payment_id: PaymentId);
}

/// A trait defining behavior for routing an [`Invoice`] payment.
pub trait Router {
        /// Finds a [`Route`] between `payer` and `payee` for a payment with the given values.
        fn find_route(
                &self, payer: &PublicKey, route_params: &RouteParameters, payment_hash: &PaymentHash,
                first_hops: Option<&[&ChannelDetails]>, inflight_htlcs: InFlightHtlcs
        ) -> Result<Route, LightningError>;
        /// Lets the router know that payment through a specific path has failed.
        fn notify_payment_path_failed(&self, path: &[&RouteHop], short_channel_id: u64);
        /// Lets the router know that payment through a specific path was successful.
        fn notify_payment_path_successful(&self, path: &[&RouteHop]);
        /// Lets the router know that a payment probe was successful.
        fn notify_payment_probe_successful(&self, path: &[&RouteHop]);
        /// Lets the router know that a payment probe failed.
        fn notify_payment_probe_failed(&self, path: &[&RouteHop], short_channel_id: u64);
}

/// Strategies available to retry payment path failures for an [`Invoice`].
///
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub enum Retry {
        /// Max number of attempts to retry payment.
        ///
        /// Note that this is the number of *path* failures, not full payment retries. For multi-path
        /// payments, if this is less than the total number of paths, we will never even retry all of the
        /// payment's paths.
        Attempts(usize),
        #[cfg(feature = "std")]
        /// Time elapsed before abandoning retries for a payment.
        Timeout(Duration),
}

impl Retry {
        fn is_retryable_now<T: Time>(&self, attempts: &PaymentAttempts<T>) -> bool {
                match (self, attempts) {
                        (Retry::Attempts(max_retry_count), PaymentAttempts { count, .. }) => {
                                max_retry_count >= &count
                        },
                        #[cfg(feature = "std")]
                        (Retry::Timeout(max_duration), PaymentAttempts { first_attempted_at, .. } ) =>
                                *max_duration >= T::now().duration_since(*first_attempted_at),
                }
        }
}

/// An error that may occur when making a payment.
#[derive(Clone, Debug)]
pub enum PaymentError {
        /// An error resulting from the provided [`Invoice`] or payment hash.
        Invoice(&'static str),
        /// An error occurring when finding a route.
        Routing(LightningError),
        /// An error occurring when sending a payment.
        Sending(PaymentSendFailure),
}

impl<P: Deref, R: Router, L: Deref, E: EventHandler, T: Time> InvoicePayerUsingTime<P, R, L, E, T>
where
        P::Target: Payer,
        L::Target: Logger,
{
        /// Creates an invoice payer that retries failed payment paths.
        ///
        /// Will forward any [`Event::PaymentPathFailed`] events to the decorated `event_handler` once
        /// `retry` has been exceeded for a given [`Invoice`].
        pub fn new(
                payer: P, router: R, logger: L, event_handler: E, retry: Retry
        ) -> Self {
                Self {
                        payer,
                        router,
                        logger,
                        event_handler,
                        payment_cache: Mutex::new(HashMap::new()),
                        retry,
                }
        }

        /// Pays the given [`Invoice`], caching it for later use in case a retry is needed.
        ///
        /// [`Invoice::payment_hash`] is used as the [`PaymentId`], which ensures idempotency as long
        /// as the payment is still pending. Once the payment completes or fails, you must ensure that
        /// a second payment with the same [`PaymentHash`] is never sent.
        ///
        /// If you wish to use a different payment idempotency token, see
        /// [`Self::pay_invoice_with_id`].
        pub fn pay_invoice(&self, invoice: &Invoice) -> Result<PaymentId, PaymentError> {
                let payment_id = PaymentId(invoice.payment_hash().into_inner());
                self.pay_invoice_with_id(invoice, payment_id).map(|()| payment_id)
        }

        /// Pays the given [`Invoice`] with a custom idempotency key, caching the invoice for later use
        /// in case a retry is needed.
        ///
        /// Note that idempotency is only guaranteed as long as the payment is still pending. Once the
        /// payment completes or fails, no idempotency guarantees are made.
        ///
        /// You should ensure that the [`Invoice::payment_hash`] is unique and the same [`PaymentHash`]
        /// has never been paid before.
        ///
        /// See [`Self::pay_invoice`] for a variant which uses the [`PaymentHash`] for the idempotency
        /// token.
        pub fn pay_invoice_with_id(&self, invoice: &Invoice, payment_id: PaymentId) -> Result<(), PaymentError> {
                if invoice.amount_milli_satoshis().is_none() {
                        Err(PaymentError::Invoice("amount missing"))
                } else {
                        self.pay_invoice_using_amount(invoice, None, payment_id)
                }
        }

        /// Pays the given zero-value [`Invoice`] using the given amount, caching it for later use in
        /// case a retry is needed.
        ///
        /// [`Invoice::payment_hash`] is used as the [`PaymentId`], which ensures idempotency as long
        /// as the payment is still pending. Once the payment completes or fails, you must ensure that
        /// a second payment with the same [`PaymentHash`] is never sent.
        ///
        /// If you wish to use a different payment idempotency token, see
        /// [`Self::pay_zero_value_invoice_with_id`].
        pub fn pay_zero_value_invoice(
                &self, invoice: &Invoice, amount_msats: u64
        ) -> Result<PaymentId, PaymentError> {
                let payment_id = PaymentId(invoice.payment_hash().into_inner());
                self.pay_zero_value_invoice_with_id(invoice, amount_msats, payment_id).map(|()| payment_id)
        }

        /// Pays the given zero-value [`Invoice`] using the given amount and custom idempotency key,
        /// caching the invoice for later use in case a retry is needed.
        ///
        /// Note that idempotency is only guaranteed as long as the payment is still pending. Once the
        /// payment completes or fails, no idempotency guarantees are made.
        ///
        /// You should ensure that the [`Invoice::payment_hash`] is unique and the same [`PaymentHash`]
        /// has never been paid before.
        ///
        /// See [`Self::pay_zero_value_invoice`] for a variant which uses the [`PaymentHash`] for the
        /// idempotency token.
        pub fn pay_zero_value_invoice_with_id(
                &self, invoice: &Invoice, amount_msats: u64, payment_id: PaymentId
        ) -> Result<(), PaymentError> {
                if invoice.amount_milli_satoshis().is_some() {
                        Err(PaymentError::Invoice("amount unexpected"))
                } else {
                        self.pay_invoice_using_amount(invoice, Some(amount_msats), payment_id)
                }
        }

        fn pay_invoice_using_amount(
                &self, invoice: &Invoice, amount_msats: Option<u64>, payment_id: PaymentId
        ) -> Result<(), PaymentError> {
                debug_assert!(invoice.amount_milli_satoshis().is_some() ^ amount_msats.is_some());

                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                match self.payment_cache.lock().unwrap().entry(payment_hash) {
                        hash_map::Entry::Occupied(_) => return Err(PaymentError::Invoice("payment pending")),
                        hash_map::Entry::Vacant(entry) => entry.insert(PaymentInfo::new()),
                };

                let payment_secret = Some(invoice.payment_secret().clone());
                let mut payment_params = PaymentParameters::from_node_id(invoice.recover_payee_pub_key())
                        .with_expiry_time(expiry_time_from_unix_epoch(&invoice).as_secs())
                        .with_route_hints(invoice.route_hints());
                if let Some(features) = invoice.features() {
                        payment_params = payment_params.with_features(features.clone());
                }
                let route_params = RouteParameters {
                        payment_params,
                        final_value_msat: invoice.amount_milli_satoshis().or(amount_msats).unwrap(),
                        final_cltv_expiry_delta: invoice.min_final_cltv_expiry() as u32,
                };

                let send_payment = |route: &Route| {
                        self.payer.send_payment(route, payment_hash, &payment_secret, payment_id)
                };

                self.pay_internal(&route_params, payment_hash, send_payment)
                        .map_err(|e| { self.payment_cache.lock().unwrap().remove(&payment_hash); e })
        }

        /// Pays `pubkey` an amount using the hash of the given preimage, caching it for later use in
        /// case a retry is needed.
        ///
        /// The hash of the [`PaymentPreimage`] is used as the [`PaymentId`], which ensures idempotency
        /// as long as the payment is still pending. Once the payment completes or fails, you must
        /// ensure that a second payment with the same [`PaymentPreimage`] is never sent.
        pub fn pay_pubkey(
                &self, pubkey: PublicKey, payment_preimage: PaymentPreimage, amount_msats: u64,
                final_cltv_expiry_delta: u32
        ) -> Result<PaymentId, PaymentError> {
                let payment_hash = PaymentHash(Sha256::hash(&payment_preimage.0).into_inner());
                let payment_id = PaymentId(payment_hash.0);
                self.do_pay_pubkey(pubkey, payment_preimage, payment_hash, payment_id, amount_msats,
                                final_cltv_expiry_delta)
                        .map(|()| payment_id)
        }

        /// Pays `pubkey` an amount using the hash of the given preimage and a custom idempotency key,
        /// caching the invoice for later use in case a retry is needed.
        ///
        /// Note that idempotency is only guaranteed as long as the payment is still pending. Once the
        /// payment completes or fails, no idempotency guarantees are made.
        ///
        /// You should ensure that the [`PaymentPreimage`] is unique and the corresponding
        /// [`PaymentHash`] has never been paid before.
        pub fn pay_pubkey_with_id(
                &self, pubkey: PublicKey, payment_preimage: PaymentPreimage, payment_id: PaymentId,
                amount_msats: u64, final_cltv_expiry_delta: u32
        ) -> Result<(), PaymentError> {
                let payment_hash = PaymentHash(Sha256::hash(&payment_preimage.0).into_inner());
                self.do_pay_pubkey(pubkey, payment_preimage, payment_hash, payment_id, amount_msats,
                                final_cltv_expiry_delta)
        }

        fn do_pay_pubkey(
                &self, pubkey: PublicKey, payment_preimage: PaymentPreimage, payment_hash: PaymentHash,
                payment_id: PaymentId, amount_msats: u64, final_cltv_expiry_delta: u32
        ) -> Result<(), PaymentError> {
                match self.payment_cache.lock().unwrap().entry(payment_hash) {
                        hash_map::Entry::Occupied(_) => return Err(PaymentError::Invoice("payment pending")),
                        hash_map::Entry::Vacant(entry) => entry.insert(PaymentInfo::new()),
                };

                let route_params = RouteParameters {
                        payment_params: PaymentParameters::for_keysend(pubkey),
                        final_value_msat: amount_msats,
                        final_cltv_expiry_delta,
                };

                let send_payment = |route: &Route| {
                        self.payer.send_spontaneous_payment(route, payment_preimage, payment_id)
                };
                self.pay_internal(&route_params, payment_hash, send_payment)
                        .map_err(|e| { self.payment_cache.lock().unwrap().remove(&payment_hash); e })
        }

        fn pay_internal<F: FnOnce(&Route) -> Result<(), PaymentSendFailure> + Copy>(
                &self, params: &RouteParameters, payment_hash: PaymentHash, send_payment: F,
        ) -> Result<(), PaymentError> {
                #[cfg(feature = "std")] {
                        if has_expired(params) {
                                log_trace!(self.logger, "Invoice expired prior to send for payment {}", log_bytes!(payment_hash.0));
                                return Err(PaymentError::Invoice("Invoice expired prior to send"));
                        }
                }

                let payer = self.payer.node_id();
                let first_hops = self.payer.first_hops();
                let inflight_htlcs = self.create_inflight_map();
                let route = self.router.find_route(
                        &payer, &params, &payment_hash, Some(&first_hops.iter().collect::<Vec<_>>()),
                        inflight_htlcs
                ).map_err(|e| PaymentError::Routing(e))?;

                match send_payment(&route) {
                        Ok(()) => {
                                for path in route.paths {
                                        self.process_path_inflight_htlcs(payment_hash, path);
                                }
                                Ok(())
                        },
                        Err(e) => match e {
                                PaymentSendFailure::ParameterError(_) => Err(e),
                                PaymentSendFailure::PathParameterError(_) => Err(e),
                                PaymentSendFailure::AllFailedResendSafe(_) => {
                                        let mut payment_cache = self.payment_cache.lock().unwrap();
                                        let payment_info = payment_cache.get_mut(&payment_hash).unwrap();
                                        payment_info.attempts.count += 1;
                                        if self.retry.is_retryable_now(&payment_info.attempts) {
                                                core::mem::drop(payment_cache);
                                                Ok(self.pay_internal(params, payment_hash, send_payment)?)
                                        } else {
                                                Err(e)
                                        }
                                },
                                PaymentSendFailure::PartialFailure { failed_paths_retry, payment_id, results } => {
                                        // If a `PartialFailure` event returns a result that is an `Ok()`, it means that
                                        // part of our payment is retried. When we receive `MonitorUpdateInProgress`, it
                                        // means that we are still waiting for our channel monitor update to be completed.
                                        for (result, path) in results.iter().zip(route.paths.into_iter()) {
                                                match result {
                                                        Ok(_) | Err(APIError::MonitorUpdateInProgress) => {
                                                                self.process_path_inflight_htlcs(payment_hash, path);
                                                        },
                                                        _ => {},
                                                }
                                        }

                                        if let Some(retry_data) = failed_paths_retry {
                                                // Some paths were sent, even if we failed to send the full MPP value our
                                                // recipient may misbehave and claim the funds, at which point we have to
                                                // consider the payment sent, so return `Ok()` here, ignoring any retry
                                                // errors.
                                                let _ = self.retry_payment(payment_id, payment_hash, &retry_data);
                                                Ok(())
                                        } else {
                                                // This may happen if we send a payment and some paths fail, but
                                                // only due to a temporary monitor failure or the like, implying
                                                // they're really in-flight, but we haven't sent the initial
                                                // HTLC-Add messages yet.
                                                Ok(())
                                        }
                                },
                        },
                }.map_err(|e| PaymentError::Sending(e))
        }

        // Takes in a path to have its information stored in `payment_cache`. This is done for paths
        // that are pending retry.
        fn process_path_inflight_htlcs(&self, payment_hash: PaymentHash, path: Vec<RouteHop>) {
                self.payment_cache.lock().unwrap().entry(payment_hash)
                        .or_insert_with(|| PaymentInfo::new())
                        .paths.push(path);
        }

        // Find the path we want to remove in `payment_cache`. If it doesn't exist, do nothing.
        fn remove_path_inflight_htlcs(&self, payment_hash: PaymentHash, path: &Vec<RouteHop>) {
                self.payment_cache.lock().unwrap().entry(payment_hash)
                        .and_modify(|payment_info| {
                                if let Some(idx) = payment_info.paths.iter().position(|p| p == path) {
                                        payment_info.paths.swap_remove(idx);
                                }
                        });
        }

        fn retry_payment(
                &self, payment_id: PaymentId, payment_hash: PaymentHash, params: &RouteParameters
        ) -> Result<(), ()> {
                let attempts = self.payment_cache.lock().unwrap().entry(payment_hash)
                        .and_modify(|info| info.attempts.count += 1 )
                        .or_insert_with(|| PaymentInfo {
                                attempts: PaymentAttempts {
                                        count: 1,
                                        first_attempted_at: T::now(),
                                },
                                paths: vec![],
                        }).attempts;

                if !self.retry.is_retryable_now(&attempts) {
                        log_trace!(self.logger, "Payment {} exceeded maximum attempts; not retrying ({})", log_bytes!(payment_hash.0), attempts);
                        return Err(());
                }

                #[cfg(feature = "std")] {
                        if has_expired(params) {
                                log_trace!(self.logger, "Invoice expired for payment {}; not retrying ({:})", log_bytes!(payment_hash.0), attempts);
                                return Err(());
                        }
                }

                let payer = self.payer.node_id();
                let first_hops = self.payer.first_hops();
                let inflight_htlcs = self.create_inflight_map();

                let route = self.router.find_route(
                        &payer, &params, &payment_hash, Some(&first_hops.iter().collect::<Vec<_>>()),
                        inflight_htlcs
                );

                if route.is_err() {
                        log_trace!(self.logger, "Failed to find a route for payment {}; not retrying ({:})", log_bytes!(payment_hash.0), attempts);
                        return Err(());
                }

                match self.payer.retry_payment(&route.as_ref().unwrap(), payment_id) {
                        Ok(()) => {
                                for path in route.unwrap().paths.into_iter() {
                                        self.process_path_inflight_htlcs(payment_hash, path);
                                }
                                Ok(())
                        },
                        Err(PaymentSendFailure::ParameterError(_)) |
                        Err(PaymentSendFailure::PathParameterError(_)) => {
                                log_trace!(self.logger, "Failed to retry for payment {} due to bogus route/payment data, not retrying.", log_bytes!(payment_hash.0));
                                Err(())
                        },
                        Err(PaymentSendFailure::AllFailedResendSafe(_)) => {
                                self.retry_payment(payment_id, payment_hash, params)
                        },
                        Err(PaymentSendFailure::PartialFailure { failed_paths_retry, results, .. }) => {
                                // If a `PartialFailure` error contains a result that is an `Ok()`, it means that
                                // part of our payment is retried. When we receive `MonitorUpdateInProgress`, it
                                // means that we are still waiting for our channel monitor update to complete.
                                for (result, path) in results.iter().zip(route.unwrap().paths.into_iter()) {
                                        match result {
                                                Ok(_) | Err(APIError::MonitorUpdateInProgress) => {
                                                        self.process_path_inflight_htlcs(payment_hash, path);
                                                },
                                                _ => {},
                                        }
                                }

                                if let Some(retry) = failed_paths_retry {
                                        // Always return Ok for the same reason as noted in pay_internal.
                                        let _ = self.retry_payment(payment_id, payment_hash, &retry);
                                }
                                Ok(())
                        },
                }
        }

        /// Removes the payment cached by the given payment hash.
        ///
        /// Should be called once a payment has failed or succeeded if not using [`InvoicePayer`] as an
        /// [`EventHandler`]. Otherwise, calling this method is unnecessary.
        pub fn remove_cached_payment(&self, payment_hash: &PaymentHash) {
                self.payment_cache.lock().unwrap().remove(payment_hash);
        }

        /// Given a [`PaymentHash`], this function looks up inflight path attempts in the payment_cache.
        /// Then, it uses the path information inside the cache to construct a HashMap mapping a channel's
        /// short channel id and direction to the amount being sent through it.
        ///
        /// This function should be called whenever we need information about currently used up liquidity
        /// across payments.
        fn create_inflight_map(&self) -> InFlightHtlcs {
                let mut total_inflight_map: HashMap<(u64, bool), u64> = HashMap::new();
                // Make an attempt at finding existing payment information from `payment_cache`. If it
                // does not exist, it probably is a fresh payment and we can just return an empty
                // HashMap.
                for payment_info in self.payment_cache.lock().unwrap().values() {
                        for path in &payment_info.paths {
                                if path.is_empty() { break };
                                // total_inflight_map needs to be direction-sensitive when keeping track of the HTLC value
                                // that is held up. However, the `hops` array, which is a path returned by `find_route` in
                                // the router excludes the payer node. In the following lines, the payer's information is
                                // hardcoded with an inflight value of 0 so that we can correctly represent the first hop
                                // in our sliding window of two.
                                let our_node_id: PublicKey = self.payer.node_id();
                                let reversed_hops_with_payer = path.iter().rev().skip(1)
                                        .map(|hop| hop.pubkey)
                                        .chain(core::iter::once(our_node_id));
                                let mut cumulative_msat = 0;

                                // Taking the reversed vector from above, we zip it with just the reversed hops list to
                                // work "backwards" of the given path, since the last hop's `fee_msat` actually represents
                                // the total amount sent.
                                for (next_hop, prev_hop) in path.iter().rev().zip(reversed_hops_with_payer) {
                                        cumulative_msat += next_hop.fee_msat;
                                        total_inflight_map
                                                .entry((next_hop.short_channel_id, NodeId::from_pubkey(&prev_hop) < NodeId::from_pubkey(&next_hop.pubkey)))
                                                .and_modify(|used_liquidity_msat| *used_liquidity_msat += cumulative_msat)
                                                .or_insert(cumulative_msat);
                                }
                        }
                }

                InFlightHtlcs(total_inflight_map)
        }
}

fn expiry_time_from_unix_epoch(invoice: &Invoice) -> Duration {
        invoice.signed_invoice.raw_invoice.data.timestamp.0 + invoice.expiry_time()
}

#[cfg(feature = "std")]
fn has_expired(route_params: &RouteParameters) -> bool {
        if let Some(expiry_time) = route_params.payment_params.expiry_time {
                Invoice::is_expired_from_epoch(&SystemTime::UNIX_EPOCH, Duration::from_secs(expiry_time))
        } else { false }
}

impl<P: Deref, R: Router, L: Deref, E: EventHandler, T: Time> EventHandler for InvoicePayerUsingTime<P, R, L, E, T>
where
        P::Target: Payer,
        L::Target: Logger,
{
        fn handle_event(&self, event: &Event) {
                match event {
                        Event::PaymentPathFailed { payment_hash, path, ..  }
                        | Event::PaymentPathSuccessful { path, payment_hash: Some(payment_hash), .. }
                        | Event::ProbeSuccessful { payment_hash, path, .. }
                        | Event::ProbeFailed { payment_hash, path, .. } => {
                                self.remove_path_inflight_htlcs(*payment_hash, path);
                        },
                        _ => {},
                }

                match event {
                        Event::PaymentPathFailed {
                                payment_id, payment_hash, payment_failed_permanently, path, short_channel_id, retry, ..
                        } => {
                                if let Some(short_channel_id) = short_channel_id {
                                        let path = path.iter().collect::<Vec<_>>();
                                        self.router.notify_payment_path_failed(&path, *short_channel_id)
                                }

                                if payment_id.is_none() {
                                        log_trace!(self.logger, "Payment {} has no id; not retrying", log_bytes!(payment_hash.0));
                                } else if *payment_failed_permanently {
                                        log_trace!(self.logger, "Payment {} rejected by destination; not retrying", log_bytes!(payment_hash.0));
                                        self.payer.abandon_payment(payment_id.unwrap());
                                } else if retry.is_none() {
                                        log_trace!(self.logger, "Payment {} missing retry params; not retrying", log_bytes!(payment_hash.0));
                                        self.payer.abandon_payment(payment_id.unwrap());
                                } else if self.retry_payment(payment_id.unwrap(), *payment_hash, retry.as_ref().unwrap()).is_ok() {
                                        // We retried at least somewhat, don't provide the PaymentPathFailed event to the user.
                                        return;
                                } else {
                                        self.payer.abandon_payment(payment_id.unwrap());
                                }
                        },
                        Event::PaymentFailed { payment_hash, .. } => {
                                self.remove_cached_payment(&payment_hash);
                        },
                        Event::PaymentPathSuccessful { path, .. } => {
                                let path = path.iter().collect::<Vec<_>>();
                                self.router.notify_payment_path_successful(&path);
                        },
                        Event::PaymentSent { payment_hash, .. } => {
                                let mut payment_cache = self.payment_cache.lock().unwrap();
                                let attempts = payment_cache
                                        .remove(payment_hash)
                                        .map_or(1, |payment_info| payment_info.attempts.count + 1);
                                log_trace!(self.logger, "Payment {} succeeded (attempts: {})", log_bytes!(payment_hash.0), attempts);
                        },
                        Event::ProbeSuccessful { payment_hash, path, .. } => {
                                log_trace!(self.logger, "Probe payment {} of {}msat was successful", log_bytes!(payment_hash.0), path.last().unwrap().fee_msat);
                                let path = path.iter().collect::<Vec<_>>();
                                self.router.notify_payment_probe_successful(&path);
                        },
                        Event::ProbeFailed { payment_hash, path, short_channel_id, .. } => {
                                if let Some(short_channel_id) = short_channel_id {
                                        log_trace!(self.logger, "Probe payment {} of {}msat failed at channel {}", log_bytes!(payment_hash.0), path.last().unwrap().fee_msat, *short_channel_id);
                                        let path = path.iter().collect::<Vec<_>>();
                                        self.router.notify_payment_probe_failed(&path, *short_channel_id);
                                }
                        },
                        _ => {},
                }

                // Delegate to the decorated event handler unless the payment is retried.
                self.event_handler.handle_event(event)
        }
}

/// A map with liquidity value (in msat) keyed by a short channel id and the direction the HTLC
/// is traveling in. The direction boolean is determined by checking if the HTLC source's public
/// key is less than its destination. See [`InFlightHtlcs::used_liquidity_msat`] for more
/// details.
pub struct InFlightHtlcs(HashMap<(u64, bool), u64>);

impl InFlightHtlcs {
        /// Returns liquidity in msat given the public key of the HTLC source, target, and short channel
        /// id.
        pub fn used_liquidity_msat(&self, source: &NodeId, target: &NodeId, channel_scid: u64) -> Option<u64> {
                self.0.get(&(channel_scid, source < target)).map(|v| *v)
        }
}

impl Writeable for InFlightHtlcs {
        fn write<W: lightning::util::ser::Writer>(&self, writer: &mut W) -> Result<(), io::Error> { self.0.write(writer) }
}

impl lightning::util::ser::Readable for InFlightHtlcs {
        fn read<R: io::Read>(reader: &mut R) -> Result<Self, lightning::ln::msgs::DecodeError> {
                let infight_map: HashMap<(u64, bool), u64> = lightning::util::ser::Readable::read(reader)?;
                Ok(Self(infight_map))
        }
}

#[cfg(test)]
mod tests {
        use super::*;
        use crate::{InvoiceBuilder, Currency};
        use crate::utils::{ScorerAccountingForInFlightHtlcs, create_invoice_from_channelmanager_and_duration_since_epoch};
        use bitcoin_hashes::sha256::Hash as Sha256;
        use lightning::ln::PaymentPreimage;
        use lightning::ln::channelmanager;
        use lightning::ln::features::{ChannelFeatures, NodeFeatures};
        use lightning::ln::functional_test_utils::*;
        use lightning::ln::msgs::{ChannelMessageHandler, ErrorAction, LightningError};
        use lightning::routing::gossip::{EffectiveCapacity, NodeId};
        use lightning::routing::router::{PaymentParameters, Route, RouteHop};
        use lightning::routing::scoring::{ChannelUsage, LockableScore, Score};
        use lightning::util::test_utils::TestLogger;
        use lightning::util::errors::APIError;
        use lightning::util::events::{Event, EventsProvider, MessageSendEvent, MessageSendEventsProvider};
        use secp256k1::{SecretKey, PublicKey, Secp256k1};
        use std::cell::RefCell;
        use std::collections::VecDeque;
        use std::ops::DerefMut;
        use std::time::{SystemTime, Duration};
        use crate::time_utils::tests::SinceEpoch;
        use crate::DEFAULT_EXPIRY_TIME;
        use lightning::util::errors::APIError::{ChannelUnavailable, MonitorUpdateInProgress};

        fn invoice(payment_preimage: PaymentPreimage) -> Invoice {
                let payment_hash = Sha256::hash(&payment_preimage.0);
                let private_key = SecretKey::from_slice(&[42; 32]).unwrap();

                InvoiceBuilder::new(Currency::Bitcoin)
                        .description("test".into())
                        .payment_hash(payment_hash)
                        .payment_secret(PaymentSecret([0; 32]))
                        .duration_since_epoch(duration_since_epoch())
                        .min_final_cltv_expiry(144)
                        .amount_milli_satoshis(128)
                        .build_signed(|hash| {
                                Secp256k1::new().sign_ecdsa_recoverable(hash, &private_key)
                        })
                        .unwrap()
        }

        fn duration_since_epoch() -> Duration {
                #[cfg(feature = "std")]
                        let duration_since_epoch =
                        SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).unwrap();
                #[cfg(not(feature = "std"))]
                        let duration_since_epoch = Duration::from_secs(1234567);
                duration_since_epoch
        }

        fn zero_value_invoice(payment_preimage: PaymentPreimage) -> Invoice {
                let payment_hash = Sha256::hash(&payment_preimage.0);
                let private_key = SecretKey::from_slice(&[42; 32]).unwrap();

                InvoiceBuilder::new(Currency::Bitcoin)
                        .description("test".into())
                        .payment_hash(payment_hash)
                        .payment_secret(PaymentSecret([0; 32]))
                        .duration_since_epoch(duration_since_epoch())
                        .min_final_cltv_expiry(144)
                        .build_signed(|hash| {
                                Secp256k1::new().sign_ecdsa_recoverable(hash, &private_key)
                        })
                        .unwrap()
        }

        #[cfg(feature = "std")]
        fn expired_invoice(payment_preimage: PaymentPreimage) -> Invoice {
                let payment_hash = Sha256::hash(&payment_preimage.0);
                let private_key = SecretKey::from_slice(&[42; 32]).unwrap();
                let duration = duration_since_epoch()
                        .checked_sub(Duration::from_secs(DEFAULT_EXPIRY_TIME * 2))
                        .unwrap();
                InvoiceBuilder::new(Currency::Bitcoin)
                        .description("test".into())
                        .payment_hash(payment_hash)
                        .payment_secret(PaymentSecret([0; 32]))
                        .duration_since_epoch(duration)
                        .min_final_cltv_expiry(144)
                        .amount_milli_satoshis(128)
                        .build_signed(|hash| {
                                Secp256k1::new().sign_ecdsa_recoverable(hash, &private_key)
                        })
                        .unwrap()
        }

        fn pubkey() -> PublicKey {
                PublicKey::from_slice(&hex::decode("02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619").unwrap()[..]).unwrap()
        }

        #[test]
        fn pays_invoice_on_first_attempt() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                invoice_payer.handle_event(&Event::PaymentSent {
                        payment_id, payment_preimage, payment_hash, fee_paid_msat: None
                });
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 1);
        }

        #[test]
        fn pays_invoice_on_retry() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: TestRouter::path_for_value(final_value_msat),
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 2);

                invoice_payer.handle_event(&Event::PaymentSent {
                        payment_id, payment_preimage, payment_hash, fee_paid_msat: None
                });
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 2);
        }

        #[test]
        fn pays_invoice_on_partial_failure() {
                let event_handler = |_: &_| { panic!() };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let retry = TestRouter::retry_for_invoice(&invoice);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .fails_with_partial_failure(retry.clone(), OnAttempt(1), None)
                        .fails_with_partial_failure(retry, OnAttempt(2), None)
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                assert!(invoice_payer.pay_invoice(&invoice).is_ok());
        }

        #[test]
        fn retries_payment_path_for_unknown_payment() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::OnRetry(final_value_msat / 2))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(PaymentId([1; 32]));
                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: TestRouter::path_for_value(final_value_msat),
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 1);

                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 2);

                invoice_payer.handle_event(&Event::PaymentSent {
                        payment_id, payment_preimage, payment_hash, fee_paid_msat: None
                });
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 2);
        }

        #[test]
        fn fails_paying_invoice_after_max_retry_counts() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: true,
                        path: TestRouter::path_for_value(final_value_msat),
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 2);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: TestRouter::path_for_value(final_value_msat / 2),
                        short_channel_id: None,
                        retry: Some(RouteParameters {
                                final_value_msat: final_value_msat / 2, ..TestRouter::retry_for_invoice(&invoice)
                        }),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 3);

                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 3);
        }

        #[cfg(feature = "std")]
        #[test]
        fn fails_paying_invoice_after_max_retry_timeout() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));

                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                type InvoicePayerUsingSinceEpoch <P, R, L, E> = InvoicePayerUsingTime::<P, R, L, E, SinceEpoch>;

                let invoice_payer =
                        InvoicePayerUsingSinceEpoch::new(&payer, router, &logger, event_handler, Retry::Timeout(Duration::from_secs(120)));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: true,
                        path: TestRouter::path_for_value(final_value_msat),
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 2);

                SinceEpoch::advance(Duration::from_secs(121));

                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 2);
        }

        #[test]
        fn fails_paying_invoice_with_missing_retry_params() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: vec![],
                        short_channel_id: None,
                        retry: None,
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 1);
        }

        // Expiration is checked only in an std environment
        #[cfg(feature = "std")]
        #[test]
        fn fails_paying_invoice_after_expiration() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payer = TestPayer::new();
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = expired_invoice(payment_preimage);
                if let PaymentError::Invoice(msg) = invoice_payer.pay_invoice(&invoice).unwrap_err() {
                        assert_eq!(msg, "Invoice expired prior to send");
                } else { panic!("Expected Invoice Error"); }
        }

        // Expiration is checked only in an std environment
        #[cfg(feature = "std")]
        #[test]
        fn fails_retrying_invoice_after_expiration() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router,  &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let mut retry_data = TestRouter::retry_for_invoice(&invoice);
                retry_data.payment_params.expiry_time = Some(SystemTime::now()
                        .checked_sub(Duration::from_secs(2)).unwrap()
                        .duration_since(SystemTime::UNIX_EPOCH).unwrap().as_secs());
                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: vec![],
                        short_channel_id: None,
                        retry: Some(retry_data),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 1);
        }

        #[test]
        fn fails_paying_invoice_after_retry_error() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .fails_on_attempt(2)
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: TestRouter::path_for_value(final_value_msat / 2),
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 2);
        }

        #[test]
        fn fails_paying_invoice_after_rejected_by_payee() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash: PaymentHash(invoice.payment_hash().clone().into_inner()),
                        network_update: None,
                        payment_failed_permanently: true,
                        all_paths_failed: false,
                        path: vec![],
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 1);
        }

        #[test]
        fn fails_repaying_invoice_with_pending_payment() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());

                // Cannot repay an invoice pending payment.
                match invoice_payer.pay_invoice(&invoice) {
                        Err(PaymentError::Invoice("payment pending")) => {},
                        Err(_) => panic!("unexpected error"),
                        Ok(_) => panic!("expected invoice error"),
                }

                // Can repay an invoice once cleared from cache.
                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                invoice_payer.remove_cached_payment(&payment_hash);
                assert!(invoice_payer.pay_invoice(&invoice).is_ok());

                // Cannot retry paying an invoice if cleared from cache.
                invoice_payer.remove_cached_payment(&payment_hash);
                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: vec![],
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), true);
        }

        #[test]
        fn fails_paying_invoice_with_routing_errors() {
                let payer = TestPayer::new();
                let router = FailingRouter {};
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, |_: &_| {}, Retry::Attempts(0));

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                match invoice_payer.pay_invoice(&invoice) {
                        Err(PaymentError::Routing(_)) => {},
                        Err(_) => panic!("unexpected error"),
                        Ok(_) => panic!("expected routing error"),
                }
        }

        #[test]
        fn fails_paying_invoice_with_sending_errors() {
                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .fails_on_attempt(1)
                        .expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, |_: &_| {}, Retry::Attempts(0));

                match invoice_payer.pay_invoice(&invoice) {
                        Err(PaymentError::Sending(_)) => {},
                        Err(_) => panic!("unexpected error"),
                        Ok(_) => panic!("expected sending error"),
                }
        }

        #[test]
        fn pays_zero_value_invoice_using_amount() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = zero_value_invoice(payment_preimage);
                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                let final_value_msat = 100;

                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                let payment_id =
                        Some(invoice_payer.pay_zero_value_invoice(&invoice, final_value_msat).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                invoice_payer.handle_event(&Event::PaymentSent {
                        payment_id, payment_preimage, payment_hash, fee_paid_msat: None
                });
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 1);
        }

        #[test]
        fn fails_paying_zero_value_invoice_with_amount() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payer = TestPayer::new();
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router,  &logger, event_handler, Retry::Attempts(0));

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);

                // Cannot repay an invoice pending payment.
                match invoice_payer.pay_zero_value_invoice(&invoice, 100) {
                        Err(PaymentError::Invoice("amount unexpected")) => {},
                        Err(_) => panic!("unexpected error"),
                        Ok(_) => panic!("expected invoice error"),
                }
        }

        #[test]
        fn pays_pubkey_with_amount() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let pubkey = pubkey();
                let payment_preimage = PaymentPreimage([1; 32]);
                let payment_hash = PaymentHash(Sha256::hash(&payment_preimage.0).into_inner());
                let final_value_msat = 100;
                let final_cltv_expiry_delta = 42;

                let payer = TestPayer::new()
                        .expect_send(Amount::Spontaneous(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat));
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_pubkey(
                                pubkey, payment_preimage, final_value_msat, final_cltv_expiry_delta
                        ).unwrap());
                assert_eq!(*payer.attempts.borrow(), 1);

                let retry = RouteParameters {
                        payment_params: PaymentParameters::for_keysend(pubkey),
                        final_value_msat,
                        final_cltv_expiry_delta,
                };
                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: vec![],
                        short_channel_id: None,
                        retry: Some(retry),
                };
                invoice_payer.handle_event(&event);
                assert_eq!(*event_handled.borrow(), false);
                assert_eq!(*payer.attempts.borrow(), 2);

                invoice_payer.handle_event(&Event::PaymentSent {
                        payment_id, payment_preimage, payment_hash, fee_paid_msat: None
                });
                assert_eq!(*event_handled.borrow(), true);
                assert_eq!(*payer.attempts.borrow(), 2);
        }

        #[test]
        fn scores_failed_channel() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let payment_hash = PaymentHash(invoice.payment_hash().clone().into_inner());
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();
                let path = TestRouter::path_for_value(final_value_msat);
                let short_channel_id = Some(path[0].short_channel_id);

                // Expect that scorer is given short_channel_id upon handling the event.
                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2));
                let scorer = TestScorer::new().expect(TestResult::PaymentFailure {
                        path: path.clone(), short_channel_id: path[0].short_channel_id,
                });
                let router = TestRouter::new(scorer);
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = Some(invoice_payer.pay_invoice(&invoice).unwrap());
                let event = Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path,
                        short_channel_id,
                        retry: Some(TestRouter::retry_for_invoice(&invoice)),
                };
                invoice_payer.handle_event(&event);
        }

        #[test]
        fn scores_successful_channels() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let payment_hash = Some(PaymentHash(invoice.payment_hash().clone().into_inner()));
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();
                let route = TestRouter::route_for_value(final_value_msat);

                // Expect that scorer is given short_channel_id upon handling the event.
                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let scorer = TestScorer::new()
                        .expect(TestResult::PaymentSuccess { path: route.paths[0].clone() })
                        .expect(TestResult::PaymentSuccess { path: route.paths[1].clone() });
                let router = TestRouter::new(scorer);
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                let payment_id = invoice_payer.pay_invoice(&invoice).unwrap();
                let event = Event::PaymentPathSuccessful {
                        payment_id, payment_hash, path: route.paths[0].clone()
                };
                invoice_payer.handle_event(&event);
                let event = Event::PaymentPathSuccessful {
                        payment_id, payment_hash, path: route.paths[1].clone()
                };
                invoice_payer.handle_event(&event);
        }

        #[test]
        fn generates_correct_inflight_map_data() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice = invoice(payment_preimage);
                let payment_hash = Some(PaymentHash(invoice.payment_hash().clone().into_inner()));
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new().expect_send(Amount::ForInvoice(final_value_msat));
                let final_value_msat = invoice.amount_milli_satoshis().unwrap();
                let route = TestRouter::route_for_value(final_value_msat);
                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                let payment_id = invoice_payer.pay_invoice(&invoice).unwrap();

                let inflight_map = invoice_payer.create_inflight_map();
                // First path check
                assert_eq!(inflight_map.0.get(&(0, false)).unwrap().clone(), 94);
                assert_eq!(inflight_map.0.get(&(1, true)).unwrap().clone(), 84);
                assert_eq!(inflight_map.0.get(&(2, false)).unwrap().clone(), 64);

                // Second path check
                assert_eq!(inflight_map.0.get(&(3, false)).unwrap().clone(), 74);
                assert_eq!(inflight_map.0.get(&(4, false)).unwrap().clone(), 64);

                invoice_payer.handle_event(&Event::PaymentPathSuccessful {
                        payment_id, payment_hash, path: route.paths[0].clone()
                });

                let inflight_map = invoice_payer.create_inflight_map();

                assert_eq!(inflight_map.0.get(&(0, false)), None);
                assert_eq!(inflight_map.0.get(&(1, true)), None);
                assert_eq!(inflight_map.0.get(&(2, false)), None);

                // Second path should still be inflight
                assert_eq!(inflight_map.0.get(&(3, false)).unwrap().clone(), 74);
                assert_eq!(inflight_map.0.get(&(4, false)).unwrap().clone(), 64)
        }

        #[test]
        fn considers_inflight_htlcs_between_invoice_payments_when_path_succeeds() {
                // First, let's just send a payment through, but only make sure one of the path completes
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let payment_invoice = invoice(payment_preimage);
                let payment_hash = Some(PaymentHash(payment_invoice.payment_hash().clone().into_inner()));
                let final_value_msat = payment_invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::ForInvoice(final_value_msat));
                let final_value_msat = payment_invoice.amount_milli_satoshis().unwrap();
                let route = TestRouter::route_for_value(final_value_msat);
                let scorer = TestScorer::new()
                        // 1st invoice, 1st path
                        .expect_usage(ChannelUsage { amount_msat: 64, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 84, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 94, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // 1st invoice, 2nd path
                        .expect_usage(ChannelUsage { amount_msat: 64, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 74, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // 2nd invoice, 1st path
                        .expect_usage(ChannelUsage { amount_msat: 64, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 84, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 94, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // 2nd invoice, 2nd path
                        .expect_usage(ChannelUsage { amount_msat: 64, inflight_htlc_msat: 64, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 74, inflight_htlc_msat: 74, effective_capacity: EffectiveCapacity::Unknown } );
                let router = TestRouter::new(scorer);
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                // Succeed 1st path, leave 2nd path inflight
                let payment_id = invoice_payer.pay_invoice(&payment_invoice).unwrap();
                invoice_payer.handle_event(&Event::PaymentPathSuccessful {
                        payment_id, payment_hash, path: route.paths[0].clone()
                });

                // Let's pay a second invoice that will be using the same path. This should trigger the
                // assertions that expect the last 4 ChannelUsage values above where TestScorer is initialized.
                // Particularly, the 2nd path of the 1st payment, since it is not yet complete, should still
                // have 64 msats inflight for paths considering the channel with scid of 1.
                let payment_preimage_2 = PaymentPreimage([2; 32]);
                let payment_invoice_2 = invoice(payment_preimage_2);
                invoice_payer.pay_invoice(&payment_invoice_2).unwrap();
        }

        #[test]
        fn considers_inflight_htlcs_between_retries() {
                // First, let's just send a payment through, but only make sure one of the path completes
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let payment_invoice = invoice(payment_preimage);
                let payment_hash = PaymentHash(payment_invoice.payment_hash().clone().into_inner());
                let final_value_msat = payment_invoice.amount_milli_satoshis().unwrap();

                let payer = TestPayer::new()
                        .expect_send(Amount::ForInvoice(final_value_msat))
                        .expect_send(Amount::OnRetry(final_value_msat / 2))
                        .expect_send(Amount::OnRetry(final_value_msat / 4));
                let final_value_msat = payment_invoice.amount_milli_satoshis().unwrap();
                let scorer = TestScorer::new()
                        // 1st invoice, 1st path
                        .expect_usage(ChannelUsage { amount_msat: 64, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 84, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 94, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // 1st invoice, 2nd path
                        .expect_usage(ChannelUsage { amount_msat: 64, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 74, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // Retry 1, 1st path
                        .expect_usage(ChannelUsage { amount_msat: 32, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 52, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 62, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // Retry 1, 2nd path
                        .expect_usage(ChannelUsage { amount_msat: 32, inflight_htlc_msat: 64, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 42, inflight_htlc_msat: 64 + 10, effective_capacity: EffectiveCapacity::Unknown } )
                        // Retry 2, 1st path
                        .expect_usage(ChannelUsage { amount_msat: 16, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 36, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 46, inflight_htlc_msat: 0, effective_capacity: EffectiveCapacity::Unknown } )
                        // Retry 2, 2nd path
                        .expect_usage(ChannelUsage { amount_msat: 16, inflight_htlc_msat: 64 + 32, effective_capacity: EffectiveCapacity::Unknown } )
                        .expect_usage(ChannelUsage { amount_msat: 26, inflight_htlc_msat: 74 + 32 + 10, effective_capacity: EffectiveCapacity::Unknown } );
                let router = TestRouter::new(scorer);
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(2));

                // Fail 1st path, leave 2nd path inflight
                let payment_id = Some(invoice_payer.pay_invoice(&payment_invoice).unwrap());
                invoice_payer.handle_event(&Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: TestRouter::path_for_value(final_value_msat),
                        short_channel_id: None,
                        retry: Some(TestRouter::retry_for_invoice(&payment_invoice)),
                });

                // Fails again the 1st path of our retry
                invoice_payer.handle_event(&Event::PaymentPathFailed {
                        payment_id,
                        payment_hash,
                        network_update: None,
                        payment_failed_permanently: false,
                        all_paths_failed: false,
                        path: TestRouter::path_for_value(final_value_msat / 2),
                        short_channel_id: None,
                        retry: Some(RouteParameters {
                                final_value_msat: final_value_msat / 4,
                                ..TestRouter::retry_for_invoice(&payment_invoice)
                        }),
                });
        }

        #[test]
        fn accounts_for_some_inflight_htlcs_sent_during_partial_failure() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice_to_pay = invoice(payment_preimage);
                let final_value_msat = invoice_to_pay.amount_milli_satoshis().unwrap();

                let retry = TestRouter::retry_for_invoice(&invoice_to_pay);
                let payer = TestPayer::new()
                        .fails_with_partial_failure(
                                retry.clone(), OnAttempt(1),
                                Some(vec![
                                        Err(ChannelUnavailable { err: "abc".to_string() }), Err(MonitorUpdateInProgress)
                                ]))
                        .expect_send(Amount::ForInvoice(final_value_msat));

                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                invoice_payer.pay_invoice(&invoice_to_pay).unwrap();
                let inflight_map = invoice_payer.create_inflight_map();

                // Only the second path, which failed with `MonitorUpdateInProgress` should be added to our
                // inflight map because retries are disabled.
                assert_eq!(inflight_map.0.len(), 2);
        }

        #[test]
        fn accounts_for_all_inflight_htlcs_sent_during_partial_failure() {
                let event_handled = core::cell::RefCell::new(false);
                let event_handler = |_: &_| { *event_handled.borrow_mut() = true; };

                let payment_preimage = PaymentPreimage([1; 32]);
                let invoice_to_pay = invoice(payment_preimage);
                let final_value_msat = invoice_to_pay.amount_milli_satoshis().unwrap();

                let retry = TestRouter::retry_for_invoice(&invoice_to_pay);
                let payer = TestPayer::new()
                        .fails_with_partial_failure(
                                retry.clone(), OnAttempt(1),
                                Some(vec![
                                        Ok(()), Err(MonitorUpdateInProgress)
                                ]))
                        .expect_send(Amount::ForInvoice(final_value_msat));

                let router = TestRouter::new(TestScorer::new());
                let logger = TestLogger::new();
                let invoice_payer =
                        InvoicePayer::new(&payer, router, &logger, event_handler, Retry::Attempts(0));

                invoice_payer.pay_invoice(&invoice_to_pay).unwrap();
                let inflight_map = invoice_payer.create_inflight_map();

                // All paths successful, hence we check of the existence of all 5 hops.
                assert_eq!(inflight_map.0.len(), 5);
        }

        struct TestRouter {
                scorer: RefCell<TestScorer>,
        }

        impl TestRouter {
                fn new(scorer: TestScorer) -> Self {
                        TestRouter { scorer: RefCell::new(scorer) }
                }

                fn route_for_value(final_value_msat: u64) -> Route {
                        Route {
                                paths: vec![
                                        vec![
                                                RouteHop {
                                                        pubkey: PublicKey::from_slice(&hex::decode("02eec7245d6b7d2ccb30380bfbe2a3648cd7a942653f5aa340edcea1f283686619").unwrap()[..]).unwrap(),
                                                        channel_features: ChannelFeatures::empty(),
                                                        node_features: NodeFeatures::empty(),
                                                        short_channel_id: 0,
                                                        fee_msat: 10,
                                                        cltv_expiry_delta: 0
                                                },
                                                RouteHop {
                                                        pubkey: PublicKey::from_slice(&hex::decode("0324653eac434488002cc06bbfb7f10fe18991e35f9fe4302dbea6d2353dc0ab1c").unwrap()[..]).unwrap(),
                                                        channel_features: ChannelFeatures::empty(),
                                                        node_features: NodeFeatures::empty(),
                                                        short_channel_id: 1,
                                                        fee_msat: 20,
                                                        cltv_expiry_delta: 0
                                                },
                                                RouteHop {
                                                        pubkey: PublicKey::from_slice(&hex::decode("027f31ebc5462c1fdce1b737ecff52d37d75dea43ce11c74d25aa297165faa2007").unwrap()[..]).unwrap(),
                                                        channel_features: ChannelFeatures::empty(),
                                                        node_features: NodeFeatures::empty(),
                                                        short_channel_id: 2,
                                                        fee_msat: final_value_msat / 2,
                                                        cltv_expiry_delta: 0
                                                },
                                        ],
                                        vec![
                                                RouteHop {
                                                        pubkey: PublicKey::from_slice(&hex::decode("029e03a901b85534ff1e92c43c74431f7ce72046060fcf7a95c37e148f78c77255").unwrap()[..]).unwrap(),
                                                        channel_features: ChannelFeatures::empty(),
                                                        node_features: NodeFeatures::empty(),
                                                        short_channel_id: 3,
                                                        fee_msat: 10,
                                                        cltv_expiry_delta: 144
                                                },
                                                RouteHop {
                                                        pubkey: PublicKey::from_slice(&hex::decode("027f31ebc5462c1fdce1b737ecff52d37d75dea43ce11c74d25aa297165faa2007").unwrap()[..]).unwrap(),
                                                        channel_features: ChannelFeatures::empty(),
                                                        node_features: NodeFeatures::empty(),
                                                        short_channel_id: 4,
                                                        fee_msat: final_value_msat / 2,
                                                        cltv_expiry_delta: 144
                                                }
                                        ],
                                ],
                                payment_params: None,
                        }
                }

                fn path_for_value(final_value_msat: u64) -> Vec<RouteHop> {
                        TestRouter::route_for_value(final_value_msat).paths[0].clone()
                }

                fn retry_for_invoice(invoice: &Invoice) -> RouteParameters {
                        let mut payment_params = PaymentParameters::from_node_id(invoice.recover_payee_pub_key())
                                .with_expiry_time(expiry_time_from_unix_epoch(invoice).as_secs())
                                .with_route_hints(invoice.route_hints());
                        if let Some(features) = invoice.features() {
                                payment_params = payment_params.with_features(features.clone());
                        }
                        let final_value_msat = invoice.amount_milli_satoshis().unwrap() / 2;
                        RouteParameters {
                                payment_params,
                                final_value_msat,
                                final_cltv_expiry_delta: invoice.min_final_cltv_expiry() as u32,
                        }
                }
        }

        impl Router for TestRouter {
                fn find_route(
                        &self, payer: &PublicKey, route_params: &RouteParameters, _payment_hash: &PaymentHash,
                        _first_hops: Option<&[&ChannelDetails]>, inflight_htlcs: InFlightHtlcs
                ) -> Result<Route, LightningError> {
                        // Simulate calling the Scorer just as you would in find_route
                        let route = Self::route_for_value(route_params.final_value_msat);
                        let mut locked_scorer = self.scorer.lock();
                        let scorer = ScorerAccountingForInFlightHtlcs::new(locked_scorer.deref_mut(), inflight_htlcs);
                        for path in route.paths {
                                let mut aggregate_msat = 0u64;
                                for (idx, hop) in path.iter().rev().enumerate() {
                                        aggregate_msat += hop.fee_msat;
                                        let usage = ChannelUsage {
                                                amount_msat: aggregate_msat,
                                                inflight_htlc_msat: 0,
                                                effective_capacity: EffectiveCapacity::Unknown,
                                        };

                                        // Since the path is reversed, the last element in our iteration is the first
                                        // hop.
                                        if idx == path.len() - 1 {
                                                scorer.channel_penalty_msat(hop.short_channel_id, &NodeId::from_pubkey(payer), &NodeId::from_pubkey(&hop.pubkey), usage);
                                        } else {
                                                scorer.channel_penalty_msat(hop.short_channel_id, &NodeId::from_pubkey(&path[idx + 1].pubkey), &NodeId::from_pubkey(&hop.pubkey), usage);
                                        }
                                }
                        }

                        Ok(Route {
                                payment_params: Some(route_params.payment_params.clone()), ..Self::route_for_value(route_params.final_value_msat)
                        })
                }

                fn notify_payment_path_failed(&self, path: &[&RouteHop], short_channel_id: u64) {
                        self.scorer.lock().payment_path_failed(path, short_channel_id);
                }

                fn notify_payment_path_successful(&self, path: &[&RouteHop]) {
                        self.scorer.lock().payment_path_successful(path);
                }

                fn notify_payment_probe_successful(&self, path: &[&RouteHop]) {
                        self.scorer.lock().probe_successful(path);
                }

                fn notify_payment_probe_failed(&self, path: &[&RouteHop], short_channel_id: u64) {
                        self.scorer.lock().probe_failed(path, short_channel_id);
                }
        }

        struct FailingRouter;

        impl Router for FailingRouter {
                fn find_route(
                        &self, _payer: &PublicKey, _params: &RouteParameters, _payment_hash: &PaymentHash,
                        _first_hops: Option<&[&ChannelDetails]>, _inflight_htlcs: InFlightHtlcs
                ) -> Result<Route, LightningError> {
                        Err(LightningError { err: String::new(), action: ErrorAction::IgnoreError })
                }

                fn notify_payment_path_failed(&self, _path: &[&RouteHop], _short_channel_id: u64) {}

                fn notify_payment_path_successful(&self, _path: &[&RouteHop]) {}

                fn notify_payment_probe_successful(&self, _path: &[&RouteHop]) {}

                fn notify_payment_probe_failed(&self, _path: &[&RouteHop], _short_channel_id: u64) {}
        }

        struct TestScorer {
                event_expectations: Option<VecDeque<TestResult>>,
                scorer_expectations: RefCell<Option<VecDeque<ChannelUsage>>>,
        }

        #[derive(Debug)]
        enum TestResult {
                PaymentFailure { path: Vec<RouteHop>, short_channel_id: u64 },
                PaymentSuccess { path: Vec<RouteHop> },
        }

        impl TestScorer {
                fn new() -> Self {
                        Self {
                                event_expectations: None,
                                scorer_expectations: RefCell::new(None),
                        }
                }

                fn expect(mut self, expectation: TestResult) -> Self {
                        self.event_expectations.get_or_insert_with(|| VecDeque::new()).push_back(expectation);
                        self
                }

                fn expect_usage(self, expectation: ChannelUsage) -> Self {
                        self.scorer_expectations.borrow_mut().get_or_insert_with(|| VecDeque::new()).push_back(expectation);
                        self
                }
        }

        #[cfg(c_bindings)]
        impl lightning::util::ser::Writeable for TestScorer {
                fn write<W: lightning::util::ser::Writer>(&self, _: &mut W) -> Result<(), lightning::io::Error> { unreachable!(); }
        }

        impl Score for TestScorer {
                fn channel_penalty_msat(
                        &self, _short_channel_id: u64, _source: &NodeId, _target: &NodeId, usage: ChannelUsage
                ) -> u64 {
                        if let Some(scorer_expectations) = self.scorer_expectations.borrow_mut().as_mut() {
                                match scorer_expectations.pop_front() {
                                        Some(expectation) => {
                                                assert_eq!(expectation.amount_msat, usage.amount_msat);
                                                assert_eq!(expectation.inflight_htlc_msat, usage.inflight_htlc_msat);
                                        },
                                        None => {},
                                }
                        }
                        0
                }

                fn payment_path_failed(&mut self, actual_path: &[&RouteHop], actual_short_channel_id: u64) {
                        if let Some(expectations) = &mut self.event_expectations {
                                match expectations.pop_front() {
                                        Some(TestResult::PaymentFailure { path, short_channel_id }) => {
                                                assert_eq!(actual_path, &path.iter().collect::<Vec<_>>()[..]);
                                                assert_eq!(actual_short_channel_id, short_channel_id);
                                        },
                                        Some(TestResult::PaymentSuccess { path }) => {
                                                panic!("Unexpected successful payment path: {:?}", path)
                                        },
                                        None => panic!("Unexpected notify_payment_path_failed call: {:?}", actual_path),
                                }
                        }
                }

                fn payment_path_successful(&mut self, actual_path: &[&RouteHop]) {
                        if let Some(expectations) = &mut self.event_expectations {
                                match expectations.pop_front() {
                                        Some(TestResult::PaymentFailure { path, .. }) => {
                                                panic!("Unexpected payment path failure: {:?}", path)
                                        },
                                        Some(TestResult::PaymentSuccess { path }) => {
                                                assert_eq!(actual_path, &path.iter().collect::<Vec<_>>()[..]);
                                        },
                                        None => panic!("Unexpected notify_payment_path_successful call: {:?}", actual_path),
                                }
                        }
                }

                fn probe_failed(&mut self, actual_path: &[&RouteHop], _: u64) {
                        if let Some(expectations) = &mut self.event_expectations {
                                match expectations.pop_front() {
                                        Some(TestResult::PaymentFailure { path, .. }) => {
                                                panic!("Unexpected failed payment path: {:?}", path)
                                        },
                                        Some(TestResult::PaymentSuccess { path }) => {
                                                panic!("Unexpected successful payment path: {:?}", path)
                                        },
                                        None => panic!("Unexpected notify_payment_path_failed call: {:?}", actual_path),
                                }
                        }
                }
                fn probe_successful(&mut self, actual_path: &[&RouteHop]) {
                        if let Some(expectations) = &mut self.event_expectations {
                                match expectations.pop_front() {
                                        Some(TestResult::PaymentFailure { path, .. }) => {
                                                panic!("Unexpected payment path failure: {:?}", path)
                                        },
                                        Some(TestResult::PaymentSuccess { path }) => {
                                                panic!("Unexpected successful payment path: {:?}", path)
                                        },
                                        None => panic!("Unexpected notify_payment_path_successful call: {:?}", actual_path),
                                }
                        }
                }
        }

        impl Drop for TestScorer {
                fn drop(&mut self) {
                        if std::thread::panicking() {
                                return;
                        }

                        if let Some(event_expectations) = &self.event_expectations {
                                if !event_expectations.is_empty() {
                                        panic!("Unsatisfied event expectations: {:?}", event_expectations);
                                }
                        }

                        if let Some(scorer_expectations) = self.scorer_expectations.borrow().as_ref() {
                                if !scorer_expectations.is_empty() {
                                        panic!("Unsatisfied scorer expectations: {:?}", scorer_expectations)
                                }
                        }
                }
        }

        struct TestPayer {
                expectations: core::cell::RefCell<VecDeque<Amount>>,
                attempts: core::cell::RefCell<usize>,
                failing_on_attempt: core::cell::RefCell<HashMap<usize, PaymentSendFailure>>,
        }

        #[derive(Clone, Debug, PartialEq, Eq)]
        enum Amount {
                ForInvoice(u64),
                Spontaneous(u64),
                OnRetry(u64),
        }

        struct OnAttempt(usize);

        impl TestPayer {
                fn new() -> Self {
                        Self {
                                expectations: core::cell::RefCell::new(VecDeque::new()),
                                attempts: core::cell::RefCell::new(0),
                                failing_on_attempt: core::cell::RefCell::new(HashMap::new()),
                        }
                }

                fn expect_send(self, value_msat: Amount) -> Self {
                        self.expectations.borrow_mut().push_back(value_msat);
                        self
                }

                fn fails_on_attempt(self, attempt: usize) -> Self {
                        let failure = PaymentSendFailure::ParameterError(APIError::MonitorUpdateInProgress);
                        self.fails_with(failure, OnAttempt(attempt))
                }

                fn fails_with_partial_failure(self, retry: RouteParameters, attempt: OnAttempt, results: Option<Vec<Result<(), APIError>>>) -> Self {
                        self.fails_with(PaymentSendFailure::PartialFailure {
                                results: results.unwrap_or(vec![]),
                                failed_paths_retry: Some(retry),
                                payment_id: PaymentId([1; 32]),
                        }, attempt)
                }

                fn fails_with(self, failure: PaymentSendFailure, attempt: OnAttempt) -> Self {
                        self.failing_on_attempt.borrow_mut().insert(attempt.0, failure);
                        self
                }

                fn check_attempts(&self) -> Result<(), PaymentSendFailure> {
                        let mut attempts = self.attempts.borrow_mut();
                        *attempts += 1;

                        match self.failing_on_attempt.borrow_mut().remove(&*attempts) {
                                Some(failure) => Err(failure),
                                None => Ok(())
                        }
                }

                fn check_value_msats(&self, actual_value_msats: Amount) {
                        let expected_value_msats = self.expectations.borrow_mut().pop_front();
                        if let Some(expected_value_msats) = expected_value_msats {
                                assert_eq!(actual_value_msats, expected_value_msats);
                        } else {
                                panic!("Unexpected amount: {:?}", actual_value_msats);
                        }
                }
        }

        impl Drop for TestPayer {
                fn drop(&mut self) {
                        if std::thread::panicking() {
                                return;
                        }

                        if !self.expectations.borrow().is_empty() {
                                panic!("Unsatisfied payment expectations: {:?}", self.expectations.borrow());
                        }
                }
        }

        impl Payer for TestPayer {
                fn node_id(&self) -> PublicKey {
                        let secp_ctx = Secp256k1::new();
                        PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap())
                }

                fn first_hops(&self) -> Vec<ChannelDetails> {
                        Vec::new()
                }

                fn send_payment(
                        &self, route: &Route, _payment_hash: PaymentHash,
                        _payment_secret: &Option<PaymentSecret>, _payment_id: PaymentId,
                ) -> Result<(), PaymentSendFailure> {
                        self.check_value_msats(Amount::ForInvoice(route.get_total_amount()));
                        self.check_attempts()
                }

                fn send_spontaneous_payment(
                        &self, route: &Route, _payment_preimage: PaymentPreimage, _payment_id: PaymentId,
                ) -> Result<(), PaymentSendFailure> {
                        self.check_value_msats(Amount::Spontaneous(route.get_total_amount()));
                        self.check_attempts()
                }

                fn retry_payment(
                        &self, route: &Route, _payment_id: PaymentId
                ) -> Result<(), PaymentSendFailure> {
                        self.check_value_msats(Amount::OnRetry(route.get_total_amount()));
                        self.check_attempts()
                }

                fn abandon_payment(&self, _payment_id: PaymentId) { }
        }

        // *** Full Featured Functional Tests with a Real ChannelManager ***
        struct ManualRouter(RefCell<VecDeque<Result<Route, LightningError>>>);

        impl Router for ManualRouter {
                fn find_route(
                        &self, _payer: &PublicKey, _params: &RouteParameters, _payment_hash: &PaymentHash,
                        _first_hops: Option<&[&ChannelDetails]>, _inflight_htlcs: InFlightHtlcs
                ) -> Result<Route, LightningError> {
                        self.0.borrow_mut().pop_front().unwrap()
                }

                fn notify_payment_path_failed(&self, _path: &[&RouteHop], _short_channel_id: u64) {}

                fn notify_payment_path_successful(&self, _path: &[&RouteHop]) {}

                fn notify_payment_probe_successful(&self, _path: &[&RouteHop]) {}

                fn notify_payment_probe_failed(&self, _path: &[&RouteHop], _short_channel_id: u64) {}
        }
        impl ManualRouter {
                fn expect_find_route(&self, result: Result<Route, LightningError>) {
                        self.0.borrow_mut().push_back(result);
                }
        }
        impl Drop for ManualRouter {
                fn drop(&mut self) {
                        if std::thread::panicking() {
                                return;
                        }
                        assert!(self.0.borrow_mut().is_empty());
                }
        }

        #[test]
        fn retry_multi_path_single_failed_payment() {
                // Tests that we can/will retry after a single path of an MPP payment failed immediately
                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, None]);
                let nodes = create_network(2, &node_cfgs, &node_chanmgrs);

                create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
                create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
                let chans = nodes[0].node.list_usable_channels();
                let mut route = Route {
                        paths: vec![
                                vec![RouteHop {
                                        pubkey: nodes[1].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chans[0].short_channel_id.unwrap(),
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 10_000,
                                        cltv_expiry_delta: 100,
                                }],
                                vec![RouteHop {
                                        pubkey: nodes[1].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chans[1].short_channel_id.unwrap(),
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 100_000_001, // Our default max-HTLC-value is 10% of the channel value, which this is one more than
                                        cltv_expiry_delta: 100,
                                }],
                        ],
                        payment_params: Some(PaymentParameters::from_node_id(nodes[1].node.get_our_node_id())),
                };
                let router = ManualRouter(RefCell::new(VecDeque::new()));
                router.expect_find_route(Ok(route.clone()));
                // On retry, split the payment across both channels.
                route.paths[0][0].fee_msat = 50_000_001;
                route.paths[1][0].fee_msat = 50_000_000;
                router.expect_find_route(Ok(route.clone()));

                let event_handler = |_: &_| { panic!(); };
                let invoice_payer = InvoicePayer::new(nodes[0].node, router, nodes[0].logger, event_handler, Retry::Attempts(1));

                assert!(invoice_payer.pay_invoice(&create_invoice_from_channelmanager_and_duration_since_epoch(
                        &nodes[1].node, nodes[1].keys_manager, nodes[1].logger, Currency::Bitcoin,
                        Some(100_010_000), "Invoice".to_string(), duration_since_epoch(), 3600).unwrap())
                        .is_ok());
                let htlc_msgs = nodes[0].node.get_and_clear_pending_msg_events();
                assert_eq!(htlc_msgs.len(), 2);
                check_added_monitors!(nodes[0], 2);
        }

        #[test]
        fn immediate_retry_on_failure() {
                // Tests that we can/will retry immediately after a failure
                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, None]);
                let nodes = create_network(2, &node_cfgs, &node_chanmgrs);

                create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
                create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 1_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features());
                let chans = nodes[0].node.list_usable_channels();
                let mut route = Route {
                        paths: vec![
                                vec![RouteHop {
                                        pubkey: nodes[1].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chans[0].short_channel_id.unwrap(),
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 100_000_001, // Our default max-HTLC-value is 10% of the channel value, which this is one more than
                                        cltv_expiry_delta: 100,
                                }],
                        ],
                        payment_params: Some(PaymentParameters::from_node_id(nodes[1].node.get_our_node_id())),
                };
                let router = ManualRouter(RefCell::new(VecDeque::new()));
                router.expect_find_route(Ok(route.clone()));
                // On retry, split the payment across both channels.
                route.paths.push(route.paths[0].clone());
                route.paths[0][0].short_channel_id = chans[1].short_channel_id.unwrap();
                route.paths[0][0].fee_msat = 50_000_000;
                route.paths[1][0].fee_msat = 50_000_001;
                router.expect_find_route(Ok(route.clone()));

                let event_handler = |_: &_| { panic!(); };
                let invoice_payer = InvoicePayer::new(nodes[0].node, router, nodes[0].logger, event_handler, Retry::Attempts(1));

                assert!(invoice_payer.pay_invoice(&create_invoice_from_channelmanager_and_duration_since_epoch(
                        &nodes[1].node, nodes[1].keys_manager, nodes[1].logger, Currency::Bitcoin,
                        Some(100_010_000), "Invoice".to_string(), duration_since_epoch(), 3600).unwrap())
                        .is_ok());
                let htlc_msgs = nodes[0].node.get_and_clear_pending_msg_events();
                assert_eq!(htlc_msgs.len(), 2);
                check_added_monitors!(nodes[0], 2);
        }

        #[test]
        fn no_extra_retries_on_back_to_back_fail() {
                // In a previous release, we had a race where we may exceed the payment retry count if we
                // get two failures in a row with the second having `all_paths_failed` set.
                // Generally, when we give up trying to retry a payment, we don't know for sure what the
                // current state of the ChannelManager event queue is. Specifically, we cannot be sure that
                // there are not multiple additional `PaymentPathFailed` or even `PaymentSent` events
                // pending which we will see later. Thus, when we previously removed the retry tracking map
                // entry after a `all_paths_failed` `PaymentPathFailed` event, we may have dropped the
                // retry entry even though more events for the same payment were still pending. This led to
                // us retrying a payment again even though we'd already given up on it.
                //
                // We now have a separate event - `PaymentFailed` which indicates no HTLCs remain and which
                // is used to remove the payment retry counter entries instead. This tests for the specific
                // excess-retry case while also testing `PaymentFailed` generation.

                let chanmon_cfgs = create_chanmon_cfgs(3);
                let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
                let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, None, None]);
                let nodes = create_network(3, &node_cfgs, &node_chanmgrs);

                let chan_1_scid = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 10_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features()).0.contents.short_channel_id;
                let chan_2_scid = create_announced_chan_between_nodes_with_value(&nodes, 1, 2, 10_000_000, 0, channelmanager::provided_init_features(), channelmanager::provided_init_features()).0.contents.short_channel_id;

                let mut route = Route {
                        paths: vec![
                                vec![RouteHop {
                                        pubkey: nodes[1].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chan_1_scid,
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 0,
                                        cltv_expiry_delta: 100,
                                }, RouteHop {
                                        pubkey: nodes[2].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chan_2_scid,
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 100_000_000,
                                        cltv_expiry_delta: 100,
                                }],
                                vec![RouteHop {
                                        pubkey: nodes[1].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chan_1_scid,
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 0,
                                        cltv_expiry_delta: 100,
                                }, RouteHop {
                                        pubkey: nodes[2].node.get_our_node_id(),
                                        node_features: channelmanager::provided_node_features(),
                                        short_channel_id: chan_2_scid,
                                        channel_features: channelmanager::provided_channel_features(),
                                        fee_msat: 100_000_000,
                                        cltv_expiry_delta: 100,
                                }]
                        ],
                        payment_params: Some(PaymentParameters::from_node_id(nodes[2].node.get_our_node_id())),
                };
                let router = ManualRouter(RefCell::new(VecDeque::new()));
                router.expect_find_route(Ok(route.clone()));
                // On retry, we'll only be asked for one path
                route.paths.remove(1);
                router.expect_find_route(Ok(route.clone()));

                let expected_events: RefCell<VecDeque<&dyn Fn(&Event)>> = RefCell::new(VecDeque::new());
                let event_handler = |event: &Event| {
                        let event_checker = expected_events.borrow_mut().pop_front().unwrap();
                        event_checker(event);
                };
                let invoice_payer = InvoicePayer::new(nodes[0].node, router, nodes[0].logger, event_handler, Retry::Attempts(1));

                assert!(invoice_payer.pay_invoice(&create_invoice_from_channelmanager_and_duration_since_epoch(
                        &nodes[1].node, nodes[1].keys_manager, nodes[1].logger, Currency::Bitcoin,
                        Some(100_010_000), "Invoice".to_string(), duration_since_epoch(), 3600).unwrap())
                        .is_ok());
                let htlc_updates = SendEvent::from_node(&nodes[0]);
                check_added_monitors!(nodes[0], 1);
                assert_eq!(htlc_updates.msgs.len(), 1);

                nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &htlc_updates.msgs[0]);
                nodes[1].node.handle_commitment_signed(&nodes[0].node.get_our_node_id(), &htlc_updates.commitment_msg);
                check_added_monitors!(nodes[1], 1);
                let (bs_first_raa, bs_first_cs) = get_revoke_commit_msgs!(nodes[1], nodes[0].node.get_our_node_id());

                nodes[0].node.handle_revoke_and_ack(&nodes[1].node.get_our_node_id(), &bs_first_raa);
                check_added_monitors!(nodes[0], 1);
                let second_htlc_updates = SendEvent::from_node(&nodes[0]);

                nodes[0].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &bs_first_cs);
                check_added_monitors!(nodes[0], 1);
                let as_first_raa = get_event_msg!(nodes[0], MessageSendEvent::SendRevokeAndACK, nodes[1].node.get_our_node_id());

                nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &second_htlc_updates.msgs[0]);
                nodes[1].node.handle_commitment_signed(&nodes[0].node.get_our_node_id(), &second_htlc_updates.commitment_msg);
                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[1].node.handle_revoke_and_ack(&nodes[0].node.get_our_node_id(), &as_first_raa);
                check_added_monitors!(nodes[1], 1);
                let bs_fail_update = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());

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

                nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &bs_fail_update.update_fail_htlcs[0]);
                nodes[0].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &bs_fail_update.commitment_signed);
                check_added_monitors!(nodes[0], 1);
                let (as_second_raa, as_third_cs) = 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);
                let bs_second_fail_update = 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_third_cs);
                check_added_monitors!(nodes[1], 1);
                let bs_third_raa = get_event_msg!(nodes[1], MessageSendEvent::SendRevokeAndACK, nodes[0].node.get_our_node_id());

                nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &bs_second_fail_update.update_fail_htlcs[0]);
                nodes[0].node.handle_commitment_signed(&nodes[1].node.get_our_node_id(), &bs_second_fail_update.commitment_signed);
                check_added_monitors!(nodes[0], 1);

                nodes[0].node.handle_revoke_and_ack(&nodes[1].node.get_our_node_id(), &bs_third_raa);
                check_added_monitors!(nodes[0], 1);
                let (as_third_raa, as_fourth_cs) = 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_third_raa);
                check_added_monitors!(nodes[1], 1);
                nodes[1].node.handle_commitment_signed(&nodes[0].node.get_our_node_id(), &as_fourth_cs);
                check_added_monitors!(nodes[1], 1);
                let bs_fourth_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_fourth_raa);
                check_added_monitors!(nodes[0], 1);

                // At this point A has sent two HTLCs which both failed due to lack of fee. It now has two
                // pending `PaymentPathFailed` events, one with `all_paths_failed` unset, and the second
                // with it set. The first event will use up the only retry we are allowed, with the second
                // `PaymentPathFailed` being passed up to the user (us, in this case). Previously, we'd
                // treated this as "HTLC complete" and dropped the retry counter, causing us to retry again
                // if the final HTLC failed.
                expected_events.borrow_mut().push_back(&|ev: &Event| {
                        if let Event::PaymentPathFailed { payment_failed_permanently, all_paths_failed, .. } = ev {
                                assert!(!payment_failed_permanently);
                                assert!(all_paths_failed);
                        } else { panic!("Unexpected event"); }
                });
                nodes[0].node.process_pending_events(&invoice_payer);
                assert!(expected_events.borrow().is_empty());

                let retry_htlc_updates = SendEvent::from_node(&nodes[0]);
                check_added_monitors!(nodes[0], 1);

                nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &retry_htlc_updates.msgs[0]);
                commitment_signed_dance!(nodes[1], nodes[0], &retry_htlc_updates.commitment_msg, false, true);
                let bs_fail_update = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
                nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &bs_fail_update.update_fail_htlcs[0]);
                commitment_signed_dance!(nodes[0], nodes[1], &bs_fail_update.commitment_signed, false, true);

                expected_events.borrow_mut().push_back(&|ev: &Event| {
                        if let Event::PaymentPathFailed { payment_failed_permanently, all_paths_failed, .. } = ev {
                                assert!(!payment_failed_permanently);
                                assert!(all_paths_failed);
                        } else { panic!("Unexpected event"); }
                });
                expected_events.borrow_mut().push_back(&|ev: &Event| {
                        if let Event::PaymentFailed { .. } = ev {
                        } else { panic!("Unexpected event"); }
                });
                nodes[0].node.process_pending_events(&invoice_payer);
                assert!(expected_events.borrow().is_empty());
        }
}