[rust-lightning] / lightning / src / ln / outbound_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.

//! Utilities to send payments and manage outbound payment information.

use bitcoin::hashes::Hash;
use bitcoin::hashes::sha256::Hash as Sha256;
use bitcoin::secp256k1::{self, Secp256k1, SecretKey};

use crate::chain::keysinterface::{EntropySource, NodeSigner, Recipient};
use crate::events::{self, PaymentFailureReason};
use crate::ln::{PaymentHash, PaymentPreimage, PaymentSecret};
use crate::ln::channelmanager::{ChannelDetails, HTLCSource, IDEMPOTENCY_TIMEOUT_TICKS, PaymentId};
use crate::ln::onion_utils::HTLCFailReason;
use crate::routing::router::{InFlightHtlcs, PaymentParameters, Route, RouteHop, RouteParameters, RoutePath, Router};
use crate::util::errors::APIError;
use crate::util::logger::Logger;
use crate::util::time::Time;
#[cfg(all(not(feature = "no-std"), test))]
use crate::util::time::tests::SinceEpoch;
use crate::util::ser::ReadableArgs;

use core::cmp;
use core::fmt::{self, Display, Formatter};
use core::ops::Deref;

use crate::prelude::*;
use crate::sync::Mutex;

/// Stores the session_priv for each part of a payment that is still pending. For versions 0.0.102
/// and later, also stores information for retrying the payment.
pub(crate) enum PendingOutboundPayment {
        Legacy {
                session_privs: HashSet<[u8; 32]>,
        },
        Retryable {
                retry_strategy: Option<Retry>,
                attempts: PaymentAttempts,
                payment_params: Option<PaymentParameters>,
                session_privs: HashSet<[u8; 32]>,
                payment_hash: PaymentHash,
                payment_secret: Option<PaymentSecret>,
                payment_metadata: Option<Vec<u8>>,
                keysend_preimage: Option<PaymentPreimage>,
                pending_amt_msat: u64,
                /// Used to track the fee paid. Only present if the payment was serialized on 0.0.103+.
                pending_fee_msat: Option<u64>,
                /// The total payment amount across all paths, used to verify that a retry is not overpaying.
                total_msat: u64,
                /// Our best known block height at the time this payment was initiated.
                starting_block_height: u32,
        },
        /// When a pending payment is fulfilled, we continue tracking it until all pending HTLCs have
        /// been resolved. This ensures we don't look up pending payments in ChannelMonitors on restart
        /// and add a pending payment that was already fulfilled.
        Fulfilled {
                session_privs: HashSet<[u8; 32]>,
                payment_hash: Option<PaymentHash>,
                timer_ticks_without_htlcs: u8,
        },
        /// When we've decided to give up retrying a payment, we mark it as abandoned so we can eventually
        /// generate a `PaymentFailed` event when all HTLCs have irrevocably failed.
        Abandoned {
                session_privs: HashSet<[u8; 32]>,
                payment_hash: PaymentHash,
                /// Will be `None` if the payment was serialized before 0.0.115.
                reason: Option<PaymentFailureReason>,
        },
}

impl PendingOutboundPayment {
        fn increment_attempts(&mut self) {
                if let PendingOutboundPayment::Retryable { attempts, .. } = self {
                        attempts.count += 1;
                }
        }
        fn is_auto_retryable_now(&self) -> bool {
                match self {
                        PendingOutboundPayment::Retryable {
                                retry_strategy: Some(strategy), attempts, payment_params: Some(_), ..
                        } => {
                                strategy.is_retryable_now(&attempts)
                        },
                        _ => false,
                }
        }
        fn is_retryable_now(&self) -> bool {
                match self {
                        PendingOutboundPayment::Retryable { retry_strategy: None, .. } => {
                                // We're handling retries manually, we can always retry.
                                true
                        },
                        PendingOutboundPayment::Retryable { retry_strategy: Some(strategy), attempts, .. } => {
                                strategy.is_retryable_now(&attempts)
                        },
                        _ => false,
                }
        }
        pub fn insert_previously_failed_scid(&mut self, scid: u64) {
                if let PendingOutboundPayment::Retryable { payment_params: Some(params), .. } = self {
                        params.previously_failed_channels.push(scid);
                }
        }
        pub(super) fn is_fulfilled(&self) -> bool {
                match self {
                        PendingOutboundPayment::Fulfilled { .. } => true,
                        _ => false,
                }
        }
        pub(super) fn abandoned(&self) -> bool {
                match self {
                        PendingOutboundPayment::Abandoned { .. } => true,
                        _ => false,
                }
        }
        fn get_pending_fee_msat(&self) -> Option<u64> {
                match self {
                        PendingOutboundPayment::Retryable { pending_fee_msat, .. } => pending_fee_msat.clone(),
                        _ => None,
                }
        }

        fn payment_hash(&self) -> Option<PaymentHash> {
                match self {
                        PendingOutboundPayment::Legacy { .. } => None,
                        PendingOutboundPayment::Retryable { payment_hash, .. } => Some(*payment_hash),
                        PendingOutboundPayment::Fulfilled { payment_hash, .. } => *payment_hash,
                        PendingOutboundPayment::Abandoned { payment_hash, .. } => Some(*payment_hash),
                }
        }

        fn mark_fulfilled(&mut self) {
                let mut session_privs = HashSet::new();
                core::mem::swap(&mut session_privs, match self {
                        PendingOutboundPayment::Legacy { session_privs } |
                                PendingOutboundPayment::Retryable { session_privs, .. } |
                                PendingOutboundPayment::Fulfilled { session_privs, .. } |
                                PendingOutboundPayment::Abandoned { session_privs, .. }
                        => session_privs,
                });
                let payment_hash = self.payment_hash();
                *self = PendingOutboundPayment::Fulfilled { session_privs, payment_hash, timer_ticks_without_htlcs: 0 };
        }

        fn mark_abandoned(&mut self, reason: PaymentFailureReason) {
                if let PendingOutboundPayment::Retryable { session_privs, payment_hash, .. } = self {
                        let mut our_session_privs = HashSet::new();
                        core::mem::swap(&mut our_session_privs, session_privs);
                        *self = PendingOutboundPayment::Abandoned {
                                session_privs: our_session_privs,
                                payment_hash: *payment_hash,
                                reason: Some(reason)
                        };
                }
        }

        /// panics if path is None and !self.is_fulfilled
        fn remove(&mut self, session_priv: &[u8; 32], path: Option<&Vec<RouteHop>>) -> bool {
                let remove_res = match self {
                        PendingOutboundPayment::Legacy { session_privs } |
                                PendingOutboundPayment::Retryable { session_privs, .. } |
                                PendingOutboundPayment::Fulfilled { session_privs, .. } |
                                PendingOutboundPayment::Abandoned { session_privs, .. } => {
                                        session_privs.remove(session_priv)
                                }
                };
                if remove_res {
                        if let PendingOutboundPayment::Retryable { ref mut pending_amt_msat, ref mut pending_fee_msat, .. } = self {
                                let path = path.expect("Fulfilling a payment should always come with a path");
                                let path_last_hop = path.last().expect("Outbound payments must have had a valid path");
                                *pending_amt_msat -= path_last_hop.fee_msat;
                                if let Some(fee_msat) = pending_fee_msat.as_mut() {
                                        *fee_msat -= path.get_path_fees();
                                }
                        }
                }
                remove_res
        }

        pub(super) fn insert(&mut self, session_priv: [u8; 32], path: &Vec<RouteHop>) -> bool {
                let insert_res = match self {
                        PendingOutboundPayment::Legacy { session_privs } |
                                PendingOutboundPayment::Retryable { session_privs, .. } => {
                                        session_privs.insert(session_priv)
                                }
                        PendingOutboundPayment::Fulfilled { .. } => false,
                        PendingOutboundPayment::Abandoned { .. } => false,
                };
                if insert_res {
                        if let PendingOutboundPayment::Retryable { ref mut pending_amt_msat, ref mut pending_fee_msat, .. } = self {
                                let path_last_hop = path.last().expect("Outbound payments must have had a valid path");
                                *pending_amt_msat += path_last_hop.fee_msat;
                                if let Some(fee_msat) = pending_fee_msat.as_mut() {
                                        *fee_msat += path.get_path_fees();
                                }
                        }
                }
                insert_res
        }

        pub(super) fn remaining_parts(&self) -> usize {
                match self {
                        PendingOutboundPayment::Legacy { session_privs } |
                                PendingOutboundPayment::Retryable { session_privs, .. } |
                                PendingOutboundPayment::Fulfilled { session_privs, .. } |
                                PendingOutboundPayment::Abandoned { session_privs, .. } => {
                                        session_privs.len()
                                }
                }
        }
}

/// Strategies available to retry payment path failures.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub enum Retry {
        /// Max number of attempts to retry payment.
        ///
        /// Each attempt may be multiple HTLCs along multiple paths if the router decides to split up a
        /// retry, and may retry multiple failed HTLCs at once if they failed around the same time and
        /// were retried along a route from a single call to [`Router::find_route_with_id`].
        Attempts(usize),
        #[cfg(not(feature = "no-std"))]
        /// Time elapsed before abandoning retries for a payment. At least one attempt at payment is made;
        /// see [`PaymentParameters::expiry_time`] to avoid any attempt at payment after a specific time.
        ///
        /// [`PaymentParameters::expiry_time`]: crate::routing::router::PaymentParameters::expiry_time
        Timeout(core::time::Duration),
}

impl Retry {
        pub(crate) fn is_retryable_now(&self, attempts: &PaymentAttempts) -> bool {
                match (self, attempts) {
                        (Retry::Attempts(max_retry_count), PaymentAttempts { count, .. }) => {
                                max_retry_count > count
                        },
                        #[cfg(all(not(feature = "no-std"), not(test)))]
                        (Retry::Timeout(max_duration), PaymentAttempts { first_attempted_at, .. }) =>
                                *max_duration >= std::time::Instant::now().duration_since(*first_attempted_at),
                        #[cfg(all(not(feature = "no-std"), test))]
                        (Retry::Timeout(max_duration), PaymentAttempts { first_attempted_at, .. }) =>
                                *max_duration >= SinceEpoch::now().duration_since(*first_attempted_at),
                }
        }
}

#[cfg(feature = "std")]
pub(super) fn has_expired(route_params: &RouteParameters) -> bool {
        if let Some(expiry_time) = route_params.payment_params.expiry_time {
                if let Ok(elapsed) = std::time::SystemTime::UNIX_EPOCH.elapsed() {
                        return elapsed > core::time::Duration::from_secs(expiry_time)
                }
        }
        false
}

pub(crate) type PaymentAttempts = PaymentAttemptsUsingTime<ConfiguredTime>;

/// Storing minimal payment attempts information required for determining if a outbound payment can
/// be retried.
pub(crate) struct PaymentAttemptsUsingTime<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 not known yet.
        pub(crate) count: usize,
        /// This field is only used when retry is `Retry::Timeout` which is only build with feature std
        #[cfg(not(feature = "no-std"))]
        first_attempted_at: T,
        #[cfg(feature = "no-std")]
        phantom: core::marker::PhantomData<T>,

}

#[cfg(not(any(feature = "no-std", test)))]
type ConfiguredTime = std::time::Instant;
#[cfg(feature = "no-std")]
type ConfiguredTime = crate::util::time::Eternity;
#[cfg(all(not(feature = "no-std"), test))]
type ConfiguredTime = SinceEpoch;

impl<T: Time> PaymentAttemptsUsingTime<T> {
        pub(crate) fn new() -> Self {
                PaymentAttemptsUsingTime {
                        count: 0,
                        #[cfg(not(feature = "no-std"))]
                        first_attempted_at: T::now(),
                        #[cfg(feature = "no-std")]
                        phantom: core::marker::PhantomData,
                }
        }
}

impl<T: Time> Display for PaymentAttemptsUsingTime<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()
                );
        }
}

/// Indicates an immediate error on [`ChannelManager::send_payment`]. Further errors may be
/// surfaced later via [`Event::PaymentPathFailed`] and [`Event::PaymentFailed`].
///
/// [`ChannelManager::send_payment`]: crate::ln::channelmanager::ChannelManager::send_payment
/// [`Event::PaymentPathFailed`]: crate::events::Event::PaymentPathFailed
/// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
#[derive(Clone, Debug)]
pub enum RetryableSendFailure {
        /// The provided [`PaymentParameters::expiry_time`] indicated that the payment has expired. Note
        /// that this error is *not* caused by [`Retry::Timeout`].
        ///
        /// [`PaymentParameters::expiry_time`]: crate::routing::router::PaymentParameters::expiry_time
        PaymentExpired,
        /// We were unable to find a route to the destination.
        RouteNotFound,
        /// Indicates that a payment for the provided [`PaymentId`] is already in-flight and has not
        /// yet completed (i.e. generated an [`Event::PaymentSent`] or [`Event::PaymentFailed`]).
        ///
        /// [`PaymentId`]: crate::ln::channelmanager::PaymentId
        /// [`Event::PaymentSent`]: crate::events::Event::PaymentSent
        /// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
        DuplicatePayment,
}

/// If a payment fails to send with [`ChannelManager::send_payment_with_route`], it can be in one
/// of several states. This enum is returned as the Err() type describing which state the payment
/// is in, see the description of individual enum states for more.
///
/// [`ChannelManager::send_payment_with_route`]: crate::ln::channelmanager::ChannelManager::send_payment_with_route
#[derive(Clone, Debug)]
pub enum PaymentSendFailure {
        /// A parameter which was passed to send_payment was invalid, preventing us from attempting to
        /// send the payment at all.
        ///
        /// You can freely resend the payment in full (with the parameter error fixed).
        ///
        /// Because the payment failed outright, no payment tracking is done and no
        /// [`Event::PaymentPathFailed`] or [`Event::PaymentFailed`] events will be generated.
        ///
        /// [`Event::PaymentPathFailed`]: crate::events::Event::PaymentPathFailed
        /// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
        ParameterError(APIError),
        /// A parameter in a single path which was passed to send_payment was invalid, preventing us
        /// from attempting to send the payment at all.
        ///
        /// You can freely resend the payment in full (with the parameter error fixed).
        ///
        /// Because the payment failed outright, no payment tracking is done and no
        /// [`Event::PaymentPathFailed`] or [`Event::PaymentFailed`] events will be generated.
        ///
        /// The results here are ordered the same as the paths in the route object which was passed to
        /// send_payment.
        ///
        /// [`Event::PaymentPathFailed`]: crate::events::Event::PaymentPathFailed
        /// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
        PathParameterError(Vec<Result<(), APIError>>),
        /// All paths which were attempted failed to send, with no channel state change taking place.
        /// You can freely resend the payment in full (though you probably want to do so over different
        /// paths than the ones selected).
        ///
        /// Because the payment failed outright, no payment tracking is done and no
        /// [`Event::PaymentPathFailed`] or [`Event::PaymentFailed`] events will be generated.
        ///
        /// [`Event::PaymentPathFailed`]: crate::events::Event::PaymentPathFailed
        /// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
        AllFailedResendSafe(Vec<APIError>),
        /// Indicates that a payment for the provided [`PaymentId`] is already in-flight and has not
        /// yet completed (i.e. generated an [`Event::PaymentSent`] or [`Event::PaymentFailed`]).
        ///
        /// [`PaymentId`]: crate::ln::channelmanager::PaymentId
        /// [`Event::PaymentSent`]: crate::events::Event::PaymentSent
        /// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
        DuplicatePayment,
        /// Some paths that were attempted failed to send, though some paths may have succeeded. At least
        /// some paths have irrevocably committed to the HTLC.
        ///
        /// The results here are ordered the same as the paths in the route object that was passed to
        /// send_payment.
        ///
        /// Any entries that contain `Err(APIError::MonitorUpdateInprogress)` will send once a
        /// [`MonitorEvent::Completed`] is provided for the next-hop channel with the latest update_id.
        ///
        /// [`MonitorEvent::Completed`]: crate::chain::channelmonitor::MonitorEvent::Completed
        PartialFailure {
                /// The errors themselves, in the same order as the paths from the route.
                results: Vec<Result<(), APIError>>,
                /// If some paths failed without irrevocably committing to the new HTLC(s), this will
                /// contain a [`RouteParameters`] object for the failing paths.
                failed_paths_retry: Option<RouteParameters>,
                /// The payment id for the payment, which is now at least partially pending.
                payment_id: PaymentId,
        },
}

/// Information which is provided, encrypted, to the payment recipient when sending HTLCs.
///
/// This should generally be constructed with data communicated to us from the recipient (via a
/// BOLT11 or BOLT12 invoice).
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct RecipientOnionFields {
        /// The [`PaymentSecret`] is an arbitrary 32 bytes provided by the recipient for us to repeat
        /// in the onion. It is unrelated to `payment_hash` (or [`PaymentPreimage`]) and exists to
        /// authenticate the sender to the recipient and prevent payment-probing (deanonymization)
        /// attacks.
        ///
        /// If you do not have one, the [`Route`] you pay over must not contain multiple paths as
        /// multi-path payments require a recipient-provided secret.
        ///
        /// Note that for spontaneous payments most lightning nodes do not currently support MPP
        /// receives, thus you should generally never be providing a secret here for spontaneous
        /// payments.
        pub payment_secret: Option<PaymentSecret>,
        /// The payment metadata serves a similar purpose as [`Self::payment_secret`] but is of
        /// arbitrary length. This gives recipients substantially more flexibility to receive
        /// additional data.
        ///
        /// In LDK, while the [`Self::payment_secret`] is fixed based on an internal authentication
        /// scheme to authenticate received payments against expected payments and invoices, this field
        /// is not used in LDK for received payments, and can be used to store arbitrary data in
        /// invoices which will be received with the payment.
        ///
        /// Note that this field was added to the lightning specification more recently than
        /// [`Self::payment_secret`] and while nearly all lightning senders support secrets, metadata
        /// may not be supported as universally.
        pub payment_metadata: Option<Vec<u8>>,
}

impl_writeable_tlv_based!(RecipientOnionFields, {
        (0, payment_secret, option),
        (2, payment_metadata, option),
});

impl RecipientOnionFields {
        /// Creates a [`RecipientOnionFields`] from only a [`PaymentSecret`]. This is the most common
        /// set of onion fields for today's BOLT11 invoices - most nodes require a [`PaymentSecret`]
        /// but do not require or provide any further data.
        pub fn secret_only(payment_secret: PaymentSecret) -> Self {
                Self { payment_secret: Some(payment_secret), payment_metadata: None }
        }

        /// Creates a new [`RecipientOnionFields`] with no fields. This generally does not create
        /// payable HTLCs except for spontaneous payments, i.e. this should generally only be used for
        /// calls to [`ChannelManager::send_spontaneous_payment`].
        ///
        /// [`ChannelManager::send_spontaneous_payment`]: super::channelmanager::ChannelManager::send_spontaneous_payment
        pub fn spontaneous_empty() -> Self {
                Self { payment_secret: None, payment_metadata: None }
        }

        /// When we have received some HTLC(s) towards an MPP payment, as we receive further HTLC(s) we
        /// have to make sure that some fields match exactly across the parts. For those that aren't
        /// required to match, if they don't match we should remove them so as to not expose data
        /// that's dependent on the HTLC receive order to users.
        ///
        /// Here we implement this, first checking compatibility then mutating two objects and then
        /// dropping any remaining non-matching fields from both.
        pub(super) fn check_merge(&mut self, further_htlc_fields: &mut Self) -> Result<(), ()> {
                if self.payment_secret != further_htlc_fields.payment_secret { return Err(()); }
                if self.payment_metadata != further_htlc_fields.payment_metadata { return Err(()); }
                // For custom TLVs we should just drop non-matching ones, but not reject the payment.
                Ok(())
        }
}

pub(super) struct OutboundPayments {
        pub(super) pending_outbound_payments: Mutex<HashMap<PaymentId, PendingOutboundPayment>>,
        pub(super) retry_lock: Mutex<()>,
}

impl OutboundPayments {
        pub(super) fn new() -> Self {
                Self {
                        pending_outbound_payments: Mutex::new(HashMap::new()),
                        retry_lock: Mutex::new(()),
                }
        }

        pub(super) fn send_payment<R: Deref, ES: Deref, NS: Deref, IH, SP, L: Deref>(
                &self, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields, payment_id: PaymentId,
                retry_strategy: Retry, route_params: RouteParameters, router: &R,
                first_hops: Vec<ChannelDetails>, compute_inflight_htlcs: IH, entropy_source: &ES,
                node_signer: &NS, best_block_height: u32, logger: &L,
                pending_events: &Mutex<Vec<events::Event>>, send_payment_along_path: SP,
        ) -> Result<(), RetryableSendFailure>
        where
                R::Target: Router,
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                L::Target: Logger,
                IH: Fn() -> InFlightHtlcs,
                SP: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>,
        {
                self.send_payment_internal(payment_id, payment_hash, recipient_onion, None, retry_strategy,
                        route_params, router, first_hops, &compute_inflight_htlcs, entropy_source, node_signer,
                        best_block_height, logger, pending_events, &send_payment_along_path)
        }

        pub(super) fn send_payment_with_route<ES: Deref, NS: Deref, F>(
                &self, route: &Route, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields,
                payment_id: PaymentId, entropy_source: &ES, node_signer: &NS, best_block_height: u32,
                send_payment_along_path: F
        ) -> Result<(), PaymentSendFailure>
        where
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                F: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                let onion_session_privs = self.add_new_pending_payment(payment_hash, recipient_onion.clone(), payment_id, None, route, None, None, entropy_source, best_block_height)?;
                self.pay_route_internal(route, payment_hash, recipient_onion, None, payment_id, None,
                        onion_session_privs, node_signer, best_block_height, &send_payment_along_path)
                        .map_err(|e| { self.remove_outbound_if_all_failed(payment_id, &e); e })
        }

        pub(super) fn send_spontaneous_payment<R: Deref, ES: Deref, NS: Deref, IH, SP, L: Deref>(
                &self, payment_preimage: Option<PaymentPreimage>, recipient_onion: RecipientOnionFields,
                payment_id: PaymentId, retry_strategy: Retry, route_params: RouteParameters, router: &R,
                first_hops: Vec<ChannelDetails>, inflight_htlcs: IH, entropy_source: &ES,
                node_signer: &NS, best_block_height: u32, logger: &L,
                pending_events: &Mutex<Vec<events::Event>>, send_payment_along_path: SP
        ) -> Result<PaymentHash, RetryableSendFailure>
        where
                R::Target: Router,
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                L::Target: Logger,
                IH: Fn() -> InFlightHtlcs,
                SP: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>,
        {
                let preimage = payment_preimage
                        .unwrap_or_else(|| PaymentPreimage(entropy_source.get_secure_random_bytes()));
                let payment_hash = PaymentHash(Sha256::hash(&preimage.0).into_inner());
                self.send_payment_internal(payment_id, payment_hash, recipient_onion, Some(preimage),
                        retry_strategy, route_params, router, first_hops, inflight_htlcs, entropy_source,
                        node_signer, best_block_height, logger, pending_events, send_payment_along_path)
                        .map(|()| payment_hash)
        }

        pub(super) fn send_spontaneous_payment_with_route<ES: Deref, NS: Deref, F>(
                &self, route: &Route, payment_preimage: Option<PaymentPreimage>,
                recipient_onion: RecipientOnionFields, payment_id: PaymentId, entropy_source: &ES,
                node_signer: &NS, best_block_height: u32, send_payment_along_path: F
        ) -> Result<PaymentHash, PaymentSendFailure>
        where
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                F: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                let preimage = payment_preimage
                        .unwrap_or_else(|| PaymentPreimage(entropy_source.get_secure_random_bytes()));
                let payment_hash = PaymentHash(Sha256::hash(&preimage.0).into_inner());
                let onion_session_privs = self.add_new_pending_payment(payment_hash, recipient_onion.clone(),
                        payment_id, Some(preimage), &route, None, None, entropy_source, best_block_height)?;

                match self.pay_route_internal(route, payment_hash, recipient_onion, Some(preimage),
                        payment_id, None, onion_session_privs, node_signer, best_block_height, &send_payment_along_path
                ) {
                        Ok(()) => Ok(payment_hash),
                        Err(e) => {
                                self.remove_outbound_if_all_failed(payment_id, &e);
                                Err(e)
                        }
                }
        }

        pub(super) fn check_retry_payments<R: Deref, ES: Deref, NS: Deref, SP, IH, FH, L: Deref>(
                &self, router: &R, first_hops: FH, inflight_htlcs: IH, entropy_source: &ES, node_signer: &NS,
                best_block_height: u32, pending_events: &Mutex<Vec<events::Event>>, logger: &L,
                send_payment_along_path: SP,
        )
        where
                R::Target: Router,
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                SP: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>,
                IH: Fn() -> InFlightHtlcs,
                FH: Fn() -> Vec<ChannelDetails>,
                L::Target: Logger,
        {
                let _single_thread = self.retry_lock.lock().unwrap();
                loop {
                        let mut outbounds = self.pending_outbound_payments.lock().unwrap();
                        let mut retry_id_route_params = None;
                        for (pmt_id, pmt) in outbounds.iter_mut() {
                                if pmt.is_auto_retryable_now() {
                                        if let PendingOutboundPayment::Retryable { pending_amt_msat, total_msat, payment_params: Some(params), payment_hash, .. } = pmt {
                                                if pending_amt_msat < total_msat {
                                                        retry_id_route_params = Some((*payment_hash, *pmt_id, RouteParameters {
                                                                final_value_msat: *total_msat - *pending_amt_msat,
                                                                payment_params: params.clone(),
                                                        }));
                                                        break
                                                }
                                        } else { debug_assert!(false); }
                                }
                        }
                        core::mem::drop(outbounds);
                        if let Some((payment_hash, payment_id, route_params)) = retry_id_route_params {
                                self.retry_payment_internal(payment_hash, payment_id, route_params, router, first_hops(), &inflight_htlcs, entropy_source, node_signer, best_block_height, logger, pending_events, &send_payment_along_path)
                        } else { break }
                }

                let mut outbounds = self.pending_outbound_payments.lock().unwrap();
                outbounds.retain(|pmt_id, pmt| {
                        let mut retain = true;
                        if !pmt.is_auto_retryable_now() && pmt.remaining_parts() == 0 {
                                pmt.mark_abandoned(PaymentFailureReason::RetriesExhausted);
                                if let PendingOutboundPayment::Abandoned { payment_hash, reason, .. } = pmt {
                                        pending_events.lock().unwrap().push(events::Event::PaymentFailed {
                                                payment_id: *pmt_id,
                                                payment_hash: *payment_hash,
                                                reason: *reason,
                                        });
                                        retain = false;
                                }
                        }
                        retain
                });
        }

        pub(super) fn needs_abandon(&self) -> bool {
                let outbounds = self.pending_outbound_payments.lock().unwrap();
                outbounds.iter().any(|(_, pmt)|
                        !pmt.is_auto_retryable_now() && pmt.remaining_parts() == 0 && !pmt.is_fulfilled())
        }

        /// Errors immediately on [`RetryableSendFailure`] error conditions. Otherwise, further errors may
        /// be surfaced asynchronously via [`Event::PaymentPathFailed`] and [`Event::PaymentFailed`].
        ///
        /// [`Event::PaymentPathFailed`]: crate::events::Event::PaymentPathFailed
        /// [`Event::PaymentFailed`]: crate::events::Event::PaymentFailed
        fn send_payment_internal<R: Deref, NS: Deref, ES: Deref, IH, SP, L: Deref>(
                &self, payment_id: PaymentId, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields,
                keysend_preimage: Option<PaymentPreimage>, retry_strategy: Retry, route_params: RouteParameters,
                router: &R, first_hops: Vec<ChannelDetails>, inflight_htlcs: IH, entropy_source: &ES,
                node_signer: &NS, best_block_height: u32, logger: &L,
                pending_events: &Mutex<Vec<events::Event>>, send_payment_along_path: SP,
        ) -> Result<(), RetryableSendFailure>
        where
                R::Target: Router,
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                L::Target: Logger,
                IH: Fn() -> InFlightHtlcs,
                SP: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                #[cfg(feature = "std")] {
                        if has_expired(&route_params) {
                                return Err(RetryableSendFailure::PaymentExpired)
                        }
                }

                let route = router.find_route_with_id(
                        &node_signer.get_node_id(Recipient::Node).unwrap(), &route_params,
                        Some(&first_hops.iter().collect::<Vec<_>>()), &inflight_htlcs(),
                        payment_hash, payment_id,
                ).map_err(|_| RetryableSendFailure::RouteNotFound)?;

                let onion_session_privs = self.add_new_pending_payment(payment_hash,
                        recipient_onion.clone(), payment_id, keysend_preimage, &route, Some(retry_strategy),
                        Some(route_params.payment_params.clone()), entropy_source, best_block_height)
                        .map_err(|_| RetryableSendFailure::DuplicatePayment)?;

                let res = self.pay_route_internal(&route, payment_hash, recipient_onion, None, payment_id, None,
                        onion_session_privs, node_signer, best_block_height, &send_payment_along_path);
                log_info!(logger, "Result sending payment with id {}: {:?}", log_bytes!(payment_id.0), res);
                if let Err(e) = res {
                        self.handle_pay_route_err(e, payment_id, payment_hash, route, route_params, router, first_hops, &inflight_htlcs, entropy_source, node_signer, best_block_height, logger, pending_events, &send_payment_along_path);
                }
                Ok(())
        }

        fn retry_payment_internal<R: Deref, NS: Deref, ES: Deref, IH, SP, L: Deref>(
                &self, payment_hash: PaymentHash, payment_id: PaymentId, route_params: RouteParameters,
                router: &R, first_hops: Vec<ChannelDetails>, inflight_htlcs: &IH, entropy_source: &ES,
                node_signer: &NS, best_block_height: u32, logger: &L,
                pending_events: &Mutex<Vec<events::Event>>, send_payment_along_path: &SP,
        )
        where
                R::Target: Router,
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                L::Target: Logger,
                IH: Fn() -> InFlightHtlcs,
                SP: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                #[cfg(feature = "std")] {
                        if has_expired(&route_params) {
                                log_error!(logger, "Payment params expired on retry, abandoning payment {}", log_bytes!(payment_id.0));
                                self.abandon_payment(payment_id, PaymentFailureReason::PaymentExpired, pending_events);
                                return
                        }
                }

                let route = match router.find_route_with_id(
                        &node_signer.get_node_id(Recipient::Node).unwrap(), &route_params,
                        Some(&first_hops.iter().collect::<Vec<_>>()), &inflight_htlcs(),
                        payment_hash, payment_id,
                ) {
                        Ok(route) => route,
                        Err(e) => {
                                log_error!(logger, "Failed to find a route on retry, abandoning payment {}: {:#?}", log_bytes!(payment_id.0), e);
                                self.abandon_payment(payment_id, PaymentFailureReason::RouteNotFound, pending_events);
                                return
                        }
                };
                for path in route.paths.iter() {
                        if path.len() == 0 {
                                log_error!(logger, "length-0 path in route");
                                self.abandon_payment(payment_id, PaymentFailureReason::UnexpectedError, pending_events);
                                return
                        }
                }

                const RETRY_OVERFLOW_PERCENTAGE: u64 = 10;
                let mut onion_session_privs = Vec::with_capacity(route.paths.len());
                for _ in 0..route.paths.len() {
                        onion_session_privs.push(entropy_source.get_secure_random_bytes());
                }

                macro_rules! abandon_with_entry {
                        ($payment: expr, $reason: expr) => {
                                $payment.get_mut().mark_abandoned($reason);
                                if let PendingOutboundPayment::Abandoned { reason, .. } = $payment.get() {
                                        if $payment.get().remaining_parts() == 0 {
                                                pending_events.lock().unwrap().push(events::Event::PaymentFailed {
                                                        payment_id,
                                                        payment_hash,
                                                        reason: *reason,
                                                });
                                                $payment.remove();
                                        }
                                }
                        }
                }
                let (total_msat, recipient_onion, keysend_preimage) = {
                        let mut outbounds = self.pending_outbound_payments.lock().unwrap();
                        match outbounds.entry(payment_id) {
                                hash_map::Entry::Occupied(mut payment) => {
                                        let res = match payment.get() {
                                                PendingOutboundPayment::Retryable {
                                                        total_msat, keysend_preimage, payment_secret, payment_metadata, pending_amt_msat, ..
                                                } => {
                                                        let retry_amt_msat: u64 = route.paths.iter().map(|path| path.last().unwrap().fee_msat).sum();
                                                        if retry_amt_msat + *pending_amt_msat > *total_msat * (100 + RETRY_OVERFLOW_PERCENTAGE) / 100 {
                                                                log_error!(logger, "retry_amt_msat of {} will put pending_amt_msat (currently: {}) more than 10% over total_payment_amt_msat of {}", retry_amt_msat, pending_amt_msat, total_msat);
                                                                abandon_with_entry!(payment, PaymentFailureReason::UnexpectedError);
                                                                return
                                                        }
                                                        (*total_msat, RecipientOnionFields {
                                                                        payment_secret: *payment_secret,
                                                                        payment_metadata: payment_metadata.clone(),
                                                                }, *keysend_preimage)
                                                },
                                                PendingOutboundPayment::Legacy { .. } => {
                                                        log_error!(logger, "Unable to retry payments that were initially sent on LDK versions prior to 0.0.102");
                                                        return
                                                },
                                                PendingOutboundPayment::Fulfilled { .. } => {
                                                        log_error!(logger, "Payment already completed");
                                                        return
                                                },
                                                PendingOutboundPayment::Abandoned { .. } => {
                                                        log_error!(logger, "Payment already abandoned (with some HTLCs still pending)");
                                                        return
                                                },
                                        };
                                        if !payment.get().is_retryable_now() {
                                                log_error!(logger, "Retries exhausted for payment id {}", log_bytes!(payment_id.0));
                                                abandon_with_entry!(payment, PaymentFailureReason::RetriesExhausted);
                                                return
                                        }
                                        payment.get_mut().increment_attempts();
                                        for (path, session_priv_bytes) in route.paths.iter().zip(onion_session_privs.iter()) {
                                                assert!(payment.get_mut().insert(*session_priv_bytes, path));
                                        }
                                        res
                                },
                                hash_map::Entry::Vacant(_) => {
                                        log_error!(logger, "Payment with ID {} not found", log_bytes!(payment_id.0));
                                        return
                                }
                        }
                };
                let res = self.pay_route_internal(&route, payment_hash, recipient_onion, keysend_preimage,
                        payment_id, Some(total_msat), onion_session_privs, node_signer, best_block_height,
                        &send_payment_along_path);
                log_info!(logger, "Result retrying payment id {}: {:?}", log_bytes!(payment_id.0), res);
                if let Err(e) = res {
                        self.handle_pay_route_err(e, payment_id, payment_hash, route, route_params, router, first_hops, inflight_htlcs, entropy_source, node_signer, best_block_height, logger, pending_events, send_payment_along_path);
                }
        }

        fn handle_pay_route_err<R: Deref, NS: Deref, ES: Deref, IH, SP, L: Deref>(
                &self, err: PaymentSendFailure, payment_id: PaymentId, payment_hash: PaymentHash, route: Route,
                mut route_params: RouteParameters, router: &R, first_hops: Vec<ChannelDetails>,
                inflight_htlcs: &IH, entropy_source: &ES, node_signer: &NS, best_block_height: u32, logger: &L,
                pending_events: &Mutex<Vec<events::Event>>, send_payment_along_path: &SP,
        )
        where
                R::Target: Router,
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                L::Target: Logger,
                IH: Fn() -> InFlightHtlcs,
                SP: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                match err {
                        PaymentSendFailure::AllFailedResendSafe(errs) => {
                                Self::push_path_failed_evs_and_scids(payment_id, payment_hash, &mut route_params, route.paths, errs.into_iter().map(|e| Err(e)), logger, pending_events);
                                self.retry_payment_internal(payment_hash, payment_id, route_params, router, first_hops, inflight_htlcs, entropy_source, node_signer, best_block_height, logger, pending_events, send_payment_along_path);
                        },
                        PaymentSendFailure::PartialFailure { failed_paths_retry: Some(mut retry), results, .. } => {
                                Self::push_path_failed_evs_and_scids(payment_id, payment_hash, &mut retry, route.paths, results.into_iter(), logger, pending_events);
                                // 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.
                                self.retry_payment_internal(payment_hash, payment_id, retry, router, first_hops, inflight_htlcs, entropy_source, node_signer, best_block_height, logger, pending_events, send_payment_along_path);
                        },
                        PaymentSendFailure::PartialFailure { failed_paths_retry: None, .. } => {
                                // 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.
                        },
                        PaymentSendFailure::PathParameterError(results) => {
                                log_error!(logger, "Failed to send to route due to parameter error in a single path. Your router is buggy");
                                Self::push_path_failed_evs_and_scids(payment_id, payment_hash, &mut route_params, route.paths, results.into_iter(), logger, pending_events);
                                self.abandon_payment(payment_id, PaymentFailureReason::UnexpectedError, pending_events);
                        },
                        PaymentSendFailure::ParameterError(e) => {
                                log_error!(logger, "Failed to send to route due to parameter error: {:?}. Your router is buggy", e);
                                self.abandon_payment(payment_id, PaymentFailureReason::UnexpectedError, pending_events);
                        },
                        PaymentSendFailure::DuplicatePayment => debug_assert!(false), // unreachable
                }
        }

        fn push_path_failed_evs_and_scids<I: ExactSizeIterator + Iterator<Item = Result<(), APIError>>, L: Deref>(
                payment_id: PaymentId, payment_hash: PaymentHash, route_params: &mut RouteParameters,
                paths: Vec<Vec<RouteHop>>, path_results: I, logger: &L, pending_events: &Mutex<Vec<events::Event>>
        ) where L::Target: Logger {
                let mut events = pending_events.lock().unwrap();
                debug_assert_eq!(paths.len(), path_results.len());
                for (path, path_res) in paths.into_iter().zip(path_results) {
                        if let Err(e) = path_res {
                                if let APIError::MonitorUpdateInProgress = e { continue }
                                log_error!(logger, "Failed to send along path due to error: {:?}", e);
                                let mut failed_scid = None;
                                if let APIError::ChannelUnavailable { .. } = e {
                                        let scid = path[0].short_channel_id;
                                        failed_scid = Some(scid);
                                        route_params.payment_params.previously_failed_channels.push(scid);
                                }
                                events.push(events::Event::PaymentPathFailed {
                                        payment_id: Some(payment_id),
                                        payment_hash,
                                        payment_failed_permanently: false,
                                        failure: events::PathFailure::InitialSend { err: e },
                                        path,
                                        short_channel_id: failed_scid,
                                        #[cfg(test)]
                                        error_code: None,
                                        #[cfg(test)]
                                        error_data: None,
                                });
                        }
                }
        }

        pub(super) fn send_probe<ES: Deref, NS: Deref, F>(
                &self, hops: Vec<RouteHop>, probing_cookie_secret: [u8; 32], entropy_source: &ES,
                node_signer: &NS, best_block_height: u32, send_payment_along_path: F
        ) -> Result<(PaymentHash, PaymentId), PaymentSendFailure>
        where
                ES::Target: EntropySource,
                NS::Target: NodeSigner,
                F: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                let payment_id = PaymentId(entropy_source.get_secure_random_bytes());

                let payment_hash = probing_cookie_from_id(&payment_id, probing_cookie_secret);

                if hops.len() < 2 {
                        return Err(PaymentSendFailure::ParameterError(APIError::APIMisuseError {
                                err: "No need probing a path with less than two hops".to_string()
                        }))
                }

                let route = Route { paths: vec![hops], payment_params: None };
                let onion_session_privs = self.add_new_pending_payment(payment_hash,
                        RecipientOnionFields::spontaneous_empty(), payment_id, None, &route, None, None,
                        entropy_source, best_block_height)?;

                match self.pay_route_internal(&route, payment_hash, RecipientOnionFields::spontaneous_empty(),
                        None, payment_id, None, onion_session_privs, node_signer, best_block_height, &send_payment_along_path
                ) {
                        Ok(()) => Ok((payment_hash, payment_id)),
                        Err(e) => {
                                self.remove_outbound_if_all_failed(payment_id, &e);
                                Err(e)
                        }
                }
        }

        #[cfg(test)]
        pub(super) fn test_set_payment_metadata(
                &self, payment_id: PaymentId, new_payment_metadata: Option<Vec<u8>>
        ) {
                match self.pending_outbound_payments.lock().unwrap().get_mut(&payment_id).unwrap() {
                        PendingOutboundPayment::Retryable { payment_metadata, .. } => {
                                *payment_metadata = new_payment_metadata;
                        },
                        _ => panic!("Need a retryable payment to update metadata on"),
                }
        }

        #[cfg(test)]
        pub(super) fn test_add_new_pending_payment<ES: Deref>(
                &self, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields, payment_id: PaymentId,
                route: &Route, retry_strategy: Option<Retry>, entropy_source: &ES, best_block_height: u32
        ) -> Result<Vec<[u8; 32]>, PaymentSendFailure> where ES::Target: EntropySource {
                self.add_new_pending_payment(payment_hash, recipient_onion, payment_id, None, route, retry_strategy, None, entropy_source, best_block_height)
        }

        pub(super) fn add_new_pending_payment<ES: Deref>(
                &self, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields, payment_id: PaymentId,
                keysend_preimage: Option<PaymentPreimage>, route: &Route, retry_strategy: Option<Retry>,
                payment_params: Option<PaymentParameters>, entropy_source: &ES, best_block_height: u32
        ) -> Result<Vec<[u8; 32]>, PaymentSendFailure> where ES::Target: EntropySource {
                let mut onion_session_privs = Vec::with_capacity(route.paths.len());
                for _ in 0..route.paths.len() {
                        onion_session_privs.push(entropy_source.get_secure_random_bytes());
                }

                let mut pending_outbounds = self.pending_outbound_payments.lock().unwrap();
                match pending_outbounds.entry(payment_id) {
                        hash_map::Entry::Occupied(_) => Err(PaymentSendFailure::DuplicatePayment),
                        hash_map::Entry::Vacant(entry) => {
                                let payment = entry.insert(PendingOutboundPayment::Retryable {
                                        retry_strategy,
                                        attempts: PaymentAttempts::new(),
                                        payment_params,
                                        session_privs: HashSet::new(),
                                        pending_amt_msat: 0,
                                        pending_fee_msat: Some(0),
                                        payment_hash,
                                        payment_secret: recipient_onion.payment_secret,
                                        payment_metadata: recipient_onion.payment_metadata,
                                        keysend_preimage,
                                        starting_block_height: best_block_height,
                                        total_msat: route.get_total_amount(),
                                });

                                for (path, session_priv_bytes) in route.paths.iter().zip(onion_session_privs.iter()) {
                                        assert!(payment.insert(*session_priv_bytes, path));
                                }

                                Ok(onion_session_privs)
                        },
                }
        }

        fn pay_route_internal<NS: Deref, F>(
                &self, route: &Route, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields,
                keysend_preimage: Option<PaymentPreimage>, payment_id: PaymentId, recv_value_msat: Option<u64>,
                onion_session_privs: Vec<[u8; 32]>, node_signer: &NS, best_block_height: u32,
                send_payment_along_path: &F
        ) -> Result<(), PaymentSendFailure>
        where
                NS::Target: NodeSigner,
                F: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                if route.paths.len() < 1 {
                        return Err(PaymentSendFailure::ParameterError(APIError::InvalidRoute{err: "There must be at least one path to send over".to_owned()}));
                }
                if recipient_onion.payment_secret.is_none() && route.paths.len() > 1 {
                        return Err(PaymentSendFailure::ParameterError(APIError::APIMisuseError{err: "Payment secret is required for multi-path payments".to_owned()}));
                }
                let mut total_value = 0;
                let our_node_id = node_signer.get_node_id(Recipient::Node).unwrap(); // TODO no unwrap
                let mut path_errs = Vec::with_capacity(route.paths.len());
                'path_check: for path in route.paths.iter() {
                        if path.len() < 1 || path.len() > 20 {
                                path_errs.push(Err(APIError::InvalidRoute{err: "Path didn't go anywhere/had bogus size".to_owned()}));
                                continue 'path_check;
                        }
                        for (idx, hop) in path.iter().enumerate() {
                                if idx != path.len() - 1 && hop.pubkey == our_node_id {
                                        path_errs.push(Err(APIError::InvalidRoute{err: "Path went through us but wasn't a simple rebalance loop to us".to_owned()}));
                                        continue 'path_check;
                                }
                        }
                        total_value += path.last().unwrap().fee_msat;
                        path_errs.push(Ok(()));
                }
                if path_errs.iter().any(|e| e.is_err()) {
                        return Err(PaymentSendFailure::PathParameterError(path_errs));
                }
                if let Some(amt_msat) = recv_value_msat {
                        total_value = amt_msat;
                }

                let cur_height = best_block_height + 1;
                let mut results = Vec::new();
                debug_assert_eq!(route.paths.len(), onion_session_privs.len());
                for (path, session_priv) in route.paths.iter().zip(onion_session_privs.into_iter()) {
                        let mut path_res = send_payment_along_path(&path, &payment_hash, recipient_onion.clone(),
                                total_value, cur_height, payment_id, &keysend_preimage, session_priv);
                        match path_res {
                                Ok(_) => {},
                                Err(APIError::MonitorUpdateInProgress) => {
                                        // While a MonitorUpdateInProgress is an Err(_), the payment is still
                                        // considered "in flight" and we shouldn't remove it from the
                                        // PendingOutboundPayment set.
                                },
                                Err(_) => {
                                        let mut pending_outbounds = self.pending_outbound_payments.lock().unwrap();
                                        if let Some(payment) = pending_outbounds.get_mut(&payment_id) {
                                                let removed = payment.remove(&session_priv, Some(path));
                                                debug_assert!(removed, "This can't happen as the payment has an entry for this path added by callers");
                                        } else {
                                                debug_assert!(false, "This can't happen as the payment was added by callers");
                                                path_res = Err(APIError::APIMisuseError { err: "Internal error: payment disappeared during processing. Please report this bug!".to_owned() });
                                        }
                                }
                        }
                        results.push(path_res);
                }
                let mut has_ok = false;
                let mut has_err = false;
                let mut pending_amt_unsent = 0;
                let mut max_unsent_cltv_delta = 0;
                for (res, path) in results.iter().zip(route.paths.iter()) {
                        if res.is_ok() { has_ok = true; }
                        if res.is_err() { has_err = true; }
                        if let &Err(APIError::MonitorUpdateInProgress) = res {
                                // MonitorUpdateInProgress is inherently unsafe to retry, so we call it a
                                // PartialFailure.
                                has_err = true;
                                has_ok = true;
                        } else if res.is_err() {
                                pending_amt_unsent += path.last().unwrap().fee_msat;
                                max_unsent_cltv_delta = cmp::max(max_unsent_cltv_delta, path.last().unwrap().cltv_expiry_delta);
                        }
                }
                if has_err && has_ok {
                        Err(PaymentSendFailure::PartialFailure {
                                results,
                                payment_id,
                                failed_paths_retry: if pending_amt_unsent != 0 {
                                        if let Some(payment_params) = &route.payment_params {
                                                Some(RouteParameters {
                                                        payment_params: payment_params.clone(),
                                                        final_value_msat: pending_amt_unsent,
                                                })
                                        } else { None }
                                } else { None },
                        })
                } else if has_err {
                        Err(PaymentSendFailure::AllFailedResendSafe(results.drain(..).map(|r| r.unwrap_err()).collect()))
                } else {
                        Ok(())
                }
        }

        #[cfg(test)]
        pub(super) fn test_send_payment_internal<NS: Deref, F>(
                &self, route: &Route, payment_hash: PaymentHash, recipient_onion: RecipientOnionFields,
                keysend_preimage: Option<PaymentPreimage>, payment_id: PaymentId, recv_value_msat: Option<u64>,
                onion_session_privs: Vec<[u8; 32]>, node_signer: &NS, best_block_height: u32,
                send_payment_along_path: F
        ) -> Result<(), PaymentSendFailure>
        where
                NS::Target: NodeSigner,
                F: Fn(&Vec<RouteHop>, &PaymentHash, RecipientOnionFields, u64, u32, PaymentId,
                        &Option<PaymentPreimage>, [u8; 32]) -> Result<(), APIError>
        {
                self.pay_route_internal(route, payment_hash, recipient_onion, keysend_preimage, payment_id,
                        recv_value_msat, onion_session_privs, node_signer, best_block_height,
                        &send_payment_along_path)
                        .map_err(|e| { self.remove_outbound_if_all_failed(payment_id, &e); e })
        }

        // If we failed to send any paths, remove the new PaymentId from the `pending_outbound_payments`
        // map as the payment is free to be resent.
        fn remove_outbound_if_all_failed(&self, payment_id: PaymentId, err: &PaymentSendFailure) {
                if let &PaymentSendFailure::AllFailedResendSafe(_) = err {
                        let removed = self.pending_outbound_payments.lock().unwrap().remove(&payment_id).is_some();
                        debug_assert!(removed, "We should always have a pending payment to remove here");
                }
        }

        pub(super) fn claim_htlc<L: Deref>(
                &self, payment_id: PaymentId, payment_preimage: PaymentPreimage, session_priv: SecretKey,
                path: Vec<RouteHop>, from_onchain: bool, pending_events: &Mutex<Vec<events::Event>>, logger: &L
        ) where L::Target: Logger {
                let mut session_priv_bytes = [0; 32];
                session_priv_bytes.copy_from_slice(&session_priv[..]);
                let mut outbounds = self.pending_outbound_payments.lock().unwrap();
                let mut pending_events = pending_events.lock().unwrap();
                if let hash_map::Entry::Occupied(mut payment) = outbounds.entry(payment_id) {
                        if !payment.get().is_fulfilled() {
                                let payment_hash = PaymentHash(Sha256::hash(&payment_preimage.0).into_inner());
                                let fee_paid_msat = payment.get().get_pending_fee_msat();
                                pending_events.push(
                                        events::Event::PaymentSent {
                                                payment_id: Some(payment_id),
                                                payment_preimage,
                                                payment_hash,
                                                fee_paid_msat,
                                        }
                                );
                                payment.get_mut().mark_fulfilled();
                        }

                        if from_onchain {
                                // We currently immediately remove HTLCs which were fulfilled on-chain.
                                // This could potentially lead to removing a pending payment too early,
                                // with a reorg of one block causing us to re-add the fulfilled payment on
                                // restart.
                                // TODO: We should have a second monitor event that informs us of payments
                                // irrevocably fulfilled.
                                if payment.get_mut().remove(&session_priv_bytes, Some(&path)) {
                                        let payment_hash = Some(PaymentHash(Sha256::hash(&payment_preimage.0).into_inner()));
                                        pending_events.push(
                                                events::Event::PaymentPathSuccessful {
                                                        payment_id,
                                                        payment_hash,
                                                        path,
                                                }
                                        );
                                }
                        }
                } else {
                        log_trace!(logger, "Received duplicative fulfill for HTLC with payment_preimage {}", log_bytes!(payment_preimage.0));
                }
        }

        pub(super) fn finalize_claims(&self, sources: Vec<HTLCSource>, pending_events: &Mutex<Vec<events::Event>>) {
                let mut outbounds = self.pending_outbound_payments.lock().unwrap();
                let mut pending_events = pending_events.lock().unwrap();
                for source in sources {
                        if let HTLCSource::OutboundRoute { session_priv, payment_id, path, .. } = source {
                                let mut session_priv_bytes = [0; 32];
                                session_priv_bytes.copy_from_slice(&session_priv[..]);
                                if let hash_map::Entry::Occupied(mut payment) = outbounds.entry(payment_id) {
                                        assert!(payment.get().is_fulfilled());
                                        if payment.get_mut().remove(&session_priv_bytes, None) {
                                                pending_events.push(
                                                        events::Event::PaymentPathSuccessful {
                                                                payment_id,
                                                                payment_hash: payment.get().payment_hash(),
                                                                path,
                                                        }
                                                );
                                        }
                                }
                        }
                }
        }

        pub(super) fn remove_stale_resolved_payments(&self, pending_events: &Mutex<Vec<events::Event>>) {
                // If an outbound payment was completed, and no pending HTLCs remain, we should remove it
                // from the map. However, if we did that immediately when the last payment HTLC is claimed,
                // this could race the user making a duplicate send_payment call and our idempotency
                // guarantees would be violated. Instead, we wait a few timer ticks to do the actual
                // removal. This should be more than sufficient to ensure the idempotency of any
                // `send_payment` calls that were made at the same time the `PaymentSent` event was being
                // processed.
                let mut pending_outbound_payments = self.pending_outbound_payments.lock().unwrap();
                let pending_events = pending_events.lock().unwrap();
                pending_outbound_payments.retain(|payment_id, payment| {
                        if let PendingOutboundPayment::Fulfilled { session_privs, timer_ticks_without_htlcs, .. } = payment {
                                let mut no_remaining_entries = session_privs.is_empty();
                                if no_remaining_entries {
                                        for ev in pending_events.iter() {
                                                match ev {
                                                        events::Event::PaymentSent { payment_id: Some(ev_payment_id), .. } |
                                                                events::Event::PaymentPathSuccessful { payment_id: ev_payment_id, .. } |
                                                                events::Event::PaymentPathFailed { payment_id: Some(ev_payment_id), .. } => {
                                                                        if payment_id == ev_payment_id {
                                                                                no_remaining_entries = false;
                                                                                break;
                                                                        }
                                                                },
                                                        _ => {},
                                                }
                                        }
                                }
                                if no_remaining_entries {
                                        *timer_ticks_without_htlcs += 1;
                                        *timer_ticks_without_htlcs <= IDEMPOTENCY_TIMEOUT_TICKS
                                } else {
                                        *timer_ticks_without_htlcs = 0;
                                        true
                                }
                        } else { true }
                });
        }

        // Returns a bool indicating whether a PendingHTLCsForwardable event should be generated.
        pub(super) fn fail_htlc<L: Deref>(
                &self, source: &HTLCSource, payment_hash: &PaymentHash, onion_error: &HTLCFailReason,
                path: &Vec<RouteHop>, session_priv: &SecretKey, payment_id: &PaymentId,
                probing_cookie_secret: [u8; 32], secp_ctx: &Secp256k1<secp256k1::All>,
                pending_events: &Mutex<Vec<events::Event>>, logger: &L
        ) -> bool where L::Target: Logger {
                #[cfg(test)]
                let (network_update, short_channel_id, payment_retryable, onion_error_code, onion_error_data) = onion_error.decode_onion_failure(secp_ctx, logger, &source);
                #[cfg(not(test))]
                let (network_update, short_channel_id, payment_retryable, _, _) = onion_error.decode_onion_failure(secp_ctx, logger, &source);

                let payment_is_probe = payment_is_probe(payment_hash, &payment_id, probing_cookie_secret);
                let mut session_priv_bytes = [0; 32];
                session_priv_bytes.copy_from_slice(&session_priv[..]);
                let mut outbounds = self.pending_outbound_payments.lock().unwrap();

                // If any payments already need retry, there's no need to generate a redundant
                // `PendingHTLCsForwardable`.
                let already_awaiting_retry = outbounds.iter().any(|(_, pmt)| {
                        let mut awaiting_retry = false;
                        if pmt.is_auto_retryable_now() {
                                if let PendingOutboundPayment::Retryable { pending_amt_msat, total_msat, .. } = pmt {
                                        if pending_amt_msat < total_msat {
                                                awaiting_retry = true;
                                        }
                                }
                        }
                        awaiting_retry
                });

                let mut full_failure_ev = None;
                let mut pending_retry_ev = false;
                let attempts_remaining = if let hash_map::Entry::Occupied(mut payment) = outbounds.entry(*payment_id) {
                        if !payment.get_mut().remove(&session_priv_bytes, Some(&path)) {
                                log_trace!(logger, "Received duplicative fail for HTLC with payment_hash {}", log_bytes!(payment_hash.0));
                                return false
                        }
                        if payment.get().is_fulfilled() {
                                log_trace!(logger, "Received failure of HTLC with payment_hash {} after payment completion", log_bytes!(payment_hash.0));
                                return false
                        }
                        let mut is_retryable_now = payment.get().is_auto_retryable_now();
                        if let Some(scid) = short_channel_id {
                                // TODO: If we decided to blame ourselves (or one of our channels) in
                                // process_onion_failure we should close that channel as it implies our
                                // next-hop is needlessly blaming us!
                                payment.get_mut().insert_previously_failed_scid(scid);
                        }

                        if payment_is_probe || !is_retryable_now || !payment_retryable {
                                let reason = if !payment_retryable {
                                        PaymentFailureReason::RecipientRejected
                                } else {
                                        PaymentFailureReason::RetriesExhausted
                                };
                                payment.get_mut().mark_abandoned(reason);
                                is_retryable_now = false;
                        }
                        if payment.get().remaining_parts() == 0 {
                                if let PendingOutboundPayment::Abandoned { payment_hash, reason, .. }= payment.get() {
                                        if !payment_is_probe {
                                                full_failure_ev = Some(events::Event::PaymentFailed {
                                                        payment_id: *payment_id,
                                                        payment_hash: *payment_hash,
                                                        reason: *reason,
                                                });
                                        }
                                        payment.remove();
                                }
                        }
                        is_retryable_now
                } else {
                        log_trace!(logger, "Received duplicative fail for HTLC with payment_hash {}", log_bytes!(payment_hash.0));
                        return false
                };
                core::mem::drop(outbounds);
                log_trace!(logger, "Failing outbound payment HTLC with payment_hash {}", log_bytes!(payment_hash.0));

                let path_failure = {
                        if payment_is_probe {
                                if !payment_retryable {
                                        events::Event::ProbeSuccessful {
                                                payment_id: *payment_id,
                                                payment_hash: payment_hash.clone(),
                                                path: path.clone(),
                                        }
                                } else {
                                        events::Event::ProbeFailed {
                                                payment_id: *payment_id,
                                                payment_hash: payment_hash.clone(),
                                                path: path.clone(),
                                                short_channel_id,
                                        }
                                }
                        } else {
                                // If we miss abandoning the payment above, we *must* generate an event here or else the
                                // payment will sit in our outbounds forever.
                                if attempts_remaining && !already_awaiting_retry {
                                        debug_assert!(full_failure_ev.is_none());
                                        pending_retry_ev = true;
                                }
                                events::Event::PaymentPathFailed {
                                        payment_id: Some(*payment_id),
                                        payment_hash: payment_hash.clone(),
                                        payment_failed_permanently: !payment_retryable,
                                        failure: events::PathFailure::OnPath { network_update },
                                        path: path.clone(),
                                        short_channel_id,
                                        #[cfg(test)]
                                        error_code: onion_error_code,
                                        #[cfg(test)]
                                        error_data: onion_error_data
                                }
                        }
                };
                let mut pending_events = pending_events.lock().unwrap();
                pending_events.push(path_failure);
                if let Some(ev) = full_failure_ev { pending_events.push(ev); }
                pending_retry_ev
        }

        pub(super) fn abandon_payment(
                &self, payment_id: PaymentId, reason: PaymentFailureReason, pending_events: &Mutex<Vec<events::Event>>
        ) {
                let mut outbounds = self.pending_outbound_payments.lock().unwrap();
                if let hash_map::Entry::Occupied(mut payment) = outbounds.entry(payment_id) {
                        payment.get_mut().mark_abandoned(reason);
                        if let PendingOutboundPayment::Abandoned { payment_hash, reason, .. } = payment.get() {
                                if payment.get().remaining_parts() == 0 {
                                        pending_events.lock().unwrap().push(events::Event::PaymentFailed {
                                                payment_id,
                                                payment_hash: *payment_hash,
                                                reason: *reason,
                                        });
                                        payment.remove();
                                }
                        }
                }
        }

        #[cfg(test)]
        pub fn has_pending_payments(&self) -> bool {
                !self.pending_outbound_payments.lock().unwrap().is_empty()
        }

        #[cfg(test)]
        pub fn clear_pending_payments(&self) {
                self.pending_outbound_payments.lock().unwrap().clear()
        }
}

/// Returns whether a payment with the given [`PaymentHash`] and [`PaymentId`] is, in fact, a
/// payment probe.
pub(super) fn payment_is_probe(payment_hash: &PaymentHash, payment_id: &PaymentId,
        probing_cookie_secret: [u8; 32]) -> bool
{
        let target_payment_hash = probing_cookie_from_id(payment_id, probing_cookie_secret);
        target_payment_hash == *payment_hash
}

/// Returns the 'probing cookie' for the given [`PaymentId`].
fn probing_cookie_from_id(payment_id: &PaymentId, probing_cookie_secret: [u8; 32]) -> PaymentHash {
        let mut preimage = [0u8; 64];
        preimage[..32].copy_from_slice(&probing_cookie_secret);
        preimage[32..].copy_from_slice(&payment_id.0);
        PaymentHash(Sha256::hash(&preimage).into_inner())
}

impl_writeable_tlv_based_enum_upgradable!(PendingOutboundPayment,
        (0, Legacy) => {
                (0, session_privs, required),
        },
        (1, Fulfilled) => {
                (0, session_privs, required),
                (1, payment_hash, option),
                (3, timer_ticks_without_htlcs, (default_value, 0)),
        },
        (2, Retryable) => {
                (0, session_privs, required),
                (1, pending_fee_msat, option),
                (2, payment_hash, required),
                // Note that while we "default" payment_param's final CLTV expiry delta to 0 we should
                // never see it - `payment_params` was added here after the field was added/required.
                (3, payment_params, (option: ReadableArgs, 0)),
                (4, payment_secret, option),
                (5, keysend_preimage, option),
                (6, total_msat, required),
                (7, payment_metadata, option),
                (8, pending_amt_msat, required),
                (10, starting_block_height, required),
                (not_written, retry_strategy, (static_value, None)),
                (not_written, attempts, (static_value, PaymentAttempts::new())),
        },
        (3, Abandoned) => {
                (0, session_privs, required),
                (1, reason, option),
                (2, payment_hash, required),
        },
);

#[cfg(test)]
mod tests {
        use bitcoin::network::constants::Network;
        use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};

        use crate::events::{Event, PathFailure, PaymentFailureReason};
        use crate::ln::PaymentHash;
        use crate::ln::channelmanager::{PaymentId, RecipientOnionFields};
        use crate::ln::features::{ChannelFeatures, NodeFeatures};
        use crate::ln::msgs::{ErrorAction, LightningError};
        use crate::ln::outbound_payment::{OutboundPayments, Retry, RetryableSendFailure};
        use crate::routing::gossip::NetworkGraph;
        use crate::routing::router::{InFlightHtlcs, PaymentParameters, Route, RouteHop, RouteParameters};
        use crate::sync::{Arc, Mutex};
        use crate::util::errors::APIError;
        use crate::util::test_utils;

        #[test]
        #[cfg(feature = "std")]
        fn fails_paying_after_expiration() {
                do_fails_paying_after_expiration(false);
                do_fails_paying_after_expiration(true);
        }
        #[cfg(feature = "std")]
        fn do_fails_paying_after_expiration(on_retry: bool) {
                let outbound_payments = OutboundPayments::new();
                let logger = test_utils::TestLogger::new();
                let network_graph = Arc::new(NetworkGraph::new(Network::Testnet, &logger));
                let scorer = Mutex::new(test_utils::TestScorer::new());
                let router = test_utils::TestRouter::new(network_graph, &scorer);
                let secp_ctx = Secp256k1::new();
                let keys_manager = test_utils::TestKeysInterface::new(&[0; 32], Network::Testnet);

                let past_expiry_time = std::time::SystemTime::UNIX_EPOCH.elapsed().unwrap().as_secs() - 2;
                let payment_params = PaymentParameters::from_node_id(
                                PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap()),
                                0
                        ).with_expiry_time(past_expiry_time);
                let expired_route_params = RouteParameters {
                        payment_params,
                        final_value_msat: 0,
                };
                let pending_events = Mutex::new(Vec::new());
                if on_retry {
                        outbound_payments.add_new_pending_payment(PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(),
                                PaymentId([0; 32]), None, &Route { paths: vec![], payment_params: None },
                                Some(Retry::Attempts(1)), Some(expired_route_params.payment_params.clone()),
                                &&keys_manager, 0).unwrap();
                        outbound_payments.retry_payment_internal(
                                PaymentHash([0; 32]), PaymentId([0; 32]), expired_route_params, &&router, vec![],
                                &|| InFlightHtlcs::new(), &&keys_manager, &&keys_manager, 0, &&logger,
                                &pending_events, &|_, _, _, _, _, _, _, _| Ok(()));
                        let events = pending_events.lock().unwrap();
                        assert_eq!(events.len(), 1);
                        if let Event::PaymentFailed { ref reason, .. } = events[0] {
                                assert_eq!(reason.unwrap(), PaymentFailureReason::PaymentExpired);
                        } else { panic!("Unexpected event"); }
                } else {
                        let err = outbound_payments.send_payment(
                                PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(), PaymentId([0; 32]),
                                Retry::Attempts(0), expired_route_params, &&router, vec![], || InFlightHtlcs::new(),
                                &&keys_manager, &&keys_manager, 0, &&logger,
                                &pending_events, |_, _, _, _, _, _, _, _| Ok(())).unwrap_err();
                        if let RetryableSendFailure::PaymentExpired = err { } else { panic!("Unexpected error"); }
                }
        }

        #[test]
        fn find_route_error() {
                do_find_route_error(false);
                do_find_route_error(true);
        }
        fn do_find_route_error(on_retry: bool) {
                let outbound_payments = OutboundPayments::new();
                let logger = test_utils::TestLogger::new();
                let network_graph = Arc::new(NetworkGraph::new(Network::Testnet, &logger));
                let scorer = Mutex::new(test_utils::TestScorer::new());
                let router = test_utils::TestRouter::new(network_graph, &scorer);
                let secp_ctx = Secp256k1::new();
                let keys_manager = test_utils::TestKeysInterface::new(&[0; 32], Network::Testnet);

                let payment_params = PaymentParameters::from_node_id(
                        PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap()), 0);
                let route_params = RouteParameters {
                        payment_params,
                        final_value_msat: 0,
                };
                router.expect_find_route(route_params.clone(),
                        Err(LightningError { err: String::new(), action: ErrorAction::IgnoreError }));

                let pending_events = Mutex::new(Vec::new());
                if on_retry {
                        outbound_payments.add_new_pending_payment(PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(),
                                PaymentId([0; 32]), None, &Route { paths: vec![], payment_params: None },
                                Some(Retry::Attempts(1)), Some(route_params.payment_params.clone()),
                                &&keys_manager, 0).unwrap();
                        outbound_payments.retry_payment_internal(
                                PaymentHash([0; 32]), PaymentId([0; 32]), route_params, &&router, vec![],
                                &|| InFlightHtlcs::new(), &&keys_manager, &&keys_manager, 0, &&logger,
                                &pending_events, &|_, _, _, _, _, _, _, _| Ok(()));
                        let events = pending_events.lock().unwrap();
                        assert_eq!(events.len(), 1);
                        if let Event::PaymentFailed { .. } = events[0] { } else { panic!("Unexpected event"); }
                } else {
                        let err = outbound_payments.send_payment(
                                PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(), PaymentId([0; 32]),
                                Retry::Attempts(0), route_params, &&router, vec![], || InFlightHtlcs::new(),
                                &&keys_manager, &&keys_manager, 0, &&logger,
                                &pending_events, |_, _, _, _, _, _, _, _| Ok(())).unwrap_err();
                        if let RetryableSendFailure::RouteNotFound = err {
                        } else { panic!("Unexpected error"); }
                }
        }

        #[test]
        fn initial_send_payment_path_failed_evs() {
                let outbound_payments = OutboundPayments::new();
                let logger = test_utils::TestLogger::new();
                let network_graph = Arc::new(NetworkGraph::new(Network::Testnet, &logger));
                let scorer = Mutex::new(test_utils::TestScorer::new());
                let router = test_utils::TestRouter::new(network_graph, &scorer);
                let secp_ctx = Secp256k1::new();
                let keys_manager = test_utils::TestKeysInterface::new(&[0; 32], Network::Testnet);

                let sender_pk = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[42; 32]).unwrap());
                let receiver_pk = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&[43; 32]).unwrap());
                let payment_params = PaymentParameters::from_node_id(sender_pk, 0);
                let route_params = RouteParameters {
                        payment_params: payment_params.clone(),
                        final_value_msat: 0,
                };
                let failed_scid = 42;
                let route = Route {
                        paths: vec![vec![RouteHop {
                                pubkey: receiver_pk,
                                node_features: NodeFeatures::empty(),
                                short_channel_id: failed_scid,
                                channel_features: ChannelFeatures::empty(),
                                fee_msat: 0,
                                cltv_expiry_delta: 0,
                        }]],
                        payment_params: Some(payment_params),
                };
                router.expect_find_route(route_params.clone(), Ok(route.clone()));
                let mut route_params_w_failed_scid = route_params.clone();
                route_params_w_failed_scid.payment_params.previously_failed_channels.push(failed_scid);
                router.expect_find_route(route_params_w_failed_scid, Ok(route.clone()));
                router.expect_find_route(route_params.clone(), Ok(route.clone()));
                router.expect_find_route(route_params.clone(), Ok(route.clone()));

                // Ensure that a ChannelUnavailable error will result in blaming an scid in the
                // PaymentPathFailed event.
                let pending_events = Mutex::new(Vec::new());
                outbound_payments.send_payment(
                        PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(), PaymentId([0; 32]),
                        Retry::Attempts(0), route_params.clone(), &&router, vec![], || InFlightHtlcs::new(),
                        &&keys_manager, &&keys_manager, 0, &&logger, &pending_events,
                        |_, _, _, _, _, _, _, _| Err(APIError::ChannelUnavailable { err: "test".to_owned() }))
                        .unwrap();
                let mut events = pending_events.lock().unwrap();
                assert_eq!(events.len(), 2);
                if let Event::PaymentPathFailed {
                        short_channel_id,
                        failure: PathFailure::InitialSend { err: APIError::ChannelUnavailable { .. }}, .. } = events[0]
                {
                        assert_eq!(short_channel_id, Some(failed_scid));
                } else { panic!("Unexpected event"); }
                if let Event::PaymentFailed { .. } = events[1] { } else { panic!("Unexpected event"); }
                events.clear();
                core::mem::drop(events);

                // Ensure that a MonitorUpdateInProgress "error" will not result in a PaymentPathFailed event.
                outbound_payments.send_payment(
                        PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(), PaymentId([0; 32]),
                        Retry::Attempts(0), route_params.clone(), &&router, vec![], || InFlightHtlcs::new(),
                        &&keys_manager, &&keys_manager, 0, &&logger, &pending_events,
                        |_, _, _, _, _, _, _, _| Err(APIError::MonitorUpdateInProgress)).unwrap();
                assert_eq!(pending_events.lock().unwrap().len(), 0);

                // Ensure that any other error will result in a PaymentPathFailed event but no blamed scid.
                outbound_payments.send_payment(
                        PaymentHash([0; 32]), RecipientOnionFields::spontaneous_empty(), PaymentId([1; 32]),
                        Retry::Attempts(0), route_params.clone(), &&router, vec![], || InFlightHtlcs::new(),
                        &&keys_manager, &&keys_manager, 0, &&logger, &pending_events,
                        |_, _, _, _, _, _, _, _| Err(APIError::APIMisuseError { err: "test".to_owned() }))
                        .unwrap();
                let events = pending_events.lock().unwrap();
                assert_eq!(events.len(), 2);
                if let Event::PaymentPathFailed {
                        short_channel_id,
                        failure: PathFailure::InitialSend { err: APIError::APIMisuseError { .. }}, .. } = events[0]
                {
                        assert_eq!(short_channel_id, None);
                } else { panic!("Unexpected event"); }
                if let Event::PaymentFailed { .. } = events[1] { } else { panic!("Unexpected event"); }
        }
}