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Fix missing PaymentSent::fee_paid_msat #4651

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Fix missing PaymentSent::fee_paid_msat #4651

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3 changes: 2 additions & 1 deletion lightning/src/events/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1201,7 +1201,8 @@ pub enum Event {
/// If the recipient or an intermediate node misbehaves and gives us free money, this may
/// overstate the amount paid, though this is unlikely.
///
/// This is only `None` for payments initiated on LDK versions prior to 0.0.103.
/// This is only `None` for payments abandoned but ultimately claimed when using LDK versions
/// prior to 0.3, 0.2.3, or 0.1.10.
///
/// [`Route::get_total_fees`]: crate::routing::router::Route::get_total_fees
fee_paid_msat: Option<u64>,
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2 changes: 1 addition & 1 deletion lightning/src/ln/functional_tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8579,7 +8579,7 @@ pub fn test_inconsistent_mpp_params() {
pass_along_path(&nodes[0], path_b, real_amt, hash, Some(payment_secret), event, true, None);

do_claim_payment_along_route(ClaimAlongRouteArgs::new(&nodes[0], &[path_a, path_b], preimage));
expect_payment_sent(&nodes[0], preimage, Some(None), true, true);
expect_payment_sent(&nodes[0], preimage, Some(Some(2000)), true, true);
}

#[xtest(feature = "_externalize_tests")]
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7 changes: 7 additions & 0 deletions lightning/src/ln/outbound_payment.rs
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Expand Up @@ -164,6 +164,9 @@ pub(crate) enum PendingOutboundPayment {
/// The total payment amount across all paths, used to be able to issue `PaymentSent` if
/// an HTLC still happens to succeed after we marked the payment as abandoned.
total_msat: Option<u64>,
/// Preserved from `Retryable` so we can still report `fee_paid_msat` if an HTLC succeeds after
/// the payment was abandoned. Added in 0.3.
pending_fee_msat: Option<u64>,

@ldk-claude-review-bot ldk-claude-review-bot Jun 8, 2026

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Bug: remove() (line 335) only decrements pending_fee_msat for the Retryable variant, not for Abandoned. This means if an MPP payment is manually abandoned with multiple paths in-flight, and then some paths fail back while others succeed, the fee for the failed paths won't be subtracted — causing PaymentSent::fee_paid_msat to be overstated.

Concrete scenario:

  1. Two-path MPP, path A (fee=100) and path B (fee=200) in-flight → pending_fee_msat = Some(300)
  2. abandon_payment() → Abandoned { pending_fee_msat: Some(300) }
  3. Path A fails (intermediate node timeout) → remove() removes session_priv but doesn't decrement fee → pending_fee_msat still Some(300)
  4. Path B succeeds (recipient claims) → PaymentSent { fee_paid_msat: Some(300) } — should be Some(200)

The remove() function at line 348-363 needs an additional branch for Abandoned:

if let PendingOutboundPayment::Abandoned {
    ref mut pending_fee_msat, ..
} = self {
    let path = path.expect("Removing a failed payment should always come with a path");
    if let Some(fee_msat) = pending_fee_msat.as_mut() {
        *fee_msat -= path.fee_msat();
    }
}

Note: for the auto-abandon path (in handle_update_fail_htlc), remove() is called before mark_abandoned(), so the fee is correctly captured. This bug only affects the manual-abandon path and any future failures arriving after auto-abandonment.

@valentinewallace valentinewallace Jun 8, 2026

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See #4651 (comment)

},
}

Expand Down Expand Up @@ -252,6 +255,7 @@ impl PendingOutboundPayment {
fn get_pending_fee_msat(&self) -> Option<u64> {
match self {
PendingOutboundPayment::Retryable { pending_fee_msat, .. } => pending_fee_msat.clone(),
PendingOutboundPayment::Abandoned { pending_fee_msat, .. } => pending_fee_msat.clone(),
_ => None,
}
}
Expand Down Expand Up @@ -308,6 +312,7 @@ impl PendingOutboundPayment {
_ => new_hash_set(),
};
let total_msat = self.total_msat();
let pending_fee_msat = self.get_pending_fee_msat();
match self {
Self::Retryable { payment_hash, .. } |
Self::InvoiceReceived { payment_hash, .. } |
Expand All @@ -318,6 +323,7 @@ impl PendingOutboundPayment {
payment_hash: *payment_hash,
reason: Some(reason),
total_msat,

@tankyleo tankyleo Jun 4, 2026

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curious here if we should be setting this to the sum of the current paths currently in-flight, and not the fixed total msat ?

pending_fee_msat,
};
},
_ => {}
Expand Down Expand Up @@ -2778,6 +2784,7 @@ impl_writeable_tlv_based_enum_upgradable!(PendingOutboundPayment,
(1, reason, upgradable_option),
(2, payment_hash, required),
(3, total_msat, option),
(5, pending_fee_msat, option),
},
(5, AwaitingInvoice) => {
(0, expiration, required),
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30 changes: 30 additions & 0 deletions lightning/src/ln/payment_tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2241,6 +2241,36 @@ fn abandoned_send_payment_idempotent() {
claim_payment(&nodes[0], &[&nodes[1]], second_payment_preimage);
}

#[test]
fn abandoned_payment_fulfilled_preserves_fee_paid_msat() {
// Previously, if we abandoned a payment with HTLCs in-flight and the payment eventually
// succeeded, we would set the `Event::PaymentSent::fee_paid_msat` to None, even though we had
// docs guaranteeing that it would always be Some after 0.0.103.
let chanmon_cfgs = create_chanmon_cfgs(3);
let node_cfgs = create_node_cfgs(3, &chanmon_cfgs);
let node_chanmgrs = create_node_chanmgrs(3, &node_cfgs, &[None, None, None]);
let nodes = create_network(3, &node_cfgs, &node_chanmgrs);

create_announced_chan_between_nodes(&nodes, 0, 1);
create_announced_chan_between_nodes(&nodes, 1, 2);

let amt_msat = 10_000_000;
let (route, payment_hash, payment_preimage, payment_secret) =
get_route_and_payment_hash!(&nodes[0], nodes[2], amt_msat);
let payment_id = PaymentId(payment_hash.0);
let onion = RecipientOnionFields::secret_only(payment_secret, amt_msat);
nodes[0].node.send_payment_with_route(route, payment_hash, onion, payment_id).unwrap();
check_added_monitors(&nodes[0], 1);

let path: &[&Node] = &[&nodes[1], &nodes[2]];
pass_along_route(&nodes[0], &[path], amt_msat, payment_hash, payment_secret);

nodes[0].node.abandon_payment(payment_id);
assert!(nodes[0].node.get_and_clear_pending_events().is_empty());

claim_payment_along_route(ClaimAlongRouteArgs::new(&nodes[0], &[path], payment_preimage));
}

#[derive(PartialEq)]
enum InterceptTest {
Forward,
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