ln: add awaiting_trampoline_forwards to accumulate inbound MPP

diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index 0303483..3cacbdc 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -1313,6 +1313,12 @@ fn check_mpp_timeout<'a>(
 	timed_out
 }
 
+/// Tracks trampoline HTLCs being accumulated before forwarding.
+struct TrampolinePayment {
+	onion_fields: RecipientOnionFields,
+	htlcs: Vec<MppPart>,
+}
+
 /// Represent the channel funding transaction type.
 enum FundingType {
 	/// This variant is useful when we want LDK to validate the funding transaction and
@@ -2894,6 +2900,16 @@ pub struct ChannelManager<
 	/// [`ClaimablePayments`]' individual field docs for more info.
 	claimable_payments: Mutex<ClaimablePayments>,
 
+	/// The sets of trampoline payments which are in the process of being accumulated on inbound
+	/// channel(s).
+	///
+	/// Note that this map is currently not persisted, as there is ongoing work to refactor our
+	/// reload from disk depending only on channel managers. Until proper restart logic is added
+	/// we will "forget" about any HTLCs that are pending in this map on restart waiting for MPP
+	/// timeout. For this reason, we should not forward any trampoline HTLCs until properly
+	/// implemented.
+	awaiting_trampoline_forwards: Mutex<HashMap<PaymentHash, TrampolinePayment>>,
+
 	/// The set of outbound SCID aliases across all our channels, including unconfirmed channels
 	/// and some closed channels which reached a usable state prior to being closed. This is used
 	/// only to avoid duplicates, and is not persisted explicitly to disk, but rebuilt from the
@@ -3741,6 +3757,7 @@ impl<
 			forward_htlcs: Mutex::new(new_hash_map()),
 			decode_update_add_htlcs: Mutex::new(new_hash_map()),
 			claimable_payments: Mutex::new(ClaimablePayments { claimable_payments: new_hash_map(), pending_claiming_payments: new_hash_map() }),
+			awaiting_trampoline_forwards: Mutex::new(new_hash_map()),
 			pending_intercepted_htlcs: Mutex::new(new_hash_map()),
 			short_to_chan_info: FairRwLock::new(new_hash_map()),
 
@@ -9098,6 +9115,26 @@ impl<
 				},
 			);
 
+			self.awaiting_trampoline_forwards.lock().unwrap().retain(|payment_hash, payment| {
+				if payment.htlcs.is_empty() {
+					debug_assert!(false);
+					return false;
+				}
+				let mpp_timeout =
+					check_mpp_timeout(payment.htlcs.iter_mut(), &payment.onion_fields);
+				if mpp_timeout {
+					let previous_hop_data =
+						payment.htlcs.drain(..).map(|claimable| claimable.prev_hop).collect();
+
+					timed_out_mpp_htlcs.push((
+						HTLCSource::TrampolineForward { previous_hop_data, outbound_payment: None },
+						*payment_hash,
+						HTLCHandlingFailureType::TrampolineForward {},
+					));
+				}
+				!mpp_timeout
+			});
+
 			for (htlc_source, payment_hash, failure_type) in timed_out_mpp_htlcs.drain(..) {
 				let failure_reason = LocalHTLCFailureReason::MPPTimeout;
 				let reason = HTLCFailReason::from_failure_code(failure_reason);
@@ -16586,6 +16623,31 @@ impl<
 				},
 			);
 
+			self.awaiting_trampoline_forwards.lock().unwrap().retain(|payment_hash, payment| {
+				if payment.htlcs.is_empty() {
+					debug_assert!(false);
+					return false;
+				}
+				let htlc_timed_out =
+					payment.htlcs.iter().any(|htlc| htlc.check_onchain_timeout(height));
+				if htlc_timed_out {
+					let previous_hop_data =
+						payment.htlcs.drain(..).map(|claimable| claimable.prev_hop).collect();
+
+					let failure_reason = LocalHTLCFailureReason::CLTVExpiryTooSoon;
+					timed_out_htlcs.push((
+						HTLCSource::TrampolineForward { previous_hop_data, outbound_payment: None },
+						*payment_hash,
+						HTLCFailReason::reason(
+							failure_reason,
+							self.get_htlc_inbound_temp_fail_data(failure_reason),
+						),
+						HTLCHandlingFailureType::TrampolineForward {},
+					));
+				}
+				!htlc_timed_out
+			});
+
 			let mut intercepted_htlcs = self.pending_intercepted_htlcs.lock().unwrap();
 			intercepted_htlcs.retain(|_, htlc| {
 				if height >= htlc.forward_info.outgoing_cltv_value - HTLC_FAIL_BACK_BUFFER {
@@ -20484,6 +20546,7 @@ impl<
 				claimable_payments,
 				pending_claiming_payments,
 			}),
+			awaiting_trampoline_forwards: Mutex::new(new_hash_map()),
 			outbound_scid_aliases: Mutex::new(outbound_scid_aliases),
 			short_to_chan_info: FairRwLock::new(short_to_chan_info),
 			fake_scid_rand_bytes: fake_scid_rand_bytes.unwrap(),