Allow users to accept skimmed fees in calling peel_payment_onion

LSP users who wish to use `peel_payment_onion` to understand if
they'd accept an HTLC prior to receit should be able to check the
skimmed fees just like they would for full payment receipt. Thus,
we need to expose the fee-skimming acceptance bool to
`peel_payment_onion`, which we do here, in addition to some doc
cleanups.

Author: TheBlueMatt

lightning/src/ln/onion_payment.rs

@@ -1,6 +1,7 @@
-//! Utilities for channelmanager.rs
+//! Utilities to decode payment onions and do contextless validation of incoming payments.
 //!
-//! Includes a public [`peel_payment_onion`] function for use by external projects or libraries.
+//! Primarily features [`peel_payment_onion`], which allows the decoding of an onion statelessly
+//! and can be used to predict whether we'd accept a payment.
 
 use bitcoin::hashes::Hash;
 use bitcoin::hashes::sha256::Hash as Sha256;
@@ -179,7 +180,9 @@ pub(super) fn create_recv_pending_htlc_info(
 	})
 }
 
-/// Peel one layer off an incoming onion, returning [`PendingHTLCInfo`] (either Forward or Receive).
+/// Peel one layer off an incoming onion, returning a [`PendingHTLCInfo`] which provides details on
+/// where the sender intended the HTLC to go.
+///
 /// This does all the relevant context-free checks that LDK requires for payment relay or
 /// acceptance. If the payment is to be received, and the amount matches the expected amount for
 /// a given invoice, this indicates the [`msgs::UpdateAddHTLC`], once fully committed in the
@@ -188,7 +191,7 @@ pub(super) fn create_recv_pending_htlc_info(
 /// [`Event::PaymentClaimable`]: crate::events::Event::PaymentClaimable
 pub fn peel_payment_onion<NS: Deref, L: Deref, T: secp256k1::Verification>(
 	msg: &msgs::UpdateAddHTLC, node_signer: &NS, logger: &L, secp_ctx: &Secp256k1<T>,
-	cur_height: u32, accept_mpp_keysend: bool,
+	cur_height: u32, accept_mpp_keysend: bool, allow_skimmed_fees: bool,
 ) -> Result<PendingHTLCInfo, InboundOnionErr>
 where
 	NS::Target: NodeSigner,
@@ -227,6 +230,10 @@ where
 					err_data: Vec::new(),
 				});
 			}
+
+			// TODO: If this is potentially a phantom payment we should decode the phantom payment
+			// onion here and check it.
+
 			create_fwd_pending_htlc_info(
 				msg, next_hop_data, next_hop_hmac, new_packet_bytes, shared_secret,
 				Some(next_packet_pubkey)
@@ -235,7 +242,7 @@ where
 		onion_utils::Hop::Receive(received_data) => {
 			create_recv_pending_htlc_info(
 				received_data, shared_secret, msg.payment_hash, msg.amount_msat, msg.cltv_expiry,
-				None, false, msg.skimmed_fee_msat, cur_height, accept_mpp_keysend,
+				None, allow_skimmed_fees, msg.skimmed_fee_msat, cur_height, accept_mpp_keysend,
 			)?
 		}
 	})
@@ -410,7 +417,7 @@ mod tests {
 		let msg = make_update_add_msg(amount_msat, cltv_expiry, payment_hash, onion);
 		let logger = test_utils::TestLogger::with_id("bob".to_string());
 
-		let peeled = peel_payment_onion(&msg, &&bob, &&logger, &secp_ctx, cur_height, true)
+		let peeled = peel_payment_onion(&msg, &&bob, &&logger, &secp_ctx, cur_height, true, false)
 			.map_err(|e| e.msg).unwrap();
 
 		let next_onion = match peeled.routing {
@@ -421,7 +428,7 @@ mod tests {
 		};
 
 		let msg2 = make_update_add_msg(amount_msat, cltv_expiry, payment_hash, next_onion);
-		let peeled2 = peel_payment_onion(&msg2, &&charlie, &&logger, &secp_ctx, cur_height, true)
+		let peeled2 = peel_payment_onion(&msg2, &&charlie, &&logger, &secp_ctx, cur_height, true, false)
 			.map_err(|e| e.msg).unwrap();
 
 		match peeled2.routing {