Add total-MPP-value storage in pending payments

In some uses of LDK we need the ability to send HTLCs for only a
portion of some larger MPP payment. This allows payers to make
single payments which spend funds from multiple wallets, which may
be important for ecash wallets holding funds in multiple mints or
graduated wallets which hold funds across a trusted wallet and a
self-custodial wallet.

In the previous commits we moved the total-MPP-value we set in
onions from being manually passed through onion-building to passing
it via `RecipientOnionFields`. This introduced a subtle bug, though
 - payments which are retried will get a fresh
`RecipientOnionFields` built from the data in
`PendingOutboundPayment::Retryable`, losing any custom
total-MPP-value settings and causing retries to fail.

Here we fix this by storing the total-MPP-value directly in
`PendingOutboundPayment::Retryable`.

Author: TheBlueMatt

lightning/src/ln/outbound_payment.rs

@@ -133,6 +133,11 @@ pub(crate) enum PendingOutboundPayment {
 		pending_fee_msat: Option<u64>,
 		/// The total payment amount across all paths, used to verify that a retry is not overpaying.
 		total_msat: u64,
+		/// The total payment amount which is set in the onion.
+		///
+		/// This is generally equal to [`Self::Retryable::total_msat`] but may differ when making
+		/// payments which are sent MPP from different sources.
+		onion_total_msat: u64,
 		/// Our best known block height at the time this payment was initiated.
 		starting_block_height: u32,
 		remaining_max_total_routing_fee_msat: Option<u64>,
@@ -1700,7 +1705,7 @@ impl OutboundPayments {
 					match payment.get() {
 						PendingOutboundPayment::Retryable {
 							total_msat, keysend_preimage, payment_secret, payment_metadata,
-							custom_tlvs, pending_amt_msat, invoice_request, ..
+							custom_tlvs, pending_amt_msat, invoice_request, onion_total_msat, ..
 						} => {
 							const RETRY_OVERFLOW_PERCENTAGE: u64 = 10;
 							let retry_amt_msat = route.get_total_amount();
@@ -1720,7 +1725,7 @@ impl OutboundPayments {
 								payment_secret: *payment_secret,
 								payment_metadata: payment_metadata.clone(),
 								custom_tlvs: custom_tlvs.clone(),
-								total_mpp_amount_msat: *total_msat,
+								total_mpp_amount_msat: *onion_total_msat,
 							};
 							let keysend_preimage = *keysend_preimage;
 							let invoice_request = invoice_request.clone();
@@ -2036,6 +2041,7 @@ impl OutboundPayments {
 			custom_tlvs: recipient_onion.custom_tlvs,
 			starting_block_height: best_block_height,
 			total_msat: route.get_total_amount(),
+			onion_total_msat: recipient_onion.total_mpp_amount_msat,
 			remaining_max_total_routing_fee_msat:
 				route.route_params.as_ref().and_then(|p| p.max_total_routing_fee_msat),
 		};
@@ -2743,6 +2749,7 @@ impl OutboundPayments {
 					pending_amt_msat: path_amt,
 					pending_fee_msat: Some(path_fee),
 					total_msat: path_amt,
+					onion_total_msat: path_amt,
 					starting_block_height: best_block_height,
 					remaining_max_total_routing_fee_msat: None, // only used for retries, and we'll never retry on startup
 				}
@@ -2825,6 +2832,21 @@ impl_writeable_tlv_based_enum_upgradable!(PendingOutboundPayment,
 		(9, custom_tlvs, optional_vec),
 		(10, starting_block_height, required),
 		(11, remaining_max_total_routing_fee_msat, option),
+		(12, onion_total_msat, (custom, u64,
+			// Once we get here, `total_msat` will have been read (or we'll fail to read)
+			|read_val: Option<u64>| Ok(read_val.unwrap_or(total_msat.0.unwrap())),
+			|us: &PendingOutboundPayment| {
+				match us {
+					PendingOutboundPayment::Retryable { total_msat, onion_total_msat, .. } => {
+						if total_msat != onion_total_msat {
+							Some(*onion_total_msat)
+						} else {
+							None
+						}
+					},
+					_ => unreachable!(),
+				}
+			})),
 		(13, invoice_request, option),
 		(15, bolt12_invoice, option),
 		(not_written, retry_strategy, (static_value, None)),