Extract send_to_static_invoice util

Makes an upcoming commit cleaner.

In the util we allow manually passing in the first_hops that should be used
when paying a static invoice, which is useful for only passing in next-hop
channels that support the hold_htlcs feature.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -5449,23 +5449,11 @@ where
 	fn send_payment_for_static_invoice(
 		&self, payment_id: PaymentId,
 	) -> Result<(), Bolt12PaymentError> {
-		let best_block_height = self.best_block.read().unwrap().height;
 		let mut res = Ok(());
+		let first_hops = self.list_usable_channels();
 		PersistenceNotifierGuard::optionally_notify(self, || {
-			let outbound_pmts_res = self.pending_outbound_payments.send_payment_for_static_invoice(
-				payment_id,
-				&self.router,
-				self.list_usable_channels(),
-				|| self.compute_inflight_htlcs(),
-				&self.entropy_source,
-				&self.node_signer,
-				&self,
-				&self.secp_ctx,
-				best_block_height,
-				&self.logger,
-				&self.pending_events,
-				|args| self.send_payment_along_path(args),
-			);
+			let outbound_pmts_res =
+				self.send_payment_for_static_invoice_no_persist(payment_id, first_hops);
 			match outbound_pmts_res {
 				Err(Bolt12PaymentError::UnexpectedInvoice)
 				| Err(Bolt12PaymentError::DuplicateInvoice) => {
@@ -5481,6 +5469,27 @@ where
 		res
 	}
 
+	/// Useful if the caller is already triggering a persist of the `ChannelManager`.
+	fn send_payment_for_static_invoice_no_persist(
+		&self, payment_id: PaymentId, first_hops: Vec<ChannelDetails>,
+	) -> Result<(), Bolt12PaymentError> {
+		let best_block_height = self.best_block.read().unwrap().height;
+		self.pending_outbound_payments.send_payment_for_static_invoice(
+			payment_id,
+			&self.router,
+			first_hops,
+			|| self.compute_inflight_htlcs(),
+			&self.entropy_source,
+			&self.node_signer,
+			&self,
+			&self.secp_ctx,
+			best_block_height,
+			&self.logger,
+			&self.pending_events,
+			|args| self.send_payment_along_path(args),
+		)
+	}
+
 	/// If we are holding an HTLC on behalf of an often-offline sender, this method allows us to
 	/// create a path for the sender to use as the reply path when they send the recipient a
 	/// [`HeldHtlcAvailable`] onion message, so the recipient's [`ReleaseHeldHtlc`] response will be