Payment session and error handling on onion packet forwarding

[chenyukang]

This PR contains these changes:

1. Add payment session and payment status, and get_payment rpc will query the payment status.
2. Add error handling on sending onion packet and TLC forwarding (without onion encryption right now)
3. Some code clean up and refactor

[contrun]

Getting a default fee value here effectively makes a non-public channel forwarding TLCs? Is my assumption correct? If it is, is this kind of behavior desirable?

[chenyukang]

if this node broadcasting a fee rate 100, then a update on fee rate is done, but the latest fee rate haven't been received by the graph before a payment is sent.

now there are two scenarios:

1. the new fee is lower than previous one, but we don't report error here [maybe need to discussed here]
2. the new fee is higher than previous one, we need to report error here.

if there is no configuration for the fee(means the node removed fee rate), I think it's same with scenario #1.

How can we find out a route with this node if this node is non-public? At least our current code will not do this. maybe we should add a check that only public node will forward a TLC right now.

[contrun]

The public_channel_info is set while creating the channel, we deem every channel with this information as public channel https://github.com/contrun/cfn-node/blob/4033caaae18933ed922fa05543b6d686407628c0/src/fiber/channel.rs#L2224 .

[chenyukang]

I resolve this issue with 9fc0a35

[quake]

suggest to extract two helper functions build_route and send_onion_packet to simplifying the main function.

and setting final error to `None`` outside the loop and then assigning it within the loop lead to unnecessary state management and logic complexity.

async fn try_payment_session(
    &self,
    state: &mut NetworkActorState<S>,
    mut payment_session: PaymentSession,
) -> Result<PaymentSession, Error> {
    let payment_data = payment_session.request.clone();
    let payment_hash = payment_data.payment_hash;

    while payment_session.can_retry() {
        payment_session.retried_times += 1;

        let hops_infos = self.build_route(payment_data.clone()).await?;
        let first_channel_outpoint = hops_infos[0]
            .channel_outpoint
            .clone()
            .expect("first hop channel outpoint");

        let tlc_id = self.send_onion_packet(state, hops_infos).await?;

        payment_session.set_status(PaymentSessionStatus::Inflight);
        payment_session.set_first_hop_info(first_channel_outpoint, tlc_id);
        self.store.insert_payment_session(payment_session.clone());

        return Ok(payment_session);
    }

    let error_msg = format!("Exhausted retries for payment: {:?}", payment_hash);
    payment_session.set_status(PaymentSessionStatus::Failed);
    payment_session.set_failed_status(&error_msg);
    self.store.insert_payment_session(payment_session);
    Err(Error::SendPaymentError(error_msg))
}

[chenyukang]

When there is any error happened in build_route or send_onion_packet, instead of just throwing an exception to upper level, we need to set payment session status and store into db.

let me think how to make the code simpler.

[chenyukang]

Will refactor on PR #228, merge now.